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Organic Photovoltaic Cells and Organic Up-conversion Devices

Permanent Link: http://ufdc.ufl.edu/UFE0024091/00001

Material Information

Title: Organic Photovoltaic Cells and Organic Up-conversion Devices
Physical Description: 1 online resource (173 p.)
Language: english
Creator: Kim, Do
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: infrared, interlayer, light, molybdenum, organic, solar
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic electronic devices such as organic light emitting diodes, organic photovoltaic cells and organic photodetectors are attracting a great deal of attention because of their compatibility with flexible substrates, low manufacturing cost processes, and large area applications. In this work, we study several key factors affecting the power efficiency of organic photovoltaic cells and the use of novel organic up-conversion devices for infrared imaging applications. In the area of organic photovoltaics, the power efficiency has been limited to about 5%. The major limitations to power conversion efficiency are narrow spectral response, low open circuit voltages and small fill factors. Here, we present our results how to increase the open circuit voltage using materials with deep highest occupied molecular orbital (HOMO) energy, extend the photoresponse using infrared absorbing organic semiconductors, and increase the fill factor using molybdenum oxide (MoO3) interlayer. In the area of up-conversion devices, we will present our results on organic thin film photodetectors as well as on integrating organic photodetector with organic light emitting device (OLED) to demonstrate up-conversion for infrared imaging. Aluminum phthalocyanine chloride (AlPcCl) planar and bulk heterojunction cells were fabricated and the results were compared with heterojunction cells fabricated using copper phthalocyanine (CuPc). We demonstrated that in both AlPcCl and CuPc cells, the device performance was enhanced due to the bulk heterojunction effect. Comparing with the CuPc cells, the open-circuit voltages of AlPcCl cells is almost doubled compared with CuPc cells due to the deeper HOMO energy in AlPcCl. Tin phthalocyanine (SnPc) bulk heterojunction cells were fabricated. The absorption of CuPc is limited to wavelength below 800 nm. On the other hand, SnPc can extend the absorption wavelength to about 1000 nm. We demonstrated SnPc:C60 bulk heterojunction cells with photoresponse extended to about 900 nm. However, due to the suppression of SnPc dimer formation, the photoresponse of SnPc:C60 bulk heterojunction cells beyond 900 nm is significantly reduced. The effect of MoO3 interlayer on small molecule and polymer photovoltaic cells was studied. We found that it has a strongest effect on fill factors. In polymer cells, we found that the MoO3 interlayer increase the fill factor by 10-20%. On the other hand, in cells with small molecules such as CuPc, the enhancement in fill factor due to the MoO3 interlayer can be as large as 30%. Our photoelectron spectroscopy results show there is a strong band bending at the organic/interlayer interface and the enhancement of fill factor is due to the strong built-in field at the interface leading to enhancement in carrier extraction. Organic photodetectors were demonstrated using both SnPc and CuPc. With bathocuproine (BCP) and MoO3 as the charge carrier blocking layers, the dark current is significantly reduced and external quantum efficiencies exceeding 90% were obtained. Novel infrared-to-visible up-conversion devices were demonstrated by fabricating an organic light emitting device in series with a photodetector. With SnPc as the infrared absorber and fac-tris(2-phenylpyridinato) iridium (III) (Irpy3) as an emitter, an infrared-to-green up-conversion device with a current efficiency exceeding 105 cd/A was demonstrated under 830 nm irradiation. The maximum photon-to-photon conversion efficiency is 2.7% at 15V. These results are consistent with the fact that the external quantum efficiency of the Irppy3 based green emitting OLED is about 20% and the external quantum efficiency of the infrared photodetector is 10%. The maximum on/off ratio exceeds 1500 at an operating voltage of 12.7 V. The current efficiency of the OLED part of the device exceeds 100 cd/A. The high current efficiency in the OLED is due to photo-injected carriers resulting in enhanced charge balance.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Do Kim.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: So, Franky.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024091:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024091/00001

Material Information

Title: Organic Photovoltaic Cells and Organic Up-conversion Devices
Physical Description: 1 online resource (173 p.)
Language: english
Creator: Kim, Do
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: infrared, interlayer, light, molybdenum, organic, solar
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic electronic devices such as organic light emitting diodes, organic photovoltaic cells and organic photodetectors are attracting a great deal of attention because of their compatibility with flexible substrates, low manufacturing cost processes, and large area applications. In this work, we study several key factors affecting the power efficiency of organic photovoltaic cells and the use of novel organic up-conversion devices for infrared imaging applications. In the area of organic photovoltaics, the power efficiency has been limited to about 5%. The major limitations to power conversion efficiency are narrow spectral response, low open circuit voltages and small fill factors. Here, we present our results how to increase the open circuit voltage using materials with deep highest occupied molecular orbital (HOMO) energy, extend the photoresponse using infrared absorbing organic semiconductors, and increase the fill factor using molybdenum oxide (MoO3) interlayer. In the area of up-conversion devices, we will present our results on organic thin film photodetectors as well as on integrating organic photodetector with organic light emitting device (OLED) to demonstrate up-conversion for infrared imaging. Aluminum phthalocyanine chloride (AlPcCl) planar and bulk heterojunction cells were fabricated and the results were compared with heterojunction cells fabricated using copper phthalocyanine (CuPc). We demonstrated that in both AlPcCl and CuPc cells, the device performance was enhanced due to the bulk heterojunction effect. Comparing with the CuPc cells, the open-circuit voltages of AlPcCl cells is almost doubled compared with CuPc cells due to the deeper HOMO energy in AlPcCl. Tin phthalocyanine (SnPc) bulk heterojunction cells were fabricated. The absorption of CuPc is limited to wavelength below 800 nm. On the other hand, SnPc can extend the absorption wavelength to about 1000 nm. We demonstrated SnPc:C60 bulk heterojunction cells with photoresponse extended to about 900 nm. However, due to the suppression of SnPc dimer formation, the photoresponse of SnPc:C60 bulk heterojunction cells beyond 900 nm is significantly reduced. The effect of MoO3 interlayer on small molecule and polymer photovoltaic cells was studied. We found that it has a strongest effect on fill factors. In polymer cells, we found that the MoO3 interlayer increase the fill factor by 10-20%. On the other hand, in cells with small molecules such as CuPc, the enhancement in fill factor due to the MoO3 interlayer can be as large as 30%. Our photoelectron spectroscopy results show there is a strong band bending at the organic/interlayer interface and the enhancement of fill factor is due to the strong built-in field at the interface leading to enhancement in carrier extraction. Organic photodetectors were demonstrated using both SnPc and CuPc. With bathocuproine (BCP) and MoO3 as the charge carrier blocking layers, the dark current is significantly reduced and external quantum efficiencies exceeding 90% were obtained. Novel infrared-to-visible up-conversion devices were demonstrated by fabricating an organic light emitting device in series with a photodetector. With SnPc as the infrared absorber and fac-tris(2-phenylpyridinato) iridium (III) (Irpy3) as an emitter, an infrared-to-green up-conversion device with a current efficiency exceeding 105 cd/A was demonstrated under 830 nm irradiation. The maximum photon-to-photon conversion efficiency is 2.7% at 15V. These results are consistent with the fact that the external quantum efficiency of the Irppy3 based green emitting OLED is about 20% and the external quantum efficiency of the infrared photodetector is 10%. The maximum on/off ratio exceeds 1500 at an operating voltage of 12.7 V. The current efficiency of the OLED part of the device exceeds 100 cd/A. The high current efficiency in the OLED is due to photo-injected carriers resulting in enhanced charge balance.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Do Kim.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: So, Franky.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024091:00001


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1 ORGANIC PHOTOVOLTAIC CELLS AND ORGANIC UP -CONVERTION DEVICES By DO YOUNG KIM A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009

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2 2009 Do Young Kim

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3 To my wife

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4 ACKNOWLEDGMENTS My sincere thanks go to my advisor, Dr. Franky So for having given me a chance to work in his organic electro nic materials and device group and for supporting me academically and financially throughout my graduate study. I truly appreciate his feedback in research, life, technical writing and for inculcating the qualities of perseverance and creativity in scienti fic research. I would like to thank Dr. Andrew Rinzler for exposing me on having a professional outlook to life, for his research feedback. Thanks go to Dr. David Norton and Dr. Stephen Pearton for their interest in my research and for serving on my disser tation committee. Thanks also go to Dr. Jiangeng Xue. I truly appreciate his guidance and concern in my research I would like to express my great thanks to all my colleagues in Dr. So s group, Dr. Kaushik Roy Choudhury, Dr. Subbiah Jegadesan, Dr. Jiyeon Song Jaewon Lee, Neetu Chopra, Galileo S arasqueta, Jong Hyuk Yoon, Alok Gupta, Cephas Small, Dongwoo Song, Michael Hartel, Mikail Shaikh, Song Chen, Pieter De Somer, Verena Giese, Daniel S. Duncan, I feel fortunate to work with them. They were willing to help me in conducting my experiment s. I would have never finished my research projects without them. I would like to express my special thanks to Korean students in UF for the beautiful times in G ainesville Joo Ro (Dr. Kim ), he help ed me from the first da y of my USA life. I really thanks to all Korean friends who made my best year (2005 2006 ) in UF ( Seok Kim, Seung Young Son, Seung Hun You, Se Yeon Kim, Chan -Woo Lee, Wan Tae Lim, Ji -Won Kang, Jae Won Lee, Dong Hwa Lee, Jae Seok Lee, Ho -Sang Ahn, HyungJun Park, JongHyuk Yoon, DongJo Oh, Jung Hun Jang, JunWoo Son ). And, I hope they will do their best and achieve what they want. I thank to my parent and sister for their forever support and trust. I would also like to thank to my wifes parents and sisters for welcoming me into their family, entrusting their precious daughter to me. I also thank my daughter, Daeun (Lucy) for bringing me the joy of life. Finally, I am entirely

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5 grateful to my beautiful wife, So Youn. It is hard to believe this time has fi nal ly come to an end. Thank you for your love, your patience and your enthusiasm. Your influence can be found on every page of this thesis. If it were not for your constant support and encouragement, I would not be obtaining this degree. You remain the inspir ation for my life and work, and I love you. I should also thank and praise to my Lord, Jesus Christ.

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6 TABLE OF CONTENTS ACKNOWLEDGMENTS .................................................................................................................... 4 page LIST OF TAB LES ................................................................................................................................ 8 LIST OF FIGURES .............................................................................................................................. 9 ABSTRACT ........................................................................................................................................ 13 1 INTRODUCTION ....................................................................................................................... 16 1.1 Organic Electronics ............................................................................................................... 16 1.1.1 Organic Semiconductors ............................................................................................ 16 1.1.2 Advantage s and Disadvantages ................................................................................. 17 1.1.3 Material Purification .................................................................................................. 19 1.1.4 Device Fabrication ..................................................................................................... 20 1.2 Organic Photovoltaic Cells ................................................................................................... 20 1.2.1 Background ................................................................................................................. 20 1.2.2 Photovoltaic Cell Characterization............................................................................ 22 1.2.2.1 Current -Voltage Measurement System .......................................................... 23 1.2.2.2 Spectral Response Measurement System ....................................................... 24 1.2.3 Enhancement of VOC due to Donor Materials with Deep HOMO Energy ............. 27 1.2.4 Infrared Absorbing Organic Photovoltaic Cells ....................................................... 28 1.2.5 Interlayer on Organic Photovoltaic Cells .................................................................. 30 1.3 Organic Photodetectors and Up-conversion Devices ......................................................... 31 1.3. 1 Background ................................................................................................................. 31 1.3.2 Photodetector Characterization ................................................................................. 32 1.3.3 High Gain Organic Photodetectors ........................................................................... 33 1.3.4 Low Operating Voltage Organic Photodetectors ..................................................... 34 1.3.5 Infrared Organic Photodetectors ................................................................................ 35 1.3.6 Light Up-conversion Devices .................................................................................... 36 1.4 Dissertation Organization ..................................................................................................... 37 2 MEASUREMENT SETUP FOR SMALL AREA ORGANIC PHOTOVOLTAIC CELLS ......................................................................................................................................... 50 2.1 Introduction ........................................................................................................................... 50 2.2 Experimental Details ............................................................................................................. 51 2.3 Results and Discussions........................................................................................................ 52 2.4 Conclusions ........................................................................................................................... 55 3 ENHNANCEMENT IN VOC DUE TO A DONOR MATERIAL WITH DEEP HOMO ENERGY ..................................................................................................................................... 62 3.1 Introduction ........................................................................................................................... 62

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7 3.2 Experimental Details ............................................................................................................. 63 3.3 Results and Discussions ........................................................................................................ 65 3.4 Conclusions ........................................................................................................................... 68 4 INFRARED ABSORBING ORGANIC PHOTOVOLTAIC CELLS WITH MOLYBDENUM OXIDE INTERLAYER ............................................................................... 74 4.1 Introduction ........................................................................................................................... 74 4.2 Experimental Detail .............................................................................................................. 76 4.3 Results and Discussion ......................................................................................................... 77 4.4 Conclusion ............................................................................................................................. 80 5 T HE EFFECT OF INTERLAYER ON ORGANIC PHOTOVOLTAIC CELLS .................. 88 5.1 Introduction ........................................................................................................................... 88 5.2 Experimental Details ............................................................................................................. 90 5.3 Results and Discussions ........................................................................................................ 92 5.3.1 The Effect of MoO3 Interlayer on Organic Photovoltaic Cells ............................... 92 5.3.2 The Effect of TFB Interlayer on Polymer Cells ....................................................... 96 5.4 Conclusions ........................................................................................................................... 98 6 ORGANIC PHOTODETECTORS .......................................................................................... 112 6.1 Introduction ......................................................................................................................... 112 6.2 Experimental Details ........................................................................................................... 114 6.3 Results and Discussions ...................................................................................................... 116 6.3.1 High Gain Organic Photodetectors ......................................................................... 116 6.3.2 Low Operating Voltage Organic Photodetector ..................................................... 118 6.3.3 Infrared organic photodetector ................................................................................ 121 6.4 Conclusions ......................................................................................................................... 123 7 L IGHT UP CONVERSION DEVICE ..................................................................................... 136 7.1 Introduction ......................................................................................................................... 136 7.2 Experimental Details ........................................................................................................... 138 7.3 Results and Discussions ...................................................................................................... 139 7.3.1 Redto green Light Up -conv ersion Devices ........................................................... 139 7.3.2 IR -to -green Light Up -conversion Devices ............................................................. 142 7.4 Conclusions ......................................................................................................................... 145 8 CONCLUSION ......................................................................................................................... 161 LIST OF REFERENCES ................................................................................................................. 164 BIOGRAPHICAL SKETCH ........................................................................................................... 171

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8 LIST O F TABLES Table page 1 1. PV characteristics measured under 100 mW/cm2 AM1.5G illumination at 298K, including the power conversion efficiency P), open-circuit voltage (VOC), short c ircuit current density (JSC), and fill factor (FF). ................................................................. 45 1 2 T he characteristics of organic up -conversion devices compared with conventional night vision goggles. .............................................................................................................. 49 3 1. Summary of CuPc and AlPcCl based organic PV cells; (a) PHJ CuPc cell, (b) BHJ CuPc cell, (c) PHJ AlPcCl cell, and (d) BHJ AlPcCl cell (PHJ:planar heterojunction, BHJ: bulk heterojunction) ...................................................................................................... 71 4 1. Summary of the infrared absorbing organic PV cells without and with the MoO3 interlayer ................................................................................................................................. 84 5 1. Summary of infrared absorbing polymer photov oltaic cells without and with the MoO3 interlayer .................................................................................................................... 1 01 5 2 Summary of infrared absorbing small molecule organic photovoltaic cells without and with the MoO3 interlayer .............................................................................................. 103 5 3 Summary of MDMO -PPV/PCBM cell with various interlayers ....................................... 109

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9 LIST OF FIGURE S Figure page 1 1 A ............. 39 1 2 Ir complexes used in the present study and n ormalized phosphorescence spectra. [8] ..... 40 1 3 Absorption spectra of several small molecule organic materials [10] ............................... 41 1 4 The photo image of the purification system used in thermal grad ient sublimation purification. ............................................................................................................................. 42 1 5 The photo image of the evaporator used in fabricating light up -conversion devices. ....... 43 1 6 The photo image of the evaporator used in fabricating organic photovoltaic cells and organic photodetectors. .......................................................................................................... 44 1 7 Typical J -V curves of photovoltaic cells in the dark and under illumin ation. Various device parameters are also shown, including the VOC, the JSC, and the FF. ....................... 46 1 8 Open -circuit voltage (Voc) of different bulk heterojunction solar cells plotted versus the oxi dation potential/HOMO position of the donor polymer used in each individual device. The straight line represents a linear fit with a slope of 1. ....................................... 47 1 9 Typical J -V curves of organic photodet ectors in the dark and under illumination. ........... 48 2 1 ASTM G173 03 Reference spectrum. .................................................................................. 56 2 2 Expected short circuit current den sities corresponding to IPCE spectra. ........................... 57 2 3 Setup for calibration of power density on actual device from solar simulator. .................. 58 2 4 Evaluation of positional beam uniformity using J -V measurement. ................................... 59 2 5 Beam uniformity on JSC, VOC, FF, and P. ........................................................................... 60 2 6 Confirmation of JSC on J -V characteristics using IPCE spectrum. .................................... 61 3 1. Schematic cross -section view of CuPc based reference organic PV cells with (a) planar heterojunction and (b) bulk hete rojunction and AlPcCl based organic PV cells with (c) planar heterojunction and (d) bulk heterojunction ................................................ 69 3 2. (a) P hoto J -V characteristics and (b) IPCE of organic PV cells with structures of Fig. 3 1 (a) PHJ CuPc cell, (b) BHJ CuPc cell, (c) PHJ AlPcCl cell, and (d) BHJ AlPcCl cell (PHJ:planar heterojunction, BHJ: bulk heterojunction) .............................................. 70 3 3 Energy level alignment of (a) th e ITO/ Cu Pc and (b) ITO/AlPcCl .................................... 72

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10 3 4 (a) J -V characteristics, (b) VOC, FF, and (c) JSC, nP of bulk heterojunction AlPcCl cell with high VOC as a function of incident light intensities .................................................... 73 4 1. Schematic cross -section view of the infrared absorbing OPV cells (a) without and (b) with the SnPc:C60 bulk heterojunction layer ....................................................................... 82 4 2 (a) The photo J -V characteristics and (b) the IPCE spectra of the infrared absorbing OPV cells as a function of thicknesses of the SnPc:C60 bulk heterojunction layer .......... 83 4 3 (a) The photo J -V characteristics and (b) the IPCE spectra of the infrared absorbing OPV cells without and with the MoO3 interlayer ............................................................... 85 4 4 (a) JSC, (b) fill factor, (c) VOC, and (d) power c onversion efficiency of the infrared absorbing OPV cells without and with the MoO3 interlayer as a function of thicknesses of the SnPc:C60 bulk heterojunction layer ....................................................... 86 4 5 Surface morpholog y of (a) bare ITO, (b) 10 nm thick MoO3 layer on ITO, (c) light absorbing layer (10 nm CuPc/10 nm SnPc:C60) on ITO, and (d) light absorbing layer (10 nm CuPc/10 nm SnPc:C60) on 10 nm thick MoO3 layer on ITO ................................. 87 5 1 Schematic diagrams of (a) the P3HT:PCBM cell and the MDMO PPV:PCBM cell with the PEDOT:PSS interlayers, (b) the P3HT:PCBM cell and the MDMO PPV:PCBM cell with the MoO3 interlayer, (c) the CuPc:C60 cell with the CuPc interlayers an d the AlPcCl:C60 cell with the AlPcCl interlayers, and (d) the CuPc:C60 cell and the AlPcCl:C60 cell with the MoO3 interlayer. ....................................................... 99 5 2 The photo J -V characteristics of the P3HT:PCBM cells wit h the PEDOT:PSS interlayer (Black line) and with the MoO3 interlayer (Red line). ...................................... 100 5 3 The photo J -V characteristics of (a) the CuPc:C60 cells with the CuPc interlayer (Black line) and wit h the MoO3 interlayer (Red line) and (b) the AlPcCl:C60 cells with the AlPcCl interlayer (Black line) and with the MoO3 interlayer (Green line). ...... 102 5 4 UPS spectra at (a) low cut -off e nergy and (b) high HOMO energy on ITO/AlPcCl interfaces. .............................................................................................................................. 104 5 5 UPS spectra at (a) low cut -off e nergy and (b) high HOMO energy on ITO/MoO3/AlPcCl interfaces. ............................................................................................. 105 5 6 Energy level alignment at (a) the ITO/AlPcCl and (b) ITO/MoO3/AlPcCl interfaces. ... 106 5 7 (a) Schematic diagram showing energy level of electrode, various interface layer s (MoO3, TFB, PEDOT:PSS) and active layer. (b) Molecular structure of the polymer MDMO -PPV and (c) TFB. .................................................................................................. 107 5 8 The photo J -V characteristics of a MDMO -PPV/PCBM based polyme r cell s with various interlayers such as ITO, ITO/PEDOT:PSS, ITO/MoO3 and ITO/MoO3/TFB. ................................................................................................................... 108

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11 5 9 The IPCE spectra of the MDMO -PPV:PCBM device with MoO3 and MoO3+TFB interlayers ............................................................................................................................ 110 5 10. (a) Surface morphology of PEDOT PSS inter layer and (b) MoO3/TFB inter layer. ........ 111 6 1 Schematic diagram of high gain organic photodetector. ................................................... 125 6 2 Gain -voltage characteristics of high gain organic photodetector. ..................................... 126 6 3. Dark and photo -I -V characteris tics of high gain organic photodetector ......................... 127 6 4. Proposed band structure for PTCDA high gain photodetector on (a) Dark and (b) Photo. .................................................................................................................................... 128 6 5. (a) External quantum efficiency of organic photodetectors with planar heterojunction structure (dotted line) and bulk heterojunction structure (solid line) as a function of applied voltages, (b) current density-voltage characteristics on dark (solid line) and photo (dash line) situation as a function of applied voltages on organic photodetectors with planar heterojunction structure (black line) and bulk heterojunction structure (red line). ...................................................................................... 129 6 6. (a) Current density voltage characteristics of the bulk heterojunction organic photodetector as a function of applied voltages under various input light intensities; dark (black), 0.23 mW/cm2 (red), 1.70 mW/cm2 (green), and 13.75 mW /cm2 (yellow) and (b) external quantum efficiency of the bulk heterojunction organic photodetector as a function of applied voltages under various input light intensities; (a) 0.23 mW/cm2 (red), 1.70 mW/cm2 (green), and 13.75 mW/cm2 (yellow). .............................. 130 6 7. Schematic diagram of device operation; (a) low operating voltage and no light irradiation, (b) low operating voltage and light irradiation, (c) high operating voltage and no light irradiation, an d (d) high operating voltage and light irradiation. ................. 131 6 8. Schematic diagram of near IR photodetector (a) without any interlayers, (b) with MoO3 interlayer, (c) with BCP interlayer, and (d) with both MoO3 and BCP interlayers. ............................................................................................................................ 132 6 9. Dark current density-voltage characteristics on IR photodetector without any interlayers (Black line), with MoO3 (Red line), with BCP (Green line ), and with MoO3 and BCP (Blue line). ................................................................................................. 133 6 10. (a) Spectral responsivities and (b) External quantum efficiencies of infrared organic photodetector as a function of wavelength under different applied voltages; 0 V (Black), 1 V (Red), 3 V (Green), and 5 V (Blue) and optical absorbance of 100 nm thick SnPc film (Dotted line) and 100 nm thick SnPc:C60 mixed film (Dashed line). .... 134 6 11. (a) Responsivity and external quantum efficiency as a function of applied voltages at light irradiation with different wavelength; 740 nm (Black), 830 nm (Red), and 900 nm (Green). ........................................................................................................................... 135

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12 7 1. Schematic diagram of light up conversion device. ............................................................ 146 7 2. Absorbance of SnPc thin film with infrared sensitivity. .................................................... 147 7 3. (a) Schematic band diagram, (b) quantum efficiency, and (c) J -V characteristics of red organic photodetector. ................................................................................................... 148 7 4. (a) Schematic band diagram, (b) current efficiency, and (c) L -I -V ch aracteristics of inverted top emitting OLED. ............................................................................................... 149 7 5. (a) Schematic band diagram and (b) L I -V characteristics of red -to -green light up conversion device. ................................................................................................................ 150 7 6. Schematic band diagram of red -to -green light up -conversion device under dark and photo (630 nm laser) irradiation. ......................................................................................... 151 7 7. (a) Current efficiency and (b ) photon -to -photon conversion efficiency of redto -green light up -conversion device. .................................................................................................. 152 7 8. Calculation of photonto photon conversion efficiency for light up-conversion devices. .................................................................................................................................. 153 7 11. Schematic band diagram, quantum efficiency, and J -V characteristics of infrared organic photodetector. .......................................................................................................... 155 7 12. L -I -V charact eristics of infrared to green light up conversion device under dark and photo (830 nm infrared) irradiation. ................................................................................... 156 7 13. (a) Current efficiency and (b) photon -to -photon conversion efficiency of IR -to -green light up -conversion device. .................................................................................................. 158 7 14. On/off ratio as a function of current densities on infrared to green light up conversion device. ................................................................................................................................... 159 7 15. The images (a) without and (b) with 830 nm infrared irradiation in infrared to green light up conversion device under 10 V. .............................................................................. 160

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13 Abstract of Dissertation Presented to the Graduate School of t he University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ORGANIC PHTOVOLTAIC CELLS AND ORGANIC PHOTODETECTORS By Do Young Kim May 2009 Chair: Franky So Major: Materials Science and Engine ering Organic electronic devices such as organic light emitting diodes, organic photovoltaic cells and organic photodetectors are attracting a great deal of attention because of their compatibility with flexible substrates, low manufacturing cost process es, and large area applications. In this work, we study several key factors affecting the power efficiency of organic photovoltaic cells and the use of novel organic up-conversion devices for infrared imaging applications. In the area of organic photovolt aics, the power efficiency has been limited to about 5%. The major limitations to power conversion efficiency are narrow spectral response, low open circuit voltages and small fill factors. Here, we present our results how to increase the open circuit voltage using materials with deep highest occupied molecular orbital (HOMO) energy, extend the photoresponse using infrared absorbing organic semiconductors, and increase the fill factor using molybdenum oxide (MoO3) interlayer. In the area of up -conversion de vices, we will present our results on organic thin film photodetectors as well as on integrating organic photodetector with organic light emitting device (OLED) to demonstrate up -conversion for infrared imaging. Aluminum phthalocyanine chloride (AlPcCl) pl anar and bulk heterojunction cells were fabricated and the results were compared with heterojunction cells fabricated using copper

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14 phthalocyanine (CuPc). We demonstrated that in both AlPcCl and CuPc cells, the device performance was enhanced due to the bul k heterojunction effect. Comparing with the CuPc cells, the open-circuit voltages of AlPcCl cells is almost doubled compared with CuPc cells due to the deeper HOMO energy in AlPcCl. Tin phthalocyanine (SnPc) bulk heterojunction cells were fabricated. The a bsorption of CuPc is limited to wavelength below 800 nm. On the other hand, SnPc can extend the absorption wavelength to about 1000 nm. We demonstrated SnPc:C60 bulk heterojunction cells with photoresponse extended to about 900 nm. However, due to the suppression of SnPc dimer formation, the photoresponse of SnPc:C60 bulk heterojunction cells beyond 900 nm is significantly reduced. The effect of MoO3 interlayer on small molecule and polymer photovoltaic cells was studied. We found that it has a strongest e ffect on fill factors. In polymer cells, we found that the MoO3 interlayer increase the fill factor by 10 20%. On the other hand, in cells with small molecules such as CuPc, the enhancement in fill factor due to the MoO3 interlayer can be as large as 30%. Our photoelectron spectroscopy results show there is a strong band bending at the organic/interl a yer interface and the enhancement of fill factor is due to the strong buil t in field at the interface leading to enhancement in carrier extraction. Organic pho todetectors were demonstrated using both SnPc and CuPc. With bathocuproine (BCP) and MoO3 as the charge carrier blocking layers, the dark current is significantly reduced and external quantum efficiencies exceeding 90% were obtained. Novel infrared -to -visi ble up -conversion devices were demonstrated by fabricating an organic light emitting device in series with a photodetector. With SnPc as the infrared absorber and fac tris(2 -phenylpyridinato) iridium (III) (Irpy3) as an emitter, an infrared to -green up-con version

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15 device with a current efficiency exceeding 105 cd/A was demonstrated under 830 nm irradiation. The maximum photon -to -photon conversion efficiency is 2.7% at 15V. These results are consistent with the fact that the external quantum efficiency of the Irppy3 based green emitting OLED is about 20% and the external quantum efficiency of the infrared photodetector is 1 0%. The maximum on/off ratio exceeds 1500 at an operating voltage of 12.7 V. The current efficiency of the OLED part of the device exceeds 100 cd/A. The high current efficiency in the OLED is due to photo -injected carriers resulting in enhanced charge balance.

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16 CHAPTER 1 INTRODUCTION The purpose of this introductory chapter is to provide the reader s an introduction to the field of organic s emi conducting materials and their characteristics Also, we present an overview of the field of organic photovoltaic cells and organic photodetectors highlighting the challenges to increasing device efficiency. This introduction is intended to present an overview of the concepts which are important to the understanding of the principles and devices presented in subsequent chapters. 1.1 Organic Electronics 1 1 .1 Organic S emiconductor s Most organic materials are carbon -based compounds, with other atoms such as hydrogen, nitrogen, and oxygen. O rganic compounds contains carbon atoms form ing single, double triple bonds, rings, and chains. O rganic materials used in organic electronic devices are mostly conjugated molecules. The -electrons of carbon in these co njugated molecules are delocalized within the molecule, and the overlap -electrons between neighboring molecules determines the transport properties of molecular solids [1] While organic solids are often electrically insulating, conj ugated polymers and small molecule organic materials are semiconductors The overlapping -orbitals of double bonded carbon atoms provide a continuous path for electron transport along the molecular backbone.[1] This overlapping of orbitals creates a degen eracy which leads to the formation of fi lled and unfi lled bands, with the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of particular signi fi cance as shown in Figure 1 1. Here, the and orbitals are iden tifi ed as the HOMO and LUMO levels respectively. In the case of a large number of electrons, the and levels broaden into continuous bands, with the HOMO / LUMO energy gap analogous to the

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17 valance / conduction band gap of inorganic semiconductor materials .[ 2 ] These fi lled and unfi lled bands result in organic materials behaving as semiconductors. On the other hand, organic materials that are actively inves tigated for organic electronics can be classified into two different categories: small molecular weight materials and polymers. Small molecular weight materials generally refer to molecules with several to a few hundred atoms. A n organic small molecular compound has a well -defined molecular weight with specific chemical composition and structure. H owever, p olymers are comprised of a long chain of monomer repeating units thus showing well defined properties and very large molecular weight s In addition organic semiconductor s consist of discrete molecules held together by weak van der Waals forces. For thi s reason, organic semiconductors are generally soft unlike hard and robust inorganic semiconductors such as Si and Ge due to the strong covalent intramolecular bonding Differences between strong covalent intramolecular forces and weak van der Waals inter molecular forces make the optical and electrical properties of organic semiconductor s differ from those of inorganic semiconductors T hus, the optical and electronic properties of organic semiconductor s can be analyzed in terms of the properties of the monomer modified by effects due to intermolecular interactions. 1 1 2 Advantages and D isadvantages Organic electronic materials have a number of advantages compared with inorganic electronic materials Most organic materials are inexpensive and compatible wi th low -cost and large area manufacturing processes. S mall molecular organic thin films are usually deposited by simple v acuum thermal evaporation [3 ] and p olymer thin films are forme d by solution processes such as spin -coating and ink jet printing. [4 7 ] Th ese processes require significantly lower processing temperature compared with thin film fabrication processes such as plasma enhanced chemical vapor deposition (PECVD) and a sputtering for most inorganic electronic materials.

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18 Some of the polymer deposition techniques such as slit -coating and flexo printing are compatible with large area, roll to roll processing, leading to very high throughput manufacturing for organic electronic devices. Hence various low -cost substrates such as plastic can be used for o rganic devices. On the other hand, a nother advantage of the organic electronic materials is the ability to engineer their electronic and optical properties by chemistry to suit a special application. For instance as shown Figure 1 2 the emission spectrum of the cyclometalated platinum complex for phosphorescent organic light -emitting devices can be tuned throughout the visible spectrum by simply changing the functional cyclometalating ligand conjugation. [8 9 ] In addition t he absorption coefficient of t hese organic materials can be very high i n the visible spectral region as shown in Figure 1 3 [10] Therefore they are well suited for making high efficiency photovoltaic cells and photodetectors. However, in spite of the above advantages, organic electro nic materials have some disadvantage s for use in electronic and optoelectronic devices. As a result of the weak intermolecular interactions, organic electronic materials generally have much lower carrier mobilities than inorganic electronic materials. In a ddition, due to the low carrier concentrations in organic semiconductors, t he electrical conductivity of organic semiconductor s is low and the corresponding devices have high series resistance thus limiting the device performance. The purification of or ganic materials is also problematic, as no available purification method can achieve organic materials with purities reaching that of silicon. A high density of traps, due to impuriti es or structural defects, exists in most organic electronic materials. Fi nally, most organic materials are susceptible to degradation when exposed to water vapor, oxygen, and other contaminants

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19 1 1 3 Material P urification The purity of organic materials for electronic or optoelectronic devices is an important factor in determi ning the electrical and opt ical properties of organic thin film s, and th us the performance of electronic and optoelectronic devices based upon organic thin films [11] The purity of precursor organic material s that can be comm ercially available rarely have purity levels greater than 99.9%. V arious methods for purif ying small molecul e organic materials such as chromatography, zone refining, and sublimation have been extensively reviewed in the literature [1 2 1 4 ] Th e purification method of precursor organic materials used in this dissertation was thermal gradient sublimation [3 ] The experimental setup used for thermal gradient sublimation is shown in Figure 1 4 The source material is loaded into the high temperature end of a quartz tube and the other end is connected to a turbomolecular pump for prevent ing further contamination through oil backstreaming. In the quartz tube as vacuum chamber another small glass tube is placed somewhere in the middle of the furnace to collect the sublimed and purified materia l. Glass wool is placed at the open and cold ends for prevent ing volatile impurities from entering the pump. The temperature at the source material is raised above the sublimation point of the source material, while the pressure in the quartz tube is maint ained at below 1 10 6 Torr. The temperature gradient that is established within the quartz tube allows volatile impurities to be transported to the cold end of the purification tube while less volatile impurities remain in the hottest end of the tube. This process typically takes several days to complete, and repeated runs of this procedure results in an increas ed purity of the organic material. The organic material s used in this dissertation are purified in three times

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20 1 1 4 Device F abrication The glass substrates with pre -patterned indium tin oxide (ITO) are used for organic electronic dev ices Prior to deposition of the organic layers, the glass substrates are cleaned via wet solvent processes, described in the experimental sections of this dissertation followed by expos ure to an ultraviolet/O3 cleaner ( Figure 1 5 ). For coating polyme r thin films spin -coating was used and the polymer solution is dispensed onto a glass substrate which wa s then spun at high speed to obtain uniform films on the substrate s T he polymer sampl e s were baked to remove any remaining solvent. For depositing thin films of small molecular weight materials, there are also a number of options available: organic molecular beam deposition (OMBD) [ 3 ], vacuum thermal evaporation (VTE) [ 3 ], organic vapor phase deposition (OVPD) [15 ], and organic vapor jet printing (OVJP) [1 6 ]. The devices used in this dissertation were all grown using VTE (Figure 1 6 and 1 7 ), which involves the placement of purified organic material in a baffled Ta or W boat which is locate d between electrodes in a vacuum chamber with a base pressure below 10 6 Torr. When current is passed through the boat, the temperature is increased beyond the sublimation temperature of the material, the material is evaporated, and deposited everywhere on the chamber walls as well as on the target substrate s A quartz cryst al sensor is used to monitor the growth rate (typically 0.5 2 /s) and thickness of the thin film s A shutter is placed below the substrate to control the deposition process Unlike spin -coating, the VTE process is solvent -free, and thus provides the abi lity of high degree of layer thickness and composition control 1.2 Organic P hotovoltaic C ells 1 2 .1 Background Photovoltaic cells are considered as an important source of renewable energy to solve the worlds energy shortage today. Table 1 1 provides a co mparison of the the state of the art

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21 photovoltaic cells fabricated using a variety of materials. T he interested reader is pointed to other, more comprehensiv e reviews of inorganic photovoltaic cells [1 7 1 9 ]. From the table, it is apparent that the power c onversion efficiencies of inorganic solar cells are substantially higher than that of organic solar cells. O n the other hand, organic photovoltaic cells are attractive for the next generation photovoltaics because of their compatibility with flexible subst rates, low manufacturing costs and large area applications. Unlike inorganic solar cells such as Si and CIGS solar cells, organic solar cells are characterized by strongly bound electron hole pairs (excitons) that are formed upon excitation with light. [2 6 ] Typical exciton binding energy in organic semiconductors is the range of 0.2 eV to 1 eV. The s trongly bound excitons are insufficient to affect direct electron hole dissociation Thus exciton dissoci ation occurs almost at the interface between two mater ials with different electron affinities such as the electron donor and the electron acceptor To collect effective photo -generated charge carriers an appropriate donor / acceptor materials and device architecture must be selected. Since Tang et. al reported the first bilayer organic photovoltaic cell at more than 20 years ago[2 7 ], organic photovoltaic cells have been improved gradual ly, leading to power conversion efficiencies of more than 5 %.[ 2 8 3 3 ] On the other hand, t wo main approaches of small molecule o rganic materials and polymers have been explored for achieving high efficiency devices T he small molecule organic solar cell is commonly fabricated by vacuum deposition of molecular materials [3 3 38] Polymer solar cell s are processed in solution via spin coating and printing. In conventional planar heterojunction cells or bilayer cells, exciton dissociation occurs at the donor acceptor interface. T hus, if the exciton diffusion length (LD) of donor or acceptor is shorter than the thickness of the donor or acceptor layer most excitons generated will recombine before diffusing to the donor acceptor interface [ 3 9 ]. The

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22 biggest advance made in organic photovoltaic cells in the last 10 years is the development of bulk heterojunction. In the so -called bulk h eterojunction structure s have a substantially large r interfacial area compared with the planar heterojunction cells, thus creating an interpenetrating network with a spatially distributed interface such that excitons generated lie within LD of the photo -g enerated excitons [3 9 ]. T herefore, the bulk heterojunction structure increases the e xciton dissociation rate ( ED) leading to an increase in short -circuit current [3 9 ]. A lso, t he real advantage of these bulk heterojunction structure s in polymer cells whic h can be processed in solution, is the ability to process the composite active layer from solution in a single step, by using a variety of techniques that range from inkjet printing to spin coating and flexo -printing. The state -of the art in the field of o rganic photovoltaic cell s are all made with bulk heterojunction structure s in both small molecule organic and polymer solar cells with reproducible efficiencies approaching 5%.[ 2 8 2 9 ] To attain efficiencies approaching 10% in such organic solar cells, m uch effort is required to extend the absorption wavelengths to the infrared region of organic materials, identify the improved donor/acceptor system for increasing open circuit voltages, and understand the interface energetics at the ITO or metal electrode interfaces an. In Chapters 2 5 of this dissertation we will show various organic photo voltaic cells including infrared sensitive organic photovoltaic cells, high open circuit cells, and organic photovoltaic cells with MoO3 interlayer. 1 2 2 Photovoltaic C ell C haracterization P hotovoltaic cells are simply photo diodes which operate at zero bias T hat means that the t ypical current density-voltage (J -V ) characteristics show rectifying characteristic s [40] Typical J -V characteristics in the dark and under i ncident illumination are s hown in Figure 1 7 T he

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23 power conversion e fficiency is the most important parameter in solar cells and is calculated by the following formula (1 1) VOC is the open circuit voltage, typically measured in V, JSC is the short circuit current density in m A/ c m2, FF is the fill factor and Pinc is the incident solar power intensity in m W/ c m2. Also, Figure 1 7 shows the JSC, the VOC, and the FF of a device under illumination. The fill factor is calculated by the max imum converted power density divided by the JSC and the VOC as shown in the following equation. FF = JmaxVmax/ JSCVOC (1 2 ) In equation (1 2), the product JmaxVmax corresponds to the maximum converted power density in the solar cell For a solar cell, the standard testing conditions are 100 mW/cm2 under air mass 1.5 global illumination (1 sun, AM1.5G ) at 298 K. 1 2 2.1 Current V oltage M easurement S ystem The construction of our I -V measurement system and measurement procedure is based on t he standard test method for photovoltaic cells [references] A xenon lamp, which is classified as a class A solar simulator, with 1.5G air mass filter was used as a solar simulator in the experimental setup. A Newport 70260 radiant power meter, combined wi th the 70268 probe was used to measure the power level of the white light illumination. Using neutral density filters the illumination levels were varied between 30 mW/cm2 and 120 mW/cm2. In addition, a calibrated CIGS solar cell was used as a reference c ell and the global reference spectrum provided by NREL to used to set the illumination level of the solar simulator to provide a first order correction for the spectral mismatch in the photo I -V measurement system. The solar simulator intensity was adjuste d by changing the power of the solar simulator so that the measured short inc SC OC PP FF J V

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24 circuit current of the reference cell is equal to its calibrated value at the standard measurement intensity of 100 mW/cm2 while the distance between the solar simulator and the test plane was adjusted For photo I -V measurements, the illumination intensity of the solar simulator was first set by the power meter and the reference cell depicted above. The photoI -V curve was then swept from the reverse bias to the forward bias using the voltage mode in a step of 10 mV using a Keithley semiconductor analyzer. The basic parameters such as the VOC, JSC, F.F., and power conversion efficiency P of test cells can be calculated from the measured photoI -V curves. 1 2 2.2 Spectral R esponse M easurement S ystem The incident photon-to -current collection efficiency (IPCE) spectra l measurement which is defined as the ratio of the collect ed charge carriers per incident photon, of the solar cell can be calculated from the measured absolute spectra l response curve. A computer -controlled spectral response measurement system to measure the spectral response was designed and constructed and the IPCE data were computed from the photoresponse data A computer program based on LabVIEW was used to control the measurement procedure and data acquisition. Two types of measurement systems (the filter wheel and grating monochromator systems) are used to measure the spectral response of the solar cells. The grating monochromator system has an advantage of flexib ility to select wavelength, but has disadvantages of low light intensity, poor beam uniformity, and small beam size. While the filter wheel system has advantages of higher light intensity, better beam uniformity, and larger beam size, but has disadvantage s of limited and fixed wavelengths in spectral response measurements. With the small area of the organic solar cells, we have constructed a spectral response measurement system using a grating monochromator to analyze the spectral response and the IPCE dat a of our solar cells. The

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25 measurement system scanning the spectral range from 4 00 nm to 1000 nm with an increment step of 10 nm has the capability of applying voltage bias to the test cell. A 150 W ozone free xenon arc lamp combined with 1.5G air mass fi lter, which produces the characteristic Class A spectra, was coupled with the monochromator as a light source to produce a monochromatic light in the wavelength range from 200 nm to 2800 nm. A resulting beam size on the test plane is large enough to cover the test cell and photodetectors. The monochromator holds two gratings simultaneously. Grating 1 is operated in the primary wavelength region from 200 nm to 1400 nm. Grating 2 is operated in the primary wavelength region from 900 nm to 2800 nm. The location and selection of the optical components (lens, mirror, and optical diffuser) are arranged in such a way that the entire area of the cell on the test plane is covered with a uniform and adequate illumination -level monochromatic light. The entrance slit width of the monochromator was opened to its close to maximum (3 mm) to increase the throughput of light intensity. The exit slit width of the monochromator was opened to 2 mm. Applying a high -transmission (>85%) optical diffuser over the test cell makes the monochromatic light more uniform. Two order -sorting filters were used to block the undesired harmonic terms from the monochromator. One with the cut on wavelength of about 610 nm and the other with the cut -on wavelength of about 830 nm were u sed for the ranges of wavelength from 630 nm to 1000 nm and from 1000 nm to 1400 nm, respectively. A lock in amplifier in conjunction with an optical chopper was to measure the spectral response. Errors can occur for the inadequate use of chopped light method when the test cell and reference detector are of different size and/or shape. These errors can be minimized by locating the chopper blade in the narrowest location of the monochromatic beam pathway. Therefore, we put the chopper right next to the exit of the monoch romator in the measuremen t system to reduce

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26 the errors. The chopper is connected to a lock in amplifier that receives the electrical signal from the solar cell after being passed through a current to voltage amplifier. A chopping frequency of 400 Hz is use d in the measurement. An NIST calibrated silicon -UV enhanced and germanium photodetectors together with a lock -in amplifier were used to measure the incident power density of the frequency-chopped monochromatic light beam on the test plane. A c urrent ampli fier was used to amplify current signal from the photodetectors or the tested cell, which was then fed into the lock in amplifier. The monochromator, which is controlled by LabVIEW scans the spectral range from 4 00 nm to 1000 nm with 10 nm as an increment al step. The incident power density on the test plane was also measured by the photodetectors. The spectral response measurement was operated at the short -circuit mode. The photocurrent Itest cell( ) of the test cell is converted into photovoltage and is m easured by using a lock in amplifier. The spectral response is calculated from the measured photocurrent of the photodetectors and the measured photocurrent of the test cell using the following equation: ST( ) = Jtest cell( )/Pinc( ) (1 3) w here Jtest cell( ) and Pinc( ) are the photocurrent of the test cell and incident power density measured by dividing the measured photocurrent of the detector by responsivity of the detector. The IPCE as a function of wavelength can be converted from the s pectral response using the following expression: (1 4) % 100 ) ( inc P q ) ( J c h % 100 ) ( S q c h ) ( IPCEcell test T

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27 where h, c, q, and are the Plank constant, speed of light, electronic charge, and the photon wavelength, respectively. On the other hand, we can calculate the JSC from the IPCE spectra us ing the following equation : JSC = Ref( )ST( )d (1 5 ) where ERef ( ) is the reference spectral irradiance and ST ( ) is the spectral responsivity of the test cell. This equation yield s the collected current density in the wavelength region of interest. The reference spectral irradiance used in the calculation is American Society for Testing and Materials ( ASTM ) Standard G173 which is a recent improvement to the spectrum of ASTM Standard G159. The JSC obtained from the calculation using the spectral responsivity must be e qual to JSC measured under the solar simulator. 1 2 3 Enhancement of VOC due to D onor M aterials with D eep HOMO E nergy Photovoltaic cells are considered as an important source of renewable energy to solve the worlds energy shortage today. Organic photovolt aic (OPV) cells are attractive for the next generation photovoltaics because of their compatibility with flexible substrates, low manufacturing costs and large area applications [ 27 30]. While there has been improvement in power conversion efficiencies of both small molecules and polymer OPV cells [ 4 1 42], the power conversion efficiency ( P) in OPV cells has been limited to about 5%. One of the reasons limiting the power conversion efficiency of OPV cells is the small open circuit voltage (VOC). The typic al VOC value of conventional CuPc based small molecule OPV cells is less than 0.45 V [ 33]. The origin of VOC in OPV cells is not fully understood. Most reports support that the VOC value depends on the energy difference between the LUMO energy of the accep tor and the HOMO energy of the donor [ 434 5 ]. Brabec group reported that there is a linear relationship

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28 between the donor HOMO energy and the open-circuit voltage of polymer PVs as shown in Figure 1 8 [113] Additionally, the VOC appears to depend on the w ork functions of the electrodes. Therefore, i t is desirable to have a low work function cathode and large work function anode to optimize the VOC [4 3 4 6 ]. A very large VOC of 0.87 V has been reported with polymer OPV cells using poly[2-meth -oxy 5 (3,7 -dim ethyloctyloxy)] 1,4 -phenylenenvinylene (MDMO PPV) as a donor [ 4 7 ]. Recently, a very large VOC of 0.98 V has been reported with small molecule OPV cells using boron subphthalocyanine chloride (SubPc) as a donor [ 4 8 ]. Previously, Forrest group reported that large VOC from planar heterojunction cells using aluminum phthalocyanine chloride (AlPcCl). [ 94] In this dissertation we also report bulk heterojunction PV cells using AlPcCl with high VOC using AlPcCl as a donor. Our recent ultraviolet photoemission spe ctroscopy (UPS) measurements show that AlPcCl has a HOMO energy of 5.3 eV which is about 0.5 eV higher than that of CuPc, suggesting that OPV cells using AlPcCl instead of CuPc as a donor should give a higher VOC. We will discuss the high VOC OPV cells in these high gain organic photodetectors in more details in Chapter 3 of this dissertation 1 2 4 I nfrared A bsorbing O rganic P hotovoltaic C ells In addition to the low open -circuit voltage, another reason for the low power conversion efficiency is the limitat ion of usable wavelength in the solar spectrum. Most organic materials have strong absorption in the visible spectrum and their absorption in the infrared region (> 700 nm) is rather limited. Since the photon flux in the visible region only accounts for ab out 25% of the entire solar spectrum it is important to extend the usable wavelength to the infrared region in order to maximize the power conversion efficiency of organic photovoltaic cells [ 59]. For example c opper phthalocyanine ( CuPc ), a donor materia l used in small molecule organic photovoltaic cells, has a band gap of 1. 7 eV (730 nm ) and thus even if CuPc cell has a high

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29 incident photon -to -current collection efficiency (IPCE) value its power conversion efficiency is still limited. Extending the usab le wavelength of organic materials to beyond 1000 nm will substantially increase the power conversion efficiency [59]. Therefore low band gap organic materials will improve the efficiency of organic photovoltaic cell s [ 59 6 1 ]. In addition, the realization of near infrared absorbing organic photovoltaic cells also enables the fabrication of translucent photovoltaic s which have recently been demonstrated [ 62]. Near infrared sensitive small molecule organic photovoltaic cells with planar heterojunction struct ure using tin (II) phthalocyanine (SnPc) have previously been reported [ 6365]. While enhanced exciton dissociation has been reported in both small molecule and polymer bulk heterojunction photovoltaic cells [66], bulk heterojunction organic photovoltaic c ells using SnPc as a donor have not been reported. In this dissertation we fabricated near infrared absorbing small molecule organic photovoltaic cells using SnPc:C60 bulk heterojunction structure. In addition to the bulk heterojunction effects, we have a lso studied the effect of the interlayer on the SnPc bulk heterojunction cells. Recently, it has been demonstrated that the interlayer between the indium tin oxide (ITO) anode and the light absorbing layer could play an important role determining the efficiency of o rganic photovoltaic cells [67]. Shrotriya et al. demonstrated enhanced cell performance us ing vanadium oxide (V2O5) and molybdenum oxide (MoO3) as the interlayer for polymer photovoltaic cells [ 68]. Similar result has also been demonstrated by Ir win et al. using a p type nickel oxide (NiO) interlayer [ 69 ]. Here, t o enhance the device performance of near infrared absorbing organic photovoltaic cells used in this study, we incorporated a MoO3 interlayer between the ITO anode and the light absorbing layers and found that the interlayer increases both the short -circuit current ( JSC) and the open circuit voltage

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30 (VOC) resulting in a 4 5 % enhancement in power conversion efficiency. We will discuss the near infrared absorbing organic photovoltaic cells in more details in Chapter 4 of this dissertation 1 2 5 I nterlayer on O rganic P hotovoltaic C ells Another key factor affecting the PV cell power conversion efficiency is the low fill factor. Many OPV, specifically, small molecule PVs, the fill factor is ofte n less than 0.5. The low fill factor is partially due to low carrier mobility in organic semiconductors. Recently, it has been demonstrated that the fill factor in polymer PV cells can be enhanced by inserting an interlayer between an indium tin oxide (ITO ) anode and the light absorbing layer [67]. In polymer photovoltaic cells, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interlayer has been demonstrated to be effective to enhance both the short -circuit current (JSC) and the open-c ircuit voltage (VOC) [70]. Brabec et al. demonstrated that there is a correlation between the anode work function and the VOC of organic photovoltaic cells and that the enhancement due to the PEDOT:PSS interlayer is attributed to its large work function co mpared with the ITO workfunction [46]. While PEDOT:PSS is a commonly used interlayer material for organic solar cells its acidity can cause chemical instability at the ITO interface which degrad e s the device performance [71, 72] In organic light emittin g diode s (OLED s ), interfacial engineering of the ITO surface has been used to enhanc e hole injection. In polymer OLED s, the insertion of a poly(9,9dioctylfluorene co N -(4 -butylphenyl) diphenylamine) (TFB) interlayer between a PEDOT:PSS and the light emitt ing polymer layer improve s both device efficiency and stability [73]. The enhancement due to the TFB interlayer was attributed to enhanc ement of hole injection and improvement of charge balance [74 76]. Recently, Wang et al. reported small molecule OLED s w ith improved hole injection and stability using a molybdenum oxide (MoO3) inter layer between the ITO anode and the hole transporting layer [77].

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31 For organic photovoltaic cells transition metal oxide interlayer has also been used [68, 69] Shrotriya et al. demonstrated enhanced device performance using both vanadium oxide (V2O5) and MoO3 interlayer s for polymer photovoltaic cells [68] More recently, Irwin et al. reported enhanced cell performance using a p type nickel oxide interlayer [69] In this dissert ation we study the effect of MoO3 interlayer on both small molecule and polymer organic photovoltaic cells, and correlate the device results with the interface electronic structure measured by ultraviolet photoelectron spectroscopy (UPS). We will discuss the effect of MoO3 interlayer on OPV cells in more details in Chapter 5 of this dissertation In addition to the MoO3 interlayer effects, we have also studied the effect of the polymer interlayer on the polymer c ells. Recently, Hains et. al showed that the performance of polymer cells can be enhanced with a cross linked blend of TFB and 4,4 -bis[( p trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi) interlayer. The enhanced performance was attributed to improved hole extraction as well as the electron blocking function of the interlayer [67]. In this dissertation we demonstrate that thin double layers of MoO3 and TFB as an anode interlayer substantially enhances the photovoltaic performance of polymer cells through efficient hole extraction. 1.3 Organi c P hotodetectors and Up -conversion Devices 1 3 1 Background O rganic photodetectors have the advantages of being compatible with flexible substrates, and can be fabricated in large areas with relatively low cost. Both small molecules [3 6 7 8 ] and conjugated polymers [79, 80] have been used as the active materials in organic photodetectors. In particular, o rganic photodetector s with the high external quantum efficiency (EQE) of over 70% at 10 V have already been reported by a multi layer method of copper phtha locyanine (CuPc) and 3,4,9,10perylenetetracarboxylic bisbenzimidazole (PTCBI) as donor and acceptor respectively

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32 [78] T he light absorbing layer with a total thickness of 320 consisted of a multilayer stack of alternating donor (CuPc) and acceptor (PTC BI) small molecular layers which was varied between 160 and 5 Thus, p hoto generated excitons efficiently dissociate into free electrons and holes by increased do nor/acceptor interfaces, while the multilayer stack of alternating donor and acceptor lay ers has a disadvantage to deteriorate charge transport property. [81] Whole molecules of donor or acceptor within the multilayer stack are not connected continuously, thus blocking the charge transport as well as interfering with the charge transport due t o opponent acceptor or donor which makes the energy barrier s. Therefore organic photodetector s using multilayers of CuPc and PTCBI showed low external quantum efficiencie s of below 10 % at low applied voltages (< 1 V) exhibiting strong field dependence. [82] The organic photodetector with the mixed layer of CuPc and PTCBI also showed similar performance However, s mall molecule organic photovoltaic cell s with 2 times higher power conversion efficiency were already reported by substitut ing C60 with an exci ton diffusion length of about 77 for PTCBI with an ex c iton diffusion length of about 30 [83] Therefore, the low operating voltage organic photodetector by substituting an acceptor from PTCBI to C60 can be expected. On the other hand, organic photodete ctors, which were reported up to now, have showed the limited photosensitive spectra of visible range wavelength. [7 8 ] Therefore, the realization of organic photodetector with infrared (IR) sensitivity can extend the applications of organic electronics to large area sensing and detection. In this dissertation (chapter 6 ~ 7 ), we will show various organic photodetectors including infrared sensitive organic photodetector s and their new applications 1 3 2 Photodetector C haracterization Photodetectors are pho todiode with rectifying current density-voltage (J -V ) characteristics Typical J -V characteristics in the dark and under incident illumination are s hown in Figure 1 9

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33 T he responsivity is the most important parameters in photodetector s and is calculated by the following formula R = Jnet photo/Pinc = (JphotoJdark)/Pinc (1 6) where Jnet photo( ) and Pinc( ) are the net photocurrent which subtracts the measured dark current density Jdark( ) from the measured photo current density Jphoto( ), of the photodetector The quantum efficiency can be calculated from the responsivity using the following expression: QE = (hc/q ) R 100% (1 7) where h, c, q, and are the Plank constant, speed of light, electronic charge, and the photon wavelength, respectively. The current density versus voltage (J -V) characteristic s of the photodetectors were measured using a Keithley 2400 source meter 520 nm, 630 nm, 830 nm laser s were used as incident power source with high power densit ies. A Newport Optical Power Meter 840 E was used to measure the power densities of laser s Using neutral density filters, the incident power densities for the test cell were varied. In addition to lasers a n Oriel monochromator were also used as the incide nt power source for low power densities. The incident light was irradiated through the ITO electrode. On the other hand, the spectral response was measured with a 150 W ozone free xenon arc lamp, a monochromater and a lock in amplifier under voltage bias of 0 V, 1V, 3 V, and 5 V The measurement systems were explained at Chapter 1.2.2.2. 1 3 3 High Gain Organic Photodetectors Organic p hotodetectors and organic photovoltaic cells are attracting a great deal of attention because of compatibility with fle xible substrates, low cost process, and large area

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34 applications. [36, 4 4, 84, 85] O rganic photodetector s with high quantum efficiency of over 70% have already been achieved by mixed layer method o f donor and acceptor. Furthermore organic photodetector s wi th gain (quantum efficiency >100 %) have been also reported from several research groups. [8688] Gain in p hotodetector means that the conversion efficiency from incident photon to electrical carrier exceeds unity. Daubler et. al. observed the gain of about 20 in organic thin film diode using arylamino poly(p -phenylene vinylene ). [9 ] Also Hiramoto et. al. had reported organic photodetector s with gain of greater than 1,000 in organic diodes based on evaporated perylene pigments. [88] The origin of g ain was interpreted by the photo controlled tunneling injection of electrons due to hole trapping at the interface between a metal electrode and a perylene pigment film. The report of the ultra high gain organic photodetector is very interesting as the sensitivity in a photosensor is one of the most important properties. In this dissertation we also have demonstrate d organic photodetector with high gain of more than 1,000. In addition, we also found that the ultra h igh gain organic photodetector s always show a h ig h dark current We will discuss the origin of gain and high dark current in these high gain organic photodetectors in more details in Chapter 6 of this dissertation 1 3 4 Low O perating V oltage O rganic P hotodetector s O rganic thin film photodiodes have attr acted a great interest for use in organic photovoltaic (OPV) cells and organic photodetectors because of compatibility with flexible substrates, low cost process, and large area application s. [36, 7 8 80] Organic photodetector s with the high external quantum efficiency (EQE) of over 70% at 10 V have already been reported by a multi layer method of copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI) as donor and acceptor respectively [7 8 ] However, organic photodetector s using multilayers of CuPc and PTCBI showed low EQEs of below 10 % at low applied voltages (< 1 V) exhibiting strong field dependence. [82] The strong field dependence is

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35 due to the short exciton diffusion length of PTCBI deteriorating the dissociation rate of photogenerated excitons. [83] However, s mall molecule OPV cells with 2 times higher power conversion efficiency were reported by substituting a C60 with an exciton diffusion length of about 77 for a PTCBI with an ex c iton diffusion length of about 30 [4 2 ] Therefore, it can be expected that organic p hotodetector with high external quantum efficiency at low operating voltages of about 1 V is realized by substituting PTCBI to a C60. In this dissertation we demonstrated the organic photodetector s with high EQEs even at an operating voltage as low as 1 V using a CuPc:C60 bulk heterojunction layer. A lso, the g ain, which means that the conversion efficiency fr om incident photon to charge carrier exceeds unity was observed when the high bias was applied o n the organic photodetector s under low input light power intensities. We will discuss the low operating voltage organic photodetectors in more details in Chapter 6 of this dissertation 1 3 5 Infrared O rganic P hotodetector s O rganic thin film photodiodes ha ve attracted great interest for use in organic photovoltaic cells and organic photodetectors. [51 81, 8 9 9 0 ] Recently, organic photodetector s with high quantum efficiency of over 70% have already been reported [8 9 ] T heir compatibility with lightweight, rugged, or flexible plastic substrates opens up many applications that cannot be addressed using other conventional detector technologies. However, organic photodetectors, which were reported up to now, have showed the limited photosensitive spectra of vis ible range wavelength. [ 8 9 ] Therefore, the realization of organic photodetector with infrared (IR) sensitivity can extend the applications of organic electronics to large area sensing and detection. IR sensitive small molecule organic material ha s previou sly been reported. [5 2 ] SnPc ha s strong absorption in the near infrared (NIR) region (700 nm ~ 1000 nm) [5 2 ] In this dissertation we demonstrated IR organic photodetector using SnPc:C60 mixed layer O n the other hand, a low

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36 dark current is quite important in photodetector for increasing sensitivity. W e incorporated molybdenum oxide (MoO3) and bathocuproine (BCP) interlayers at indium tin oxide (ITO) interface and Al interface, respectively, and found that the interlayer de creases a dark current. We will d iscuss the infrared organic photodetectors in more details in Chapter 6 of this dissertation 1 3 6 Light U p -conversion D evice s Organic light -emitting diode s (OLED s ) are attracting a great deal of attention because of compatibility with flexible substrates low cost process, and large area application s [49, 50, 91] Also, organic photodetector s with high quantum efficiency of over 70% have already been reported. [89] Their compatibility with lightweight, rugged, or flexible plastic substrates opens up many applications that cannot be addressed using other conventional photodetector technologies. Integrating both OLED and IR organic photodetector enable the realization of IR up -conversion devices for infrared image sensing applications. On the other hand, t h ere are many applications for devices capable of detecting IR radiation. IR can refer to radiation having wavelengths longer than visible light (> 0.7 m) up to about 14 m, with near IR (NIR) being a subset referring to wavelengths from about 0.7 m to ab out 1.4 m and short -wave infrared (SWIR) being referred to wavelengths from 1.4 to 3.0 m. One important application is the detection of IR in environments with low ambient light conditions. Depending on the wavelength range of the IR radiation, there are many applications for IR imaging systems. Listed below are some applications for IR imaging: (i ) NIR and SWIR (0.7 m to 3.0 m). This wavelength range might require external IR illumination light source for imaging. The long wavelength of the SWIR spectrum c an be used for high temperature thermal imaging. Applications include surveillance, security monitoring, night vision, infrared inspection of structures, fire and rescue.

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37 (ii) Mid IR (3.0 m to 12 m). IR image sensors in this wavelength wave are capable of the rmal imaging. Applications include a wide range of medical imaging such as thermal imaging of human body temperature, night vision for military, night vision for automobiles, surveillance, security monitoring for law enforcement and homeland security. One common device for detecting IR images and displaying the detected images to a user is night -vision goggles. Conventional night vision goggles are complex electro -optical devices that require very high operating voltages (5,000 V ) and cost thousands of dol lars. A typical night vision camera has a photocathode which converts the photons into electrons. The electrons are then accelerated onto the phosphor screen by a high voltage ( 5,000 V), thus generating a visible image on a phosphor screen. It is highly de sirable to have an IR imaging system that operates at low operating power, lightweight and cost -effective to manufacture. In this dissertation we demonstrated IR -to -green light up-conversion device using SnPc:C60 IR photodetector and fac tris(2 -phenylpyri dinato)iridium(III) (Irppy3) green phosphorescent organic light emitting diode (OLED) Table 1 2 shows the characteristics of organic up -conversion devices compared with conventional night vision goggles. We will discuss the light up -conversion device s in more details in Chapter 7 of this dissertation 1.4 Dissertation O rganization This dissertation covers topics on organic photovoltaic cells, organic p hotodetector s, and organic light up -conversion devices The work on organic photovoltaic cells is covered in Chapters 2 5 In Chapter 2, we describe the calibrat ion of the photovoltaic measurement setup for photovoltaic cells Chapter 3 focuses on the AlPcCl photovoltaic cells with high VOC. Chapter 4 focuses on the infrared absorbing SnPC organic photovolta ic cells Chapter 5 describes the effect of the interlayer

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38 between the ITO anode and the light absorbing layers The results are correlated with interface energetics measurements using ultra -violet photoelectron spectroscopy, The subject of organic photo -sensors and up-conversion devices will be discussed in Chapters 6 C hapter 6 focuses on the high efficiency organic p hoto detector such as the high gain organic photoconductor, low operating organic photodetector, and the infrared organic photodetector C ha pter 7 describes the details the infrared to -visible light up -conversion device integrating the green emitting OLED on the infrared organic photodetector Finally, we present our conclusions in Chapter 8

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39 Figure 1 1 Atomic orbitals (AOs) (s, p) combine to form molecular orbitals (MOs) ( ).

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40 Figure 1 2 Ir complexes used in the present study and norma lized phosphorescence spectra. [8 ]

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41 Figure 1 3 Absorption spectra o f several small molecule organic materials [10]

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42 Figure 1 4 The photo image of the purification system use d in thermal gradient sublim ation purification

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43 Figure 1 5 The photo image of the evaporator used in fabricating light up -conversion devices.

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44 Figure 1 6 The photo image of the evaporator used in fabricating organic photovoltaic cell s and organic photodetectors.

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45 Table 1 1. PV characteristics measured under 100 mW/cm2 AM1.5G illumination at 298K, including the power conversion efficiency P), open-circuit voltage (VOC), short circuit current density (JSC), and fill factor (FF). P (%) JSC (mA/cm2) VOC (V) FF (%) Ref. Organic5.15 9.4 0.876 62.5 18 Dye sensitized 10.4 21.8 0.729 65.2 19 Si (crystalline) 24.7 42.2 0.706 82.8 20 Si (amorphorse) 9.5 17.5 0.859 63 21 GaAs 25.1 28.2 1.022 87.1 22 CIGS 18.8 34 0.703 78.7 23

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46 Figure 1 7 Typical J -V curves of photovoltaic cell s in the dark and under illumination. V arious device parameters are also shown, including the VOC, the JSC, and the FF.

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47 Figure 1 8 Open -circuit voltage (Voc) of different bulk -heterojunction solar cells plotted versus the oxidation potential/HOMO position of the donor polyme r used in each individual device. The straight line represents a linear fit with a slope of 1

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48 Figure 1 9 Typical J -V curves of organic photodetectors in the dark and under illumination. Voltage (V) -10 -8 -6 -4 -2 0 Current density (mA/cm2) 10-510-410-310-210-1100101102103 Dark current Photo current

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49 Table 1 2 T he characteristics of organic up-conversion devices compared with conventional night vision goggles Device characteristics GaAs photocathode based NVG Organic up -conversion device Spectral sensitivity 450840 nm 4001000 nm Resolution 10 m < 1 m Operating voltage >3000 V 10 V P ower consumption 200 mW 1 5 mW Fabrication on plastics No Yes Weight 5001000 gm < 50 gm (even lighter if made on plastics) Size (mm) 110 x 110 x 50 110 x 50 x 20 Cost $5,000 $10,000 $20

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50 CHAPTER 2 M EASUREMENT SETUP FOR SMALL AREA ORGANIC P HO TOVOLTAIC CELLS Most photovoltaic measurement systems have been designed for large area cells. For small area cells, careful calibration is required to accurately measure the photovoltaic characteristics. In this Chapter, we describe the photovoltaic measurement setup for organic photovoltaic cells with small area cells (area = 0.04 cm2). T he positional uni formity of this measurement set up is below 5 % within 5 mm 3.5 mm The corresponding JSC with the JSC measured by the photo J V characteristics was also cross -checked with the Jsc values estimated from the IPCE spectrum. 2.1 Introduction Photovoltaic cells are considered as an important source of renewable energy to solve the worlds energy shortage today. Organic photovoltaic (OPV) cells are attractive for the next generation photovoltaics because of their compatibility with flexible substrates, low manufacturing cost s and large area applications [ 2 7 30]. Unlike rigid inorganic solar cells, o rganic cells absorbing photons first form excitons, which are s trongly bound electron hole pairs that are formed after excitation with light. [2 6 ] The s trongly bound excitons are insufficient to affect direct electron hole dissociation and t hus are mostly dissoci at ed at the interface between a donor and an acceptor F rom this reason, s ince Tang et. al reported the first bilayer organic photovoltaic cell at more than 20 years ago [2 7 ], organic photovoltaic cells have used the donor/acceptor system such as the planar heterojunction (bilayer) structure and the bulk heteroj unction structures and have been improved gradual ly, leading to power conversion efficiencies of more than 5 %.[ 2 8 3 3 ] S everal organic material s such as copper p hthalocyanine (CuPc),[4 1, 92, 93] aluminum p hthalocyanine chloride (AlPcCl),[ 94] tin (II) phthal ocyanine (SnPc),[52 5 4 ] zinc pthalocyanine (ZnPc),[ 95, 96] tetracene,[ 97], pentacene [98] poly[2 methoxy5 -(3,7 -dimethyloctyloxy) 1,4 -phenylene vinylene] (MDMO -PPV),[ 99 101 ] and

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51 regioregular poly(3 -hexylthiophene) (RR P3HT),[ 102 106] have been used as dono rs and several materials such as 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI),[ 107] buckminsterfullerene (C60),[ 83] and [6,6] -phenyl C61 butyric acid methyl ester (PCBM).[ 108] have been used as acceptors T he power conversion efficiency ( P) i n OPV cells has been achieved to more than 5%. [20, 28 30, 4 1 ] On the other hand, it is very important to accurately measure the power conversion efficiency for exactly recognizing where we are on the way to develop the photovoltaic cells thus enabl ing a fair comparison of results from different research groups. Many efforts have been performed for accurate measurement s of the power conversion efficiency, and a standard solar cell measurement has been established. [109 1 11] In the early 1980, the l aborat or ies for evaluating solar cell performance w ere established in many parts of the world. [ 110, 1 11] However, in the field of organic photovoltaic cells, the power conversion efficienc ies have been measured under various testing conditions and then reported P articularly, the device size used in organic photovoltaic cell research is usually quite small as few square millimeter s, small changes in beam nonuniformity can lead to large uncertainties in cell measurements. Therefore with this uncertainties, it i s very difficult to compar e the results between different research groups In this study we have developed a technique for accurate photovoltaic measurement s for small area organic photovoltaic cells with size of 0.04 cm2. 2.2 Experimental Detail s For evaluating the photovoltaic measurement setup, bulk heterojunction OPV cells using AlPcCl and C60 were fabricated. All OPV cells were fabricated on patterned ITO substrates with The ITO substrates were cleaned by aceton e and isopropanol in ultrasonic cleaner and then were rinsed by de -ionized water blow n by nitrogen,

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52 and subsequently treated by UV ozone for 1 5 minutes All layers in OPV cells were vacuum deposited at a pressure of 1 106 Torr. AlPcCl and C60 w ere at least purified three times by train sublimation technique [1 1 2 ]. The d evice structure of the OPV cells used in this study is Al Pc Cl (10 nm)/AlPcCl :C60 (2 0 nm) /C60 (30 nm) / bathocuproine (BCP) (7.5 nm)/Al (100). The bulk heterojunction layers were deposited b y co -evaporation. The deposition rates were 0.5 /s and 1 /s for organic materials and Al, respectively. The area of the device is 0.04 cm2. T he current density versus voltage (J -V) characteristics were measured in the dark and under simulated a ir m ass 1. 5 g lobal (AM1.5G) solar illumination. A 150 W ozone free xenon arc lamp with 1.5G air mass filter was utilized as a solar simulator in the experimental setup. F or calibration study, a Newport 70260 radiant power meter combined with a 70268 probe was used t o measure the power density The incident photonto -current collection efficiency (IPCE) spectra l measurements were done with the solar simulator a monochromater and a lock -in amplifier. National Institute of Standards and Technology ( NIST ) calibrated UV-enhanced silicon and germanium photodetectors were used to calibrate the measurements Th e devices were tested in air without encapsulation 2.3 Results and Discussion s Conventional small molecule organic solar cell using CuPc and C60 as a donor and an acc eptor respectively absorbs photon s with wavelength from 400 nm to 800 nm wavelength. [83] Assuming an organic solar cell have 100% photonto -electron conver sion efficiency within this wavelength range, the theoretical maximum short circuit current density (JSC) can be calculate d from the IPCE spectra using the following equations : JSC = Ref( )ST( )d (2 1) ST ( ) = (q IPCE ( )/100hc) (2 2)

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53 ERef ( ) is reference solar intensity at AM 1.5 condit i on shown in Figure 2 1. ST( ) is the spectral responsivity in A/W. This spectral responsivity can be calculated by equation (2 2) Thus i f we make solar cell with 100% IPCE values from 400 nm to 800 nm we can have the JSC of 26.4 mA/cm2 as shown in Figure 2 2. Also, if we have a solar cell with 60% IPCE values from 400 nm to 800 nm we can have a JSC of 15. 8 mA/cm2. For a solar cel l with 40 % IPCE, we have a JSC value of 10. 5 mA/cm2. Therefore, we can compare the JSC by the photo J -V characteristics under AM1.5 condition with the calculated JSC by the IPCE spectra to verify the measurement data Before the calibration procedure was established, we made the small molecule organic solar cell using AlPcCl: C60 bulk heterojunction structure and had higher than 5 % power conversion efficiency under AM 1.5 conditions showing t he JSC of 14.5 mA/cm2. However, we also measure d spectral respo nse of this organic solar cell and calculated the JSC by using equation (2 1) The calculated JSC was 8.5 mA/cm2. The difference between the JSC by the photo J -V characteristics under AM1.5 condition and the calculated JSC by the IPCE spectrum is significa nt The results motivated us to carefully calibrated the system again Figure 2 3 (a) is the schematic diagram of the initial calibration method for photo J -V measurement system F or calibration of power density, we put the detector of power meter on the same surface as actual device at the photo J -V measurement. T he size of detector in power meter is larger than that of active device area (0.04 cm2) and smaller than that of incident beam as shown in Figure 2 3 Thus the power density is calculated by div iding the power measured by power meter by detector area. On the other hand, a metal mask was used to determine the power density This mask has square hole with exactly same size compared with sample active area Therefore, the new method using metal mask can measure the real power density on an actual

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54 device. We put this mask on the same surface as the actual device for the photo J -V measurement and then the detector of power meter back of this square hole. The detector area of power meter is larger than t he incident stimulated sun light beam area and it is expected that the detector should be able to collect the all incident sun light which pass through this square hole. Hence, the power density is calculated by dividing the power measured by power meter by square hole area on metal mask The power density measured with metal mask is 2.5 times higher than that without metal mask In addition, the difference of incident power density between the center of the beam and beam edge is more than 5 times. A s desc ribed previously, actual devices were always put at the center of light irradiation beam. T he actual incident power density in photo J -V measurement is 2.5 times higher than the measured incident power density. Our sample size is roughly 100 times small er than the beam size and the samples always put at the center of the incident solar irradiation, and it is expected that highest power density among whole beam areas of incident light irradiation Thus, the power density is likely to be underestimated leadi ng to significant difference between the JSC by the photo J -V characteristics under AM1.5 condition and the calculated JSC by the IPCE spectrum. However, when w e use the new calibration method using metal mask, the difference between the JSC by the photo J -V characteristics under AM1.5 condition and the calculated JSC by the IPCE spectrum becomes negligible as shown in Figure 2 6 O n the other hand, we check ed the positional uniformity of this measurement set up as shown in Figure 2 4 because the sample po sition can be misaligned from the center position, at which the incident power density measured with metal mask is reliable We measured the photo J -V characteristics at twenty -one different positions. The fluctuation of the JSC is below 5 % within 5 mm 3.5 mm Also, the fluctuation of other parameters such as VOC, FF, and P also

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55 are insignificant as shown in Figure 2 5. F inally we measured spectral response and t he corresponding JSC determined from direct photo J -V is consistent with the value determin ed from the IPCE spectrum as shown in Figure 2 6. 2.4 Conclusions In conclusion, we have developed a photovoltaic measurement setup for small area organic photovoltaic cell measurements T he positional nonuni formity of this measurement set up is below 5 % within a 5 mm 3.5 mm area The corresponding JSC with the JSC measured by the photo J -V characteristics is obtained from the IPCE spectrum.

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56 Figure 2 1 ASTM G173 03 Reference spectrum.

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57 Figure 2 2 Expected short circ uit current densities corresponding to IPCE spectra.

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58 (a) (b) Figure 2 3 Setup for calibration of power density on actual device from solar simulator.

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59 Figure 2 4 Evaluation of positional beam uniformity us ing J -V measurement

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60 Figure 2 5 Beam uniformity on JSC, VOC, FF, and P.

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61 Figure 2 6 Confirmation of JSC on J -V characteristics using IPCE spectrum.

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62 CHAPTER 3 ENHNANCEMENT IN VOC DUE TO A DONOR MATERIAL WITH DEE P HOMO ENERGY Small molecule organic planar and bulk heterojunction photovoltaic cells were fabricated using aluminum phthalocyanine chloride (AlPcCl) or copper phthalocyanine (CuPc) as a donor. While both AlPcCl and CuPc cells have sim ilar short circuit current, the power conversion efficiency of the AlPcCl cells is about 1.8 times higher than that of the CuPc cells because of the higher open circuit voltage in the AlPcCl cells. The large open -circuit voltage is due to the large highest occupied molecular orbital (HOMO) energy of AlPcCl. The maximum power conversion efficiency is 2 0 % under 1 sun standard AM1.5G solar illumination of 100mW/cm2. 3.1 Introduction Photovoltaic cells are considered as an important source of renewable energy to solve the worlds energy shortage today. Organic photovoltaic (OPV) cells are attractive for the next generation photovoltaics because of their compatibility with flexible substrates, low manufacturing costs and large area applications [ 2 7 30]. While t here has been improvement in power conversion efficiencies of both small molecules and polymer OPV cells [ 4 1 4 2 ], the power conversion efficiency ( P) in OPV cells has been limited to about 5%. One of the reasons limiting the power conversion efficiency of OPV cells is the small open circuit voltage (VOC). The typical VOC value of conventional CuPc based small molecule OPV cells is less than 0.45 V [ 3 3 ]. The origin of VOC in OPV cells is not fully understood. Most reports support that the VOC value depends on the energy difference between the lowest unoccupied molecular orbital (LUMO) energy of the acceptor and the HOMO energy of the donor [ 4 3 4 5 ]. Additionally, the VOC appears to depend on the work functions of the electrodes. It is desirable to have a low work function cathode and large work function anode to optimize the VOC [4 3 4 6 ]. A very large VOC of 0.87 V has been reported with polymer OPV cells using poly[2 -meth -oxy 5 (3,7 -

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63 dimethyloctyloxy)] 1,4 -phenylenenvi nylene (MDMO -PPV) as a donor [ 4 7 ]. Recent ly, a very large VOC of 0.98 V has been reported with small molecule OPV cells using boron subphthalocyanine chloride (SubPc) as a donor [ 4 8 ]. In this letter, we report OPV cells using AlPcCl as a donor. Our recent ultraviolet photoemission spectroscopy (U PS) measurements show that AlPcCl has a HOMO energy of 5.3 eV which is about 0.5 eV higher than that of CuPc, suggesting that OPV cells using AlPcCl instead of CuPc as a donor should give a higher VOC. Furthermore, AlPcCl has an absorption peak at waveleng th of 755 nm, extending the photores ponse into the near infrared [ 94]. 3.2 Experimental Detail s Both planar heterojunction and bulk heterojunction OPV cells using AlPcCl or CuPc as a donor and C60 as an acceptor were fabricated. All OPV cells were fabricat ed on pa tterned ITO The ITO substrates were cleaned by acetone and isopropanol in ultrasonic cleaner and then were rinsed by de ionized water blow n by N2 gas and subsequently treated by UV ozone for 1 5 minut es All layers in OPV cells were vacuum deposited at a pressure of 1 106 Torr. CuPc, AlPcCl and C60 w ere at least purified three times by train sublimation technique [1 1 2 ]. Figure 3 1 (a) show s the device structures of the OPV cells used in this study. Figure s 3 1 (a) and (b) show the structure of the CuPc OPV cells with planar heterojunction and bulk heterojunction, respectively. Figure s 3 1 (c) and (d) are the AlPcCl OPV cells with planar heterojunction and bulk heterojunction, respectively. An 100 nm thick bathocuproine (BCP) and an 1000 nm thick Al were used as exciton blocking layer and cathode, respectively. The bulk heterojunction layers were deposited by co-evaporation. The deposition rates were 0.5 /s and 1 /s for organic materials and Al, r espectively. The area of the device is 0.04 cm2.

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64 T he current density versus voltage (J -V) characteristics were measured in the dark and under simulated a ir m ass 1.5 g lobal (AM1.5G) solar illumination. A 150 W ozone free xenon arc lamp with 1.5G air mass fi lter was utilized as a solar simulator in the experimental setup. A Newport 70260 radiant power meter combined with a 70268 probe was used to measure the power level of the white light illumination used for the photovoltaic measurements Using neutral den sity filters, the illumination levels for the test cell were varied between 35 mW/cm2 and 12 8 mW/cm2. The incident photon -to -current collection efficiency (IPCE) spectra l measurements were done with the solar simulator a monochromater and a lock in amplif ier. National Institute of Standards and Technology ( NIST ) calibrated UV-enhanced silicon and germanium photodetectors were used to calibrate the measurements Th e devices were tested in air without encapsulation For e nergy alignment study, u ltraviolet ph otoemission spectroscopy studies were performed on the Cu Pc and AlPc Cl using a VG ESCA Lab system equipped with a He I (21.2 eV) gas discharge lamp as the UV source. The integrated ultrahigh vacuum (UHV) system for UPS measurements consists of a UPS spectr ometer chamber and an evaporation chamber in which thin film deposition was done in ultrahigh vacuum and transferred directly into the spectrometer chamber without exposing the sample to atmosphere. The base pressure of the spectrometer chamber was typical ly 81011 torr. For energy alignment study, a 20 nm thick CuPc and AlPcCl layer s w ere deposited onto a UV ozone -treated ITO substrate by thermal evaporation. Charging was not observed from the UPS spectra at different UV intensities during the measuremen ts. The UPS spectra were taken with incremental thickness of the CuPc and AlPcCl film to evaluate the evolution of the interface formation. The UPS spectra were recorded with the samples biased at 5.0 V to observe the true, low energy secondary cut off. The typical

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65 instrumental resolution for UPS measurements ranges from ~0.03 to 0.1 eV with photon energy dispersion of less than 20 meV. All measurements were done at room temperature. 3.3 Results and Discussion s Figure 3 2 (a ) shows the photo J -V characteristics of the CuPc and AlPcCl cells. The device performances of these OPV cells are also summarized in Table 3 1 For the CuPc cells, t he short circuit current density ( JSC) values are 4.32 mA/cm2 and 7.19 mA/cm2 for the planar heterojunction and bulk het erojunction cell s respectively. The JSC of the bulk heterojunction cell is 1.65 times higher than that of the planar heterojunction cell. For AlPcCl cells, t he JSC is 4 52 mA/cm2 and 6.18 mA/cm2 for the planar heterojunction and bulk heterojunction cells, respectively. Here, t he JSC of the bulk heterojunction cell is 1.37 times larger than that of the planar heterojunction cell and this enhancement in JSC due to bulk heterojunction is similar to that of the CuPc cells I n planar heterojunction cells, excit on dissociation occurs at the donor acceptor interface. T hus, if the exciton diffusion length (LD) of donor or acceptor is shorter than the thickness of the donor or acceptor layer most excitons generated will recombine before diffusing to the donor accep tor interface [ 39]. One the other hand bulk heterojunction cells have large r interfacial area compared with the planar heterojunction cells, thus creating an interpenetrating network with a spatially distributed interface that lies within LD of the photogenerated excitons [39]. T herefore, the bulk heterojunction structure increases the e xciton dissociation rate ( ED) leading to an increase in JSC [3 9 ]. The enhancement in JSC is also consistent with the IPCE spectra for both the CuPc and AlPcCl cells shown in Figure 3 2 (b) where the photoresponse of the CuPc and AlPcCl bulk heterojunction cell s are about 2 times larger than those of the planar heterojunction cells at wavelengths between 550 nm and 800 nm. The data indicate the enhanced dissociation of exci ton generated in CuPc and AlPcCl. It should

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66 be noted that the increase of JSC due to the bulk heterojunction effect is smaller in AlPcCl cells compared with CuPc cells suggesting that the exciton diffusion length of AlPcCl is smaller than that of CuPc Ano ther possible reason is due to the difference in carrier transport between the two materials. A third reason might be due to optical effects. T he peak wavelength s of optical absorption on CuPc and AlPcCl are 62 0 nm and 755 nm, respectively [ 94]. Optical in terference due to microcavity effect depends on the wavelengths of the incident photons Since AlPcCl absorbs at longer wavelength than CuPc, the AlPcCl cell geometry might not be optimized for optical absorption. The f ill Factor s of the CuPc cells are 0. 54 and 0.42 for the planar heterojunction and bulk heterojunction cells respectively and the fill factors of the AlPcCl cells are 0.52 and 0.42 for the planar heterojunction and bulk heterojunction cells respectively. In both CuPc and AlPcCl cells, the f ill factor of the bulk heterojunction cell is slightly lower than that of the planar heterojunction cell due to lowering of carrier mobilities of the mix ed donor / acceptor network It should be noted t hat both CuPc and AlPcCl cells have shown similar fill f actors, indicating that the difference between the charge transport properties of CuPc and AlPcCl is not significant. The VOC of the CuPc cells are 0.44 V and 0.43 V for the planar heterojunction and bulk heterojunction cells, respectively and the VOC of A lPcCl cells are 0.84 V and 0.82 V for the planar heterojunction and bulk heterojunction cells, respectively. In both planar heterojunction and bulk heterojunction cells the VOC of AlPcCl cells is about 2 times larger than that of CuPc cells. This increase in VOC with respect to CuPc based OPV cells is due to the larger HOMO energy of AlPcCl compared to CuPc To understand the increase in VOC, we measured the electronic structure of the CuPc and AlPcCl by UPS. Figure 3 3 shows the energy level alignment of (a) the ITO /CuPc and (b) ITO/AlPcCl obtained from the UPS measurements The

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67 UPS data show that the HOMO energy of A l PcCl is 5. 30 0.1 eV which is significantly higher than the HOMO energy of CuPc ( 0.48 0.1 eV). T he correction between VOC and HOMO energy for polymer devices has been well documented by the work done by Brabec and coworker [ 113]. Our CuPc and AlPcCl (small molecules) device results show that there is a strong correlation between the donor HOMO energy and the VOC value, which is consistent w ith the trend reported for polymer devices. It should be noted that our Voc values for the AlPcCl cells reported here are higher than what ha ve been reported previously [94]. The reason for the difference is not clear. We have made more than 200 devices an d confirmed that the high VOC data are highly reproducible. The Ps on CuPc cells are 1.02 % and 1.29 % for the planar heterojunction and bulk heterojunction cells respectively and the Ps on AlPcCl cells are 1.86 % and 2.00 % for the planar heterojuncti on and bulk heterojunction cells, respectively. The higher P for the AlPcCl cells as compared to the CuPc cells is due to the higher VOC value of the AlPcCl ce lls Figure 3 4 shows (a) the J -V characteristics, (b) VOC and fill factor, and (c) JSC and P of the AlPcCl based OPV cell as a function of various incident light intensities. The VOC value of the AlPcCl cell increases with increasing the incident light intensity which is consistent with what have been reported previously for CuPc cells [3 3 ]. The JSC of the AlPcCl cell increases linearly with increasing the incident light intensity indicating that the cell efficiency is constant within the range of power density tested here. On the other hand, the fill factor decreases slightly with increasi ng incident light intensity, indicating the series resistance effect at high power densities

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68 3.4 Conclusions W e have fabricated both planar heterojunction and bulk heterojunction OPV cells with CuPc and AlPcCl. For both CuPc and AlPcCl cells, the bulk he terojunction cells show higher JSC than the planar hetero junction cells indicating the enhance d exciton dissociation due to the donor/acceptor interpenetrating network. Both CuPc and AlPcCl cells show very similar device performance in terms of short -circu it current as well as fill factors. This is true for both planar and bulk heterojunction. The major difference between the CuPc and AlPcCl cells are the open circuit voltages. The open -circuit voltages of AlPcCl cells are significantly higher than those of the CuP c cells due to higher HOMO energy of AlPcCl, leading to higher overall power efficiency. In fact, there is a strong correlation between the HOMO energy and the open -circuit voltage of an OPV cell. Since organic semiconductors have fairly large band gaps, there are still rooms for further improvement in OPV performance by enhancing the open-circuit voltage.

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69 (a) (b) (c) (d) Figure 3 1. Schematic cross -section view of CuPc based reference organic PV cells with (a) planar heterojunction and (b) bulk heterojunction and AlPcCl based organic PV cells with (c) planar heterojunction and (d) bulk heterojunction

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70 (a ) ( b ) Figure 3 2 (a) P h oto J -V characteristics and (b) IPCE of organic PV cells with structures of Fig. 3 1 (a) PHJ CuPc cell, (b) BHJ CuPc cell, (c) PHJ AlPcCl cell, and (d) BHJ AlPcCl cell (PHJ:planar heterojunction, BHJ: bulk heterojunction) Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Current density (mA/cm2) -10 -5 0 5 10 PHJ CuPc cell BHJ CuPc cell PHJ AlPcCl cell BHJ AlPcCl cell Wavelength (nm) 400 500 600 700 800 900 IPCE (%) 0 20 40 60 80 100 PHJ CuPc cell BHJ CuPc cell PHJ AlPcCl cell BHJ AlPcCl cell

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71 Table 3 1. Summary of CuPc and AlPcCl based organic PV cells ; (a) PHJ CuPc cell, (b) BHJ CuPc cell, (c) PHJ AlPcCl cell, and (d) BHJ AlPcCl cell (PHJ:planar heterojunction, BHJ: bulk heterojunction) Buffer layer Jsc (mA/cm2) Voc (V) FF np (%) PHJ CuPc cell 4.32 0.44 0.54 1.02 BHJ CuPc cell 7.19 0.43 0.42 1.29 PHJ AlPcCl cell 4.52 0.84 0.52 1.86 BHJ AlPcCl cell 6.18 0.82 0.42 2.00

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72 (a ) ( b ) Figure 3 3 Energy l evel alignment of (a) the ITO/ Cu Pc and (b) ITO/ AlPcCl

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73 (a) (b) (c) Figure 3 4 (a) J -V characteristics, (b) VOC, FF, and (c) JSC, nP of bulk heterojunction AlPcCl cell with high VOC as a function of incident light int ensities Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Current density (mA/cm2) 10-410-310-210-1100101102 Dark 33 mW/cm2 54 mW/cm2 90 mW/cm2 107 mW/cm2 116 mW/cm2 PO (mW/cm2) 20 40 60 80 100 120 FF 0.40 0.42 0.44 0.46 PO (mW/cm2) 20 40 60 80 100 120 JSC (mA/cm2) 1 2 3 4 5 6 7 JSC nP (%) 1.96 1.98 2.00 2.02 2.04 2.06 2.08 2.10 nP VOC (V) 0.78 0.80 0.82 0.84

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74 CHAPTER 4 INFRARED ABSORBING OR GANIC PHOTOVOLTAIC C ELLS WITH MOLYBDENUM OXIDE INTERLAYER Infrared sensitive s mall molecule organic photovoltaic cells were fabricated using a tin (II) phthalocyanine (SnPc) and C60 bulk heterojunction layer. Mol ybdenum oxide (MoO3) w as used as an interlayer between ITO anode and light absorbing layers We found that the short circuit current of the photovoltaic cells with MoO3 interlayer was substantially enhanced resulting in a 45 % enhancement in power convers ion efficiency The maximum power conversion efficiency is 1.31 % under 1 sun standard AM1.5G solar illumination of 100mW/cm2. 4.1 Introduction Photovoltaic cells are considered as an important source of renewable energy to solve the worlds energy shortag e today. V arious photovoltaic cell technologies have been developed and on e of the most promising technologies is organic photovoltaic s [49, 50]. O rganic photovoltaic cells are attracting a great deal of attention because of their compatibility with flexib le substrates, low manufacturing cost processes and large area application s [39, 5158]. While there has been improvement in power conversion efficiencies of both small molecules and polymer organic photovoltaic cells [41, 42], the power conversion effic iency of single junction organic photovoltaic cells has been limited to about 5%. One of the reasons limiting the power conversion efficiency is the limitation of usable wavelength in the solar spectrum. Most organic materials have strong absorption in the visible spectrum and their absorption in the infrared region (> 700 nm) is rather limited. Since the photon flux in the visible region only accounts for about 25% of the entire solar spectrum it is important to extend the usable wavelength to the infrare d region in order to maximize the power conversion efficiency of organic photovoltaic cells [ 59]. For example c opper phthalocyanine ( CuPc ) a donor material used in small molecule organic photovoltaic cells, has a band gap of 1. 7 eV (730 nm ) and thus

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75 even if CuPc cell has a high incident photonto -current collection efficiency (IPCE) value its power conversion efficiency is still limited. Extending the usable wavelength of organic materials to beyond 1000 nm will substantially increase the power conversion efficiency [59]. Therefore low band gap organic materials will improve the efficiency of organic photovoltaic cell s [ 596 1 ]. In addition, the realization of near infrared absorbing organic photovoltaic cells also enables the fabrication of translucent p hotovoltaic s which have recently been demonstrated [62]. Near infrared sensitive small molecule organic photovoltaic cells with planar heterojunction structure using tin (II) phthalocyanine (SnPc) have previously been reported [ 6365]. While enhanced excit on dissociation has been reported in both small molecule and polymer bulk heterojunction photovoltaic cells [66], bulk heterojunction organic photovoltaic cells using SnPc as a donor have not been reported. In this study we fabricated near infrared absorb ing small molecule organic photovoltaic cells using SnPc:C60 bulk heterojunction structure. In addition to the bulk heterojunction effects, we have also studied the effect of the interlayer on the SnPc bulk heterojunction cells. Recently, it has been demonstrated that the interlayer between the indium tin oxide (ITO) anode and the light absorbing layer could play an important role determining the efficiency of o rganic photovoltaic cells [67]. Shrotriya et al. demonstrated enhanced cell performance us ing vanadium oxide (V2O5) and molybdenum oxide (MoO3) as the interlayer for polymer photovoltaic cells [ 68]. Similar result has also been demonstrated by Irwin et al. using a p type nickel oxide (NiO) interlayer [ 69 ]. Here, t o enhance the device performance of ne ar infrared absorbing organic photovoltaic cells used in this study, we incorporated a MoO3 interlayer between the ITO anode and the light absorbing layers and

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76 found that the interlayer increases both the short -circuit current ( JSC) and the open circuit voltage (VOC) resulting in a 4 5 % enhancement in power conversion efficiency. 4.2 Experimental Detail All OPV cells were fabricated per square. The ITO substrates were first cleaned with acetone and isopropanol in an ultrasonic cleaner and then rinsed with de ionized water blow n dry with N2 gas and subsequently treated with UV ozone for 1 5 minutes All layers in OPV cells were vacuum deposited at a pressure of 1 106 Torr. Copper phthalocyanine ( CuPc ), SnPc and C60 w ere purified three times by train sublimation t echnique [112]. Figure s 4 1 ( a ) and (b) show the device structure s of the small molecule organic photovoltaic cell without and with a MoO3 interlayer used in this study respectively. A mu ltilayer stack of CuPc (10 nm)/SnPc :C60 (6, 10, 15 and 20 nm) /C60 (30 nm) was used as the light absorbing layers. A 7.5 nm thick bathocuproine (BCP) and a 100 nm thick Al films were used as the exciton blocking layer and the cathode, respectively. The SnPc :C60 bulk heterojunction layers were deposited by co evaporation. In addition to the above controlled devices, a 10 nm thick MoO3 layer was used as an inter layer between the ITO anode and the absorbing layers in one set of devices. The deposition rates wer e 0.5 /s and 1 /s for organic materials and Al, respectively. The active area of the device was 0.04 cm2. The current density versus voltage (J -V) characteristics were measured in the dark and under simulated air mass 1.5 global (AM1.5G) solar illuminat ion. A 150 W ozone free xenon arc lamp with 1.5G air mass filter was used as a solar simulator. A Newport 70260 radiant power meter combined with a Newport 70268 probe was used to measure the power densities of the white light illumination used for the pho tovoltaic measurements. Using neutral density filters, the illumination level used for testing was varied between 35 mW/cm2 and 128 mW/cm2. National Institute of Standards and Technology (NIST) calibrated UV -enhanced silicon and

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77 germanium photodetectors we re used to calibrate the measurements. Th e devices were tested in air without encapsulation 4.3 Results and Discussion Figure s 4 2 (a) shows the photo J -V characteristics of the OPV cells as a function of the SnPc:C60 bulk heterojunction layer thickness. The JSC of the OPV cells increase s with increas ing thickness of the SnPc:C60 bulk heterojunction layer and its value increases from 3.3 mA/cm2 for the cell with a 6 nm thick bulk heterojunction layer to 4.6 mA/cm2 for the 20 nm thick bulk heterojunction la yer cell The increase in JSC with increasing thickness of the bulk heterojunction layer is probably due to the increase in light absorption and exciton generation. However, the fill factor decreases with increasing thickness of the bulk heterojunction lay er from 0.56 for the cell with a 6 nm thick bulk heterojunction layer to 0.46 for the 20 nm thick heterojunction layer cell T he decrease in fill factor is due to the increase in the cell series resistance On the other hand, the VOC of the SnPc:C60 cells is 0.4 5 V and is independent of the thickness of the SnPc:C60 bulk heterojunction layer. The overall power conversion efficiency of the SnPc:C60 cells increases with increasing thickness of the SnPc:C60 bulk heterojunction layer from 0.80 % for the cell wi th 6 nm thick bulk heterojunction layer to 0.95 % for the 20 nm thick heterojunction layer cell. Figure 4 2 (b) shows the optical absorbance spectra of 100 nm thick SnPc, CuPc, and SnPc:C60 mixed films and Figure 4 2 (c) shows the IPCE spectra of the orga nic photovoltaic cells for different SnPc:C60 bulk heterojunction layer thickness es T he SnPc cells have a stronger IPCE response in the near infrared region with a peak response at about 740 nm with absorption ex tending to 900 nm. Furthermore the photore sponse in the near infrared region increase s with increas ing the thickness of the SnPc:C60 bulk heterojunction layer and the results are consistent with the thickness dependence on the JSC measured from the photo J -V characteristics. T he photoresponse spec tra of the bulk heterojunction cells are also consistent with the optical

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78 absorbance spectr um for the SnPc :C60 mixed film. T he peak intensity of the IPCE value is about 30 % at 740 nm wavelengths in the organic photovoltaic cell with the 20 nm thick SnPc:C60 bulk heterojunction layer. It should be noted that the optical absorbance spectrum of the 100 nm thick SnPc film in the NIR region is different from the optical absorbance spectrum of the SnPc:C60 mixed film. T he optical absorbance spectrum of the neat SnPc film has two peaks one at 740 nm and another one at 890 nm. H owever, the SnPc:C60 organic photovoltaic cells has only one absorption maximum at 740 nm peak. It is known that the peaks at 740 nm and 890 nm are due to monomer and dimer absorption, resp ectively [92]. Thus, the absorption data suggest that the absence of absorption at 890 nm is due to the suppression of dimer formation in the mixed film To further enhance the cell performance, we incorporated a MoO3 interlayer between the ITO electrode and the light absorbing layers. Previously work by Shrotriya et al. showed that the MoO3 interlayer increases the efficiency of poly (3-hexylthiophene) (P3HT) cells by about 5% [10] Figure 4 3 shows (a) t he photo J -V characteristics and (b) the IPCE spect ra of the SnPc:C60 oganic photovoltaic cells with and with out the MoO3 inter layer. Here, for interlayer study, the thickness of the SnPc:C60 bulk heterojunction layer was kept at 10 nm The device performance of SnPc:C60 cells with and without the MoO3 int erlayer is summarized in Table 4 1 From Figure 4 3 (a ), the MoO3 interlayer increases the VO C from 0 45 V to 0.47 V The small increase of VOC might be explained by a change in interface energetics between the ITO anode and the light absorbing layers due to the MoO3 interlayer. The change in fill factor due to the MoO3 interlayer is insignificant, indicating that the interlayer does not affect the series resistance of the OPV cell. T he MoO3 interlayer has the largest effect on the JSC and its value increas e s from 4. 04 mA/cm2 to 5 31 mA/cm2. The enhancement in the JSC is also consistent with the IPCE spectra shown in Figure 4 3 (b) where the photoresponse of the organic photovoltaic cell with the MoO3

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79 interlayer is strong er than that of the organic photovolt aic cell without the MoO3 interlayer at all wavelengths between 400 nm and 9 00 nm. The JSC is determined by a combination of light absorption efficiency, exciton dissociation efficiency, and charge carrier collecting efficiency. S ince MoO3 is a thin interl ayer (10 nm thick ), it is expected that the there is no significant change in light absorption efficiency and exciton dissociation efficiency [92], and the enhancement in JSC can be explained by an enhanced charge carrier collecting efficiency due to the i nterlayer Similar enhancement in charge carrier collection by interlayer has been demonstrated recently by Irwin et al., and enhancement in JSC has been achieved by inserting a nickel oxide (NiO) interlayer between the ITO anode and the bulk heterojunction layer in P3HT cells [ 69]. Here, we observe a 30% enhancement in JSC due to the MoO3 interface indicating that the MoO3 interlayer acts as an effective electron blocker. From the J -V data we can also determine the shunt resistance of the cell which can b e calculated from the slope of the photo J -V characteristic curve close to 0 V [ 115]. Here, the shunt resistances of the SnPc:C60 organic photovoltaic cells with and without the MoO3 interlayer are 13.0 k and 7.5 k as shown in Table 4 1 respectively. Th e higher shunt resistance of photovoltaic cell with MoO3 interlayer supports that the MoO3 interlayer acts as an effective electron blocker. In order to further study the MoO3 interlayer effect we study the cell performance as a function of the thickness of the light absorption layer. F igure 4 4 shows (a) the JSC, (b) the fill factor, (c) the VOC, and (d) the power conversion efficiency of the SnPc cells with and without the MoO3 interlayer for different SnPc:C60 bulk heterojunction layer thicknesses The VOC values of the OPV cells with the MoO3 interlayer are 0 .02 V higher than those of the devices without the interlayer for all bulk heterojunction thicknesses. In addition, the insertion of the MoO3 interlayer does not affect the thickness

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80 dependence of the bulk heterojunction layer on both fill factor and short -circuit current. Specifically t he JSC values of the MoO3 interlayer cells are about 1.3 mA/cm2 larger than those of the cells without the interlayer for all bulk heterojunction layer thicknesses. These results seem to be consistent with our model that the MoO3 interlayer acts as an electron blocker thus enhancing the charge carrier collection efficiency On the other hand, t he morphology of the inter face between the ITO electrode and the light a bsorbing layer has significant influence on the performance of organic photovoltaic cells. In order to understand the interface properties, the surface morphologies of the bare ITO surface and the 10 nm thick MoO3 layer on the ITO were explored. Figure 4 5 shows the atomic force microscopy ( AFM ) image of (a) a bare ITO surface (b) a 10 nm thick MoO3 layer on ITO, (c) a 10 nm CuPc/10 nm SnPc:C60 film stack on ITO, and (d) a 10 nm CuPc/10 nm SnPc:C60 film stack on a 10 nm thick MoO3 layer on ITO T he rms rou ghnesses of bare ITO and the 10 nm thick MoO3 layer on the ITO are 1 36 nm and 1.28 nm, respectively. Also, t he rms roughnesses of the light absorbing layer on the bare ITO and the MoO3 inter layer are 1 10 nm and 1.15 nm, respectively. The AFM results indi cate that t he surface morphology of t he light absorbing layer has not been changed by introducing the MoO3 interlayer thus not significantly affecting the photovoltaic performance. 4.4 Conclusion In conclusion, we have fabricated SnPc:C60 bulk heterojunct ion OPV cells with a power efficiency of 0.92% The device short -circuit current increases with increasing thickness of the bulk heterojunction layer due to increase in light absorption while the fill factor decreases with increasing thickness of the heter ojunction layer due to increase in series resistance. With a MoO3 interlayer between the ITO anode and the light absorbing layers the short -circuit current is

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81 substantially enhanced resulting in a 45 % enhancement in power efficiency from 0.92% to 1.31%

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82 (a) (b) Figure 4 1. Schematic cross -section view of the infrared absorbing OPV cells (a) without and (b) with the SnPc:C60 bulk heterojunction layer

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83 (a) (b) Figure 4 2 (a) The photo J -V characteristics and (b) the IPCE spectra of the infrared absorbing OPV cells as a function of thicknesses of the SnPc:C60 bulk heterojunction layer Wavelength (nm) 400 500 600 700 800 900 1000 IPCE (%) 0 10 20 30 40 50 6 nm 10 nm 15 nm 20 nm Absorbance (a.u.) 0.0 0.2 0.4 0.6 0.8 1.0 SnPc:C60SnPc CuPc Voltage (V) -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Current density (mA/cm2) -6 -4 -2 0 2 4 6 6 nm 10 nm 15 nm 20 nm

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84 Table 4 1. Summary of the infrared absorbing organic PV cells with out and with the MoO3 inter layer Jsc (mA/cm2) Voc (V) FF (%) PCE (%) Rsh(k ) w/ MoO35.31 0.47 0.52 1.31 13.1 w/o MoO34.04 0.45 0.51 0.92 7.5

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85 (a) (b) Figure 4 3 (a) The photo J -V characteristics and (b) the IPCE spectra of the infrared absorbing OPV cells without and with the MoO3 interlayer Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Current density (mA/cm2) -10 -5 0 5 10 w/o MoO3 w/ MoO3 Voltage (V) 400 500 600 700 800 900 IPCE (%) 0 10 20 30 40 50 w/o MoO3 w/ MoO3

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86 (a ) (b ) ( c ) (d ) Figure 4 4 (a) JSC, (b) fill factor, (c) VOC, and (d) power conversion efficiency of the infrared absorbing OPV cells without and with the MoO3 interlayer as a function of thicknesses of the SnPc:C60 bulk heterojunction laye r. Thickness (nm) 4 8 12 16 20 VOC (V) 0.40 0.42 0.44 0.46 0.48 0.50 w/o MoOx w/ MoOx Thickness (nm) 4 8 12 16 20 FF 0.40 0.45 0.50 0.55 0.60 w/o MoOx w/ MoOx Thickness (nm) 4 8 12 16 20 JSC (mA/cm2) 3 4 5 6 7 w/o MoO3 w/o MoO3 Thickness (nm) 4 8 12 16 20 Power conversion efficiency (%) 0.8 1.0 1.2 1.4 w/o MoO3 w/o MoO3

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87 (a ) (b ) ( c ) (d ) Figure 4 5 Surface morphology of (a) bare ITO, (b) 10 nm thick MoO3 layer on ITO, (c) light absorbing layer (10 nm CuPc/10 nm SnPc:C60) o n ITO, and (d) light absorbing layer (10 nm CuPc/10 nm SnPc:C60) on 10 nm thick MoO3 layer on ITO light absorbing layer on ITO light absorbing layer on MoO 3 ITO MoO 3 on ITO

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88 CHAPTER 5 T HE EFFECT OF INTERLA YER ON ORGANIC PHOTOVOLTAIC CELLS Both small molecule photovoltaic cells and polymer photovoltaic cells were fabricated with molybdenum oxide interlayer at the ITO electrode W ith the molybdenum oxide interlayer the power conversion efficienc ies were enhanced by a maximum of 38% due to a significant enhancement in the fill factor. The improved fill factor with the molybdenum o xide interlayer is attributed to reduction of series resistance. F rom u ltraviolet photoemission spectroscopy, we found that the reduction in the series resistance is due to hole injection barrier lower ing in the AlPcCl layer with the molybdenum oxide inter layer. 5.1 Introduction Organic photovoltaic cells are attractive for next generation photovoltaics because of their compatibility with flexible substrates, low manufacturing costs and large area applications. W hile there has been improvement in power conv ersion efficiencies of both small molecules and polymer o rganic photovoltaic cells by developing new donor/acceptor materials and optimizing the bulk heterostructures [27, 39, 66] the power conversion efficiency (P) in o rganic photovoltaic cells has been limited to about 5% [28 30] Recently, it has been demonstrated that interlayer between an indium tin oxide (ITO) anode and the light absorbing layer could play an important role determining the efficiency of o rganic photovoltaic cells [67]. In polymer ph otovoltaic cells, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interlayer has been demonstrated to be effective to enhance both the short -circuit current (JSC) and the open-circuit voltage (VOC) [70]. Brabec et al. demonstrated tha t there is a correlation between the anode work function and the VOC of organic photovoltaic cells and that the enhancement due to the PEDOT:PSS interlayer is attributed to its large work function compared with the ITO workfunction [46]. While PEDOT:PSS is a commonly used interlayer

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89 material for organic solar cells its acidity can cause chemical instability at the ITO interface which degrad e s the device performance [71, 72] In organic light emitting diode s (OLED s ), interfacial engineering of the ITO surf ace has been used to enhanc e hole injection. In polymer OLED s, the insertion of a poly(9,9dioctylfluorene co N -(4 -butylphenyl) diphenylamine) (TFB) interlayer between a PEDOT:PSS and the light emitting polymer layer improve s both device efficiency and sta bility [73]. The enhancement due to the TFB interlayer was attributed to enhanc ement of hole injection and improvement of charge balance [74 76]. Recently, Wang et al. reported small molecule OLED s with improved hole injection and stability using a molybde num oxide (MoO3) inter layer between the ITO anode and the hole transporting layer [77]. For organic photovoltaic cells transition metal oxide interlayer has also been used [68, 69]. Shrotriya et al. demonstrated enhanced device performance using both vana dium oxide (V2O5) and MoO3 interlayer s for polymer photovoltaic cells [68]. More recently, Irwin et al. reported enhanced cell performance using a ptype nickel oxide interlayer [69]. In this work we study the effect of MoO3 interlayer on both small molec ule and polymer organic photovoltaic cells, and correlate the device results with the interface electronic structure measured by ultraviolet photoelectron spectroscopy (UPS). In addition to the MoO3 interlayer effects, we have also studied the effect of t he polymer interlayer on the polymer c ells. Recently, Hains et. al showed that the performance of polymer cells can be enhanced with a cross linked blend of TFB and 4,4 -bis[( p trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi) interlayer. The enhance d performance was attributed to improved hole extraction as well as the electron blocking function of the interlayer [67] In this work we demonstrate that thin double layers of MoO3 and TFB as an

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90 anode interlayer substantially enhances the photovoltaic p erformance of polymer cells through efficient hole extraction. 5.2 Experimental Detail s To fabricate o rganic photovoltaic cells, patterned ITO substrates with a sheet resistance of in an ultrasonic cleaner and then rinsed with de -ionized water, blown dry with nitr ogen gas, and subsequently treated with UV ozone. For the study to understand the effect of MoO3 interlayer on both small molecule and polymer organic photovoltaic cells f igure s 5 (a) and (b) show the structure of the polymer photovoltaic cells with a PE DOT:PSS and a MoO3 interlayer, respectively Figure s 5 (c) and (d) show the structures of the small molecule organic photovoltaic cell without and with a MoO3 interlayer, respectively. For polymer cells with a PEDOT:PSS as an interlayer, a 25 nm thick PEDO T:PSS (Baytron P VP Al 4083) layer was spin coated on the ITO substrates and the film was subsequently baked at 1 8 0 C for 15 min Both poly(3 hexylthiophene (P3HT) and poly(2 methoxy5 -(3,7 -dimethyloctyloxy) 1,4 -phenylene vinylene (MDMO -PPV) cells were fabricated for the polymer cells study. For P3HT cells, a (1:0.8 by weight) P3HT: {6,6} -phenyl C61 butyric acid methyl ester (PCBM) blend was used, and for MDMO -PPV cells, a (1:4) MDMO PPV :PCBM blend was used. To fabricate the polymer cells, the polymers wer e first dissolved in chlorobenzene and PCBM was subsequently added to the polymer solutions. The blends were then stirred for 2 0 hours at 45C in a nitrogen glove box before spin coating and LiF (1 nm) /Al (100 nm) electrode was thermally evaporated on the polymer layer as a cathode. For small molecule cells fabrication, the bulk heterojunction layers were deposited by co-evaporation of the donor materials (AlPcCl or CuPc) and C60. A 7.5 nm thick bathocuproine (BCP) was also deposited as an exciton blocking layer. For cells with MoO3 inter layer, MoO3 was thermally

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91 evaporated on patterned ITO substrates. To finish the small molecule cell fabrication, a 100 nm Al electrode was used as a cathode. The small molecule organic and metal layers were deposited in a vacuum chamber with a base pressure of 1 106 torr. The deposition rates were 0.5 /s and 1 /s for organic materials and Al, respectively. All small molecule organic materials were purified three times by train sublimation [112] The active area of the d evices used in this study was 0.04 cm2. For the study to understand the effect of the TFB interlayer on the polymer c ells t he active layer of t he polymer cell s in this study is a blend of MDMO PPV/PCBM thin film sandwiched between an ITO anode and a metal cathode. Here, we used various interlayer materials such as PEDOT:PSS, MoO3 and MoO3+TFB layers as the anode interface. To fabricate the cells, we spin coated a thin layer (30 nm) of PEDOT:PSS on top of the ITO coated glass substrate and the layer was ba ked at 180 C for 10 min. For devices with a MoO3 interlayer (10nm), a MoO3 thin film was thermally evaporated on a pre -cleaned ITO substrate under a vacuum of 106 torr. For cells with the double interlayer we spin -coated a thin film of TFB layer on the M oO3 interlayer from a 5 mg/ml TFB solution in toluene, followed by a solvent (toluene) wash. The glass/ITO/MoO3/TFB substrate was annealed in the nitrogen glove box at 75 C for 45 min. To deposit the bulk heterojunction film, a solution of MDMO PPV (4 mg) and PCBM (16 mg) was prepared from chlorobenzene (1 ml) and the blend was stirred for 16 hrs in glove box before spin coating. After the coating of active layer with a thickness of 140 nm, the substrate was annealed at 65 C for 45 min and the cathode LiF /Al (1nm/100nm) was thermally evaporated on top of active layer. The active area of the devices used in this study was 0.04 cm2. The current density versus voltage (J -V) characteristics were measured in the dark and under simulated air mass 1.5 global (AM1.5G) solar illumination. A 150 W ozone free xenon arc

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92 lamp with 1.5G air mass filter was used as a solar simulator. A Newport 70260 radiant power meter in combination with a Newport 70268 probe was used to measure the power densities of the stimulated sola r illumination. Using neutral density filters, the illumination level was varied between 35 mW/cm2 and 128 mW/cm2. National Institute of Standards and Technology (NIST) calibrated UV -enhanced silicon and germanium diodes were used to calibrate the measurem ents. The devices were tested in ambient conditions without encapsulation. For interface electron structure study, u ltraviolet photoemission spectroscopy studies were performed on the AlPcCl system using a VG ESCA Lab system equipped with a He I (21.2 eV) gas discharge lamp as the UV source. The integrated ultrahigh vacuum (UHV) system for UPS measurements consists of a UPS spectrometer chamber and an evaporation chamber in which thin film deposition was done in ultrahigh vacuum and transferred directly i nto the spectrometer chamber without exposing the sample to atmosphere. The base pressure of the spectrometer chamber was typically 81011 torr. For interlayer study, a 10 nm thick MoO3 interlayer was deposited onto a UV ozone treated ITO substrate by th ermal evaporation. Charging was not observed from the UPS spectra at different UV intensities during the measurements. The UPS spectra were taken with incremental thickness of the AlPcCl film to evaluate the evolution of the interface formation. The UPS spectra were recorded with the samples biased at 5.0 V to observe the true, low energy secondary cut -off. The typical instrumental resolution for UPS measurements ranges from ~0.03 to 0.1 eV with photon energy dispersion of less than 20 meV. All measur ements were done at room temperature. 5.3 Results and Discussion s 5 3 1 The Effect of MoO3 I nterlayer on O rganic P hotovoltaic C ells Figure 5 2 shows the photo J -V characteristics of the P3HT: PCBM cells with the PEDOT: PSS and MoO3 interlayer s Here, for interlayer study, the thickness of the P3HT: PCBM bulk

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93 heterojunction layer was kept at 1 4 0 nm The performance of the P3HT: PCBM cells with the PEDOT: PSS interlayer and the MoO3 interlayer is summarized in Table 5 1 From Figure 5 2, we found that for th e P3HT : PCBM cells t he VOC and the JSC of the MoO3 interlayer cells are slightly higher than that for the PEDOT: PSS interlayer cells While there is a small increase of VOC from 0 59 V to 0.61 V due to the MoO3 interlayer, there is a 10% increase in fill factor, indicating that replacing the PEDOT:PSS layer with MoO3 reduces the cell series resistance and enhances the cell shunt resistance. The s eries resistance and the shunt resistance can be calculated from the slope of the photo J -V curve close to 0 mA / cm2 and close to 0 V, respectively [115] The series resistance of the P3HT: PCBM cell with the MoO3 interlayer is about 40% lower than that with the PEDOT: PSS interlayer and the shunt resistance of the P3HT: PCBM cell with the MoO3 interlayer is about 10% higher than that with the PEDOT: PSS interlayer as shown in Table 5 1 The resulting power conversion efficiency of the P3HT: PCBM cell with the MoO3 interlayer is 16% higher than that with the PEDOT: PSS interlayer cells The effect of the MoO3 interlay er on MDMO -PPV cells are qualitatively similar and enhancements in short -circuit current and fill factor are slightly larger than those observed in the P3HT cells, resulting in about 16% enhancement in power efficiency as shown in Table 5 1 Details of the MoO3 interlayer effect on MDMO -PPV cells will be published elsewhere. Figure 5 3 shows the photo J -V characteristics of (a) the CuPc: C60 cells with and without the MoO3 interlayer and (b) the AlPcCl: C60 cells with and with out the MoO3 interlayer. The p erformance of the CuPc: C60 and AlPcCl: C60 cells is also summarized in Table 5 2 The effect of MoO3 interlayer on the small molecule cell performance is more pronounced compared with the polymer cells. T he VOC increases from 0 44 V to 0.48 V for the CuPc : C60 cells and from 0 79 V to 0.83 V for the AlPcCl: C60 cells. Compared to polymer cells the enhancement in the

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94 fill factor due to the MoO3 interlayer is more pronounced. The CuPc: C60 cell with the MoO3 interlayer has a fill factor 25% higher than that with out the MoO3 interlayer while the AlPcCl: C60 cell with the MoO3 interlayer has a fill factor 15% than that with out the MoO3 interlayer. The series resistance of the CuPc and AlPcCl cells with the MoO3 interlayer is about 35% lower than that with out t he MoO3 interlayer and the shunt resistances of the CuPc cell and the AlPcCl cell with the MoO3 interlayer are about 5 0% and about 20% higher than those without the MoO3 interlayer as shown in Table 5 2 respectively. That is, all photovoltaic cells with the MoO3 interlayer used in this study show about 40% reduction in the series resistance as shown in Table 5 2 However, the amount of the enhancement in the shunt resistances due to the MoO3 interlayer is exhibited differently with different donor/acceptor systems, showing strong correlation with the amount of the enhancement in the fill factor. The overall power conversion efficiency enhancements due to the MoO3 interlayer are 3 5 % and 2 0 % for the CuP c and AlPcCl cell s, respectively. To understand the effec t of the MoO3 interlayer, we measured the electronic structure of the AlPcCl/MoO3 interface by UPS. First, we measured ITO/AlPcCl interface without any interlayer. Figure 5 4 shows the UPS spectra at (a) low cut -off energy and (b) high HOMO energy on ITO/A lPcCl interfaces. The cut -off energy shifts for a small amount of 0. 2 eV was observed after AlPcCl is deposited onto ITO, and then becomes saturated. HOMO remains almost fixed as the AlPc Cl thickness increases. And then we measured ITO/AlPcCl interface wit h MoO3 interlayer. Figure 5 5 shows the UPS spectra at (a) low cut off energy and (b) high HOMO energy on ITO/ MoO3/ AlPcCl interfaces. The deposition of 100 MoO3 shifts the cut off energy to lower binding energy for 2 eV, leading to a n increase of the workfunction. The following deposition of AlP c Cl introduces a gradual shift back toward higher binding energy,

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95 suggesting that the workfunction is continously decreased. The shift of the cut -off energy has not become saturated even at the final step of the measurement (228 ). HOMO peak of MoO3 has a strong feature at about 18.2 eV, whose position remains fixed during the AlPcCl deposition. The strong HOMO peak of AlPcCl is observed after 16 of AlPc Cl is deposited. However, the HOMO peak shift s gradually toward higher binding e nergy as more AlPcCl is deposited. From these UPS results, we measured the electronic s tructures of the ITO/AlPcCl and the ITO/MoO3/AlPc Cl interfaces. Figure 5 6 shows the energy level alignment of (a) the ITO/AlPcCl and (b) ITO / MoO3/AlPcCl interfaces obtained from the UPS measurements For the sample without the MoO3 interlayer, a small shift of the vacuum level (~ 0.2 eV) was observed at the ITO/AlPcCl interface upon deposition of the first monolayer of AlPcCl (0.2 nm). The energy levels then remain constant with further deposition of AlPcCl, leading to a flat band situation where the highest occupied molecular orbital (HOMO) energy lies 1.0 eV below the Fermi level of ITO. O n the other hand, the insertion of t he MoO3 interlayer leads to a 2 eV increase in the effective work function of the ITO electrode as shown in Figure 5 6 (b). Here, the valence band maximum of the MoO3 layer is measur ed to be about 2.53 eV below its Fermi level indicating that MoO3 is slightly n type It should be noted that there is no significant interface dipole formation at the oxide and the organic interface due to the weak interaction between the two materials. [116] The large effective workfunction of MoO3 therefore leads to an upward shift of the HOMO level of AlPcCl, resulting in a strong band bending and hole accumulation at the MoO3/ AlPcCl interface. The band bending of the AlPcCl layer extends to about 10 nm and the bands become flattened as the film becomes thicker. [117] At the final thickness we probed (20 nm), the HOMO energy of AlPcCl only reaches about 0.5 eV below the Fermi level, which is in sharp contrast to the sample without the MoO3 interlayer wh ere the HOMO level of AlPcCl is

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96 1.0 eV below the Fermi level. In the PV cells with the MoO3 interlayer, the built in field due to band bending enhances hole extraction from the AlPcCl layer to the anode, resulting in a net reduction of the cell series resi stance and a net enhancement of the cell shunt resistance as shown in our device data Similar results have also been observed at the MoO3/ CuPc interface and details of the results will be reported elsewhere. 5 3 2 The Effect of TFB I nterlayer on P olymer C ells The energy level diagram of the interlayer materials used in the polymer cells and the molecular structures of the polymers (TFB, MDMO -PPV) used in our experiments is shown in Figure 5 5 Figure 5 6 shows the current density-voltage (J -V) characterist ics of the polymer cell s with various interlayers under illumination. From these J -V characteristics, it can be seen that the power conversion efficiency of the device without any interface layer (bare ITO surface) exhibits lower efficiency with a JSC of 2 .76 mA/cm2, VOC of 0.55V, fill factor of 0.44 and power conversion efficiency of 0.67 %. Inserting a PEDOT:PSS or MoO3 layer betwe en the ITO anode and the light absorbing layer provides a better hole extraction/electron blocking layer which leads to an inc rease in JSC, VOC and fill factor to 3.37 mA/cm2, 0.79 V and 0.49 respectively for the PEDOT:PSS device, and 3.64 mA/cm2, 0.81 V and 0.52 for the MoO3 device. Inserting both the MoO3 and TFB double -interlayer between the ITO anode and the polymer layer fur ther increases JSC, VOC and fill factor to 4.28 mA/cm2, 0.85 V and 0.55 respectively, re sulting in a 45% increase in power conversion efficiency to 2.01% compared to the PEDOT:PSS cell. It is noted that the efficiency of the device with PEDOT:PSS is sligh tly less than the reported efficiency by Hains et al [67] and may be attributed to the fact that all our measurements were done in air without any prior encapsulation. A thin second interlayer (TFB) on top of MoO3 interlayer substantially increases both JS C, and VOC. The enhancement in the JSC indicates effective hole extraction and electron blocking

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97 properties of the TFB interlayer. The shallow LUMO (2.3 eV) level of the TFB layer blocks the electrons effectively from reaching the anode (ITO) and therefore avoids recombination leading to increased in the performance. The performance of the polymer cell s with various interlayers such as MoO3, PEDOT:PSS and MoO3+TFB is summarized in table 5 3 It is noted that the fill factor of the polymer cell with MoO3+TFB layer increase by 12% (from 0.49 to 0.55) comparing to PEDOT:PSS due to effective blocking of electron leakage to anode by TFB layer. The IPCE spectra of the polymer cell s with the MoO3 and MoO3+TFB interlayers are shown in figure 5 7 T he IPCE spectrum o f the polymer cell with the MoO3+TFB interlayer is strong er than that of the polymer cell with the MoO3 interlayer at all wavelengths between 400 nm and 6 00 nm. The enhancement in the IPCE spectra is also consistent with the JSC. T he HOMO energy of the MDMO -PPV used as a donor is 5.3 eV and the HOMO energies of the inter layers such as PEDOT:PSS, MoO3 and TFB are 5.2 eV 5.3 eV and 5.3 eV respectively as shown in Figure 5 5 It was observed that the VOC of the polymer cells varies with different interlayer s as shown at Table 5 3 ). In organic photovoltaic cell, the VOC depends on the energetic difference between the HOMO of the donor and the LUMO of the acceptor [113] but also influenced by the work function difference between the electrodes. [118] Since, the donor and acceptor materials are same in all these devices, the difference in VOC should be due to the inter layer s between the light absorbing layer and the ITO electrode which modifies the work function of the anode. These results are comparable with the previous study by Brab ec et. a l in which they found that the VOC shows a linear depend e nce on work function of the anode [118] On the other hand, t he morphology of the inter layer between the ITO electrode and the light absorbing layer has significant in fluence on the performance of polymer cells. In order to understand the interface properties, the morphology of the interlayer s on the ITO substrate was

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9 8 explored. Figure 5 8 shows the AFM image of PEDOT:PSS and TFB coated MoO3 interlayers. Here, the surfac e morphology of TFB layer is very smooth with a rms roughness of 0.340 nm whereas PEDOT:PSS surface shows higher roughness with a rms roughness of 1.12 nm. These results suggest that the improved surface smoothness is responsible for enhanced device perfor mance of the MoO3+TFB double interlayer comparing to the PEDOT/ MoO3 layer. 5.4 Conclusion s In conclusion, we have demonstrated the enhancement in the power conversion efficiency due to the MoO3 interlayer in the small molecule photovoltaic cells as well as the polymer photovoltaic cells. T he power conversion efficiencies of the organic photovoltaic cells with the MoO3 interlayer were enhanced by a significant increase in the fill factor, which is due to reduction in series resistance. Our photoelectron spectroscopy data indicate that the reduction in series resistance can be attributed to band bending in the AlPcCl layer in the presence of the MoO3 interlayer and the built -in field at the interface which enhances hole extraction from the AlPcCl layer towar ds the ITO anode On the other hand, we have also demonst rated that the MoO3 + TFB double inter layer can be effectively used as efficient hole extracti ng/electron blocking layer for polymer cells

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99 (a) (b) ( c ) ( d) Figure 5 1 Schematic diagram s of (a) the P3HT:PCBM cell and the MDMO PPV:PCBM cell with the PEDOT:PSS interlayers, (b) the P3HT:PCBM cell and the MDMO PPV:PCBM cell with the MoO3 interlayer, (c) the CuPc:C60 cell and the AlPcCl:C60 cell with o ut MoO3 i nterlayers, and (d) the CuPc:C60 cell and the AlPcCl:C60 cell with the MoO3 interlayer.

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100 Figure 5 2 The photo J -V characteristics of the P3HT:PCBM cells with the PEDOT:PSS interlayer (Black line) and with the MoO3 i nterlayer (Red line). Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Current density (mA/cm2) -10 -5 0 5 10 PEDOT:PSS interlayer MoO3 interlayer

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101 Table 5 1. Summary of infrared absorbing polymer photovoltaic cells with out and with the MoO3 inter layer Donor: Acceptor Interlayer Jsc (mA/cm2) Voc (V) FF (%) np (%) RSA( cm2) RPA( cm2) PEDOT 7.950.4 0.59 60.51 2.850.1 12.91 79750 MoO38.200.4 0.61 66.51 3.310.1 7.51 87350 PEDOT 3.070.4 0.80 481 1.190.1 351 77050 MoO33.270.4 0.82 521 1.380.1 211 85050 P3HT: PCBM MDMO PPV :PCBM

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102 (a) (b) Figure 5 3 The photo J -V characteristics of (a) the CuPc:C60 cells with and without the MoO3 interlayer and (b) the AlPcCl:C60 cells with and without the MoO3 interlayer. Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Current density (mA/cm2) -10 -5 0 5 10 No interlayer MoO3 interlayer Voltage (V) -1.0 -0.5 0.0 0.5 1.0 Current density (mA/cm2) -10 -5 0 5 10 No interlayer MoO3 interlayer

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103 Table 5 2 Summary of infrared absorbing small molecule organic photovoltaic cells with out and with the MoO3 inter layer Donor: Acceptor Interlayer Jsc (mA/cm2) Voc (V) FF (%) np (%) RSA( cm2) RPA( cm2) No 6.300.4 0.45 401 1.100.1 19.31 22650 MoO36.640.4 0.46 501 1.490.1 13.21 33550 No 5.380.4 0.79 401 1.700.1 33.71 37650 MoO35.670.4 0.79 461 2.040.1 22.41 44250 AlPcCl/ AlPcCl:C60 CuPc/ CuPc:C60

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104 (a) (b) Figure 5 4 UP S spectra at (a) low cut -off e nergy and (b) high HOMO energy on ITO/ AlPcCl interfaces

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105 (a) (b) Figure 5 5 UPS spectra at (a) low cut -off e nergy and (b) high HOMO energy on ITO/MoO3/ AlPcCl interfaces

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106 (a) (b) Figure 5 6 Energy level alignment at (a) the ITO/ AlPcCl and (b) ITO/MoO3/ AlPcCl interfaces 0.4 eV 4.5 eV 2.0 eV 0.5 eV 0.8 eV 10 nm 5.3 eV 2.53 eV ITO AlPc-Cl MoO3 0.7 eV 0.4 eV 4.5 eV 2.0 eV 0.5 eV 0.8 eV 10 nm 5.3 eV 2.53 eV ITO AlPc-Cl MoO3 0.7 eV 4.5 eV = 0.2 eV 1.0 eV 5.3 eV 0.3 eVITO AlPc-Cl 4.5 eV = 0.2 eV 1.0 eV 5.3 eV 0.3 eVITO AlPc-Cl

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107 Figure 5 7 (a) Schematic diagram showing energy level of electrode, various interface layer s (MoO3, TFB, PEDOT:PSS) and active layer. (b) Molecular structure of the polymer MDMO -PPV and (c) TFB (b) (c)

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108 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -4 -2 0 2 4 6 Current Density (mA/cm2)Voltage (V) ITO ITO/Pedot ITO/MoO3 ITO/MoO3/TFB Figure 5 8 The photo J -V characteristics of a MDMO PPV/PCBM based polymer cell s with various interlayers such as ITO, ITO/PEDOT:PSS, ITO/MoO3 and ITO/MoO3/TFB

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109 Table 5 3 Summary of MDMO -PPV/PCB M cell with various interlayer s Anode Interface layer J sc (mA/cm2) V oc (V) FF (%) PCE (%) Without Interface layer 2.76 0.55 44 0.67 PEDOT 3.37 0.79 49 1.31 MoO 3 3.64 0.81 52 1.53 MoO 3 +TFB 4.28 0.85 55 2.01

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110 Figure 5 9 The IPCE spectra of the MDMO -PPV: PCBM device with MoO3 and MoO3+TFB interlayers

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111 Figure 5 10. (a) Surface morphology of PEDOT PSS inter layer and (b) MoO3/TFB inter layer

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112 CHAPTER 6 ORG A NIC PHOTODETECTORS H igh gain organic photodetector s using 3,4,9,10-perylenetetracarboxylic dianhydri de (PTCDA ) w ere fabricated W e found that high dark current is necessary for high gain organic p hotodetector s T he maximum gain of the PTCDA organic photodetector is 2866 at 1 5 V under 532 nm monochromatic light Also, low dark current o rganic photodetec tor s w ere fabricated by using co pper p hthalocyanine (CuPc) / C60 bulk heterojunction structure T he EQE of the organic photodetector is 50 % at 1 V. In addition, infrared organic photodetector s using t in (II) Phthalocyanine (SnPc) were fabricated. The infrar ed organic photodetector showed strong photoresponse at wavelengths between 600 nm and 9 00 nm T he maximum external quantum efficiency of the infrared organic photodetector is 96 % at 5 V under 740 nm monochromatic light. 6.1 Introduction Organic p hotode tectors and organic photovoltaic cells are attracting a great deal of attention because of compatibility with flexible substrates, low cost process, and large area applications. [36, 44, 84, 85] In both cases of single molecules organic and polymer solar c ells, power conversion efficiencies of more than 5% have recently been demonstrated [41, 42] Also, o rganic photodetector s with the high external quantum efficiency (EQE) of over 70% at 10 V have already been reported by a multi layer method of copper phtha locyanine (CuPc) and 3,4,9,10perylenetetracarboxylic bisbenzimidazole (PTCBI) as donor and acceptor respectively [78] However, organic photodetector s using multilayers of CuPc and PTCBI showed EQEs below 10 % at low applied voltages (< 1 V) and the devices exhibit a strong field dependence. [82] The strong field dependence is due to the short exciton diffusion length of PTCBI enhancing

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113 the dissociation rate of photo -generated excitons. [83] Small molecule OPV cells with 2 times higher power conversion e fficiency were reported by substitut ing a C60 with an exciton diffusion length of about 77 for a PTCBI with an ex c iton diffusion length of about 30 [83] Also organic photodetector s with gain (quantum efficiency >100 %) have been also reported from s everal research groups. [86 88] Gain in p hotodetector means that the conversion efficiency from incident photon to electrical carrier exceeds unity. Especially, Hiramoto et. al. had reported organic photodetector s with gain of greater than 103 in organic diodes based on evaporated perylene pigments. [88] The origin of g ain was interpreted as photo controlled tunneling injection of electrons due to hole trapping at the interface between the metal electrode and the perylene film. The report of the ultra high gain organic photodetector is very interesting as the sensitivity in photoconductors is one of the most important properties. In addition t heir lightweight, ruggedness, and compatibility with flexible substrates and large area processing opens up many applications that cannot be addressed using other conventional detector technologies. However, organic photodetectors, which were reported up to now, have show n s spectral response only limited to the visible wavelength range. [90] Therefore, realization of organic photodetector with infrared (IR) sensitivity can extend the applications of organic electronics to large area sensing and detection. In this study we have demonstrate d organic photodetector with high gain of more than 103. In addition we also f ound that the ultra high gain organic photodetector s always associate with h igh dark current in the organic photodetectors I n addition, we demonstrated the organic photodetector s with high EQEs even at an operating voltage as low as 1 V using a CuPc:C60 b ulk heterojunction layer. A lso, g ain, which means that the conversion efficiency fr om incident photon to charge carrier exceeds unity was observed when the high bias was applied on the

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114 organic photodetector s under low input light power intensities. F inall y, we fabricated IR organic photodetector using SnPc:C60 mixed layer SnPc ha s a strong absorption in the near infrared (NIR) region (700 nm ~ 1000 nm) [52] O n the other hand, a low dark current is important in photodetector s for increasing sensitivity. W e incorporated molybdenum oxide (MoO3) and bathocuproine (BCP) carrier blocking layers at indium tin oxide (ITO) interface and Al interface, respectively, and found that the charge carrier blocking layer de creases a dark current. 6.2 Experimental Detail s A ll organic photodetector s were fabricated on patterned ITO substrates with a sheet isopropanol in an ultrasonic cleaner and then rinsed with de ionized water, blown dry w ith N2 gas, and subsequently treated with UV ozone for 15 minutes. PTCDA, CuPc, SnPc, and C60 w ere purified at least two times by train sublimation technique. [112] All layers in organic photodetector s were vacuum deposited at a pressure of 1 106 Torr. The area of the all devices is 0.04 cm2. For high gain organic photodetectors, a PTCDA film with thickness of 500 nm were vacuum d eposited as light absorbing layer A 100 nm thick A u w as used as a top electrode. The deposition rate w as 1 /s for both orga nic materials and A u The current density versus voltage (J -V) characteristics were measured in the dark and under green laser of 532 nm wavelength from Lasermate Group The light intensity of 250 W/cm2 was controlled by neutral density filter and Newport Optical Power Meter 840 -E. The gain was calculated as the ratio of the number of charge carriers flowing through the device by the light illumination to the number of photons absorbed by the organic film. Au electrode was always ground and the voltage bia s was applied on ITO electrode. The devices were tested in air without encapsulation.

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115 For low operating organic photodetectors, t he planar heterojunction OPD with the structure of ITO/CuPc (20nm)/C60 (40nm)/BCP (10nm)/Al (100nm) and the bulk heterojunction OPD with the structure of ITO/CuPc (10nm)/CuPc:C60 (40nm)/C60 (10nm)/BCP (10nm)/Al (100nm) were fabricated. A CuPc and C60 were used as donor and acceptor, respectively A 10 nm thick bathocuproine (BCP) and a 100 nm thick Al top electrode were used as exciton blocking layer and cathode, respectively. The Cu Pc:C60 mixed layer s were deposited by co-evaporation. The deposition rates were 0.5 /s and 1 /s for organic materials and Al, respectively. The current density versus voltage (J -V) characteristics we re measured in the dark and under 520 nm and 630 nm laser using a Keithley 2400 source meter ITO electrode was always ground and the Al electrode was applied by positive bias. A Newport Optical Power Meter 840 E was used to measure the power densities of the 520 nm and 630 nm laser Using neutral density filters, the incident power densities for the test cell were varied to 0.23 mW/cm2, 1.70 mW/cm2, and 13.75 mW/cm2. The incident light was irradiated through the ITO electrode. The devices were tested in ai r without encapsulation For IR organic p hotodetector s, A ll IR organic photodetectors had 100 nm thick SnPc:C60 mixed layer. A 10 nm thick MoO3 and 10 nm thick BCP were used as hole and electron blocking layer, respectively A 100 nm thick Al w as used as c athode The SnPc:C60 mixed layer s were deposited by co -evaporation. The deposition rates were 0.5 /s and 1 /s for organic materials and Al, respectively. The spectral responsivities and the external quantum efficiencies were measured with a 150 W ozone free xenon arc lamp, a monochromater and a lock-in amplifier under voltage bias of 0 V, 1V, 3 V, and 5 V National Institute of Standards and Technology (NIST) calibrated UV -enhanced silicon and germanium photodetectors were used to calibrate the measu rements. The current density versus voltage (J -V) characteristics were measured in the

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116 dark and under monochromatic lights of 740 nm, 830 nm and 900 nm wavelengths. Al electrode was always ground and the voltage bias was applied on ITO electrode. The devic es were tested in air without encapsulation. 6.3 Results and Discussion s 6 3 1 High G ain O rganic P hotodetector s Figure 6 2 show s the g ain as a function of applied voltage. Under negative voltages, the maximum gain of 2866 at 15 V was obtained. A lso, the h igh gain of 1100 at 12 V was also achieved under positive voltages These results are similar with p reviously work by Hiramoto et al. which showed the maximum gain of about 10,000 at 16 V using a N -methyl 3,4,9,10perylenetetracarboxyl diimide ( M e -PTC) pi gment film [88] This phenomenon can be explain ed by a tunneling injection of electrons from an ITO electrode to the PTCDA organic layer due to the photo accumulated space charges by trapped holes near the organic/ITO interface. [88] On the other hand, the high gain organic photodetector using the PTCDA film showed high dark current density as Figure 6 3. T he dark current densit ies w ere 977.5 mA/cm2 and 1225 mA/cm2 at 15 V and 12 V, respectively. Also, the photo current density was 1442.5 mA/cm2 and 1297.5 mA/cm2 at 15 V and 12 V, respectively. The dark current densities were close to the photo current densities under same applied voltages. The l owest u noccupied m olecular o rbital (LUMO ) and the h ighest o ccupied m olecular o rbital (HOMO ) of a PTCDA were 4.5 e V and 6.8 eV, respectively and w ork function s of an ITO and a Au also were 4.7 eV and 5.1 eV, respectively Thus, the energy barriers of 0.2 eV at the ITO/PTCDA interface and 0.6 eV at the Au/PTCDA interface were expected. T he small energy barriers can eas ily inject electrons from both the ITO electrode and the Au electrode. However, t he dark current density under positive bias on ITO electrode is higher than that under negative bias on ITO electrode T his means that the height of the energy barriers in act ual device might be different due to the presence of

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117 interface dipole. T hat is, the energy barrier of the ITO/PTCDA interface might be higher that of the Au/PTCDA interface. I n addition, t he dark current density increases exponentially with increasing appl ied voltages This exponential increase of the dark current density indicates that the dark current density is governed by the height of the energy barrier and applied voltages with Fowler -N ordheim equation. T he schematic band diagram as shown in Figure 6 4 (a) illustrate the situation when the device is under bias and under photoexcitation. T he origin of dark current density can be explained by el ectron injection at the ITO/PTCDA and the Au/PTCDA interface s as shown in Figure 6 4 (a). As shown at Figure 6 3, the net photo generated current density, the differenc e of photo current densities and dark current densities increase exponentially with increased dark current densities as shown at linear scale of Figure 6 3. That is, the number of charge carriers fl owing through the device under the light illumination increase exponentially with increased dark current. T he 532 nm incident green light with the same intensity of 250 W/cm2 was irradiated on high gain organic photodetector under all applied voltages. T h at is, the number of photons absorbed by the organic film is same under all applied voltages. Therefore, the small change of the electron injection barrier by trapping of photogenerated holes brings a huge difference in the amount of electrons injected by tunneling barrier under high dark current as shown at Figure 6 4 (b). This is similar with photocurrent gain mechanism in a Si:H schottky diode [119] However, even though the dark current density under positive bias on ITO electrode is larger than that u nder negative bias on ITO electrode, the gain on negative voltage is higher than that on positive voltage. T his is because the PTCDA is thick film with 500 nm thickness and the 532 nm green light is irradiated through ITO side. W hen light was irradiated th rough the ITO electrode, the incident light was absorbed at near ITO/PTCDA interface and the excess holes

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118 were generated at near the ITO/PTCDA interface due to the thick PTCDA film. Under negative bias on the ITO electrode, the photogenerated holes are tra pped at the ITO/PTCDA interface,. Thus, t he trapped holes cause huge influence on electron injection barrier height. H owever, under positive bias on the ITO electrode, the photogenerated holes at ITO/PTCDA interface have to travel through the bulk PTCDA fi lm to the Au/PTCDA interface. T he some photogenerated hole s are lost during transport across the organic layer thus caus ing relatively small influence on electron injection barrier height. 6 3 2 Low O perating V oltage O rganic P hotodetector Figure 6 5 (a) shows the EQE as a function of applied voltage s T he dotted line and the solid line are the EQE of the organic photodetector s with planar heterojunction and bulk heterojunction structures respectively. The EQE values under no applied bias (0V) are 6 % and 34 % for the planar heterojunction and the bulk heterojunction organic photodetectors respectively. The EQE of the bulk heterojunction organic photodetector is 5 times higher than that of the planar heterojunction organic photodetector The enhancement i n the EQE of the bulk heterojunction organic photodetector is due to an increase in the photo current density under zero applied bias as shown in Figure 6 5 (b) In planar heterojunction organic photodetector s, exciton dissociation occurs at donor acceptor interface s Thus, if the exciton diffusion length (LD) of donor or acceptor is shorter than the thickness of the donor or acceptor layer, most excitons generated will recombine before diffusing to the donor acceptor interface s [39] On the other hand, bul k heterojunction organic photodetector s have larger interfacial area compared with the planar heterojunction organic photodetector s, thus creating an interpenetrating network with a spatially distributed interface that lies within LD of the photo-generated excitons. [39] Therefore, the bulk heterojunction structure increases the photo current density even at no applied bias due to an increase of the exciton dissociation efficiency leading to an increase in EQE. [39]

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119 On the other hand, while the EQE of the planar heterojunction organic photodetector is almost constant with increasing the applied voltage up to 7 V, t he EQE of the bulk heterojunction organic photodetector increases drastically to 56 % with increasing the applied voltage up to 2 V and is then sa turated to 61 % with increasing the applied voltage up to 7 V. T his strong field dependence in the bulk heterojunction organic photodetector at applied voltages up to 2 V can be explained by the charge collection efficiency which increases with increasing the applied voltage. A s mentioned before the bulk heterojunction structure enhances the exciton dissociation efficiency leading to an increase in the photo-generated charge carriers. However, the bulk heterojunction structure reduces the charge collection efficiency recombining some of the photo -generated charge carriers as the bulk heterojunction structure degrades the charge transport due to lowering of carrier mobilities of the mixed donor/acceptor network. On the other hand, the charge collection effic iency in the bulk heterojunction organic photodetector with the poor charge transport property increases with increasing the applied bias as the number of the recombined charge carriers decreases with increasing the applied bias due to the reduction in tra nsport time of the charge carriers accelerated along the field direction. T herefore, the strong field dependence in the bulk heterojunction organic photodetector at applied voltages up to 2 V is due to the enhanced charge collection efficiency with increas ing the applied voltage. Also, after a significant increase of the EQE up to 2 V, t he saturated EQE values up to 7 V imply that the charge collection efficiency increases with the increased bias reaching 100 % at 2 V and is then maintained to 100 % up to 7 V. T he EQE value s and the photo current densities b eyond a n applied bias of 7 V increase exponentially for both planar heterojunction and bulk heterojunction organic photodetector s. It has been proposed that the rapid increase of the EQE value s and the ph oto current densities

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120 under high applied voltages is strongly related to enhancement in tunneling injection currents due to a high dark current level compared to a photo current level. [88, 119] This point will be discussed later. W hile the dark current de nsity in the bulk heterojunction organic photodetector is lower than that in the planar heterojunction organic photodetector at 0 V, the dark J -V characteristics in the bulk heterojunction organic photodetector shows the stronger bias dependence than that in the planar heterojunction organic photodetector leading to higher dark current densities beyond 3 V. A s mentioned before the bulk heterojunction structure degrades the charge transport due to lowering of carrier mobilities of the mixed donor/acceptor network. The bulk heterojunction structure shows very different device characteristics compared with the planar heterojunction structure In the bulk heterostructure the donor and acceptor mixture leads to charge trapping and inhibit carrier transport. Carrier trapping results in a field dependent mobility and a fielddependent dark current Figure 6 6 (a ) shows the J -V characteristics of bulk heterojunction organic photodetector in the dark and under 630 nm laser light with incident power densities of 0.23 mW/cm2, 1.70 mW/cm2 and 13.75 mW/cm2. T he photo -current densities increase exponentially beyond a n applied bias of 4 V 6 V, and 8 V at incident power densities of 0.23 mW/cm2, 1.70 mW/cm2 and 13.75 mW/cm2 respectively, exhibiting comparable levels with d ark current density. This means that the tunneling injection current by high dark current is the dominating factor for the bulk heterojunction organic photodetector u nder low input light intensit ies such as 0.23 mW/cm2 and 17.00 mW/cm2. The intervals betwe en photo current density and dark current density under log scale are almo st same under high applied bias of more than 5V at 0.23 mW/cm2, and more than 7V at 17.00 mW/cm2. That is, the net difference between photo currents and dark currents

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121 increase expone ntially with increased dark current under high applied bias. This means that the currents by the tunneling injection are more dominant than that by photogeneration under both low input light intensity and high applied bias. Figure 6 6 (b ) e xhibits the EQE of the bulk heterojunction organic photodetector as a function of applied voltage s under different input power intensities T he voltage s at which the EQE increases exponentially, are lowered by decreasing input power intensity to 6 V and 2 V at input powe r intensities of 1 7 0 mW/cm2 and 0.23 mW/cm2, respectively. F urthermore t he g ain which means that the conversion efficiency fr om incident photon to charge carrier exceeds unity is obtained on input light intensities of 0.23 mW/cm2, and 1 70 mW/cm2 at 6 V and 8 V, respectively T he schematic band diagram s as shown in Figure 6 7 illustrate the situation when the device is under bias and under photoexcitation. T he origin of the gain under high operating voltage and light irradiation can be explained by el ectr on injection at the ITO/ CuPc interface s due to the change of the electron injection barrier by trapping of photogenerated holes as shown in Figure 6 7 ( d ). 6 3 3 Infrared organic photodetector F igures 6 8 show s s chematic diagrams of IR organic photodetector s (a) with no charge blocking layers, (b) with MoO3 blocking layer, (c) with BCP blocking layer, and (d) with both MoO3 and BCP blocking layers. Figure 6 9 shows the dark J -V characteristics on IR organic photodetector s. T he organic photodetector with bot h MoO3 and BCP blocking layer s exhibit s lowest dark current density, with negligible voltage dependence. T he dark current density increases just half order from 0 V to 5 V. The dark current density with both MoO3 and BCP blocking layers is 4 orders lower than that without any interlayers at 5 V. On the other hand, as shown in F igure 6 9 (b) and (c), we found that most reduction of dark current density is due to BCP blocking layer. T he organic photodetector with only MoO3 blocking layer shows similar dar k current density to that with no charge blocking layers at low voltages and one order lower

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122 dark current density even at 5 V. However, the dark current density of IR organic photodetector with only BCP blocking layer is close to that with both MoO3 and BCP blocking layers. T he low dark current in photodetector is important for detecting signal clearly because the high dark current close to photo current will make a signal be hardly distinguishable from a background noise. [120] Figure 6 10 shows (a) the spectral responsivities and (b) the external quantum efficiencies of the IR organic photodetector with both MoO3 and BCP blocking layer s under 0 V, 1V, 3 V, and 5 V. The IR organic photodetector with both charge blocking layers shows strong photorespo nse at wavelengths between 600 nm and 9 00 nm exhibit ing IR detect ing ability. The spectral photoresponse spectra of the IR organic photodetector are also consistent with the optical absorbance spectr um for a 100 nm thick SnPc :C60 mixed film. T he peak inte nsity of the external quantum efficiency is 96 % at 740 nm wavelengths under 5 V. Also, the external quantum efficiencies under 5 V are 62 % and 22 % at 830 nm and 900 nm, respectively. On the other hand, the optical absorbance spectrum of the 100 nm thick SnPc film is different with the optical absorbance spectrum of the 100 nm thick SnPc:C60 mixed film as well as the spectral photoresponse of the IR organic photodetector. T he 100 nm thick SnPc film has two peaks of 740 nm and 890 nm at the absorbance s pectrum. H owever, the IR organic photodetector has only 740 nm peak at the spectral photoresponse. It is known that the peaks at 740 nm and 890 nm are due to monomer and dimmer absorption, respectively. [53] Thus, this means that SnPc in the SnPc:C60 mixed layer only exist in monomer phase due to the presence of C60. Figure 6 11 shows (a ) r esponsivit ies and external quantum efficienc ies of the IR organic photodetector with both MoO3 and BCP blocking layers as a function of applied voltages under input lig ht irradiation with different wavelength s of 740 nm, 830 nm, and 900 nm. T he external

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123 quantum efficiencies of the IR organic photodetector increase with increasing applied voltages. The external quantum efficiencies increase from 22 %, 14 %, and 4 % at 0 V to 95 %, 66 %, and 25 % at 5 V under input light irradiation of 740 nm, 830 nm, and 900 nm, respectively. In addition, the increase rate of external quantum efficiencies decreases at higher applied voltages. That is, the external quantum efficiency of th e IR organic photodetector is saturated at 5 V. A donor/acceptor mixed layer structure has an advantage to enhance the exciton dissociation efficiency due to increased donor/acceptor interfaces, while the mixed layer degrades the charge transport due to lowering of carrier mobilities of the mixed donor/acceptor network [81] In addition, a SnPc has low hole mobility of 2 1010 cm2/Vs and the thick SnPc:C60 mixed layer of 100 nm is used in this experiment. T hus, it is expected that the photo -generated ch arge carriers are not collected fully under 0 V. However, the external electric fields can help the charge carriers go over the shallow energy traps, leading to the enhancement in the collection yield of the photo -generated charge carriers with increasing applied voltages, and thus leading to enhancement in external quantum efficiency. F rom the F igure 4, we know that most of photogenerated charge carriers are collected even at relatively low applied voltages below 3V and thus the external quantum efficie ncy is saturated at relatively higher applied voltages until 5 V. 6.4 Conclusion s T he organic photodetector with gain of greater than 103 was fabricated. This phenomenon was explain ed by a mechanism based on the tunneling injection of electrons at the IT O/PTCDA interface and Au/ PTCDA interface H owever, the high gain organic photodetector s showed high dark current. It is thought that the small change of the electron injection barrier by trapping of photogenerated holes brings the huge difference in the amount of electrons injected by tunneling barrier under high dark current. Also, low operating voltages organic photo detector with high EQE of 50 % a t 1 V was demonstrated by using bulk heterojunction of CuPc and C60 as donor

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124 and accepter. The strong field dependence of t he EQE o n bulk heterojunction OPD at low applied voltages of below 3V is due to increase of charge collection rate by increased applied voltages. T he EQE of both bulk heterojunction OPD and planar heterojunction OPD increases exponentially a t high a pplied voltages by increasing tunneling injection current by high dark current. T he applied voltages, at which the EQE increases exponentially, are reduced by decreasing input power density. In addition w e fabricated the infrared organic photodete ctor using the SnPc:C60 mixed layer with low dark current and high external quantum efficiency, exhibiting high photoresponse at wavelength from 600 nm to 900 nm. Both MoO3 and BCP interlayers decreases drastically the dark current density even at high app lied voltages. T he maximum external quantum efficiency is 96 % a t 740 nm wavelength under applied voltage of 5 V. Also, the IR organic photodetector shows the strong electric field dependence on the external quantum efficiency.

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125 Fi gure 6 1 Schematic diagram of high gain organic photodetector.

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126 Figure 6 2 Gain -voltage characteristics of high gain organic photodetector

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127 Figure 6 3 Dark and photo I -V characteristics of high gain organic ph otodetector

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128 (a) (b) Figure 6 4. Proposed band structure for PTCDA high gain photodetector on (a) Dark and (b) Photo.

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129 (a) (b) Figure 6 5. (a) External quantum effici ency of organic photodetectors with planar heterojunction structure ( d otted line) and bulk heterojunction structure ( s olid line) as a function of applied voltages, (b) current density-voltage characteristics on dark ( s olid line) and photo ( d ash line) situa tion as a function of applied voltages on organic photodetectors with planar heterojunction structure ( b lack line) and bulk heterojunction structure ( r ed line). Voltage (V) 0 2 4 6 8 QE (%) 0 20 40 60 80 100 Mixed Planar Voltage (V) 0 2 4 6 8 Current density (mA/cm2) 10-510-410-310-210-1100101102 Dark 630 nm Planar Bulk

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130 (a) (b) Figure 6 6. (a) Current density -voltage characteristics of the bulk heterojunction organic photodetector as a function of applied voltages under various input light intensities; dark (black), 0.23 mW/cm2 (red), 1.70 mW/cm2 (green), and 13.75 mW/cm2 (yellow) and (b) external quantum efficiency of the bulk heterojunction organic photodetector as a function of applied voltages under various input light intensities; (a) 0.23 mW/cm2 (red), 1.70 mW/cm2 (green), and 13.75 mW/cm2 (yellow). Voltage (V) 0 2 4 6 8 QE (%) 10 100 1000 0.23 mW/cm2 1.70 mW/cm2 13.75 mW/cm2 Input light intensity (630 nm) Voltage (V) 0 2 4 6 8 Current density (mA/cm2) 10-510-410-310-210-1100101102 Dark 0.23 mW/cm2 1.70 mW/cm2 13.75 mW/cm2

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131 Figure 6 7. Schematic diagram of device operation; (a) low ope rating voltage and no light irradiation, (b) low operating voltage and light irradiation, (c) high operating voltage and no light irradiation, and (d) high operating voltage and light irradiation. (a) (b) (c) (d)

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132 (a) (b) ( c ) ( d) Figure 6 8. Schematic diagram of near IR photodetector (a) with out any interlayers, (b) with MoO3 interlayer, (c) with BCP interlayer, and ( d ) with both MoO3 and BCP interlayers

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133 Figure 6 9. Dark c urrent density-voltage characteristics on IR photodetector without any interlayers (Black line), with MoO3 (Red line), with B CP (Green line), and with MoO3 and BCP (Blue line) Voltage (V) -4 -2 0 2 Current density (mA/cm2) 10-410-310-210-1100101102103 No interlayer MoO 3 BCP MoO 3 and BCP

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134 (a) (b) Figure 6 10. (a) Spectral responsivities and (b) External quantum efficienc ies of infrared organic photodetector as a function of wavelength under different applied voltages ; 0 V (Black), 1 V (Red), 3 V (Green), and 5 V (Blue) and optical absorbance of 100 nm thick SnPc film (Dotted line) and 100 nm thick SnPc:C60 mix ed film (Dashed line) Wavelenght (nm) 400 500 600 700 800 900 1000 Spectral Responsivity (A/W) 0.0 0.2 0.4 0.6 0.8 0 V -1 V -3 V -5 V Absorbance (a.u.) 0.0 0.2 0.4 0.6 0.8 1.0 SnPc:C60SnPc Wavelength (nm) 400 500 600 700 800 900 1000 External Quantum Efficiency (%) 0 20 40 60 80 100 0 V -1 V -3 V -5 V Absorbance (a.u.) 0.0 0.2 0.4 0.6 0.8 1.0 SnPc:C60SnPc

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135 (a) (b) Figure 6 11. (a ) Responsivity and external quantum efficiency as a function of applied voltages at light irradiation with different wavelength ; 740 nm (Black), 830 nm (Red), and 900 nm (Green) Voltage (V) -5 -4 -3 -2 -1 0 Responsivity (A/W) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 740 nm 830 nm 900 nm Voltage (V) -5 -4 -3 -2 -1 0 External quantum efficiency (%) 0 20 40 60 80 100 740 nm 830 nm 900 nm

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136 CHAP TER 7 L IGHT UP CONVERSION DEVICE W e demonstrated infrared to -green light up -conversion devices using phosphorescent OLEDs with an infrared sensitizing layer A SnPc and a 4,4 N,Ndicarbazole -biphenyl (CBP) host with 7 % fac tris(2 -phenylpyridinato)iridium(I II) (Irppy3) dopant were used as an infrared sensitizing layer and as a phosphorescent light e mitting layer respectively The device shows a current efficiency 106 c d/A at a brightness of 100 c d/m2 under 830 nm IR irradiation. The maximum photonto -photon conversion efficiency is 2.7% at 15V. We also demonstrated red -to green light up -conversion device s using CuPc:C60 bulk heterojunction layer as the absorbing layer in the photodetector in conjunction with an inverted top emission OLED 7.1 Introduction T he realization of organic photodetector with infrared (IR) sensitivity can extend the applications of organic electronics to large area sensing and detection. In addition, the novel light up -conversion devices realized by integrating organic light emitting diode (OLED) on IR photodetector can be used a n IR imaging device, which can convert invisible infrared information to visible image s In this chapter we will describe the fabrication and the characteristics of a red to green as well as an infrared -to -gr een light up conversion device. A schematic diagram of such a novel IR imaging device is shown in Figure 7 -1. I n previous studies (Chapter 6) we already reported high quantum efficiency infrared organic photodetector s using tin (II) phthalocyanine (SnPc) absorbing in the 600 ~ 1000 nm of the spectrum as shown Figure 7 2 and this SnPc photodetector will be used as the light detection part of the up-conversion device T here are many applications for devices capable of detecting IR radiation. IR can refer to radiation having wavelengths longer than visible light (> 0.7 m) up to about 14 m, with near -

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137 IR (NIR) being a subset referring to wavelengths from about 0.7 m to about 1.4 m and short wave infrared (SWIR) being referred to wavelengths from 1.4 to 3.0 m One important application is the detection of IR in environments with low ambient light conditions. Depending on the wavelength range of the IR radiation, there are many applications for IR imaging systems. Listed below are some applications for IR imaging: (iii) NIR and SWIR (0.7 m to 3.0 m). This wavelength range might require external IR illumination light source for imaging. The long wavelength of the SWIR spectrum can be used for high temperature thermal imaging. Applications include surveillance, securi ty monitoring, night vision, infrared inspection of structures, fire and rescue. (iv ) Mid IR (3.0 m to 12 m). IR image sensors in this wavelength are capable of thermal imaging. Applications include a wide range of medical imaging such as thermal imaging of human body temperature, night vision for military, night vision for automobiles, surveillance, security monitoring for law enforcement and homeland security. One common device for detecting IR images and displaying the detected images to a user is night -vi sion goggles. Conventional night vision goggles are complex electro -optical devices that require very high operating voltages (5,000 V ) and cost thousands of dollars. A typical night vision camera has a GaAs photocathode which converts the photons into ele ctrons. The electrons are then accelerated onto the phosphor screen by a high voltage ( 5,000 V), thus generating a visible image on a phosphor screen. It is highly desirable to have an IR imaging system that operates at low operating power, lightweight and cost effective to manufacture. In this study, we fabricated IR to -green light up -conversion device using SnPc:C60 IR photodetector and fac -

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138 tris(2 -phenylpyridinato)iridium(III) (Irppy3) green phosphorescent organic light emitting diode (OLED) 7.2 Experime ntal Detail s All organic light up-conversion devices were fabricated on patterned ITO substrates with a isopropanol in an ultrasonic cleaner and then rinsed with de ionized water, blown dry with N2 gas, and subsequently treated with UV ozone for 15 minutes. All layers in light up-conversion devices were vacuum deposited at a pressure of 1 106 Torr. Cu Pc SnPc and C60 were purified three times by train sublimation technique. [112] The deposition rates were 0.5 /s and 1 /s for organic materials and Al, respectively. The area of the device is 0.04 cm2. For red -to -green light up-conversion devices, a CuPc and C60 were used as donor and acceptor, respectively in the photodetector part of the device A 10 nm thick bathocuproine (BCP) w as used as exciton blocking layer. The interconnection layer consisted of a 15 nm thick layer of Al followed by a 1 nm thick LiF. An Alq3 (5 0 nm) was used as a n electron transporting laye r (ETL) and an emitting layer (EML). A NPB (60 nm) was used as a hole transporting layer (HTL). The transparent top anode consisted of a 5 nm thick layer of a molybdenum oxide (MoO3) followed by a 15 nm thick A u Luminance -current -voltage (LIV) characteris tics of the light up -conversion device s were measured using a Keithley 2400 sourcemeter for current voltage measurements coupled with a Keithley 6485 Picoammeter connected to a calibrated Si photodiode for photocurrent measurement s under dark and photo (630 nm red laser) irradiation For IR to -green up -conversion devices, a ll devices had 2 0 nm thick SnPc:C60 mixed layer for infrared detection The SnPc:C60 mixed layer s were deposited by co-evaporation. A 50 nm thick TAPC layer was used a hole transporting material and 4,4 N,Ndicarbazole biphenyl (CBP) host (20 nm) doped with Irppy3 is used the emitting layer T ris[3 (3 pyridyl) -mesityl]borane

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139 (3TPYBM) (50 nm) is used as hole blocker/ electron transporting layer (ETL). The cathode consisted of a 1 nm thick l ayer of LiF followed by a 100 nm thick Al. Luminance-current voltage (LIV) characteristics of the light up -conversion device s were measured using the same Keithley 2400 sourcemeter for current voltage measurements under dark and photo (830 nm IR) irradiati on The data was acquired using Lab view interface. All measurements were carried out at room temperature under ambient atmosphere. The devices were tested in air without encapsulation. 7.3 Results and Discussion s 7 3 1 Red -to -green Light U p -conversion D e vice s Before fabricating IR to -green up -conversion devices, we fabricated red -to -green up conversion devices. In order to fabricate red to green up -conversion device s we need to fabricate red light sensitive organic p hotodetector s and inverted top emission OLED. Figure 7 3 shows (a) the schematic band diagram (b) the external quantum efficiency, and (c) the J -V characteristics of the organic photodetector. W e fabricated organic photodetectors with a double heterostructure. Copper phthalocyanine (CuPc) and C60 were used as donor and acceptor, respectively for the photodetector part of the device Figure 7 3 (a) shows the schematic diagram of an organic photodetector with the structure: ITO/CuPc (40nm)/C60 (40nm )/BCP (10nm)/Al (100nm). T he J -V characteristics from 0V to 10 V were measured in the dark and under 630 nm irradiation with 7 mW /cm2 power density. ITO electrode was always ground and the Al electrode was applied by positive bias. Also, the quantum efficiency of the organic photodetector was 10% at 5 V To fabricate semi -transparent up -conversion devices, an inverted top emission OLED with semi transparent Au anode was used The inverted top emission OLED has the following

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140 structure: Al (20nm)/LiF (1nm)/Alq3 (50nm)/NPD (60nm)/MoO3 (5nm)/Au (15nm) as sho wn in Figure 7 4 (a). Alq3, NPD a nd MoO3 were used as an emitting layer, a hole transport layer and a hole injection layer, respectively. Al/LiF and Au were used as the cathode and semi transparent anode, respectively. I n spite of the inverted structure an d the semi -transparent Au anode, the L I V characteristics of the inverted top emission OLED is similar with that of conventional bottom emission OLED and the current efficiency of the inverted top emission OLED is slightly higher than that of conventional OLED with Alq3 as emitting layer showing maximum current efficiency of 4.2 cd/A The transmission of the semi -transparent Au electrode is about 50%. The apparent high current efficiency of the Alq3 device in spite of the low transparency of the Au electr ode is probably due to the microcavity effect in the OLED. To fabricate the final red -to -green up-conversio n device s, the CuPc:C60 red organic photo detector and the inverted top emission OLED are fabricated in a single stack Figure 7 5 (a) shows schematic band diagram of the basic structure of the red -to -green up -conversion device : ITO/CuPc (40nm)/C60 (40nm)/BCP (10nm)/Al ( 15nm)/LiF (1nm)/Alq3 (50nm)/NPD (60nm)/MoO3 (5nm)/Au (15nm). The L I -V characteristics (0V ~ 15V) were measured in the dark and under 630 nm red light with 7 mW /cm2 power intensity. Figure 7 5 (b) shows the L I -V characteristics of red to green up -conversion device. The on/off effect of the green light emitting by 630 nm red light was clearly show n at applied voltages from 5V to 8V. Figur e 7 6 shows the operation of redto -green light up -conversion devices using the schematic energy band diagram. The current density and the luminescence under red light irradiation increase sharply up to 6 V. However, t he current density and the luminescence under red light irradiation w ere saturated from 6 V and was limited to below 2 mA/cm2 even at 15V. However, the current density in the previous inverted top emitting OLED should exceed 50 mA/cm2 at 12V. The limitation of t he

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141 current density and the lumines cence under red light irradiation is probably due to the saturation of photocurrent density of the CuPc / C60 red light sensitive organic photodetector which was saturated to 3 mA/cm2 even at 8V as shown in Figure 7 3 (c). I n case of this redto green up co nversion device, the CuPc:C60 red light sensitive organic p hotodetector plays a role of current suppl ier (specially electron supplier) for the inverted top emission OLED. T herefore, the total current density passing the inverted top emission OLED can not b e higher than the amount of current in the organic p hotodetector It should be noted that the current density of the up conversion device is about 1 mA/cm2 at voltages between 6 V and 15 V. The low current indicate that the cathode ( Cu Pc/C60/BCP /Al /LiF ) in jection efficiency is very low. T he maximum current efficiency of red to -green light up -conversion device is 0.36 cd/A as shown in Figure 7 7 (a) and is 12 times smaller than that of the inverted top emitting OLED as shown in Figure 7 4 (c). The low curren t efficiency can be explained by charge imbalance due to poor electron injection efficiency On the other hand, t he photonto -photon conversion efficiency from injected red light photon s to extracted green light photons can be calculated by the equations in Figure 7 8. [123] The photon to -photon conversion efficiency increases with increasing the applied voltages and the maximum conversion efficiency is 0.3% at 15V as shown in Figure 7 7 (b). The photon-to photon conversion process from incident red light photons to output green light photons is via a two -step process : 1) from incident red light photons to photogenerated charge carriers in the photodetector part of the device and 2) from the photogenerated charge carriers to output green light photons in th e OLED part of the device The external quantum efficiency of the Alq3 based green emitting OLED is 1~2% [121, 122] and the external quantum efficiency of the red organic photodetector was 10~20% in Figure 7 3 (b), indicating the expected conversion effici ency of

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142 0.1 ~ 0.4 % which is consistent with the experimental conversion efficiency. Therefore, the photodetector and OLED with high efficiency are required for enhancing the conversion efficiency of light up-conversion devices. However, the yield of th e red to green light up -conversion device is low and most of devices were electrically short s in the OLED part of the devic e Also, the spatial resolution of up -conversion devices is poor due to high conducti vity of the Al electrode. W hen the incident red light with a small beam size is irradiated upon the up-conversion devices, the output green light is emitted over the entire active area of up -conversion device due to current spreading in the Al electrode Therefore, an improved device structures are req uired. 7 3 2 IR-to -green Light U p -conversion D evice s In order to solve the drawback s of the device structure used in the red -to -green light up conversion device, we used different device architecture for infrar ed -to -green up -conversion device s Figure 7 9 shows the schematic cross -section view of the new up -conversion device structure without the middle Al layer. T h is up-conversion device structure has a simpler device structure compared to the previous device and is similar with conventional OLED structure This OLED has a poor hole injection layer The poor injection and hole transport of this hole injection layer allows the device to have a low dark current in the absence of IR irradiation. As a result the OLED will be essentially off when there is no IR excitation but will be on when it is photo excited. F irst, an infrared sensitive layer is needed for infrared -to -green up-conversion devices Figure 7 1 0 shows the schematic band diagram and the external quantum efficiency of the organic photodetector. W e fabricated organic photodetectors with a bulk heterostru cture. Sn Pc and C60 were used as donor and acceptor, respectively for the photodetector part of the device Here, a mixed donor/acceptor layer instead of a planar heterostructure layer was used to enhance

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143 the photodetector efficiency. Figure 7 1 0 shows the schematic diagram of an organic IR photodetector with the structure: ITO/ MoO3 (10 nm)/Sn Pc : C60 (2 0nm )/BCP (10nm)/Al (100nm). T he J -V characteristics from 0V to 10V were measured in the dark and under 8 30 nm irradiation at 14.1 mW /cm2 power density ITO electrode was always ground and the Al electrode was applied by positive bias. T he external quantum efficienc y of the IR photodetector was about 5~2 0% at below 10V On the other hand, we had previou sly used an Alq3 based OLED with a low external quantum efficiency of 1~2% for the red to -green up -conversion devices. As discussed in the previous section, high efficiency OLED is required for up -conversion device. Here, an Irppy3 based green emitting pho sphorescent OLED was chosen as the OLED part of the up-conversion device F inally, an Irppy3 based green emitting phosphorescent OLED with SnPc:C60 IR sensitive hole injection layer was fabricated Figure 7 11 shows the L I -V characteristics of the IR-to -g reen up-conversion device. Without infrared light irradiation, emission was not observed until 1 3 V showing the maximum luminance (1 cd/m2) at 15 V. The high turnon voltage indicates the poor hole injection from the SnPc:C60 layer and a good off -state in the absence of IR irradiation. Poor hole injection from the SnPc:C60 layer is ideal for the up -conversion device because the device should stay off in the absence of IR light. T he current density increased linearly with increasing the applied voltage Sin ce no light was generated, the current is probably dominated by majority carriers (electrons), mainly due to injection of electron from the cathode contact. On the other hand, when the infrared laser was irradiated, green emission was observed from 2.7 V, showing the maximum luminance (853 cd/m2) at 15 V. T he current density and the luminance w ere enhanced instantaneously.

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144 On the other hand, Figure 7 12 shows the current efficiency of (a) control OLED and (b) IR-to -green up-conversion device as a function of current densities. The current efficiency of the control OLED is shown in Figure 7 12 (a), showing maximum current efficiency of 80 cd/A which is comparable with published data In case of IR to -green up -conversion device, however, w ithout infrared lig ht irradiation, emission was not observed until 1 3 V and the device show ed a very low current efficiency of 6 c d/A compared with the controlled device with a maximum current efficiency of 80 cd/A as shown in Figure 7 12 (a) The low efficiency is due to po or hole injection from the SnPc:C60 layer and hence the device is extremely electron dominant On the contrary, when the device was irradiated with infrared light, the IR -to -green up -conversion device turned on at 2.7 V with a maximum current efficiency of 107 c d/A at a brightness of 100 c d/m2. This efficiency is even higher than that of the control device The high er efficiency in upconversion device under IR laser irradiation means that the control OLED is still charge imbalance and slightly electron dom inant. On the other hand, t he photonto -photon conversion efficiency from injected IR light photon s to extracted green light photons can be calculated by the equations in Figure 7 8. The photon to photon conversion efficiency increases with increasing the applied voltages and the maximum conversion efficiency is 2.7% at 15V as shown in Figure 7 13. Given t he external quantum efficiency of the Irppy3 based green emitting OLED is about 20% [124] and the external quantum efficiency of the infrared photodetect or is 5~20% as shown in Figure 7 1 0 the expected conversion efficiency should be 1 ~ 4 % which is consistent with the experimental data This conversion efficiency is roughly 10 times higher than that of the red to -green up -conversion devices.

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145 In addition, the switching effect by IR light is significant as shown in Figure 7 14. The maximum ON/OFF ratio of EL intensity was about 1 666 at 12 .7 V Therefore, the device acts as a n infrared s witch as well as infrared up-converter, which converts low energy infr ared light (1.49 eV) to high energy green light (Max. 2.75 eV). Figure 7 15 shows the operation image s (a) without and (b) with IR irradiation in IR to -green l ight up-conversion device under 10 V. The on/off effect of green light emitting by 830 nm near IR light irradiation was clearly showed. 7.4 Conclusions H ere, we fabricated infrared to green light up-conversion devices using the SnPc:C60 IR organic photodetector and the Irppy3 green phosphorescent OLEDs The maximum photon-to photon conversion efficien cy is 2.7% at 15V. The device shows 107 c d/A current efficiency at a brightness of 100 c d/m2 under 830 nm IR irradiation, showing higher efficiency than the control OLED. The higher efficiency in up -conversion device under IR laser irradiation means that t he control OLED is still charge imbalance. In addition, the red -to -green light up -conversion device was fabricated by integrating the red light sensitive CuPc:C60 organic photodetector and the inverted top emission OLED The maximum conversion efficiency w as 0.3% at 15V.

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146 Figure 7 1. Schematic diagram of light up-conversion device

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147 Figure 7 2. Absorbance of SnPc thin film with infrared sensitivity. WAVELENGTH (nm) 400 500 600 700 800 900 1000 ABSORBANCE 0.0 0.2 0.4 0.6 0.8 1.0 CuPc SnPc

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148 (a) (b) (c) Figure 7 3. (a) Schematic band diagram (b) quantum efficiency, and (c) J -V characteristics of red organic photodetector Voltage (V) 0 2 4 6 8 QE (%) 0 20 40 60 80 100 Voltage (V) 0 2 4 6 8 Current density (mA/cm2) 10-510-410-310-210-1100101102 Dark current Photo current

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149 (a) (b) (c) Figure 7 4. (a) Schematic band diagram, (b) current efficiency, and (c) L I -V characteristics of inverted top emitting OLED VOLTAGE (V) -10 -5 0 5 10 CURRENT DENSITY (mA/cm2) 10-410-310-210-1100101102103 Current density (mA/cm2) LUMINESCENCE (Cd/m2) 100101102103104 Luminescence (Cd/m2) CURRENT DENSITY (mA/cm2) 20 40 60 80 100 CURRENT EFFICIENCY (Cd/A) 0 1 2 3 4 5

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150 (a) (b) Figure 7 5. (a) Schematic band diagram and (b) L I -V characteristics of red -to -green light up conversion device C 60 (40 nm) 4.7 6.9 ITO 4.7 CuPc (40 nm) 3.2 5.1 BCP (10 nm) NPD (60 nm) 2.1 5.4 Alq 3 (50nm) 2.6 5.7 Al/LiF MoOx (5nm) Au (15 nm) L-I-V characteristicsVOLTAGE (V) 0 2 4 6 8 10 12 14 CURRENT DENSITY (mA/cm2) 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 1e+0 1e+1 LUMINESCENCE (Cd/m2) 0.1 1 10 Dark Photo (630 nm) Curr. Lumi.

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151 Figure 7 6. Schematic band diagram of red to -green light up -conve rsion device under dark and photo (630 nm laser) irradiation

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152 (a) (b) Figure 7 7. (a) Current efficiency and (b) photon to photon conversion efficiency of red-to -green light up -conversion device VOLTAGE (V) 0 2 4 6 8 10 12 14 CONVERSION EFFICIENCY (%) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 CURRENT DENSITY (mA/cm2) 0.4 0.6 0.8 1.0 1.2 1.4 CURRENT EFFICIENCY (Cd/A) 0.0 0.1 0.2 0.3 0.4 0.5

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153 Figure 7 8. Calculation of photonto -photon conversion efficiency for light up-conversion devices.

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154 Figure 7 9. Schematic cross -section view of infrared to green light up -conversion device.

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155 Figure 7 1 0 Schematic band diagram and qu antum efficiency of infrared organic photodetector

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156 Figure 7 1 1 L -I -V characteristics of infrared to green light up conversion device under dark and photo (830 nm infrared) irradiation. L-I-V characteristicsVOLTAGE (V) 0 2 4 6 8 10 12 14 CURRENT DENSITY (mA/cm2) 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 1e+0 1e+1 LUMINESCENCE (Cd/m2) 0.1 1 10 100 1000 Dark IR (830 nm) Curr. Lumi.

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157 (a) (b) Figure 7 12. Current eff iciency of (a) control OLED and (b) IR to -green light up conversion device

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158 Figure 7 13. The photon to -photon conversion efficiency of IR -to -green light up-conversion device

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159 Figure 7 14. On/off ratio as a function of current densities on infrared to green light up conversion device VOLTAGE (V) 0 2 4 6 8 10 12 14 ON/OFF RATIO 0 200 400 600 800 1000 1200 1400

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160 (a) (b) Figure 7 15. The images (a) without and (b) with 830 nm infrared irradiation in infrared to green light up conversion device under 10 V

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161 CHAPTER 8 CONCLUSION This dissertation has focused on studying organic photovoltaic cells, organic p hotodetector s, and light up -conversion devices using the organic photodetectors. In the study of organic photovoltaic cells, first, we have found the appropriate photovoltaic measurement setup for organic photovoltaic cells with very small size of 0.04 cm2 for obtaining the reliability of the photovoltaic data. W hen we used the new measurement setup using the metal mask, which has the hole of the e xact same size as the actual device, we could have t he positional uni formity of below 5 % within 5 mm 3.5 mm and also the corresponding JSC calculated from the IPCE spectrum compared with the JSC measured by the photo J -V characteristics. A fter that, w e have fabricated the small molecule organic photovoltaic cells with very high VOC. W hile b oth CuPc and AlPcCl cells show very similar device performance in terms of short -circuit current as well as fill factors the AlPcCl solar cells had almost two times h igher VOC, leading to roughly two times higher overall power conversion efficiency. A lso, we have fabricated SnPc:C60 bulk heterojunction organic photovoltaic cells with infrared sensitivity W ith a MoO3 interlayer between the ITO anode and the light absor bing layer, the short -circuit current is substantially enhanced resulting in a 45 % enhancement in power conversion efficiency I n addition, we have demonstrated the enhancement in the power conversion efficiency due to the MoO3 interlayer in the small mol ecule photovoltaic cells as well as the polymer photovoltaic cells. T he power conversion efficiencies of the organic photovoltaic cells with the MoO3 interlayer were enhanced by the significant increase in the fill factor, which might be due to the reducti on in the series resistance. T he reduction in the series resistance is due to the hole injection barrier lowered by the hole accumulation in MoO3/AlPcCl interface and the band bending in the AlPcCl layer.

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162 In the study of organic photodetectors, on the othe r hand, we have fabricated t he organic photoconductor with gain of greater than 103. This phenomenon was explain ed by a mechanism based on the tunneling injection of electrons at the ITO/PTCDA interface and Au/ PTCDA interface H owever, the high gain organi c photodetector s showed high dark current. It is thought that the small change of the electron injection barrier by trapping of photogenerated holes brings the huge difference in the amount of electrons injected by tunneling barrier under high dark current H owever, a low dark current in photodetector s is important for detecting signal clearly because the high dark current close to photo current will make a signal be hardly distinguishable from a background noise. Therefore the high gain organic photocondu ctor is not good for further application. A fter that we studied a diode type organic p hotodetector with low dark current but no gain. T he organic Photodetector demonstrated the high external quantum efficiency of 50 % a t 1 V. Also, we fabricated t he red t o -green light up-conversion device integrating an inverted top emitting OLED on low operating organic photodetector The inverted top emission OLED with semi transparent Au anode was used as green light emitting part I n spite of inverted structure and semi -transparent Au anode, L -I -V characteristics of the inverted top emission OLED is similar with that of conventional bottom emission OLED and the current efficiency of the inverted top emission OLED is slightly higher than that of conventional OLED with Al q3 as emitting layer. The on/off e ffect of green light emitting by 630 nm red light was clearly showed at applied voltages from 5V to 8V. However, these organic photodetectors discussed in Chapter 6 Chapter 8 have showed the limited photosensitive spectr a of visible range wavelength. Therefore, the realization of organic photodetector with infrared (IR) sensitivity can extend the applications of organic electronics to large area sensing and detection. Then, w e fabricated the infrared organic photodetector using the SnPc:C60 mixed layer with low dark current and high

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163 external quantum efficiency, exhibiting high photoresponse at wavelength from 600 nm to 900 nm. Both MoO3 and BCP interlayers decreases drastically the dark current density even at high applied voltages. T he maximum external quantum efficiency is 96 % a t 740 nm wavelength under applied voltage of 5 V. After that, we fabricated infrared -to -green light up-conversion devices using the SnPc:C60 IR organic photodetector and the Irppy3 green phosphorescent OLEDs The device shows 106. 82 c d/A current efficiency at a brightness of 100 c d/m2 under 830 nm IR irradiation. The maximum of ON/OFF ratio of EL intensity was about 1 666 at 12 .7 V Hence the device acts as a n infrared s witch as well as infrared up-converter, which converts low energy infrared light (1.49 eV) to high energy green light (Max. 2.75 eV).

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171 BIOGRAPHICAL SKETCH Long time ago, when I was a boy who had lots of dreams, I naturally had the question, where am I from and from that time I want to know and u nderstand more the nature, the world, and the universe. When I entered Kyung Hee University, I selected physics as my college major 1 8 years ago. It was a choice born of a nave desire to resolve my longtime curiosity about the material world. Physics to m e seemed to be a treasure chest full of answers to all my lifelong questions about the universe. At the time, those around me tried to dissuade me from majoring in physics, saying I would starve. However, 18 years later, I have no regrets. The 4 years I spent studying physics was pure joy, and not even a three -year break to serve in the army could dampen my enthusiasm. The tangible results of my love for physics showed in the fact that I received performance based scholarships 6 out of my 8 semesters, and in my graduating at the top of my department. My pursuit of excellence led me to begin graduate studies at the laboratory of Professor Jang, which appealed to me due to the cutting edge research being done there. I received an MS degree at Kyung H ee Un iversity for work I did under Professor Jin Jang. It was at the laboratory of Professor Jang that I first encountered semiconductor thin films and devices. The laboratory was very active, and I was able to acquire a broad range of experimental skills such as thin film deposition and photolithography, and have used these skills to fabricate countless thin film transistors ( TFTs ) and even higher -level systems such as liquid crystal ( LC ) cells for TFT LCD panels. I have also used simulation tools to design 2 -inch TFT LCD panels. These experiences are still benefiting me to this day. A fter getting MS degree, I had work ed as a research scientist at the Samsung Advanced Institute of Technology (SAIT). My work there involves research in fabricating polycrystalline silicon (poly Si) TFTs on plastic substrates, a core technology in realizing flexible displays.

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172 Among the many aspects of this research, I have concentrated on studying the room temperature deposition of amorphous silicon (a Si) films on plastic substra tes and the excimer laser crystallization of these films. Using these poly-Si films as the active channel, I have fabricated working TFTs and measured their electrical characteristics. In the course of this research, I have been exposed to a Si and poly Si precursor films deposited in various ways, and this experience has led to a deeper understanding of Si thin films and the issues related to process ing them. Also, through my deep involvement in the procurement and set up of the excimer laser annealing sys tem at SAIT, I am familiar with the requirements and complexities involved in such equipment systems. Finally, through my experience with fabricating TFTs on plastic substrates, I have an intimate understanding of the issues and potential problems that can occur when applying semiconductor processes to plastic substra tes. I had learned many things at SAIT for working on semiconductor materials and devices. However, I recognized that I need to have wider eyes and learn creative and logical thinking, and I de cided to enter Ph. D course in University of Florida, U.S.A. Now, I finished the Ph. D course. I ha d had the experience fabricating electronic devices using organic electronic materials such as organic solar cell organic p hotodetector (particularly infrared p hotodetector ), and infrared to -visible light up -conversion device I had work ed as a research assistant at organic electronic materials and devices laboratory, University of Florida. Looking back, physics ha s been very good to me. It has satisfied m y endless desire to know, opened up new worlds to me, and gave me a purpose that enabled me to overcome adversity. At times, economic hardship or despair at the long hard road ahead of me made me want to give up my dreams and try an easier, more practical road, but I always overcame the

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173 despair to move on to the next stage. I can only thank God for the strength and courage to make it this far in my quest, without losing sight of the goal.