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Fabrication of Organic Solar Palm Tree with Flexible Substrate

Permanent Link: http://ufdc.ufl.edu/UFE0042304/00001

Material Information

Title: Fabrication of Organic Solar Palm Tree with Flexible Substrate
Physical Description: 1 online resource (64 p.)
Language: english
Creator: Li, Zhifeng
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Artificial plants have been used for landscaping in commercial and residential applications because of their durability and virtually maintenance free nature. Polymer solar cells (PSCs) are an excellent candidate in the landscaping application to substitute artificial plants such as grasses, flowers or trees because they have the capability to provide choices of different colors and transparencies. During the day, these solar plants convert sunlight into energy and stored in batteries. During the night, this energy can be used to provide lighting or to power other electrical appliances. They have excellent economic potentials not only preserving the aesthetic appearance of the artificial plants but also adding a function of converting sunlight into electricity. This thesis is dedicated to this concept. First, we have investigated the PSCs fabricated on flexible plastic substrates in both the normal and inverted architectures. Second, we have demonstrated an efficient inverted structure device (a ZnO nanoparticles layer on transparent conducting oxide layer as the cathode and MoOx/Al or Ag as the anode) with 3.40 % power conversion efficiency. Third, we have successfully fabricated functional solar leaf devices on plastic substrates with excellent stability. After 90 days, the efficiency of the encapsulated leaf device with active area of 6.45 cm2 was only reduced by 16%. Fourth, we have demonstrated a functional prototype with twelve leaves which can be used to power either a mini-fan or organic light emitted diodes displays. Under 0.8 sun condition, the Voc and Isc for the solar palm tree were 1.76 V and 32.5 mA, respectively, for configuration A (in parallel) to power the mini-fan. In configuration B (in series) for powering the OLEDs, the Voc and Isc for the solar palm tree were 5.65 V and 9.62 mA, respectively.
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 Zhifeng Li.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Xue, Jiangeng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042304:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042304/00001

Material Information

Title: Fabrication of Organic Solar Palm Tree with Flexible Substrate
Physical Description: 1 online resource (64 p.)
Language: english
Creator: Li, Zhifeng
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Artificial plants have been used for landscaping in commercial and residential applications because of their durability and virtually maintenance free nature. Polymer solar cells (PSCs) are an excellent candidate in the landscaping application to substitute artificial plants such as grasses, flowers or trees because they have the capability to provide choices of different colors and transparencies. During the day, these solar plants convert sunlight into energy and stored in batteries. During the night, this energy can be used to provide lighting or to power other electrical appliances. They have excellent economic potentials not only preserving the aesthetic appearance of the artificial plants but also adding a function of converting sunlight into electricity. This thesis is dedicated to this concept. First, we have investigated the PSCs fabricated on flexible plastic substrates in both the normal and inverted architectures. Second, we have demonstrated an efficient inverted structure device (a ZnO nanoparticles layer on transparent conducting oxide layer as the cathode and MoOx/Al or Ag as the anode) with 3.40 % power conversion efficiency. Third, we have successfully fabricated functional solar leaf devices on plastic substrates with excellent stability. After 90 days, the efficiency of the encapsulated leaf device with active area of 6.45 cm2 was only reduced by 16%. Fourth, we have demonstrated a functional prototype with twelve leaves which can be used to power either a mini-fan or organic light emitted diodes displays. Under 0.8 sun condition, the Voc and Isc for the solar palm tree were 1.76 V and 32.5 mA, respectively, for configuration A (in parallel) to power the mini-fan. In configuration B (in series) for powering the OLEDs, the Voc and Isc for the solar palm tree were 5.65 V and 9.62 mA, respectively.
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 Zhifeng Li.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: Xue, Jiangeng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042304:00001


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1 FABRICATION OF ORGANIC SOLAR PALM TREE WITH FLEXIBLE SUBSTRATE By ZHIFENG LI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCI ENCE UNIVERSITY OF FLORIDA 2010

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2 2010 Zhifeng Li

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3 To my family and friends

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4 ACKNOWLEDGMENTS I would like to thank Dr. Jiangeng Xue for the opportunity to work in his group and to learn the important elements (engineering skills, mindset) du ring the completion of this master thesis. I would also like to tha nk my supervisory committee, Dr. Franky So and Dr. Elliot Douglas for their support on my research. I would like to thank Dr. Jegadesan Subblah and Fred Steffy for collaboration in the Se star Project. I would like to thank everyone in our research group William Hammond, Jason David Myers, Yixing Yang, Weiran Cao, Edward Wrzesniewski, Renjia Zhou, Sang Hyun Eom, John P. Mudrick, Wei Zhao, Zheng Ying for their active collaboration I would like to thank Yukun Feng and Wei Zhao for the support in designing the overall appearance for the solar palm tree prototype Weiran Cao for the soldering, Jason David Myers for the drilling, William Hammond for the circuit design and Zhen g Ying for overall extensive discussion. I would like to thank Lei Qian and Zheng Ying for synthesis ZnO nanoparticles Without the support from everyone who mentioned above there will not be a functional solar palm tree prototype. Finally, I would also like to acknowledge the financial support from S estar Technologies, LLC.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 4 LIST OF FIGURES ................................ ................................ ................................ ......................... 7 ABSTRACT ................................ ................................ ................................ ................................ ..... 9 CHAPTER 1 INTRODUCTION TO ORGANIC PHOTOVOLTAIC CELLS ................................ ........... 11 1.1 Background ................................ ................................ ................................ ....................... 11 1.2 Organic Photovoltaic Cells ................................ ................................ ............................... 11 1.2.1 Progress of Organic Photovoltaic Cells ................................ ................................ .. 11 1.2.2 Bulk Heterojunc tion OPV ................................ ................................ ...................... 12 1.2.3 Polymer Solar Cell ................................ ................................ ................................ 13 1.3 Photocurrent Generation ................................ ................................ ................................ ... 13 1. 3.1 Incoupling of Photon ................................ ................................ .............................. 13 1.3.2 Photon Absorption ................................ ................................ ................................ .. 14 1.3.3 Exciton Formation and Migration ................................ ................................ .......... 14 1.3.4 Exciton Dissociation ................................ ................................ ............................... 15 1.3.5 Charge Transport and Collection ................................ ................................ ............ 15 1.4 Measurement and Characterization of the OPV Device Performance ............................. 16 1.4.1PCE Measurement of OPV Devices ................................ ................................ ........ 16 1.4.2 EQE Measurement of OPV Devices ................................ ................................ ...... 17 1.5 Progress in Printing Solar Cells ................................ ................................ ........................ 18 1.5.1 Spin C oating ................................ ................................ ................................ ........... 18 1.5.2 Doctor Blading ................................ ................................ ................................ ....... 19 1.5.3 Inkjet Printing ................................ ................................ ................................ ......... 19 1.5.4 Spray Coating ................................ ................................ ................................ ......... 20 1.6 Overview of This Thesis ................................ ................................ ................................ ... 20 2 FABRICATION OF FUNCTIONAL SOLAR LEAF DEVICE ON PLASTIC SUBSTRATE ................................ ................................ ................................ ......................... 25 2.1 Background ................................ ................................ ................................ ....................... 25 2.2 Sheet Resistance of PET Substrates ................................ ................................ ................. 26 2.3 Normal Structure Devices with PET and Glass Substrates ................................ .............. 27 2.3.1 Experimental Pr ocedure ................................ ................................ ......................... 27 2.3.2 Results and Discussion ................................ ................................ ........................... 28 2.4 Normal Structure Device with Solvent Annealing on the Active Layer .......................... 29 2.4.1 Experimental Procedure ................................ ................................ ......................... 29 2.4.2 Results and Discussion ................................ ................................ ........................... 29 2.5 Devices with Different PEDOT: PSS ................................ ................................ .............. 30

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6 2.5.1 Experimental Procedure ................................ ................................ ......................... 30 2.5.2 Results and Discussion ................................ ................................ ........................... 31 2.6 Inverted Structure PPV Device with ZnO nano Parti cle Layer ................................ ........ 32 2.6.1 Experimental Procedure ................................ ................................ ......................... 32 2.6.2 Results and Discussion ................................ ................................ ........................... 32 2.7 The Effect of Different Thickness in ZnO NPs Layer ................................ ...................... 33 2.8 Effect of the Different Thickness in MoO x and Metal Electrode ................................ ..... 34 3 F ABRICATION OF SOLAR PALM TREE ................................ ................................ .......... 48 3.1 Background ................................ ................................ ................................ ....................... 48 3.2 Leaf Device Fabrication ................................ ................................ ................................ ... 49 3.3 Leaf Device Stability ................................ ................................ ................................ ........ 50 3.4 Solar Palm Tree Prototype ................................ ................................ ................................ 51 3.5 Solar Palm Tree under the Real Sun ................................ ................................ ................. 51 4 CONCLUSION AND FUTURE WORK ................................ ................................ ............... 58 4.1 Conclusion ................................ ................................ ................................ ........................ 58 4.2 Future Work and Improvement ................................ ................................ ........................ 59 4.2.1 PET Substrate ................................ ................................ ................................ ......... 59 4.2.2 PEN Substrate ................................ ................................ ................................ ......... 60 4.2.3 Metal Bus Line ................................ ................................ ................................ ....... 60 4.2.4 Shutter Design ................................ ................................ ................................ ........ 60 4.2.5 Processing Technique ................................ ................................ ............................. 60 LIST OF REFERENCES ................................ ................................ ................................ ............... 62 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ......... 64

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7 LIST OF FIGURES Figure page 1 1 A cross section of a typical polymer bulk heterojunction solar cell and the schematic energy diagram ................................ ................................ ................................ .. 22 1 2 S chematic diagram of power conversion efficiency measur ement setup [Courtesy of Ying Zhen g ] ................................ ................................ ................................ .. 22 1 3 Current Voltage (I V) characteristic of a PV cell in the dark and under illumination ................................ ................................ ................................ ........................ 22 1 4 The schematic diagram of EQE measurement setup [Courtesy of Ying Zhen g ] ................................ ................................ ................................ ................................ 23 1 5 Schematic diagram of the spin coating [14 ] ................................ ................................ ...... 23 1 6 A photograph show ing doctor blading of MEHPPV [14 ] ................................ ................. 24 1 7 Schematic diagram of organic film formation by inkjet printing [18 ] ............................... 24 1 8 Sch ematic illustration of the spray process [ 20 ] ................................ ................................ 24 2 1 (a) Schematic cross section view of the normal structure of P3HT: PCBM PSCs (b) Energy level diagram of the P3HT:PCBM device (c) The shadow ma sk pattern of the metal electrode.. ................................ ................................ ................. 35 2 2 (a) Schematic cross section view of the inverted structure of P3HT: PCBM based PSCs (b) Energy level diagram of the inverted P3HT: PCBM based PSCs ................................ ................................ ................................ ................................ ... 35 2 3 I V characteristic of device and the shadow mask pattern of the metal electrode ................................ ................................ ................................ ............................. 36 2 4 Current density voltage (J V) characteristics of P3HT: PCBM based PSCs un der 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 ................................ ..... 37 2 5 Voc, FF, Jsc and PCE of P3HT: PCBM based PSCs as a function of device areas. ................................ ................................ ................................ ................................ .. 38 2 6 Current density voltage (J V) characteristics of P3HT: PCBM based PSCs under 1 sun AM 1.5 illumination with intensity of 100 mW/cm2 with solvent annealing ................................ ................................ ................................ ............................ 39 2 7 V oc FF J sc a nd PCE of P3HT: PCBM based PSCs as a function of device areas. ................................ ................................ ................................ ................................ .. 40

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8 2 8 Current density voltage (J V) characteristics of P3HT: PCBM based PSCs under 1 sun AM 1.5 illumination with intensity of 10 0 mW/cm 2 with different PEDOT: PSS ................................ ................................ ................................ ....... 41 2 9 V oc FF J sc and PCE of P3HT: PCBM based PSCs as a function of device areas. ................................ ................................ ................................ ................................ .. 42 2 10 TEM image and the particle size distribution histogram of the ZnO N Ps ......................... 42 2 11 Current density voltage (J V) characteristics of inverted PSCs under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 ................................ ......................... 43 2 12 (a) J sc and V oc (b) PCE and FF of P3HT: PCBM based PSCs (c) and (d) device performance of PET substrate with and without acetone cleaning, (e) and (f) device performance of glass substrate with and without UV treatment. ................ 44 2 13 (a) Current density voltage (J V) characteristics of inverted structure with different ZnO layer thicknesses under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 (b) J sc and PCE (c) V oc and FF ................................ ................. 45 2 14 Current density voltage (J V) characteristics of inverted PSCs devices fabricated on glass substrate under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 ................................ ................................ ................................ .................. 46 2 15 The effect of different MoO 3 thickness in the inverted devices with aluminum electrode VS. silver electrode: (a) J sc and V oc (b) PCE and FF ................................ .......... 47 3 1 (a) A GPS charge pad by Solarmer [ 5 ] (b) Solarmer [ 5 ] (c) Solar bag panel by Konarka [ 23 ] ................................ ............................ 53 3 2 Leaf device fabrication procedures and cross section of conducting copper stem ................................ ................................ ................................ ................................ .... 53 3 3 Schematic of vacuum ther mal evaporation (VTE) system, C ring fixture, sample holder, metal electrode mask and metal bus line mask. ................................ ........ 54 3 4 Leaf device fabrication procedures in a step by step ................................ ......................... 54 3 5 Leaf device performance and stability: (a) unencapsulated small area device (b) encapsulated leaf device ................................ ................................ ............................... 55 3 6 Circuit configurations for prototype demons tration ................................ .......................... 55 3 7 A solar palm tree prototype with twelve leaves ................................ ................................ 56 3 8 Current voltage (I V) characteristics of the solar palm tree un der 0.8 sun (a) 4 units of leaf shaped devices in parallel (b) 4 units of leaf shaped devices in series [Couetesy of Ying Zheng] ................................ ................................ ........................ 57

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science FABRICATION OF ORGANIC SOLAR PALM TREE WITH FLEXIBLE SUBSTRATE By Zhifeng Li Decembe r 2010 Chair: Jiangeng Xue Major: Material s Science and Engineering Artificial plant s have been used for landscaping in commercial and residential applications because of their durability and virtually maintenance free nature. Polymer so lar cell s (PSC s) are an excellent candidate in the landscaping application to substitute artificial pla nts such as grasses, flowers or trees because they have the capability to provide choice s of different colors and t ransparencies. During the day, these solar plants convert sunl ight into energy and stored in batteries. During the night, this energy can be used t o provide lighting or to power other el ectrical appliances. They have excellent economic potentials not only preserving the aesthetic appearance of the artificial plants but also adding a function of converti ng sunlight into electricity. This thesis is dedicated t o this concept. First, we have investigated the PSCs fabricated on flexible plastic substrates in both the normal and inverted architectures Second, we have demonstrated an efficient inv erted structure device (a ZnO nanoparticle s layer on t ransparent conducting oxide layer as the cathode and MoO x /Al or Ag as the anode ) with 3.40 % power conversion efficiency Third we have successfully fabricated functi onal solar leaf devices on plastic substrates with excellent stability. After 90 days,

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10 th e efficiency of the encapsulated leaf device with active area of 6.45 cm 2 was only reduced by 16% Fourth we have demonstrated a functional prototype with twel ve leaves which can be used to power either a mini fan or organic light emitted diodes disp lays Under 0.8 sun condition, the open circuit voltage ( V oc ) and the short circuit current ( I sc ) for the solar palm tree were 1.76 V and 3 2.5 mA respectively for configuratio n A ( in parallel) to power the mini fan. In configuration B (in series) for powerin g the OLED s the V oc and I sc for the solar palm tree were 5.65 V and 9.62 mA respectively.

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11 CHAPTER 1 INTRODUCTION TO ORGA NIC PHOTOVOLTAIC CEL LS 1.1 Background Organic photovoltaic cells (OPV) are one of the widely studies optoelectronic o rganic semiconductor devices due to their potential to provide large area, low cost solar energy conversion to resolve the global warming and the increasing energy demand. This chapter will present an introduction to the organic photovoltaic cells, des cr ibe their working principle and important parameters to increase device efficiency and finally present the measurement and ch aracterization of these devices All of these are important for the understand ing of the subsequent chapter s 1.2 Organic Photovol taic Cells 1. 2 1 Progress of Organic Photovoltaic Cells The first generation of OPV devices was d iscovered by Kallman and Pope using anthracene [1] These devices are composed of a single active layer sandwiched between two metal electrodes with different work functions. The device efficiencies were poor due to the low dielectric constant of organic materials which would limit the number of photo generated free charge carriers. The power conversion efficiency (PCE) was in the range of 10 3 to 10 2 % with a 0.2V photovoltage [ 1 2 ] A breakthrough comes in 1986 when Tang first introduced the concept of heterojunction solar cell using a bilayer structure composed of copper phthalocyanine (CuPc) and 3,4,9,10 perylenetetracarboxylic bis benzimidazole (PTCBI) with a 1% PCE [3 ]. In this bilayer device two organic layer s, one p type (or electron donor) and one n type (or electron acceptor) are sandwiched between two electrodes. The energy level

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12 misalignment at the interface between these two materials provides the driving force for charge separation. A limiting factor of this type of devices is the small area of the charge generating interface. Further development on this heterojunction concept comes in 1992 when Sariciftci et al. develop a bulk heterojunction struc ture which was formed by mixing conjugated polymer with fullerene to yield a spastically distributed interface [ 4] Afterward with c ontin ue development in the bulk heterojunction concept through control of the morphology of the phase separation into an int erpenetrating network, optimizing processing techniques and development new materials, the most efficient polymer photovoltaic (PPV) device is recently approaching 8.13 % from Solar mer Energy certified by national renewable energy laboratory ( NREL ) [5] 1. 2 2 Bulk H eterojunction OPV A cross section of a typical polymer bulk heterojunction solar cell and the schematic ener gy diagram are shown in Figure 1 1 The bottom contact is a transparent conducting oxi de (TCO) such as indium tin oxide (ITO) with a high w ork function and can be deposited on either glass or flexible plastics substrate. The top contact is usually a low work function metal electrode such as aluminum or silver. The active layer which is sandwiched between the two electrodes is a mixture of don or and acceptor material s The donor material has a lower ionization potential and the a cceptor material has a higher electron affinity. When an exciton is created in the photoactive layer, it diffuses and may reach the donor acceptor interface. Then the electron will be transferred from the donor to the acceptor material and collected by the low work function metal electrode. The hole on the other hand will recede to the donor material and collected by the high work function conducting oxide. The collecti on of both charge carriers will lead to the

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13 occurrence of photovoltage. If the top and bottom contact are connected through an external circuit, then a photocurrent is generated. 1. 2 3 Polymer Solar C ell Polymer solar cell s (PSC s ) are an excellent candidat e for commercial application e specially for the architects and designers due to its capability to provide choice of different colors and transparencies. The color of PSC s can be tuned by selecting different polymer for the active layer with different abs orption bands The transparencies can be turned by choosing different thickness for the polymer active layer. An important advantage of PSC s is their capability with low cost roll to roll fabrication on large area device s, and compatible 1. 3 Photocurrent G eneration It is very important to understand the physics behind the photocurren t generation in OPV cells. This photovoltaic process of OPVs has five consecutive steps: incoupling of photon, photon absorption exciton formation and migration, exciton disso ciation and charge transport and collection. 1. 3 1 Incoupling of P hoton For a typical device, the substrate is the first material which light first pass through. T he reflection losses at th e air substrate interfaces need to be minimize d by matching the op tical refractive indices of the two different materials on the air substrate interface. Other reflection losses are from all l ayers on top of the substrate. To minimize the reflection losses, it is important to control the a ntireflection properties of the device. There are several ways to do this such as manipulating layer ordering, layer thickness or

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14 applying microlens or p yramids on the top electrode. When comes to designing layer ordering and layer thickness, optical simulation is an invaluable tool to u se 1. 3 2 Photon A bsorption The active layer is the layer whe re beneficial absorption occurs; hence, it is important to foc us as much energy (photons) as possible to the active layer. The higher the absorption coefficient of the active layer material has the more photons it can absorb for a given thickness Form a material perspective, the absorption spectrum of the active material should match the spectrum of s olar radiation for maximum absorption. Therefore, in the recent years a continuous effect has been applied to develop low band gap material to increase photon absorption in the near infrared region. 1. 3 3 Exciton Formation and M igration An exciton which consists of a pair of Coulomb attracted electron and hole is formed after photon absorption to ok place within the organic material s An e xciton has a finite lifetime and those formed but fail to diffuse to the donor acceptor interface will decay either thermally converted to heat or phonons or radiatively (i.e. is a photon will be re emitted in lum inescence ) In conventional bilayer heterojunction cells, exciton dissociation occurs only at the two dimensional interface between the donor and acceptor lay If the exciton diffusion length of the donor or acceptor is shorter than the thickness of the do nor or acceptor layer, most generated excitons will recombine before diffusing to the donor acceptor interface [ 7 ] On the other hand, a bulk heterojunction composed of donor acceptor mixture offers an interpenetrating network leading to a substantially l arger interfacial area compared to the bilayer herojunction Therefore, regardless of

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15 location where the exciton generated inside the bulk, the ge nerated exciton can be collected with a high probability [ 8 ]. Depending on the different structure of the mat erials and dielectric environment the exciton diffusion length may vary A typical exciton diffusion length for organic active material is 5 10 nm [ 8 9 ] 1. 3 4 Exciton D issociation Photocurrent is generated by exciton dissociation into free electrons and holes. The counterforce for exciton dissociation is germinate recombination and non geminate bimolecular recombination [ 2 ] In germinate recombination, the separated electrons and holes recombine back to exciton. In non geminate bimolecular recombination, the separated electrons and holes from different exciton recombine. 1. 3 5 Charge Transport and C ollection The free charges must be allowed to travel to the electrodes and collected by the electrodes so that a photocurrent is generated. The efficiency of c harge transport depends on charge carrier mobility since electrons and holes have different mobility in the material and also trapping sites within the material where charge will diminish during the transport process. Even if a charge carries either a e lectron or hole transport close to the electrode, they may no t necessarily be direct ed to the external circuit. Thus, it is important to match the energy level at the interface between the electrodes and the polymer to achieve efficient charge transport an d collection

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16 1. 4 Measurement and Characterization of the OPV Device Performance 1. 4 1 PCE Measurement of OPV D evices A schematic diagram of the PCE measurement of OPV devices is shown in Figure 1 2 The light source is provided by an Oriel solar simulator equipped with a 150 W Xenon arc lamp which has a spectral coverage from 250 nm to 1100 nm. An AM 1.5G air mass filter is placed in front of the Xenon arc lamp to simulate the standard AM1.5G solar illumination After going through a series of irises and n eutral density filter s the beam generated by the AM 1.5 solar simulator will form a uniform light spot By adjusting the power on the solar simulator and distance between the solar simulator and sample plane, the measured short circuit current of the refer ence cell will be equal to its c alibrated value measured at 100 mW/cm 2 The current v oltage (I V) characteristics of the PV device in the dark or under illumination are then taken by a semic onductor analyzer using a voltage mode from the reverse bias to the forward bias The important PV performance parameter the test cells will be calcula ted from the measured I V curve [ see Figure 1 3 ] The power conversion efficiency (PCE) is the most important parameter in solar cells which defined as the ratio between the maximum electrical power generated P m and the incident optical power P inc (1 1) Here I S C is the short circuit current, V oc is the open circuit voltage, P inc is the incident solar power intensity which is standardized as 100 mW/cm 2 under air mass 1.5 global illumination at 298 K and FF is the fill facto r defined as (1 2)

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17 The fill factor is the ratio between the maximum power delivered to an external circuit and potential power. It is the ratio of the maximum power rec tangle which is the product of the I mp and V mp to the rectangle of I sc *V oc where I mp and V mp are the current and voltage at the maxim um power output point respectively. To maximize the efficiency, we c ould maximize all three item s in the numerator of Equ ation 1 individually or collectively 1. 4 2 EQE Measurement of OPV Devic es There are two primary parameter used in describing the effectiveness of a PV cell corresponding to incident light: the internal and external quantum efficiencies. Internal quantum e fficiency ( IQE ) is the ratio of number of collected charges to the number of phot ons absorbed, whereas external quantum efficiency ( EQE ) of the OPV cell is the ratio of the number of electrons extracted from the device to the number of incident photon s The extern al quantum efficiency is described by the equation [10] : (1 3) Where A is the light absorption efficiency, ED is the exciton diffusion efficiency, CT is the charge transfer efficiency and CC is the charge collection efficiency r espectively. The relationship and EQE follows the equation: (1 4) The schematic diagram of EQE measur ement setup is shown in Figure 1 4 To measure the external quantum efficiency, a monochromatic beam of light was chopped at 400 Hz area. A current amplifier was used to amplify the current signal from the photodetectors. A lock in amplifier was used to collect the electrical signal from the device. Together

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18 with a silicon photodetector (Newport 818 UV), they were used to measure the incident power density of the chopped monochromatic light. A white light bias was used during the EQE measurement to manipulate the normal operating con ditions. T he EQE can be c alculated t hrough equation: (1 5) w here is the power of the incident light, is the short circuit current, is the spectral responsivity of the detector. The short circuit current can be ba ck calculated through equation: (1 6) 1. 5 Progress in Printing Solar Cells Roll to Roll fabrication of OPVs offers great potential for reducing fabrication costs on large, flexible area applications. This section will discuss some of the p rocess techniques aim ing for large scale printing. By selecting am appropriate printing technique and adjusting the process parameter, a wide variety of materials can be processed and a wide ran ge of desired film properties can be accessed. 1. 5 .1 Spin coat ing Spin coating as shown in Figure 1 5 is the most common and important coating method in research laboratories Spin coating contains four stages: deposition, spin up, spin off and evaporation. In the first stage, an excess amount of liquid is deposited on the surface of a substrate. The liquid is forced to flow outward because of the centrifugal force the liquid experiences during the spin up stage and liquid droplets are flung from the edges of the substrate. As the excess liquid is removed, the film b ecomes thinner and

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19 thinner, the rate of removing liquid slows down. Evaporation becomes dominant over thinning in the fourth stage [ 1 1 ]. Spin coating is a n excellent laboratory method for device optimization and materials screening because it can apply to almost all soluble material s with only minor adjustm ents of process parameters [1 2 ]. Some major disadvantages are that most of the solution is wasted during spin coating, it is not compatible with the roll to roll technology and it does not allow for pat tern controlled on the formed film. Solar cells based on P3HT: PCBM structure can reach up to 5 % by using spin coating. 1. 5 .2 Doctor B lading Doctor blading is well known for its ability to have well defined film thickness It is an ideal technique when ver y little material is available since coating solution loss is at its minimum As shown in Figure 1 6, doctor blading t he coating solution is deposited in a narrow slip between the substrate and the blade of the doctor blade which is fixed at a certain dis tance from the substrate The smooth blade dragged by a motor then move s lineally at a constant velocity across the substrate and leaves behind a uniform wet thin film between the blade and the substrate [ 1 3 ]. Unlike spin coating, doctor blading is compat ible with roll to r oll tec hnology However, doctor blading tend to form aggregates due to its slow speed in solvent evaporation [ 1 4 ]. Solar cells based on P3HT: PCBM structure can also reach 4% by using doctor blading [ 1 5 1 6 ] 1. 5 .3 I nkjet P rinting Inkje t printing is a relative novel process technique and has been extensively used in the field of polymer light emitting diode (PLED) and thin film transistors [1 7 ] A schematic diagram of inkjet printing is shown in Figure 1 7 The disadvantages of i nkjet

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20 pr inting are its relatively slow printing speed and the potential clogging in the nozzle. The biggest advantage for inkjet printing is to offer high resolution printing which can used to create complex patterning. In order to achieve high quality film by the inkjet printing, critical parameters such as vapor pressure, boiling point and surface tension of the solvents all need to be carefully considered. Solar cells based on P3HT: PCBM structure can reach up to 3.5% by using inkjet printing [ 1 5 1 8 1 9 ]. 1. 5 4 Spray C oating Spray coating is widely used for painting in commercial production and is one of the cheapest process techniques for coating polymer solutions [see Figure 1 8] Spray coating is compatible with the Roll to Roll technology and is ideal for l ow cost fabrication for unpatterned deposition. In the spray coating, the printing ink was forced through a nozzle with the help of a carrier gas (N 2 ) to form aerosol and directly deposit onto the sub strate surface [ 20 ]. One disadvantage is that the spray coated film tends to have a rough surface profile. But a careful optimization on solvent, casting conditions and thermal annealing condition might help to solve that problem. Solar cells based on P3HT: PCBM structure can reach up to 2.83% by using spray co ating [ 20 2 1 ]. 1.6 Overview of This Thesis The work presented in this thesis will focus on two major parts: the fabrication of PSCs based on flexible substrates and the fabrication of a prototype solar palm tree The current progress of OSCs, its working p rinciple, measurement and characterization and processing techniques are introduced in Chapter 1. In Chapter 2, we will describe the fabrication of PSCs based on flexible substrates and demonstrate efficient device using an inverted device structure. Chap ter 3 focus es on the leaf shaped

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21 device fabrication followed by the demonstration of a functional solar palm tree prototype with twelve leaves which was used to power either a mini fan or OLED displays. Conclus ions are presented in Chapter 4, where futur e work and improvement are also discussed

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22 Figure 1 1. A cross section of a typical polymer bulk heterojunction solar cell and the schematic energy diagram (a) Polymer bulk heterojunction. (b) Typical schematic energy diagram of OPV, th e highest occupied and lowest unoccupied molecular orbitals are HOMO and LUMO respectively [ 6 ]. Figure 1 2. S chematic diagram of power conversion efficiency measurement setup [Courtesy of Ying Zhen g ] Figure 1 3. Current Voltage (I V) characteristic of a PV cell in the dark and under illumination

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23 Figure 1 4. The schematic diagram of EQE measurement setup [Courtesy of Ying Zhen g ] Figure 1 5. Schematic diagram of the spin coating [1 4 ]

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24 Figure 1 6. A photograph showing doctor blading of MEHPPV [1 4 ] Figure 1 7. Schematic diagram of organic film formation by inkjet printing [1 8 ] Figure 1 8. Schematic illus tration of the spray process [ 20 ]

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25 CHAPTER 2 FABRICATION OF FUN CTIONAL SOLAR LEAF D EVIC E ON PLASTIC SUBSTRATE 2.1 Background Indium tin o xide ( ITO ) is commonly used as a transparent electrode in organic optoelectronic devices due to its high transparency and high electrical conductivity. It is capable of collecting either holes or elect rons since its work function of approximately 4. 5 4. 8 e V depending on the method of deposition and post depos i tion surface treatments, is between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for most organic photovoltaic materials. By insertin g functional layer s with different work functions to modify the ITO surface, the polarity of ITO electrode can be changed, serving as either a cathode or as anode In the normal structure geometry [see Figure 2 1] the device usually has four or five layers on top of the su bstrate. The structure layout is ITO/hole injection interlayer/polymer blend/electron injection interlayer/metal (Al or Ag). Poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate) ( PEDOT : PSS ) is the most commonly used p type hole transport layer. I t is used to modify the ITO surface for an anode contact. However due to its acidic nature, the PEDOT: PSS layer is potentially detrimental to the polymer layer. Moreover, it could etch the ITO during the spin coating process and lead to degradation in device performance. Other p type hol e transport layer candidates are NiO MoO 3 V 2 O 5 and etc. [ 2 2 ]. It th e inverted structure geometry [see F igure 2 2] the device type usually has five layers to enable selective transport of electron to the ITO an d holes to the metal electrode. To modify the ITO surface for a cathode contact to collect electrons lower

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26 work function metal oxide layer s of TiO 2 Cs 2 CO 3 and ZnO are used By applying these electron transporting layer s on top of the ITO the ITO will serve as the cat hode for electron collection while the anode with a high work function is on the opposite side of the active layer To efficiently co llection holes on the anode side high work function transition metal oxides of PEDOT:PSS, V 2 O 5 and MoO 3 are used as the h ole transport layer beneath the metal electrode anode Al, Ag or Au. A major difference in the fabrication process between the PSCs on the flexible plastic substrates and those on the glass substrates is the annealing treatment. Because of the flexible polyethylene terephthalate ( PET ) has a lo w glass transition temperature of 7 8 C and a melting temperature of 140 C, the annealing temperature of the active layer of the PSCs fabricated on the PET substrates should be noticeably lower than 140 C. In cont rast the annealing temperature of the active layer of the conventional PSCs on the glass substrates is usually higher than 150 C to achieve the optimal morphology in the active layer for optimized charge generation and collection Thus it is important t o process device s using PET substrate processed at low temperature to maintain flexibility and device performance. In this thesis, the maximum annealing temperature is set to 110 C for all device fabricated on PET substrates. 2.2 Sheet Resistance of PET S ubstrate s Due to the soft and flexible nature of the PET substrates, the sheet resistance of the PET substrate which coa ted with transparent conductive oxide (TCO) cannot be measured by the four point probe. To get a rough estimated on the sheet resistance of the PET substrates, a new method was d eveloped In this method, a 100nm thick of aluminum layer is deposited onto the PET substrates by VTE using the shadow mask

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27 with multiple dots [Figure 2 3 b] The measured r esistance between the metal dots R is prop ortional to the sheet resistance R s (2 1) Where is the geometrical factor. Assuming the same geometry between the chosen two dots, the geometrical factor is constant. By comparing the resista nce directly measured from the T CO/g lass and the two types of substrates TCO/PET (low and high resistance) the sheet resistance can be estimated. After fitting the slope on the measured I V curve from Figure 2 1 a for three different substrates, the measured resistances are : Resistance of ITO/Glass = 58 0.3 Resistance of TCO/PET (low resistance) = 310.5 Resistance of T CO/PET (high resistance) = 1501 The sheet resistance of ITO/Glass measured by the four point probe technique is R s of T CO/PET (low resista nce) = 111 R s of T C O /PET (high resistance) =493 The estimated sheet resistances agree with the specification obtained from Delta Technologies : R s of TC R s of T C O 2. 3 Normal Stru cture D evices with PET and G lass S ubstrates 2. 3 .1 Experimental P rocedure In this thesis, the PP V devices were fabricated using two different substrates: the polyethylene terephthalate (PET) substr ate coated with indium oxide, gold and silver (In 2 O 2 /Au/Ag) and glass substrate coated with indium tin oxide (ITO) Both substrate s

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28 wer e cleaned in ultrasonic baths of detergent deionized water acetone, and isopropyl consecutively for 15 minute s each and then exposed to an ultraviolet ozone environment for 15 minu te s Poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) (Baytron P) was deposited by spin coating at 5000 rpm for 40 seconds to modify the ITO surface. After baking at 100 C for 1 hour in air, the substrates were transferred to a nitrog en filled glove box filled with N 2 (H 2 O<0.1ppm and O 2 <0.1ppm). A blend solution made of poly (3 hexylthiopene) (P3HT) (purchased from Rieke Metals, Inc.) and 1 (3 methoxycarbonyl) propyl 1 phenyl (6,6) C 61 (PCBM) (purchased from Nano C, used as received) with a weight ratio of 1:0.8 in chlorobenzene (18 mg/ml) was spin coated at 1000 rpm for 60 seconds onto the PET substrate to form the active layer The film thickness was around 100nm. The aluminum cathode was deposited in a vacuum chamber with pressu re of torr. The deposition rate was 2~3 The active area of the devices used was in the range from 0.035 cm 2 to 0.21 cm 2 [ see Figure 2 2 c ]. After cathode deposition, the films were annealed at 110 C for 1 hour. The cross section of normal structure and energy level diagram of the P3HT: PCBM based PPV are shown in Figure 2 1 a and 2 1 b 2. 3 .2 Results and D iscussion The J V characteristics of P3HT: PCBM PSCs with normal structure and a PEDOT: PSS buffer layer are shown in Figure 2 4 a Compari son of the PV performance parameters for these devices are shown in Figure 2 5. V oc J sc FF are all significantly lower than those fabricated on the glass substrate especially worse in FF and J sc [see Figure 2 5 ]. The poor FF and J sc imp ly poor charge carrier collection within the device fabricated on PET substrate. Another possibility is maybe due to the annealing process

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29 for active layer. In the next section, I will use solvent annealing on the active layer to see whether I can improve the device performance on the PET substrate, especially the FF by changing the annealing condition. 2.4 Normal Structure Device with Solvent Annealing on the Active Layer 2.4.1 Experimental Procedure Both PET and glass substrates were cleaned in ultrasoni c baths of detergent deionized water, acetone, and isopropyl consecutively for 15 minute s each and then exposed to an ultraviolet ozone environment for 15 minute s PEDOT: PSS (Baytron P) was deposited by spin coating at 5000 rpm for 40 seconds to modify th e ITO surface. After baking at 100 C for 1 hour in air, the substrates were transferred to a nitrogen filled glove box (H 2 O<0.1ppm and O 2 <0.1ppm). A blend solution made of P3HT: PCBM with a weight ratio of 1:0.8 in chlorobenzene (18 mg/ml) was spin coat ed at 1000 rpm for 60 seconds onto PET substrate to form the active layer. Subsequently, the films were transferred i nto a covered glass petri dish filled with dichlorobenzene ( DCB ) solvent for 30 minutes. The aluminum cathode was then deposited at a rate of 2~3 in a vacuum chamber with pressure of torr. The active area of the devices was in the range from 0.035 cm 2 to 0.21 cm 2 After cathode deposition, the devices were annealed at 110 C for 10 mins or 150 C for 5 mins. 2. 4 .2 Results and D iscussion As shown i n Figure 2 6 and Figure 2 7 the device fabricated on glass substrate, which post annealed at 150 C is the most efficient device in this section, however annealing temperature of 150 C was too high for processing PET substrate

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30 T he devices, which undergoing DCB solvent annealing for 30 minutes and post annealing at 110 C for 10mins show better V oc J sc and FF and overall better PCE than the ones which only post annealing at 110 C for 1 ho ur However, the improved PCE of the dev ice fabricated on PET substrates is still far from the one with glass substrate. It implied that the annealing condition on the active layer does not play a significant role to improve efficiency for the device fabricated on PET substrates. It seems that t he real problem occurred at the interface between the TCO and the PEDOT: PSS layer. 2. 5 Devices with D ifferent PEDOT: PSS 2. 5 .1 Experimental P rocedure Both substrates of PET and glass were cleaned in ultrasonic baths of detergent deionized water, acetone, and isopropyl consecutively for 15 minute s each and then exposed to an ultraviolet ozone environment for 15 minute s PEDOT: PSS (Baytron PH500 or PH500+2%DMSO)) was deposited by spin coating at 5000 rpm for 40 seconds to modify the ITO surface. After bak ing at 100 C for 1 hour in air, the substrates were transferred to a nitrogen filled glove box (H 2 O<0.1ppm and O 2 <0.1ppm). A blend solution of P3HT: PCBM with a weight ratio of 1:0.8 in chlorobenzene (18 mg/ml) was spin coated at 1000 rpm for 60 seconds onto ITO/PET substrate to form the active layer in the glove box. The aluminum cathode was deposited in a vacuum chamber with pressure of torr. The deposition rate was 2~3 The active area of the devices used in this study was from 0.035 cm 2 to 0.21 cm 2 After cathode deposition, the films were post annealed at 110 C for 1 hour.

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31 2. 5 .2 R esults and D iscussion As shown in Figure 2 8 and Figure 2 9 the device fabricated on PET substrate using high ly conductive PEDOT:PSS (PH500 or PH500+2%DM SO) buffer layer shows better J sc FF and PCE but lower V oc th ) PEDOT:PSS (P). However, it is opposite for the devices fabricated on glass substrate using high conductive ) PEDOT: PSS. They show lower V oc FF and PCE than the ones using lower conductive PEDOT: PSS and they also show very high dark current. High dark current implies high leakage within the device mostly due to the high conductive PEDOT: PSS. Even though the devices using PET substrate an d glass substrate have exactly the same device structure, the device using PET substrate with high conductive PEDOT: PSS does not show high dark current [see Figure 2 8 ]. This implies that the anode (TCO) of the PET device and the anode (ITO) of the glass substrate are very different in terms of electronic properties. The lower FF, PCE of the device fabrication on PET substrate is mostly due to the problematic interface of TCO/PEDOT: PSS with poor charge collection efficiency. It is also possible that the PEDOT: PSS damaged the TCO surface due to its acidic nature. In the next section, I will take out the PEDOT: PSS layer and use an inverted structure approach to see whether I can improve the device performance on PET substrate mostly FF by using different functional layer on the electrodes.

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32 2.6 Inverted S tructure PPV Device with ZnO nano Particle Layer 2.6.1 Experimental P rocedure Both substrates of PET and glass were cleaned in ultrasonic baths of deionized water, acetone, and isopropyl alcohol consecutive ly for 15 minute s each and then exposed to an ultraviolet ozone environment for 15 minute s The substrates were transferred to a nitrogen filled glove box (H 2 O<0.1ppm and O 2 <0.1ppm). The ZnO NPs which prepared by Ying Z hen g and Lei Qian [see Figure 2 10 ] were spin coated at 4000 rpm for 30 seconds to form a 35nm thick film on top of the substrates. The films were then annealed at 80 C for 15 minutes to remove residual solvent. A blend solution made of P3HT: PCBM with a weight ratio of 1:0.8 in chloroben zene (18 mg/ml) was spin coated at 1000 rpm for 60 seconds onto the substrate to form the active layer. The active layer was annealed at 110 C for 1 hour. A 5 nm MoO x was deposited in a vacuum chamber with pressure of torr at a deposition ra te of 0.5 /s. A 100nm thick aluminum layer was deposited at rate of 2~3 /s. The active area of the devices used in this study was from 0.035 cm 2 to 0.21 cm 2 The schematic cross section of the inverted structure and energy level diagram are shown in Fig ure 2 2 2. 6 .2 R esults and D iscussio n As shown in Figure 2 11 a b and Figure 2 12 a b the inverted devices which fabricated on PET substrates with ZnO NPs and MoO x layer as the transparent electron transport layer and the hole transport layer show great ly improved V oc J sc FF and PCE compared to those u sing normal structure geometry The PCEs of the inverte d devices are 0.78 1.45% compared to 0.11 0.16% using normal structure geometry by

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33 significant ly improving the FF Compared with interface s of TCO/ PEDOT: PSS, PEDOT: PSS/P3HT: PCBM of the normal structure, the interfaces of TC O/ZnO, ZnO/P3HT: PCBM and P3HT: PCBM/MoO x of the inverted structure greatly improve the charge carrier collection for devices fabricated on PET substrate. On the other hand with the same device strucure the efficiency of inverted devices fabricated on glass substrate is just slightly better than the ones using normal structure geometry. As shown in Figure 2 11 c and Figure 2 12 c d the device which cleaned in IPA for 30 minutes a fter the detergent and deionized water shows slightly better device performance than the one cleaned in acetone and IPA consecutive ly for 15 minutes each Acetone is a stronger solvent than IPA and can dissolve PET in somewhat degree and that may affect de vice performance. The device with glass substrate which undergoing UV treatment after cleaning shows higher J sc PCE but lower FF than the ones without UV treatment [see Figure 2 11d and Figure 2 12e f ]. UV treatment on the TCO surface will increase its work function and lower the hole injection barrier between TCO and the ZnO. That maybe leads to poor electron collection efficiency, thus lowers the FF 2. 7 The Effect of D ifferent T hickness in ZnO NPs L ayer ZnO layer functions as a transparent, hole block ing, electron transport and protection layer for the inverted device. As shown in Figure 2 13 the inverted device fabricated on PET substrates with 60 nm thick ZnO NPs layer shows better J sc and PCE than the one with 35 nm thick ZnO This implies that a t hicker ZnO buffer layer serves as a better protection layer for preventing the pinholes than a thinner one

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34 2.8 Effect of the MoO x T hickness in and Metal E lectrode MoO x serves as a hole transport, exciton blocking and protection buffer layer, which can pr event the Al or Ag metal diffusion into the active layer. As shown in Figure 2 14 and Figure 2 15, the inverted device fabricated on glass substrate with 10nm MoO x is most efficient com pared to the one with thicker MoO x layers The best device fabricated o n glass substrate with the inverted structure has a 3.40 % PCE with 10nm MoO x 100nm Ag and a device area of 0.04 cm 2 As the MoO x layer increases beyond 20 nm, the device performance starts to suffer. As the MoO x t hickness increases, the J s c drops signifi cantly, implying that a relatively thick layer causes voltage loss. This implies that as MoO x layer increases to certain thickness, the series resistance within the device will increase and device performance will suffer. To come up with the optimum thickn ess for the MoO x with choice of electrode, more future experiments are needed.

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35 Figure 2 1 (a) Schematic cross section view of the normal structure of P3HT: PCBM PSCs (b) Energy level diagram of the P3HT: PCBM device (c) The shadow mask pattern of the metal electrode. Figure 2 2 (a) Schematic cross section view of the inverted structure of P3HT: PCBM based PSCs (b) Energy level diagram of the inverted P3HT: PCBM based PSCs

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36 Figure 2 3. (a) I V characteristic of device (b) The shadow ma sk pattern of the metal electrode

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37 Figure 2 4. Current density voltage (J V) characteristics of P3HT: PCBM based PSCs under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 : (a) device with PET subst rate (b) device with glass substrate (P3HT: PCBM anneal at 110 C) (c) device with glass substrate (P3HT: PCBM anneal at 150 C

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38 Figure 2 5 (a)V oc (b) FF (c) J sc and (d) PCE of P3HT: PCBM bas ed PSCs as a function of device areas.

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39 Figure 2 6 Current density voltage (J V) characteristics of P3HT: PCBM based PSCs under 1 sun AM 1.5 illumination with intensity of 100 mW/cm2 with solvent annealing: (a) and (b) : devices undergo solvent annealing in DCB for 30 minutes after spin coating the active layer, and later post annealing at 110 C for 10 minutes for both PET and glass substrate after the deposition of electrode (c) and (d) devices undergo solvent annealing in DCB for 30 minutes after spin coating the active layer, and later post annealing at 150 C for 5 mins for both PET and glass substrate after the deposition of electrode

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40 Figure 2 7 (a) V oc (b) FF (c) J sc and (d) PCE of P3HT: PCBM based PSCs as a function of device ar eas.

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41 Figure 2 8 Current density voltage (J V) characteristics of P3HT: PCBM based PSCs under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 with different PEDOT: PSS: (a) and (b) :devices with PEDOT:PSS (PH500) on PET and glass substrate (c) and (d) :devices with PEDOT:PSS (PH500+2%DMSO) on PET and glass substrate

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42 Figure 2 9 (a) V oc (b) FF (c) J sc and (d) PCE of P3HT: PCBM based PSCs as a function of device areas. Figure 2 10 (a) Transmission electron microscope (TEM) image of the ZnO NPs which size distribution of the ZnO NPs (Courtesy of Lei Qian)

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43 Figure 2 11. Current den sity voltage (J V) characteristics of inverted PSCs under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 (a) device with PET substrate (b) device with glass substrate (unpatterned) with UV treatment (c) device with PET substrate which cleaned without acetone but cleaned in IPA for 30 minutes instead (d) device with glass substrate without UV treatment

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44 Figure 2 12. (a) J sc and V oc (b) PCE and FF of P3HT: PCBM based PSCs as a function of device areas with inve rted structure, (c) and (d) show the device performance of PET substrate with and without acetone cleaning, (e) and (f) show the device performance of glass substrate with and without UV treatment.

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45 Figure 2 13 (a) Current density voltage (J V) characteristics of inverted structure with different ZnO layer thicknesses under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 (b) J sc and PCE (c) V oc and FF

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46 Figure 2 14. Current den sity voltage (J V) characteristics of inverted PSCs devices fabricated on glass substrate under 1 sun AM 1.5 illumination with intensity of 100 mW/cm 2 (a) Device using silver electrode with various MoO x thickness (b) Device using aluminum electrode with various MoO x thicknes s

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47 Figure 2 15 The effect of different MoO 3 thickness in the inverted devices with aluminum electrode VS. silver electrode: (a) J sc and V oc (b) PCE and FF

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48 CHAPTER 3 FABRICATION OF SOLAR PAL M TREE 3.1 Background PSCs can be inte grated into fabrics for use in clothing, such as jackets and handbags. In architectural materials, they can be used in rooftops window panes, blinds and tents. To address such concept s, some prototypes had already developed by companies such as Solarmer a nd Konarka shown in Figure 3 1 These products aim to power electronic devices such as GPS, mobile phones, PDAs, or laptops [5 2 3 ] PSCs can also be used in the building integrate d photovoltaic (BIPV) market. The solar market is estimated to grow to $ 74 b illion by 2017 [ 6 ]. However, there are still many challenges for PSCs to become commercial available such as relative low PCE compared to the projected threshold efficient of 10% and low lifetime. PSCs can also be used in the landscaping application to sub stitute nature grass, flowers or trees. During the day, these solar cells convert sunlight into energy, which could be stored in batteries. During the night, this energy can be used to provide lighting or to power other electrical appliances. Therefore, th ey have excellent economic potentials besides preserving the aesthetic appearance of the artificial plants. natural plants and grass, which makes them even more appealing for regions where obtaining sufficient water supply is at a premium In this chapter, we will demonstrate a functional a solar palm tree prototype with twelve leaves which can be used to power either a mini fan or OLED displays for this c oncept.

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49 3. 2 Leaf Device F abrication T he PET substrates were first trimmed into size o f 2 inch by 2 inch square. Aqua r egia solution was then used to remove the unwanted TCO to form the desired pattern showed in Figure 3 2 a B lack electrical tape was used to cover the wanted device area to resist the etching Forming desired pattern can prevent the direct contact between the cathode and the anode within the device After etching, the substrates were cleaned in ultrasonic baths of detergent and deionized wa ter for 15 minutes each, followed by isopro pyl alcohol for 30 minute s. After cleaning, the flexible substrates were dried an d attached to a 2 inch by 2 inch glass with double sided tape to have a rigid back support for the spin coating and easy handling. A fter exposed to an ultraviolet ozone environment for 15 minute s, t he substrates were then transferred to a nitrogen filled glove box (H 2 O<0.1ppm and O 2 <0.1ppm). The ZnO NPs prepar ed by Ying Zhen g and Lei Qian were spin coated at 4000 rpm for 30 seconds to form a 35 nm thick film ont o the substrates. The films were then annealed at 80 C for 15 minutes to remove residual solvent. A blend solution made of P3HT: PCBM with a weight ratio of 1:0.8 in chlorobenzene (27mg/ml) was spin coated at 1000 rpm for 60 seconds onto the substrate to form the active layer. The active layer was annealed at 110 C for 1 hour. A 10 nm MoO x was deposited in a vacuum chamber with pressure of torr at a deposition rate of 0.5 /s. The schematic diagram of vacuum th erma l evaporation (VTE) and the supporting apparatus for deposition are shown in Figure 3 3 A 100nm aluminum layer was later deposited at a rate of 2~3 /s. The ac tive area of the leaf devices was 6.45 cm 2 Two piece of copper tape were used to extend th e cathode and th e anode contact to form

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50 the stem which shows in F igure 3 2 The metal bus line s can be deposited on the TCO surface of PET substrate to lower the series resistance T he overlap of all the layers is shown in Figure 3 2 d. After all the layer s were deposited onto the PET substrate, the device which is in 2 inch by 2 inch square was taken out form the glove box and undergoing lamination process. The device was first placed onto the steel rule die #1 which is used to cut out 2 mm around the ori ginal leaf pattern with the alignment guide. A 1 ton arbor press was used to perform the pressing. Two piece of copper tape were attached to the two electrode s to extend the contact and form the conducting stem The leaf device with t he copper conducting stem was then inserted into a lamination pouch which has a thickness of 0.127 mm. A commercial laminator (GBC Heatseal H425) was used to provide the encapsulation at a temperature range of 120 150 C. Later the encapsulated device was placed and aligned on top of the Die #2 which used to cut out another 2mm around the leaf pattern which obtained after die #1. The solar palm leaf device and its procedure flow chart are shown in Figure 3 4 3. 3 Leaf Device S tability The encapsulated leaf device show excellen t stability [see Figure 3 5 ] After 90 days, PCE of the encapsul ated leaf device remains at 84 % compared to the 75% of unencapsulated small area device after 78 hours. The major reason causing short lifetime of OPVs is that oxygen and water will attack the organic materials and the metal electrode and diffuse laterally within the device. The encapsulated leaf device which has an active area of 6.45 cm 2 shows a poor FF (29%) compared to the unencapsulated small area devices with active areas from

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51 0.04 0.21 c m 2 By increasing the device area, the impact of the series resistance on the device efficiency increase s substantially due to the increase in current High series resistance within the device will lower the efficiency in charge carrier transport and coll ection. Thus for the large area application it is important to deposit metal bus lines on the TCO electrode in order to lower the series resistance [see Figure 3 2 d]. 3. 4 Solar Palm Tree Prototype Twelve leaf devices were attached to the trunk using mask ing tape. Fine wire or button hole thread was used to wrap and fasten the masking tape around the trunk [Figure 3 7 a]. T he burlap strip was unraveled a bit and glue around the bound area to cover the masking tape using Later, brown pipe cleaners were twist ed around the trunk to cover the wires and color the trunk as well [Figure 3 7b ]. The finished solar palm tree wa s then screwed onto t he aluminum project box with wires directed underneath the box. Three DPDT toggle switches will provi de the switching between the two configurations both in p arallel and in series [Figure 3 6 a b ]. The finished solar palm tree prototype is shown in Figure 3 7 c. 3. 5 Solar Palm Tree u nder the Real S un The I V characteristics of the solar palm tree which m easured outside the real sun using the silicon ph otodetector in both cloudy and sunny condition are shown in Figure 3 8 In the configuration for the fan, the V oc and I sc for the solar palm tree are 1.70 V and 27.4 mA in cloudy condition. In su nny conditi on which is around 0.8 sun as measured using a reference silicon detector the V oc and I sc are 1.76 V an d 32.5 mA. In the configuration for the OLED, the V oc and I sc for the solar palm tree are 5.65 V and 9.62 mA in clo udy condition. During the real ru n, the solar palm tree under a 0.8 sun

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52 condition is able to generate enough power to turn on the fan which operated at 0.5 V and 25 mA at the lowest setting.

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53 Figure 3 1. Some P rototypes of PSCs (a) A GPS charge pad by Solarmer [ 5 for tent by Solarmer [ 5 ] (c) Solar bag panel by Konarka [ 2 3 ] Figure 3 2. Leaf device fabrication procedures and cross section of conducting copper stem: (a) pattern after etching (2 by 2 inches substrate) (b) Spin coating ZnO and P3HT: PCBM (c) deposi t MoOx and metal electrode by VTE (d) deposit metal bus line after step a to lower series resistance within the device

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54 Figure 3 3. Schematic of vacuum ther mal evaporation (VTE) system, C ring fixture, sample holder, metal electrode mask and metal bus li ne mask. Figure 3 4. Step by step fabrication procedures for leaf devices

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55 Figure 3 5. Leaf device performance and stability: (a) unencapsulated small area device (b) encapsulated leaf device Figur e 3 6. Circuit configurations for prototype demonstration: (a) 4 units (each with 3 cells in series ) of leaf shaped devices are placed in parallel to power a mini fan (b) 4 units (each with 3 cells in series ) of leaf shaped devices are placed in series to power OLED displays.

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56 Figure 3 7. A solar palm tree prototype with twelve leaves. It is used to power either a mini fan which operated at 0.5 V and 25mA at the lowers setting or OLED display which operating at 4 5V and 2mA at the lowest setting. Three DPDT toggle switches will provide the switchin g between the two configurations.

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57 Figure 3 8. Current voltage (I V) characteristics of the solar palm tree under 0.8 sun (a) 4 units of leaf shaped devices in parallel (b) 4 units of leaf shaped d evices in series [Courtesy of Ying Zhen g ]

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58 CHAPTER 4 CONCLUSION AND FUTURE WORK 4.1 Conclusion The wor k presented in this thesis focus es on two major parts: the fabrication of PSCs on flexible substrates and the fabrication of solar palm tree prototype. In the study of the fabrication of PSCs on flexible substrate the normal structure devices fabricated on PET have very poor PCE of 0.1 0.2% and FF of less than 25% compared to PCE of 1.0 1.3% and FF of 48 54% for devices fabricated on glass To tackle thi s problem, we had tried changing the annealing conditions for the active layer and replacing the PEDOT: PSS (P) with a more conductive one (PH500+2%DMSO) However, the improvement is relatively small The imp roved devic es have PCE of 0.3 0.5 % and FF of le ss than 35%. The poor charge transport and collection efficiency maybe be caused by the problematic interfaces of TCO/PEDOT: PSS. With the inverted device structure (a ZnO NPs layer on TCO as cathode and MoO x /Al or Ag as anode) w e have demonstrated effi cient devices fabricated on PET substrates with PCE of 0.8 1.5% and FF 46 59% compared to PCE of 1.2 2.3% and FF 44 45% for devices fabricated on glass. We have also inves tigated the cleaning process for substrates. The best device fabricated on glass subs trate with the inverted structure has a 3.40 % PCE with 10nm MoO x 100nm Ag and a device area of 0.04 cm 2 In the study of the fabrication of solar palm tree, we have fabricated functional solar leaf devices with excellent stability. After 90 days, PCE of the encapsulated leaf device remains at 83.6% compared to the 75% of unencapsulated small area device after

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59 78 hours. The encapsulated leaf device shows low FF around 29% with active area of 6.45 cm 2 To lower the series resistance we need to deposit metal bus lines on the TCO. At last, we have demonstrated a solar palm tree prototype with twelve leaves which can be used to power either a mini fan or OLED displays. Three DPDT toggle switches will provide the switching between the two configurations. Under 0 .8 sun condition, the V oc and I sc for the solar palm tree are 1.76 V and 32 .5 mA for the configuration A (Mini Fan) In the configuration B ( OLED ) the V oc and I sc for the solar palm tree are 5.65 V and 9.62 mA. During the real run, the solar palm tree under a 0.8 sun is able to generate enough power to turn on the fan which operated at 0.5 V and 25 mA at the lowest setting. 4.2 Future Work and Improvement The specific materials we used in this inverted device are clearly not optimal. Optimization on ea ch component of the device is neeeded. For instance, both the TCO (indium oxide doped with silve and gold) and the aluminum electrode can probably be replaced by better alternatives. 4.2.1 PET Substrate The PET substrates used in this thesis were purchased from Delta Technologies. The work function of the coated TCO is unknown, t hus it is very important to have a measurement done on this TCO material As a result, it can help us to better understand the device performa n ce mostly on the FF as we need to imp rove the device efficiency, especially for large area in the Future.

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60 4.2.2 PEN Substrate A major difference in the fabrication process between the PSCs on the flexible substrates and those on the glass substrates is the annealing treatment. PET melts at te mperatures above 140 C while PEN can endure temperatures as high as 200 C. There is also a significant improvement on the glass transition temperature which increases from 78 C for PET to 120 C for PEN [ 10 ]. By switching to PEN substrate, we can incre ase the process temperature of flexible PSCs and later optimize the annealing temperature for the ac tive layer the maximum efficiency. 4.2.3 Metal Bus Line The purpose of inserting metal bus lines on top of the TCO is to lower the series resistance within the device, thus to improve the charge carrier transport and collection, a better FF, especially important for large area device. Also the selection of metals and metal bus line thickness all need to be optimized. I will start with the gold which has a g ood balance in conductivity and transparency for the metal bus line. 4.2.4 Shutter Design As shown in Figure 3 3, we had not yet design a shutter in the VTE systems for deposition of MoO x and metal electrode (Al or Ag). The deposition of the first few nan ometer of material onto the substrate has a big impact on the efficiency performance for the finished device. By having a shutter, we can control the flow rate of the evaporated material and prevent the contamination during the initial evaporation. Most im portantly, we can control the deposit film thickness more precisely. 4.2.5 P rocessing Technique Roll to Roll fabrication of OPVs offers great potential for reducing fabrication costs on large, flexible area applications. Focus for the future, it is i mportant to develop a

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61 manufacturing proce ss that enables production of solar palm tree at faster speed and at a low cost A specific processing technique need to be selected and designed so that it can be compatible wi th the Roll to Roll technology.

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62 LIST OF REFERENCES 1. Wohrle, D. and Meissner, D. Advanced Material 3 129 (1991) 2. Chamberlain, G.A. Organic solar cells: a review. Solar Cells 8 47 (1983) 3. Tan g, C.W. Applied Physics Letters 48 183. (1986) 4. Sariciftci, N.s. ,Smilowitz, L Heeger, A.J. and Wu dl, F. Science, 258 1474. (1992) 5. Solarmer Energy, Inc. Portable E lectronics October 2, (2010) < http://www.solarmer.com/productpe.php > 6. Nature photonics technology Focus Macmillian Publishers Limited, 3 august ( 2009) 7. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 270, 1789 (2005). 8. P. Peumans, A. Yakimov, and S. R. Forrest, Journal of Applied Physics 93, 3693 3723 (2003). 9. C. J. Brabec, A. Cravino, D. Meissner, N. S. Sariciftci, T. From herz, M. T. Rispens, L. Sanchez and J. C. Hummelen, Adv anced Funct ional Materials 11 374. ( 2001 ) 10. MacDonald, W.A. Organic Electronics: Materials Manufacturing and Applications (ed. H. Klauk). Wiley VCH Verlag GmbH, Chapter 7. (2006) 11. Jeffrey Brinker, Geo rge W. Scherer, Sol Gel Science : The Physics and Chemistry of Sol Gel Processing Academic Press, INC ( 1990 ) 12. Yoon, M. H. et al. Journal of Applied Physics 101 024503, (2006) 13. F. Padinger, C. J. Brabec, T. Fromherz, J. C. Hummelen, N.S. Sariciftci, Op to Electron. Rev 8, 280 (2000) 14. F. C. Krebs, Solar. Energy Materials. Solar Cells 93 394 412. ( 2009 ) 15. P. Schilinsky C. Waldauf C. J. Brabec Adv anced Funct ional Mater ials 16 1669 ( 2006 )

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63 16. D. Muhlbacher M. Scharber M. Morana Z. G. Zhu D. Waller R. Gaudiana C. Brabec Adv anced Mater ials 18 2931 ( 2006 ) 17. H. Sirringhaus T. Kawase T. Shimoda M. Inbasekaran W. Wu E. P. Woo Science 290 2133 ( 2000 ) 18. C. N. Hoth S. A. Choulis P. Schilinsky C. J. Brabec Adv anced Mater ials 19 3973 ( 2007 ) 19. C. N. Hoth P. Scholonsky S. A. Choulis C. J. Brabec Nano Lett ers 8 280 6 ( 2008 ) 20. D. Vak S. Kim J. Jo S. Oh S. Na J. Kim D. Kim Appl ied Phys ics Lett er 91 081102 ( 2007 ) 21. R. Green A. Morfa A. J. Ferguson N. Kopidakis G. Rumbles S. E. Shaheen Appl ied Phys ics Lett er 92 033301 ( 2008 ) 22. V. Shrotriya, G. Li, Y. Yao, C Chu, and Y. Yang, Appl ied Phys ics Lett er 88 073508 (2006). 23. Konarka Technologies, Inc. October 2 ( 2010 ) < http://www.konarka.com/media/pd f/konarka_casestudy_travelerschoice.pdf >

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64 BIOGRAPHICAL SKETCH Zhifeng Li was born in F oshan, China. He obtained his Bachelor of Science degree in materials science and e ngineering from the Pennsylvania State University in 2008. He received his Master of Science degree in m aterials s cience and e ngineering from the University of Florida in December 2010 He worked in group for his Master thesis and mainly focused on studying the fabrication and application of o rganic photovoltaic cells based on flexible substrates.