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1 INVERTED ORGANIC ELECTRONIC AND OPTOELECTRONIC DEVICES By CEPHAS E. SMALL 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 2012
2 2012 Cephas E. Small
3 To my parents
4 ACKNOWLEDGMENTS First of all, I give all praise to my Lord and Savior, Jesus Christ, who has guided my life and has continually opened doors of opportunity for me to walk through. I continue to put Him first and look forward to the new things that He will do in my life. There are a number of people who I must thank for their contributions to my graduate studies and to this dissertation work. It starts with my PhD advisor, Dr. Franky So. From the very beginning, Dr. So showed a willingness and a desire to have me join his research group. When I joined the group in 2006, I was this energetic, enthusiastic graduate student that knew nothing about organic electronics. Believing that I could be successful, Dr. So set high expectation s for my work and was patient with me through out the entire process. He has showed me, through his own example, the level of hard work, focus, and dedication that it takes to contribute to the research work at a high level. I have become a better researcher and am better equipped to have success in t he next stage in my career as a result of his guidance Although my years working with him has been very challenging, I am gratef ul for everything he has taught me I would also like to thank Dr. Norton, Dr. Pearton, Dr. Xue, and Dr. Rinzler for serving on my committee and providing feedback concerning my research. I would also like to acknowledge several of the past and present members of Dr. So's res earch group: Dr. Kaushik Roy Choudury, Dr. Jegadesan Subbiah, Dr. DoYoung Kim, Dr. Stephen Tsang, Dr. Leo Cheung, Dr. Neetu Chopra, Dr. Jaewon Lee, Dr. Dong Woo Song Dr. Galileo Sara squeta, Alok Gupta, Frederick Steffy, Mikail Shaikh, Nikhil Bhandari, Pieter De Somer, Verena Giese, Song Chen, Michael Hartel, Jesse Manders, Tzung Han Lai, Sujin Baek, Lordania Con stantinou and Eric Klump. I thank these wonderful people for not only contributing to my research work, but also serving as an
5 extended family that has contributed to my growth and development. In addition, I thank Dr. Mikhail Noginov Dr. Carl Bonner, Dr. Suely Black, and the rest of the faculty and staff at Norfolk State University's Center for Materials Research. I thank them for giving me the opportunity to conduct basic research for the first time. It was at NSU that I discovered my passion for research and science. Lastly, I thank my family who has continuously supported me in my academic endeavors and has helped me to stay grounded in my faith. The ir love was all the motivation I would need to finish what I started.
6 TAB LE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 ABSTRACT ................................ ................................ ................................ ................... 13 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 15 1.1 Organic Semiconductors ................................ ................................ ................... 15 1.1.1 Classes of Organic Semiconductors ................................ ........................ 15 1.1.2 Advantages and Disadvantages ................................ .............................. 16 1.2 Charge Transport and Injection in Organic Semiconductors ............................. 18 1.2.1 Charge Transport ................................ ................................ .................... 18 1.2.2 Char ge Injection ................................ ................................ ...................... 19 1.3 Inverted Organic Electronic Devices ................................ ................................ 20 1.3.1 Organic Light Emitting Diodes (OLEDs) ................................ .................. 20 1.3.2 Organic Photovoltaics (OPVs) ................................ ................................ 21 1.4 Dissertation Outline ................................ ................................ ........................... 22 2 ELECTRON ACCEPTING INTERLAYERS AS SELECTIVE p CONTACTS FOR INVERTED ORGANIC ELECTRONIC DEVICES ................................ ................... 27 2.1 Introductory Remarks ................................ ................................ ........................ 27 2.2 Expe rimental Details ................................ ................................ ......................... 31 2.2.1 Device Fabrication ................................ ................................ ................... 31 2.2.2 Device Characterization ................................ ................................ .......... 32 2.3 Results and Discussions ................................ ................................ ................... 34 2.3. 1 J V Characterization ................................ ................................ ................ 34 2.3.2 DI SCLC Transient Measurement ................................ ........................... 35 2.3.3 MoO 3 doped NPB Injection Layer For Improved Hole Injection ............... 36 2.4 Concluding Remarks ................................ ................................ ......................... 39 3 INTRODUC TION TO ORGANIC PHOTOVOLTAIC CELLS ................................ ... 50 3.1 Overview and History ................................ ................................ ........................ 50 3.2 Principles of Operation ................................ ................................ ...................... 53 3.3 Photovoltaic Cell Characterization ................................ ................................ .... 54 3.3.1 Current Voltage Measurement ................................ ................................ 55 3.3.2 Spectral Response Measurement ................................ ........................... 55
7 3.4 Progress in Organic Photovoltaic Cell Performance ................................ ......... 57 3.4.1 Novel Conjugated Polymers for OPVs ................................ ..................... 58 3.4.2 Inverted Device Geometry ................................ ................................ ....... 60 4 HIGH EFFICIENCY INVERTED POLYMER PHOTOVOLTAIC CELLS .................. 67 4.1 Introductory Remarks ................................ ................................ ........................ 67 4.2 Expe rimental Details ................................ ................................ ......................... 70 4.2.1 Device Fabrication ................................ ................................ ................... 70 4.2.2 Device Characterization ................................ ................................ .......... 71 4. 3 Results and Discussions ................................ ................................ ................... 71 4.3.1 ZnO PVP Nanocomposite Film Characterization ................................ .... 71 4.3.2 Inverted Polymer Photovoltaic Cell with ZnO PVP Nanocomposite ETL ................................ ................................ ................................ ............... 74 4.3 Concluding Remarks ................................ ................................ ......................... 78 5 LOSS MECHANISMS IN THICK FILM LOW BANDAP POLYMER SOLAR CELLS ................................ ................................ ................................ .................... 89 5.1 Introductory Remarks ................................ ................................ ........................ 89 5.2 Experimental Details ................................ ................................ ......................... 92 5.2.1 Device Fabrication ................................ ................................ ................... 92 5.2.2 De vice Characterization ................................ ................................ .......... 93 5.3 Results and Discussions ................................ ................................ ................... 93 5.3.1 PDTG TPD:PC 71 BM Solar Cell Performance ................................ .......... 93 5.3.2 Loss Mechanisms in Thick Film PDTG TPD:PC 71 BM Solar Cells ........... 97 5.4 Concluding Remarks ................................ ................................ ....................... 106 6 CONCLUSIONS ................................ ................................ ................................ ... 120 LIST OF REFERENCES ................................ ................................ ............................. 122 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 136
8 LIST OF TABLES Table page 4 1 Average solar cell performance for inverted PDTG TPD:PC 71 BM devices with 5, 10, 20, or 30 minute UV ozone treated ZnO PVP composite ETLs under initial AM 1.5G solar illumination. ................................ ................................ ........ 79 5 1 Averaged solar cell performance for inverted PDTG TPD:PC 71 BM devices with various active layer thickness under initial AM 1.5G sol ar illumination. ..... 107 5 2 Averaged solar cell performance for inverted P3HT:PC 61 BM devices with various active layer thickness under initial AM 1.5G solar illumination. ............ 107
9 LIST OF FIGURES Figure page 1 1 Number of scientific publications contributing to the topics "organic photovoltaics" and "organic light emitting diodes". ................................ ............. 24 1 2 Large scale roll to roll manufacturing process for flexible organic electronic devices. ................................ ................................ ................................ .............. 24 1 3 Schematic energy diagram of a metal organic semiconductor interface with an interface dipole formed at the interface. ................................ ........................ 25 1 4 Commercially available products based high resolut ion OLED display technology. ................................ ................................ ................................ ......... 25 1 5 Comparison between a bottom emitting and top emitting OLED with a conventional or inverted struc ture. ................................ ................................ ...... 26 2 1 Molecular structure of electron acceptors and conducting polymers used as injection layers in conventional OLEDs. ................................ ............................. 41 2 2 Energy level diagram at the NPB/HAT CN/Au interface. ................................ .... 42 2 3 Schematic cross section view of NPB single carrier devices fabricated in this study. ................................ ................................ ................................ .................. 43 2 4 An ideal DI SCLC transient current response for a trap free organic semiconductor. ................................ ................................ ................................ ... 43 2 5 Current density and injection efficiency vs. electric field for conventional and inverted NPB hole only devices using MoO 3 or HAT CN as HIL. ....................... 44 2 6 DI current density transients measured at various applied voltages using MoO 3 as HIL. ................................ ................................ ................................ ...... 45 2 7 DI current density transients measured at various applied voltages using HAT CN as HIL. ................................ ................................ ................................ .. 46 2 8 Injection efficiency, extracted from DI SCLC transient current measurements vs. electric field for normal and inverted NPB single carrier devices with either MoO 3 or HAT CN as injection layer. ................................ ..... 47 2 9 Hole mobility ( DI ) as a function of electric field ( F 1/2 ) for normal and inverted NPB hole only devices with either MoO3 or HAT CN injection layer. ................. 47
10 2 10 Current density and injection efficiency plots for conventional NPB hole only devices with either a neat MoO 3 or MoO 3 doped NPB HIL of varying doping concentration (1, 5, 10 mol%). ................................ ................................ ............ 48 2 11 Current density vs. applied electric field for MoO 3 doped and HAT CN doped NPB single carrier devices. ................................ ................................ ..... 49 2 12 Current density vs. electric field for normal NPB single carrier devices with eithe r ITO or Au anode. ................................ ................................ ...................... 49 3 1 Schematic illustrations for various solution processing methods used in the fabrication of large scale and laboratory scale polymer photovoltaic cells. ........ 62 3 2 Schematic diagram illustrating an organic donor acceptor heterjunction, as well as the bilayer and bulk heterojunction concepts. ................................ ......... 62 3 3 Typical J V characteristics for a photovoltaic cell tested in the dark ( J D ) and under illumination ( J L ). ................................ ................................ ........................ 63 3 4 Chemical structure for standard electron donor and acceptor materials used in the fabrication of organic photovoltaic cells. ................................ ................... 64 3 5 Schematic illustration showing the molecular orbital hybridization for a donor acceptor (D A) copolymer and the resulting reduction in the effective bandgap. ................................ ................................ ................................ ............ 65 3 6 Chemical structure for state of the art donor acceptor copolymers used for high efficiency polymer photovoltaic cells. ................................ .......................... 65 3 7 Normal vs. inverted device architectures for organic photovoltaic cells. ............. 66 4 1 The normal and inverted device geometries for P3HT:PCBM solar cells. .......... 79 4 2 An illustration of the process and the six processing steps employed during fabrication of the R2R coated modules. ................................ .............................. 80 4 3 AFM image (measuring 5 5 m 2 ) of a knife over edge coated ZnO nanoparticle film used in the fabrication of large scale R2R processed modules. ................................ ................................ ................................ ............. 80 4 4 Tapping mode AFM images for as prepared and 10 min UV ozone treated ZnO PVP nanocomposite films. ................................ ................................ .......... 81 4 5 Tapping mode AFM phase images for ZnO PVP nanocomposite films before and after UV ozone treatment (5 m scale size). ................................ ............... 82 4 6 X ray photoemission spectra (XPS) for the as prepared and 10 minute UV ozone treated ZnO PVP nanocomposite films. ................................ ................... 83
11 4 7 UV visible NIR transmission spectra for the as prepared and 10 minute UV ozone treated ZnO PVP nanocomposite films. ................................ ................... 84 4 8 Photo J V characteristics for inverted PDTG TPD:PC 71 BM solar cells highlighting the effect of prolonged light soaking on device performance under AM 1.5G solar illumination at 100 mW cm 2 ................................ ............ 84 4 9 Photo J V characteristics and EQE spectra for inverted PDTG TPD:PC 71 BM solar cells with UV ozone treated ZnO PVP nanocomposite ETL for various treatment times. ................................ ................................ ................................ .. 85 4 10 Dark J V characteristics for inverted PDTG TPD:PC 71 BM solar cells with UV ozone treated ZnO PVP nanocomposite ETL measured as is and after being light soaked using either a 600 nm or 350 nm band pass filter. ......................... 86 4 11 Certified I V characteristics for an inverted PDTG TPD:PC 71 BM solar cell with 10 minute UV ozone treated ZnO PVP nanocomposite ETL. The device was certified by NEWPORT Corporation. ................................ ................................ .. 87 4 12 Solar cell parameters versus time for encapsulated inverted PDTG TPD:PC 71 BM with as prepared and 10 minute UV ozone treated ZnO PVP nanocomposite ETLs. ................................ ................................ ......................... 88 5 1 Device structure for PDTG TPD:PC 71 BM solar cells, P3HT:PC 61 BM solar cells, and corresponding single carrier devices studied in this work. ................ 108 5 2 Photo J V characteristics and EQE spectra for inverted PDTG TPD:PC 71 BM solar cells with increasing active layer thickness. ................................ ............. 109 5 3 Photo J V characteristics and EQE spectra for inverted P3HT:PC 61 BM solar cells with increasing active layer thickness. ................................ ...................... 110 5 4 Average fill factor and EQE versus active layer thickness for PDTG TPD:PC 71 BM and P3HT:PC 61 BM solar cells. ................................ ................... 111 5 5 Current density times active layer thickness ( J L ) versus electric field curves for PDTG TPD:PC 71 BM hole only devices with 100 nm and 409 nm thick layers. ................................ ................................ ................................ ....... 112 5 6 Field dependent EQE spectra for PDTG TPD:PC 71 BM solar cells with increasing active layer thickness. ................................ ................................ ..... 113 5 7 Normalized photocurrent density ( J L ) as a function of internal electric field for inverted PDTG TPD:PC 71 BM solar cells with increasing active layer thickness. ................................ ................................ ................................ ......... 114 5 8 UV Vis NIR spectra and fraction of photons absorbed vs. active layer thickness for PDTG TPD:PC 71 BM and P3HT:PC 61 BM solar cells. ................... 115
12 5 9 Normalized J ph V eff curves for under 100 mW cm 2 illumination for the devices studied. ................................ ................................ ................................ 116 5 10 Light intensity dependence of the photocurrent and fill factor for 105 nm thick and 409 m thick PDTG TPD:PC 71 BM solar cells. ................................ ............. 117 5 11 Calculated number of photons absorbed in the PDTG TPD:PC 71 BM layer under AM1.5G illumination and photocurrent loss as a function of layer thickness. ................................ ................................ ................................ ......... 118 5 12 The critical active layer thickness corresponding to the onset of SCL photocurrent ( L c ) in low bandgap polymer solar cells versus bandgap of the polymer. ................................ ................................ ................................ ............ 119
13 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy INVERTED ORGANIC ELECTRONIC AND OPTOELECTRONIC DEVICES By Cephas E. Small D e c ember 2012 Chair: Franky So Major: Materials Science and Engineering The research and development of organic electro nics for commercial application has received much attention due to the unique properties of organic semiconductors and the potential for low cost high throughput manufacturing. For improved large scale processing compatibility and enhanced device stability an inverted geometry has been employed for devices such as organic light emitting diodes and organic photovoltaic cells. T hese improvements are attributed to the added flexibility to incorporate more air stable materials into the inverted device geometry However, early work on organic electronic devices with an inverted geometry typically showed reduced device performance compared to device s with a conventional structure. In the case of organic light emitting diodes, inverted devices typically show high operating voltages due to insufficient carrier injection. Here, a method for enhancing hole injection in inverted organic electronic devices is presented. By incorporating an electron accepting interlayer into the inverted device, a substantial enhancement in hole injection efficiency was observed as compared to conventional devices. Through a detailed carrier injection study it is determined that the injection efficiency
14 enhancements in the inverted device s are due to enhanced charge transfer at the elect ron acceptor/organic semiconductor interface. A similar situation is observed for organic photovoltaic cells, in which devices w ith an inverted geometry show limited carrier extraction in early studies. I n this work, enhanced carrier extraction is demonst rated for inverted polymer solar cells using a surface modified ZnO polymer composite electron transporting layer. The insulating polymer in the composite layer inhibited aggregation of the ZnO nanoparticles, while t he surface modification of the composite interlayer improved the electronic coupling with the photoactive layer. As a result, inverted polymer solar cell s with power conversion efficiencies of over 8% w ere obtained. To further study carrier extraction in inverted polymer solar cells, the ac tive layer thickness dependence of the efficiency was investigated. For devices w ith active layer thickness < 2 00 nm, power conversion efficiencies over 8 % was obtained. This result is important for demonstrating improved large scale processing compatibility A bove 200 nm, significant reduction in cell efficiency were observed. A detailed study of the loss processes that contributed to the reduction in efficiency for thick film devices are presented.
15 CHAPTER 1 INTRODUCTION 1.1 Organic Semiconductors Although research focused on the use of organic semiconductors for electronic and optoelectronic a pplication started as a mere academic curiosity, today low cost commercially viable technology based on these novel materials have become a reality Rapid progr ess in the field of organic electronics and optoelectronics is in large part due to do the signi ficant research contributions from academia, research laboratories and industry. As an example, the number of scientific publications contributing to the subjects 'organic photovoltaic cells' and 'organic light emitting diodes' by year, which is displayed in Figure 1 1, clearly highlights the significant increase in contribu tions for these research areas per year Initially, the interest in organic electronics and optoelectronics was aimed at studying con jugated molecules that exhibited interesting electrical and optical properties. Development of these novel organic semiconductors has continued to be a core focus of the research. This chapter will introduce the reader to organic semiconductors: the different classes of these materials, their propert ies, and the use of these materials in optoelectronic devices. Lastly, the research scope for this dissertation will be presented. 1. 1.1 Classes of Organic Semiconductors Organic semiconductors can be categorized into two classes: small molecular weight mo lecules and conjugated polymers Small molecules are discrete units with several to a few hundred atoms. Consequently, the molecular weight for these materials are well defined. Organic small molecules are typically processed at high vacuum by the rmal evap oration/ sublimation for thin film fabrication. By controlling the substrate
16 heating temperature during processing, amorphous, polycrystalline and single crystal small molecule thin films can be produced. Due to their ease of processing and the diverse ele ctrical and optical properties of these materials, organic small molecules have been used as hole transporting, electron transporting, and photoactive materials in organic thin film transistors (OTFTs), o rganic light emitting diodes (OLEDs) and organic pho tovoltaic s (OPVs) In contrast to organic small molecules, conjugated polymers have a molecular structure consisting of long chains of repeat units called monomers. The molecular weights for conjugated polymers are typically very large and vary from polymer to polymer. These materials are processed from solution using various processing methods such as spin coating, drop casting, or ink jet printing. The use of solution processing permits the control of numerous processing conditions th at impact the thin film morphology such as the cho ice of solvent and annealing conditions. However, the solution processing of organic layer stacks can be difficult due to t he requirement of orthogonal solvents for the organic layers. Controlling the morp hology of solution processed polymer films a nd developing novel solution processing procedures for fabricating orga nic layer stacks is still a subject of ongoing research and is critical for various device applications. 1.1.2 Advantages and Disadvantag e s Organic semiconductors, due to their intrinsic properties, has many advantages that are suitable for electronic and optoelectronic applications. For one, organic semiconductors have a low material cost and can be processed using low cost fabrication met hods. For example, high vacuum thermal evaporation of small molecules is a simple process that is more energy efficient than processing methods using for thin
17 film inorganic semiconductors, such as sputtering and plasma enhanced chemical vapor deposition ( PECVD). The cost for fabrication is reduced even further when solution processing methods are employed. These low cost fabrication methods are compatible with large scale roll to roll (R2R) manufacturing on rigid or flexible substrates, allowing organic ba sed devices to be produ ced at high throughput. Figure 1 2 shows the large scale manufacture of flexible organic electronics produced by different parties. Another key advantage to organic semiconductors is the ability to tailor the properties of these mat erial through intelligent chemical modification. This has lead to increased charge carrier mobility in OTFTs, efficient color tuning OLEDs, and enhanced light harvesting in OPVs. Interestingly, the intrinsic properties of organic semiconductors also introd uce a number of disadvantages that hinder the performance of electronic and optoelectronic devices. These materials have much low er carrier mobilities and intrinsic carrier concentration s compared to inorganic semiconductors Consequently, the device performance of organic electronic and optoelectronic devices becomes limited by the low electrical conductivity of organic semiconductors. Another issue is the inability to obtain high purity levels in organic materials. As a resul t, a high trap density due to impurities and structural defects is common for organic semiconductors. Lastly, organic materials degrade in ambient condition due to oxidation and exposure to moisture. To produce devices with enhanced stability and lifetime, it becomes necessary to design suitable encapsulation technology This adds additional complexity to the fabrication of organic electronic devices.
18 1.2 Charge Transport and Injection in Organic Semiconductors 1.2.1 Charge Transport The transport of elect rons and holes in organic solids can be described by three different models based on either a microscopic or macro scopic view of charge conduction. In the microscopic view, charge carrier hop between adjacent molecules to facilitate charge transport. In th is hopping model, the charge hopping rate is a function of the reorganization energy ( ) and the electron coupling factor (V ab ) 1 If and V ab are kno wn, the diffusion coefficient ( ) can be cal c ulated as ( 1 1 ) where is the hopping distance. T he carrier mobility can then be estimated from the Einste i n relation: ( 1 2 ) where is electron charge, is the Boltzmann constant, and is the absolute temperature. In the macroscopic view, charge conduction occurs by drift via an external field along disordered hopping sites in the organic semiconductor. For t his charge transpo rt model, first proposed by Poole and Frenkel, the field dependent carrier mobility is given as ( 1 3 ) where is the Poole Frenkel slope, is the zero field mobility, is the electric f ield, and is the dielectric constant The Poole Frenkel model has been widely used to experimentally d etermine carrier mobilities in organic semiconductors and is
19 employed in Chapter 2 to quantif y carrier mobility for the hole transporting materials stud ied in that work. Another c harge transport model worth mentioning is the Gaussian disorder model (GDM), which assumes t hat the transport energy states in the organic semiconductor have a Gaussian distribution. Since the charge ho pping in this model is field assisted and thermally activated in the disordered system, the carrier mobility is a function of electric field, temperature, and energetic and positional disorder 2 The energetic disorder can be conceptualized as the width of the Gaussian distribution, while the positional disorder is the geometric randomness cause by structural defects 3 1.2.2 C harge Injection Figure 1 3 shows the energy diagram for a metal organic semiconductor interface based on the Mott Schottky model 4 When an organic semiconductor is in contact with a metal, an energy barrier is formed at the organic semiconductor metal interface. The energy barrier s for electron and hole injection ( ) are determined by the electron affinity ( ), the ionization potential ( ) and the vacuum level shift due to the formation of an interface dipole ( ): ( 1 4 ) ( 1 5 ) where is the metal work function. Reducing the electron and hole energy barriers to carrier injection is critically important to th e efficient operation of OLEDs. However, this can be a challenge since the work function of t he metal electrode typically does not match the transport level of interest for the organic semiconductor. Novel approaches for reducing the energy barrier include introducing interlayers between the metal and
20 the organic semiconductor to modify the interface energetics, or introducing electr on accepting /donating atoms or molecules into the organic semiconductor to dope the interface. These approaches will be expounded upon in Chapter 2. 1. 3 Inverted Organic E lectronic Devices 1. 3 .1 Organic Light Emitting Diodes (OLEDs) T he first demonstration of electroluminescence from an organic heterojunction was observed in the work by Tang and coworkers in 1987 5 Since that discovery, OLEDs have become a commercially via ble technology for lighting and display applications. OLEDs have the following advantages for these applications: area emission, high electron to photon conversion efficiency, wide viewing angle, low operating voltage, fast switching, vivid colors, light w eight, small thickness, color tunability, and dimmability 6 Examples of commercial products based on OLED technology is shown in Figure 1 4 Simply put, a n OLED is an electroluminescent device in which the emitting and carrier transporting layers are organic semiconductors. Most polymer LEDs consist of a single light emitting layer, while OLEDs based on small molecules contain a stack of organic thin films. In either case, the organic layer(s) are sandwiched between two electrodes When a bias is applied to the electrodes carriers are injected into the emitting layer and recombine to form an exciton (electron hole pair) Light is emitt ed from this photoactive layer when the exciton undergoes radiative decay to the ground state. To efficiently produce light, efficient OLEDs require balanced carrier injection and transport As a result, conventional OLEDs are designed to ensure efficient car rier injection and balanced charge transport. These devices consist of the following: (i) injection layers, which ensure efficient carrier injection to the organic layers; ( ii ) electron and hole transport layers, which selectively transport s the injected c harge carriers to the emitting
21 laye rs; ( iii ) blocking layers, which confine carrier s and excitons in the emitting layer; and (iv ) an emitting layer 7 10 Figure 1 5 provides a schematic diagram showing the conventional geometry for a bottom emitting and top emitting OLED. In addition to enhancing the performance of OLEDs by optimizing carrier injection and transport, a developing trend is OLED research is to employ an inverted geometry for the device. An example of the inverted device geometry for a top emitting OLED is shown in Figure 1 5 The inverted device geometry greatly improves the dev ice stability and eliminates the need for expensive encapsulation technologies. In this geometry, the air stable high work function anode is moved to the top of the device and is coupled with an air stable hole injection layer. For the bottom cathode, air stable electron injection/transporting materials such as n type metal oxides and novel polyfluorene s provide efficient electron injection from an indium tin oxide (ITO) electrode 11 15 In addition to enhanced stability, the inverted device geometry is preferential for active matrix display applications since low cost display driver circuits are based on n channel a Si TFT technology 6 The main problem with inverted OLED devices is their high drive voltage compared to conventional OLEDs due to inefficient carrier injection 6 1. 3 .2 Organic Photovoltaics (OPVs) In contrast to the rapid progress observed in OLED development progress in organic photovoltaics ( OPV s) has been somewhat slower. Despite the record efficiencies being reported in the literature for laboratory scale photovoltaic cells much research efforts are still required before the technology will be commercially viable. The main priorities for OPV research include increasing the device efficiency, extending the device lifetime to over 10 years, and developing a greater expertise in the manufacturing of large scale roll to roll (R2R) processed devices. Consequently,
22 r esearchers world wide have attempted to enhance cell efficiency through employing novel material design and device concepts. Furthermore, improving device stability by developing more effective encapsulation technology and investigating the issues involved in manufacturing large scale OPV modules have become a focus of the wor k. Interestingly, employing an inverted device geometry for OPV cells has lead to enhanced cell efficiency, increased device stability, and improved R2R processing compatibility. A more detailed introduction to organic photovoltaic cells will be provided i n Chapter 4. 1. 4 Dissertation Outline There are two main topics presented in this dissertation. First, a method for enhancing carrier injection in inverted organic electronic devices is presented. This result is of critical importance for OLEDs, since reducing the drive voltage of inverted devices is required for display applications. In the second, the factors affecting carrier extraction are determined for inverted polymer photovoltaic cells In Chapter 1 the reader is pr esented a general introduction to organic electronics. This chapter describes the properties of organic semiconductors, discusses model s used to describe charge carrier transport and injection in these materials, and highlights the motivation behind using the inverted device geometry for organic electronic devices In Chapter 2, a detailed study of carrier injection in inverted organic electronic devices using electron accepting interlayers is presented. In addition to demonstrating ohmic hole injection in inverted devices, the origin of this enhanced hole injection is experimentally investigated The subsequent chapters of this dissertation focuses on inverted organic photovolt a i c cells Chapter 3 provides an introduction to organic photovoltaics,
2 3 des cribing the history, operating principles, and experimental methods used to c haracterize photovoltaic cells. Furthermore, recent process in this research field is discussed, including the use of the inverted device geometry In Chapter 4 high efficiency i nverted polymer photovoltaic cells are demonstrated By incorporating a ZnO polymer composite film as the electron transporting layer, enhanced charge collection is observed in the devices. Using the same device geometry the thickness dependence of the ef ficiency is investigated in Chapter 5. The loss processes responsible for limited device performance in inverted polymer solar cells with thick active layer are investigated. A theoretical model designed to help explain the role that space charge effects p lay in increasing photocurrent loss is presented. Finally, Chapter 6 summarizes the results of this work and provides a brief outlook on future work.
24 Figure 1 1. Number of scientific publications contributing to the topics "organic photovoltaics" and "organic light emitting diodes". Search done through ISI, Web of Science database. Figure 1 2. Large scale roll to roll manufacturing process for flexible organic electro nic devices 16 17
25 Figure 1 3. Schematic energy diagram of a metal organic semiconductor interface with an interfa ce dipole formed at the interface 1 Figure 1 4. Commercially available products based high resolution OLED display technology 18
26 Figure 1 5. Com parison between a bottom emitting and top emitting OLED with a conventional or inverted structure. A) B ottom emitting OLED with conventional structure. B) T op emitting OLED wi th a conventional structure. C) Top emitting OLED with an invert ed structure. HTL = hole transport layer, EBL = electron blocking layer, EML = emission layer, HBL = hole blocking layer, ETL = electron transport layer, CL = capping layer 6
27 CHAPTER 2 ELECTRON ACCEPTING I NTERLAYERS AS SELECT IVE P CONTACTS FOR INVERTED ORGANIC ELE CTRONIC DEVICES 2.1 Introductory Remarks Research aimed at optimizing the performance of organic light emitting diodes (OL EDs) has received much attention in recent years due to the enormous potential of the technology in flat panel display and solid state lighting applications 19 A key requirement to demonstrating OLEDs with efficient operation is to optimize carrier injection from the electrodes to the active emitting layer 20 21 As discussed in Chapter 1, t his requiremen t is due to the low intrinsic carrier concentration of organic semiconductors and thus the need to reduce the device operating voltage. Optimizing carrier injection is not a simple task, especially since OLEDs utilize wide band gap materials whose energy l evels do not match the work function of the electrodes. The work function mismatch is especially a problem for hole injection because the work function of transparent electrodes such as indium tin oxide (ITO) is less than 5.0 eV while the highest occupied molecular orbital (HOMO) energies of wide band gap hole transporting materials commonly used in OLEDs are typically 5.5 eV or higher, resulting in low hole injection efficiencies. To overcome this limitation, interlayers are often incorporated into the d evice to enhance hole injection. Figure 2 1 shows the molecular structu re for materials commo nly used as hole injectio n layers in conventional OLEDs. In general, these materials can be grouped into two categories. One class of materials used for the hole i njecting interlayer is conducting polymers, such as poly(3,4 ethylene dioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) 22 24 Although the use of PEDOT:PSS as a hole in jection layer (HIL) has been quite popular in small molecule and polymer OLED
28 research, its work function of 5.0 eV is not adequate to provide optimum hole injection into wide band gap materials with a deep HOMO energy 25 Another class of materials used as interlayer is strong electron acceptors. These materials can facilitate enhanced hole injection in OLEDs 26 Unlike conducting polymer HILs, electron accepting interlayers modify the energy levels of the anode and wide band gap hole transporting materials when ins erted between these two layers. P reviously repor ted ultraviolet photoemission spectroscopy (UPS) results showed that the electron accepting interlayer was responsible for both the large vacuum level shift in the anode whether indium tin oxide (ITO) or Au, and the observed band bending in the HOMO level of the hole transporting layer 27 30 As an illustration, Figure 2 2 shows the schemat ic energy level alignment of the Au / HAT CN /NPB interface based on in situ ultraviolet photoemission spectroscopy (UPS) measurements The strong band bending of the hole transporting position as typical for p doping. The use of electron acceptors for controlled p doping in amorphous hole transporting material s employed in high efficiency OLEDs has been well studie d 31 37 For example, strong p type doping has also been observed when MoO 3 was used as a dopant in hole transporters with a deep HOMO level 36 Due to their ability to modify the energy levels of the anode and the wide band gap hole transporting layers, these electron acceptors can serve as efficient hole injectors for wide band gap materials with a deep HOMO level. Despite the use of conducting polymers or electron acceptors as interlayers for improved carrier injection in organic devices, truly ohmic hole injection has rarely been reported. When the hole injection contact is ohmic, the carrier transport is limited by the
29 bulk of the materials rather than the injection contact. As a result of the ohmic injection, the carrier transport is space charge limited 32 The determination between bulk limited versus injection limited transport is made by calculating the carrier injection eff iciency, which is the ratio between the measured steady state current density of the device and the theoretical space charge limited current density g iven by the Mott Gurney Law 38 Even for conventional bottom injection contact hole only devices with PEDOT:PSS as an interlayer, injection limited transport and low injection effic iency values ranging from 30 to 60% have been reported 25 Additional studies featuring MoO 3 as an electron accepting interlayers have reported even lower injection efficiencies for similar single carrier devices 39 Recently, two novel approaches have been demonstrated for achieving ohmic hole injection in OLEDs. One approach i s to utilize a composite HIL consisting of a mixture of conducting polymers with different work functions and graphene for ohmic hole injection in flexible OLEDs 40 The other approach for achieving ohmic hole injection consists of using electron accepting interl ayers in OLEDs with an inverted architecture, with a substantial enhancement in current density being observed in the device when the bottom hole injection contact using PEDOT:PSS interlayer is replaced by a top hole injection contact using MoO 3 interlayer 41 In these inverted devices, the electron accepting interlayer is deposited on top of the hole tran sporting layer to form the top hole injection contact. Using this approach, optimized hole injection into wide band gap materials can be achieved. However, the origin of this enhancement is not understood. T he mechanism of enhanced hole injection in inve rted organic devices with different electron accepting interlayers is investigated in this chapter The results
30 presented here suggest a generalized behavior of the electron acceptors MoO 3 and HAT CN to provide enhanced hole injection in devices with an in verted architecture. A widely used amorphous hole transporting material N N' di(naphthalene 1 yl) N N' diphenyl benzidine (NPB), was used for this study 42 44 Current voltage ( J V ) and space charge limited dark injection (DI SCLC) transient measurements were used to characterize the hole injection efficiency in the normal and inverted single carrier devices. It was found that HAT CN was a superior hole injector compared with MoO 3 as a hole injection interlayer used in both nor mal and inverted devices. Furthermore, inverted devices with a top hole injection contact showed a higher injection efficiency compared to the normal devices with a bottom injection contact. For inverted devices with top contact HAT CN interlayer truly ohm ic hole injection was demonstrated, with an injection efficiency close to 100% being obtained. These results suggest that a better charge transfer at the electron acceptor/NPB interface is formed in the inverted device architecture. In this work, the term from the HOMO of NPB to the lowest unoccupied molecular orbital (LUMO) of the electron acceptor as discussed in previous photoemission spectroscopy results for MoO 3 and HAT CN 27 30 To further support our charge transfer model, we fabricated normal NPB single carrier devices that consisted of either a neat MoO 3 HIL or a MoO 3 doped NPB injection layer. Not only did we observe enhanced hole injection for devices with a MoO 3 doped NPB injection layer, but we also found that the injection efficiency can be tuned by increasing the MoO 3 doping concentration. Lastly, to understand why HAT CN was the better hole injector compared to MoO 3 we fabricated hole only devices with a p doped NPB interlayer using both electron acceptors as dopants. The
31 doping effect obtained using a fixed doping concen t ration of HAT CN or MoO 3 in NPB allowed us to characterize the degree of charge transfer between the electron acceptors and NPB. Sinc e st r onger p type doping was observed when HAT CN was used as the dopant compared with MoO 3 we confirmed that HAT CN provides more efficient charge transfer with NPB compared to MoO 3 2. 2 Experimental Details 2.2.1 Device Fabrication Hole only devices were fabricated by thermal evaporation of the organic small molecule materials and MoO 3 MoO 3 (99.999% purity) and HAT CN were used for this study. Figure 2 3 shows a schematic diagram of the devices fabricated in this study. For the J V and DI SCLC transi ent measurements, the following hole only devices were fabricated: (a) ITO/electron accepting HIL (10 nm)/NPB (1.5 m)/Au (normal device) and (b) ITO/NPB (1.5 m)/electron accepting HIL (10 nm)/Au (inverted device). Both devices were fabricated in the same thermal evaporation run for ease of comparison. Prior to film deposition, pre patterned ITO coated glass substrates were cleaned by sequential baths of de ionized water, acetone, and isopropanol. The substrates were then exposed to UV ozone treatment for 15 minutes. The electron accepting HILs and NPB were thermally evaporated at a base pressure ~10 6 Torr onto the substrates at rates of 0.05 and 0.5 nm/s, respectively. The high work function (WF) metal Au was used as the electron blocking cathode in the n ormal device and as the anode in the inverted device. It should be noted that UV ozone treated ITO (WF ~ 4.7 eV) served as an effective electron blocking cathode in the inverted device. To compare the quality of hole injection for a neat MoO 3 hole injectio n layer versus a MoO 3 doped NPB hole injection layer, the following hole only devices were fabricated simultaneously: (c)
32 ITO/MoO 3 (10 nm)/NPB (1.8 m)/Au, and (d) ITO/NPB:MoO 3 (10 nm, 1 10 mol%)/NPB (1.8 m)/Au. For p doped NPB single carrier devices, the doping concentration (10 mol%) was held fixed for both MoO 3 doped and HAT CN doped NPB films. 2.2.2 Device Characterization Current Voltage ( J V ) Measurement : The hole injection behavior of NPB single carrier devices with electron accepting HILs were analyzed by J V characterization. The J V characteristics were measured using a test fixture connected to a Keithley 4200 SCS, which provided a computer controlled source measuring unit (SMU) as the steady state voltage source and current measuring device. Assuming that the organic material is trap free with an ohmic injection contact, the steady state current should follow the space charge limited current ( J SCL ) as follows 45 : ( 2 1) where 0 is the zero field mobility, 0 is the vacuum permittivity, r is the relative permittivity, and F = V/d is the applied electric field. The Poole Frenkel slope is the field d ependent mobility coefficient. Both = 1.3 10 3 (cm/V) 1/2 and 0 = 2.7 10 4 cm 2 /V s were obtained from independent time of flight (TOF) m easurements reported in the literature 39 The carrier injection efficie ncy can then be calculated by the f ollowing equation 38 : (2 2 ) where J INJ is the measured steady state current density If the e xperimentally measured current density agree s with the ideal J SCL such that 1 then the injection contact is ohmic However, the experimental J V curves often deviate from J SCL signifying that J IN J is injection limited. Fo r a non ohmic contact, the carrier injection can be described by
33 the Richardson Schottky model of thermionic emission i.e. the charge carriers are injected in to the organic semiconductor if and only if they have enough thermal en ergy 46 47 DI SCLC Measurement: DI SCLC transient measurement is a well known technique for characterizing the carrier mobility and carrier injection in organic semiconductors 48 50 For the DI SCLC measurements taken in this work a pulse generator [HP model 214B] was used to inject holes into the NPB single carrier devices by application of a rectangular voltage pulse. A digital oscilloscope captured the voltage across a current sensing resistor that was connected in series to the samples and the corresponding current density was calcu lated following the measurement The large thickness for the NPB layer (1.5 1.8 m) ensured that the RC decay did not dominate the measured current transient, making the transit time e asy to determine experimentally Figure 2 4 shows the ideal DI SCLC transient current response for a trap free organic semiconductor. The ideal transient shows a well defined maximum at DI which is the arrival time of th e leading front of the injected carriers transporting to the non injecting electrode Two critical parameters can be extracted directly from the DI SC LC transient curre nt response. First, since the position of DI is related to the space charge free transient time by DI = 0.787 tr the carrier mobility can be calculated as follows 51 : (3) where d is the thickness of the organic layer and V is the voltage applied to the sample. Second, t he carrier in jection efficiency can be extracted from the DI SCLC transient by
34 calculating the ratio between the measured peak transient current density and the theoretical peak current density, which is ~1.2 1 times the space charge limited curren t density Therefore, the ca rrier i njection efficiency is given as 45 52 : ( 4 ) where J DI is the peak transient currrent density at t = DI 2. 3 Results and Discussions 2. 3 .1 J V Characterization Figure 2 5 shows the current density vs. electric field characteristics for the normal and inverted NPB hole only devices with either MoO 3 or HAT CN as the HIL. Th e solid line shown in the figure represents the theoretical J SCL calculated from Equation 1. Two conclusions can be drawn directly from these results. First, the inverted NPB hole only devices showed enhanced current densities compared to the devices with the normal architecture with MoO 3 or HAT CN as the HIL. Second, the devices with HAT CN HIL showed a significantly a larger current density than similar devices using MoO 3 as the HIL. It should be noted that the inverted NPB de vice with HAT CN HIL was the only device to show a good agreement with the theoretical J SCL curve. The injection efficiency, was then calculated from the J V characteristics using Equation 2 and plotted as a function of el ectr ic field for the devices (s ee Figure 2 5) A marked improvement in hole injection efficiencies was observed for the inverted devices with either a MoO 3 or HAT CN HIL compared to the normal device. The injection efficiency for inverted devices with HAT CN top contact HIL is near unit y, meaning that the injection contact is truly ohmic. These results show that improved hole injection in
35 inverted NPB hole only devices using MoO 3 or HAT CN as HIL is a generalized behavior. 2. 3 .2 DI SCLC Transient Measurement Analysis of the DI SCLC trans ient data provides in depth understanding of the charge injection processes 38 39 Figure 2 6 shows the DI SCLC transients with increasing applied bias for normal and inverted NPB single carrier devices with MoO 3 injection layer. The measured transients show a well defined maximum at DI followed by decay to a saturation value. The shift in DI to shorter times with increasing applied bias indicates that the hole mobility in NPB is field dependent 25 Larger current density values can be observed in the i nverted hole only devices compared to normal devices with the same film thickness. A similar result was obtained from DI SCLC transients of normal and inverted NPB single carrier devices with HAT CN HIL, shown in Figure 2 7 These devices showed larger tra nsient current densities compared to the corresponding devices with MoO 3 HIL, which is consistent with the J V measurements highlighted in Section 2. 3. 1. Furthermore, to confirm that HAT CN HIL provides ohmic hole injection in inverted NPB single carrier d evices, the carrier injection efficiencies were calculated for the same devices based on the measured DI SCLC transients using Equation 4 This result is shown in Figure 2 8. The field dependent injection efficiencies based on the DI SCLC transients are in good agreement with the results showed in Figur e 2 5, with ohmic hole injection ( ) being obtained for the inverted NPB devices with top contact HAT CN HIL. Analyzing all of the normal and inverted NPB hole only devices under the same applied field r evealed a difference in the measured DI DI SCLC transient
36 measurements assume the injection contact for the device is truly ohmic. If the injection contact is injection limited, then the carrier mobility calculated from the transients ( DI ) will be limit ed since the transient time corresponding to the leading front charge ( DI ) will be delayed due to inefficient charge injection. Since the injection contact for our inverted device with HAT CN HIL was truly ohmic while the injection contacts corresponding to the other devices were somewhat injection limited, the measured DI for the inverted device with HAT CN HIL was shorter than those obtained for the other devices Using Equation 3, the carrier mobility was plotted as a function of electric field for each of the normal and inverted NPB devices with either MoO 3 or HAT CN HIL (see Figure 2 9 ). The field dependent mobility values extracted from previously reported time of flight (TOF) measurements are shown for comparison 53 The extracted hole mobility for the devices correlated very well with the measured injection efficiencies. Devices with higher injection efficiency showed a large r DI under the same applied field. The mobility values for inverted device with HAT CN HIL are in good agreement with previously reported time of flight (TOF) mobility values measured at the same applied electric field for NPB single carrier devices 39 53 These results provide further evidence that inverting the device architecture when using electron acceptors as HILs is necessary to improve hole injection into organic electronic devices. 2. 3.3 MoO 3 doped NPB Injection Layer For Improved Hole Injection T he enhancement in hole injection observed in the inverted devices is attributed to enhanced charge transfer at the top hole injecting contact. In the inverted device, the electron acceptor dopes the surface and subsurface of the organic semiconductor during thermal evaporation. To confirm that the improved hole injection using electro n
37 acceptors as HIL is due to interfacial doping, we fabricated normal structure devices with a 10 nm thick MoO 3 HIL and compared the results to devices with a 10 nm thick NPB:MoO 3 HIL of different doping concentrations. Figure 2 10 shows the current densit y and the hole injection efficiency vs. electric field plots for these devices. As expected, hole injection for the normal NPB device with MoO 3 HIL is contact limited. When the MoO 3 HIL is replaced with a NPB:MoO 3 HIL, significantly larger current densitie s and enhanced injection efficiencies are obtained. As the doping concentration in the NPB:MoO 3 HIL is increased from 1 10 mol%, the hole injection efficiency is greatly increased at high fields. The difference in hole injection for these devices is due to the degree of surface and subsurface doping occurring at the bottom hole injecting contact. With MoO 3 as a HIL, the surface layer of NPB is p doped due to the charge transfer at the MoO 3 / NPB interface 54 55 By doping the entire HIL with MoO 3 the resulting heavily p doped NPB enables th e formation of ohmic contacts for enhanced hole injection. This interfacial doping model characterized by the surface and subsurface doping of NPB when using electron acceptors as HIL also applies to our results for HAT CN assuming that this electron acceptor serves as a p type dopant. Based on the results of our injection study, HAT CN provides stronger surface and subsurface doping of NPB compared to MoO 3 when used as an injection layer. To compare the p type doping of NPB due to MoO 3 and HAT CN, we fabricated MoO 3 doped and HAT CN doped NPB hole only devices. Th e same doping concentration (10 mol%) was used for each electron acceptor. The current density electric field characteristics of the se devices are shown in Figure 2 11 The data for an undoped NPB hole only device with HAT CN HIL
38 is also shown for comparis on. The data for the undoped device shows the transition from ohmic to trap limited to trap free SCL transport. On the other hand, the HAT CN doped NPB devices show only trap free SCL transport over the entire voltage range with a current density 3 orders of magnitude larger than the undoped device in the ohmic regime. Compared to the HAT CN device, the MoO 3 doped NPB device shows the transition from trap limited transport to trap free SCL transport with a current density at low fields substantially lower t han that of the HAT CN doped NPB device. These results show a clear indication of p type doping effect of NPB using HAT CN as dopant which is significantly stronger than that of the devices with MoO 3 doped layer, confirming that HAT CN provides stronger ch arge transfer with NPB when used as a dopant compared to MoO 3 When used as a hole injector, HAT CN would therefore provide stronger interfacial doping of NPB compared to MoO 3 and allow for more efficient charge injection in OLED devices. Based on these re sults, we conclude that both MoO 3 and HAT CN injection layers rely on efficient surface and subsurface doping of NPB to form an ohmic contact for hole injection. Lastly, the anode/electron acceptor interface could play a critical role in t he results report ed in this chapter To address the issue of whether the metal/electron acceptor interface contributed to the injection efficiencies of the NPB singl e carrier devices, t he hole current densities for normal devices with HAT CN HIL and either Au or ITO as ano de were analyzed T he measured current densities for the devices were independent of the electrode used for the device as shown in Figure 2 12 This result is in agreement with previous studies on metal/electron acceptor interfaces, which showed that the s trong charge transfer between the electron acceptor and the anode leads to a
39 lowering of the interfacial energy barrier 56 57 The work function of the anode becomes pinned to the Fermi level of the electron a cceptor, with this Fermi level pinning being observed for ITO and various metals when paired with an electron accepting HILs 58 This behavior is consistent with the classical description of an ohm i c contact between a metal and an n type semiconductor, which requires that the work function of the n type n ) is larger than the work function of the metal m ) 59 This phenomenon is independent of the device architecture employed, since the degree of charge transfer between the anode and the electron acceptor is very strong 29 30 Based on these findings, we confirmed that the interfacial doping of NPB at the NPB/electron acceptor interface is responsible for the enhanced hole injection in inverted NPB devices. 2. 4 Concluding Remarks In conclusion, enhanced hole injection as been demonstrate d for inverted NPB hole only devices by using electron acceptors as HIL. J V and DI SCLC transient measurements were performed to cross examine the hole injection behavior in normal and inverted NPB hole only devices. The enhanced hole injection in the inv erted hole only devices was due to enhanced charge transfer at the NPB/electron accepting HIL interface. An interfacial doping model was then proposed to explain the results obtained for the normal and inverted devices. By utilizing MoO 3 doped NPB HILs, it has been demonstrated that the hole injection in the normal NPB device can be further enhanced. Increasing the doping concentration of the MoO 3 doped NPB HIL directly increased the injection efficiency, highlighting the importance of interfacial doping to obtain an ohmic contact using these electron acceptors. Lastly, it was determined that a strong degree of
40 charge transfer between the HAT CN and NPB was responsible for the large injection efficiencies obtained in devices with HAT CN injection layer.
41 Figure 2 1. Molecular structure of electron acceptors and conducting polymers used as injection layers in conventional OLEDs.
42 Figure 2 2 Energy level diagram at the NPB/HAT CN/Au interface. Reproduced with permission from Kim, Y. K et al. Appl. Phys. Lett. vol. 94, 063305, 2009 30
43 Figure 2 3 Schematic cross section view of NPB single carrier devices fabricated in this study. A) C onventional device with bottom contact MoO 3 or HAT CN HIL. B) I nverted device with top contact MoO 3 or HAT CN HIL C) Conventional device with a NPB:MoO 3 interlayer (1 10 mol% d oping concentration). Figure 2 4. An ideal DI SCLC transient current response for a trap free organic semiconductor. Reproduced with permission from Tse S. et al J Appl Phys vol. 100, 063708, 2006 43
44 Figure 2 5. Current density and injection efficiency vs. electric field for conventional and inverted NPB hole only devices using MoO 3 or HAT CN as HIL. A) J E curves for the device studied Solid line represents theoretical J SCL B) Hole injection efficiency as a function of electric field for the devices 60
45 Figure 2 6. DI current density transients measured at various applied voltages using MoO 3 as HIL A) N ormal device archit ecture B) I nverted device architecture 60
46 Figure 2 7. DI current density transients measured at various applied voltages using HAT CN as HIL A) N ormal device architecture B) I nverted device architecture 60
47 Figure 2 8. Injection efficiency, extracted from DI SCLC transient current measurements vs. electric field for normal a nd inverted NPB single carrier devices with either MoO 3 or HAT CN as injection layer 60 Figure 2 9. Hole mobility ( DI ) as a function of electric field ( F 1/2 ) for normal and inverted NPB hole only devices with either MoO3 or HAT CN injection layer. Field dependent mobility based on time of fligh t (TOF) measurements is shown for comparison 53 60
48 Figure 2 10. Current density and injection efficiency plots for conventional NPB hole only devices with either a neat MoO 3 or MoO 3 doped NPB HIL of varying doping concentration (1, 5, 10 mol%). A) J E curves for the devices. B) Injection efficiency electric field plot for the devices 60
49 Figure 2 11. Current density vs. applied electric field for MoO 3 doped and HAT CN doped NPB single carrier devices. The data for an undoped NPB hole only device with HAT CN HIL is shown for comparison 60 Figure 2 12. Current density vs. electric field for normal NPB single carrier devic es with either ITO or Au anode. HAT CN is used as injection layer for both devices 60
50 CHAPTER 3 INTRODUCTION TO ORGA NIC PHOTOVOLTAIC CEL LS 3.1 Overview and History Due to the s ignificant effort in the research and development of photovoltaic cells this renewable energy technology has become a promising candidate for meeting the world's energy requirements. The state of the art for this technology consist of p hotovoltaic cells using inorganic materials such as silicon and compound semiconduct ors. Photovoltaic cells and modules based on these materials have shown hi gh power conversion efficiencies, highlighting the potential of this technology to achieve grid parity. However, to achieve grid parity, the total cost per watt would need to be redu ced for th is technology to be competitive with non renewable energy sources. In an attempt to reduce the material and processing cost, thin film inorganic photovoltaic cells using materials such as amorphous/nanocrystal l ine Si CdTe, and CuIn 1 x Ga x Se 2 (CIGS ) or Cu 2 ZnSn(S 1 x Se x ) 4 (CZTSSe) have been adopted 61 63 Despite the potential cost savings associated with these thin film devices, their power conversion efficiency which 20 % is significantly lower than the state of the art technology 64 In addition to thin film inorganic photovoltaic cells, devices based on organic materials is a promising techno logy for low cost energy harvesting. These organic photovoltaic cells typically consist of either organic molecules of well defined molecular weight, structure and chemical composition or conjugated polymers with high molecular weight but a polydispersity index (PDI) greater than 1. O rganic photovoltaic cells are intrinsic ally fl exibility, mechanical robust and can be processed on inexpensive substrates using high throughput methods. For example, small molecule OPVs are
51 generally processed by high vacuum thermal evaporation and can be processed in high throughput using this method 65 66 Similarly devices based on conjugated polymers and novel oligomers can be fabricated by solution processing methods that are large scale roll to roll (R2R) processing compatible, such as slot die coating, doctor blading, and ink jet printing 67 70 Figure 3 1 illustrates various processing methods utilized in the fabrication of large area and laboratory scale organic solar cells. Organic photovoltaic cells also have an additional benefit associated with the design of organic small molecules and conjugated polymers: the electronic and optical properties of these organic materi als can be modified by altering their chemistry. For example, one approach for enhancing the performance of organic photovoltaic cell is to manipulate the chemical structure of the electron donor Modifications such as introducing electron withdrawing grou ps, reducing the conjugated length or enhancing the planarity of the polymer can lower the bandgap lower the HOMO level or increase the polymer's carrier mobility respectively 71 72 Such intelligent design of the conjugated polymer has lead to enhanced J sc and V oc in laboratory scale organic photovoltaic cells. Despite these advantages, the properties of the organic materials employed in organic photovoltaic cells h ave introdu ce d a number of issues that have impacted device design One of the key challenges with organic photovoltaic cells is to effic iently dissociate tightly bound electron hole pairs, called ex c i tons generated with in the organic seminconductor T he re are two types of excitons formed i n o rganic semiconductors: Frenkel excitons and charge transfer excitons (CTEs) 73 74 Frenkel excitons consist of electron hole pairs with a binding distance less than a single molecule while cha rge transfer exciton s are characterized as electron hole pairs that
52 are delocalized over adjacent molecules The bindi ng energy for these excitons are both greater than 0.1 eV 73 74 To separate these tightly bound excitons, t he first organic photovoltaic cells utilized Schottky diodes to facilitate exciton dissociation. Although t he strong electric field near the metal ele c trode or ganic interface dissociated excitons and generated charge carriers, exciton quenching at this int erface and limited exciton dissociation throughout the photoactive layer limited the overall device performance 75 77 In 1986, a major breakthrough in organic photovoltaic cell performance was achieved by Ching Tang with the demonstration of the donor acceptor (D:A) bilayer heterojunction photovoltaic 78 A sche matic diagram illustrating the D:A bilayer heterojuncti on structure is show in Figure 3 2 In the bilayer heterojunction photovoltaic cell, exciton s generated within the organic materials dissociate at the heterojunction assuming that the energy level offs et between the donor and acceptor are large enough to prov ide a sufficient driving force For efficient device operation, the thickness of the donor and acceptor layers must be less than the exciton diffusion length (L D ) 79 Although the shift to the bilayer D:A heterojunction structure lead to enhanced photocurrent pr oduced by the device, exciton dissociation was still limited since the PV cells consisted of only one D:A interface. In the 1990's, a further enhancement in exciton dissociation was demonstrated in OPVs through the development of the D:A bulk heterojunctio n (BHJ) photovoltaic cell 79 81 In BHJ photovoltaic cells, the interpenetrating network created by the D:A blend resulted in enhanced exciton dissociation since photogenerated excitons now were within L D (see Figure 3 2 ) S ignificant enhancements in photocurrent were observed for these photovoltaic cells as compared to bilayer heterojunction devices. In more recent work improvements to BHJ structure (optimizing the D:A blend
53 morphology, incorporating anode/cathode interlaye rs) and modification of the donor conjugated polymer's energy levels have led to a world recor d power conversion efficiency of 9 .21 % in single junction devices 82 3.2 Principles of Operation To better appreciate the progress that has been made in organic photovoltaic cell research, it is important to understand the o perating principles of these devices. The ability to generate power in an OPV cell is dependent on four processes that sequentially occur with the photoactive layer. These processes are as follows: photon absorption, exciton diffusion, charge transfer (exc iton dissociation), and charge carrier collection. The external quantum efficiency ( EQE, ) or the ratio of the number of charge carriers collected by the photovoltaic cell to the number of photons incident on the device, is the product of the effi ciencies associated with each of the four processes: ( 3 1) where is the photon absorption efficiency, is the exciton diffusion efficiency, is the efficiency of charge transf er, and is the charge carrier collec t ion efficiency. This expression can also be written as: ( 3 2 ) where is the internal quantum efficiency. In the ideal case, a value approaching unity can be achieved if both and are maximized. However, in the state of the art OPV cells the processes influencing absorption efficiency and internal quantum efficiency are opposing. Since organic semiconductors have a high absorption coefficient ( ) of ~ 10 4 to 10 5 cm 1 the optical absorption length ( ) in these materials is typically at least 100 nm 83 84 As a result increasing the photoactive layer thickness in
54 BHJ OPV cells above 100 nm enhances However, charge carrier collection becomes limited due to recombination losses in the thicker photoactive layer Research efforts have focused on enhancing devi ce performance despite t his tradeoff between and Chapter 4 will demonstrate an approach to enhance charge carrier collection through the use of novel interlayers, while Chapter 5 will highlight this tradeoff problem for high efficiency polymer photovoltaic cells with increased active layer thickness 3. 3 Photovoltaic Cell Characterization The key parameters used to characterize organic photovoltaic cells under operation will be discussed herein. In general, photovoltaic cells act as pho todiodes which operate when unbiased and show rectifying behavior. These devices differ from typical photodiode s in that a wide spectral response over a broad solar wavelength range is required 85 The current voltage ( J V ) characteristics for a photovoltaic cell in the dark and under illum ination is shown in Figure 3 3 The J V characteristics for the effective photocurrent ( J ph ), which is the difference between the current under illumination ( J L ) and the dark current ( J D ), is also shown. From the J L V curve, the key parameters used to characterize the device perform ance can be e xtracted T he short circuit current ( J sc ), the photocurrent at zero bias, and the open circuit voltage ( V oc ) the voltage at which the J L = 0, is highlighted in the figure. The product of the J sc and V oc is the theoretical maximum power produced from the cell. The actual maximum power output ( P m = I m V m ) is typically less than this theoretical maximum. The determination of P m is illustrated by a small rectangle in Figure 3 3 The fill factor ( FF ) measur es the sharpness of the J L V curve and is defined as the ratio between the actual and theoretical maximum power outputs 85
55 ( 3 3 ) The most important figure of merit is the power conversion efficiency ( ), which is the ratio of the maximum power output to the incident power P in c ( 3 3 ) 3 3.1 Current Voltage Measurement To experimentally determine the J V characteristics of the organic photovoltaic cell were measured in the dark and under Air Mass 1.5 global (AM 1.5G) illumination. The standard reporting conditions (SRC) for rating the performance of terrestrial photovoltaic cells are as follows: 100 mW cm 2 (1 sun) AM 1.5G reference spectrum, and 298 K cel l temperature. To measure the current voltage characteristics, a standard test method for photovoltaic cells was utilized 86 A xenon lamp with 1.5G air mass filter was used to simulate the solar spectrum. The incident light from the solar simulator was calibrated using a certified Si refer ence cell to ensure that P inc = 100 mW cm 2 To control the incident light intensity, neutral density filters were used so P inc could be varied from 10 to 100 mW cm 2 With the J V characteristics measured, the key parameters described in the previous section can be extracted. 3 3.2 Spectral Response Measurement To experimentally determine a spectral responsivity measurement was performed on the organic photovoltaic cells studied in this work. A computer controlled spectral re sponsivity measurement system that met the ASTM E1021 testing standard was used for this measurement 87 The measurement system consisted of a 150 W xenon arc lamp solar simulator with a 1.5G AM filter coupled with a monoch romator as
56 the light source, producing monochoma tic light with wavelengths from 400 nm to 1800 nm. The light beam from the monochromator is collimated and chopped using a collimating lens and a optical chopper, respectively. The optical chopper creates an alternating signal which allows the photocurrent produced by the test cell to be differentiate d from the background noise. By connecting the chopper to a lock in amplifier and using the chopper frequency (~400 Hz) as a reference t he photocurrent signal can be isolated. T he chopped monochromatic light then passes through a condensing lens to either a calibrated Si photodetector for measuring the wavelength dependent incident light power or to the test cell. The beam spot size is less than both the Si phot odetector area and the test cell area for accuracy. A labVIEW program was used to control the testing procedure and acquire the relevant data. The spectral responsivity and, ultimately, the external quantum efficiency spectrum can now be calculated The spectral responsivity for the test cells, is calculated as follows: ( 3 4 ) where is the measured photocurrent density of the test cell, is the incident light power measured with the Si photodetector is the Si photodetector current density, and is the responsivity of the reference detector The external quantum efficiency, also known as the incident photon to current efficien cy (IPCE), can now be calculated as ( 3 5 )
57 where h is Planck's constant, c is the speed of light, q is the electron charge, and is the wavelength. Lastly, the short circuit current density under AM 1.5G illumination at 100 mW cm 2 can be calculated from the external quantum efficiency spectrum as ( 3 5 ) where is the reference AM 1.5G power intensity. Ideally, the integrated J sc calculated from the EQE spectrum should match the value extracted from the J V characterization for the test cells This is only possible if the calibrations for both the J V measurement setup and the sp ectral responsivity measurement setup are done corre ctly 3. 4 Progress in Organic Photovoltaic Cell Performance Since the first demonstration of organic photovoltaic cells utilizing the BHJ structure, research efforts have focused on enhancing the power conversion efficiency of these devices. Improving the properties of the conjugated polymer s and organic small molecule s used as the donor in the D:A blend through intelligent chemical design has been largely responsible for the high efficiencies reported in the literature 88 90 These materia ls are typically blended with C 60 fullerene or its derivative s to facilitate exciton d issociation within the photoactive layer. C 60 derivatives, such as [6,6] phenyl C61 butyric acid methyl ester (PCBM), have become the standard n type materials used in OPV cells due to their high electron mobility and strong electronegativity 91 T his section will d iscuss the chemica l design and important material properties of the novel conjugate polymer s employed in Chapters 4 and 5 of this dissertation as well as similar polymers reported in the literature Furthermore, the power conversion efficiencies reported for polymer BHJ OPV cells incorporating these polymers will be highlighted
58 3 4.1 Novel Conjugated Polymer s for OPVs Figure 3 4 shows the chemical structure for standard conjugated polymer s and fullerene acceptor materials used in OPV cell s 92 Of the c onjugated polymers shown in this figure, soluble thiophenes such as poly( 3 hexylthiopene (P3HT) has become a standard for the fabrication of solution processed polymer OPV cells 93 Devic es incorporating P3HT: PCBM blends showed higher charge carrier mobilities compared to previously reported OPV cells, lead ing to enhanced FF and power conversion efficiencies in these device s 94 Despite these enhancements, the large optical bangap of P3HT (~1.9 eV) limit s the fraction of the solar spectrum that can be harvested by the photovoltaic cell. To achieve power conversion efficiencies approaching 10%, conjugated polymers with optical bandgap between 1.4 eV and 1.7 eV would be required 95 In addit i on to the previously mentioned methods for reducing the ban dgap of the conjugated polymer (i.e. increasing the conjugation length, controlling the polymer chain planarity), the most common approach is to utilize an alternating donor acceptor architecture. By incorporating alternating pairs of electron donating and accepting moieties wi thin the polymer repeat unit a push pull driving force between the se moeities allow for the bandgap of the polymer to be tuned 96 97 Figure 3 5 shows a schematic diagram illustrating the push pull interaction for a donor acceptor copolymer and the resulting bandgap. The advantage of this approac h is that not only can the polymer's bandgap be reduced, but the position of its HOMO level can be lowered. Since for BHJ OPV cells, it is possible to increase the V oc for the photovoltaic cell by lowe r ing the donor polymer's HOMO level 95 However, this
59 approach is not without its disadvantages. As shown in Figure 3 5, the HOMO level for the copolymer is centralized around the donor unit while the LUMO level resides on the acceptor un it. For efficient hole transport along the copolymer's HOMO level, the donor unit must be in close proximity to other donor units. The same is true for electron transport along the copolymer's LUMO level, which is centralized around the acceptor unit. Ther efore, interchain packing becomes critically important for sufficient carrier transport in donor acceptor copolymers. Figure 3 6 displays the chemical structure of representative donor acceptor copolymers used in state of the art polymer solar cells. Initial p rogress in the development of these novel polymers was made by incorporating the acceptor unit benzothiadiazole (BTD) into the copolym er. Examples include the copolymers poly[2,6 (4,4 bis (2 ethylhexyl) 4 H cyclopenta[2,1 b ;3,4 b ']dithiophen e) alt 4,7 (2,1,3 benzothiadiazole)] (PCPDTBT) and poly[N 9" hepta decanyl 2,7 carbazole alt 5,5 (4',7' di 2 thienyl 2',1',3' benzothiadiazole)] (PCDTBT), both of which have shown power conversion efficiencies from 5.5% to 6.1% when incorporated in optimiz ed polymer BHJ solar cells 90 98 A further enhancement in device performance was observed for donor acceptor polymers composed of either benzodithiophene (BDT) or thienopyrrolodione (TPD) as the acceptor unit 99 100 For example, Reynolds and coworkers demonstrated high efficiency polymer solar cells incorporating a dithienosilole fused (DTS) or dithienogermole fused (DTG) donor acceptor co polymer with TPD as the acceptor unit 101 R eplacing the carbon bridging atom i n t he donor unit with Si or Ge improved molecular chain packing in the BHJ film, leading to increased FF and power conversion efficiencies over 7% for the solar cell. Such manipulations to the chemical structure of
60 the conjugated polymer's has been vital to t he continued improved of polymer solar cell performance. 3 4.2 Inverted Device Geometry In addition to developing novel conjugated polymers for improved light harvesting and increased V oc researcher s have also focused on the inverted device architecture for improved polymer solar cell stability and large scale roll to roll (R2R) processing compatibility. A schematic of the conventional and inverted geometries employed for polymer solar cell fabrication is shown in Figure 3 7 In the inverted architecture, t he device stability is improved since unfavorable materials such as acidic PEDOT:PSS and low work function metals are replaced by more favorable materials. Novel anode and cathode interlayers, which modify the work function of the electrodes and thus form Ohmic contacts for improved carrier extraction have enable d high efficiency polymer solar cells with an inverted geometry to be realized. T hese interlayers also serve as selective contacts for electron or hole transport wh ile effectively bl ock ing the opposing charge carrier. Presently, novel interlayers for inverted polymer solar cells include n type materials like TiO x ZnO, and water/alcohol soluble polyfluo renes, while transition metal oxides such as MoO 3 and V 2 O 5 have been successful ly u se d as anode interlayers for improved carrier extraction 102 104 Using this approach world record certified power conversion efficiencies of over 9% has been demonstrated 82 In Chapter 4, high efficiency inverted polymer solar cells based on polydithienogermole thienopyrrolodione (PDTG TPD) will be discussed. The focus of that discussion will be on the materials characterization and electronic properties of a novel ZnO composite electron transporting layer employed in the device. The impact of this result on demonstrating the commercial viability of OPV cells with be highlighted. In
61 Chapter 5, the active layer thickness dependenc e of the power conversion efficiency will be investigated for inverted solar cells with novel low bandgap polymers Furthermore, the consequence of space charge formation on carrier extraction will be discussed.
62 Figure 3 1. Schematic illustrations for v arious solution processing methods used in the fabrication of large scale and laboratory scale polymer photovoltaic cells. A) Screen printing B) Spin coating. C) knife over edge, slot die, gravure, and meniscus coating 105 Figure 3 2. Schematic diagram illustrating an organic donor acceptor heterjunction, as well as the bilayer and bulk heterojunction concepts. A) The D:A he terojunction. B) T h e bilayer heterojunction device structure. C) The bulk heterojunction device structure Taken from: M. Riede. I CAMP 2012 Lecture, Topic: "Vacuum Processing and the p i n Concept for OPV." Boulder, CO, July 7, 2012.
63 Figure 3 3. Typical J V characteristics for a photovoltaic cell tested in the dark ( J D ) and under illumination ( J L ). The resulting photocurrent, which is given by J ph = J L J D is also shown.
64 Figure 3 4 Chemical structure for standard electron donor and acceptor materials used in the fabrication of organic photovoltaic cells.
65 Figure 3 5 Schematic illustration showing the molecular orbital hybridization for a donor acceptor (D A) copolymer and the resulting reduction in the effective bandgap 106 Figure 3 6. Chemical structure for state of the art donor acceptor copolymers used for high efficiency polymer photovoltaic cells.
66 Figure 3 7. Normal vs. inverted device architectures for organic photovoltaic cells 107
67 CHAPTER 4 HIGH EFFICIENCY INVE RTED POLYMER PHOTOVO LTAIC CELLS 4.1 Introductory Remarks Extensive efforts have been directed at developing polymer bulk heterojunction (BHJ) solar cells due to their potential f or low cost energy harvesting 108 109 High efficiency laboratory scale polyme r solar cells based on low band gap pol ymers have been demonstrated as highlighted in Chapter 3 98 110 113 The device geometry of typical laboratory scale polymer sol ar cells consists of a bottom indium tin oxide (ITO) anode, an anode interfacial layer, a photoactive layer, and a low work function top metal cathode. Because vacuum d eposition of low work function metals is required for these top cathode devices, it is n ot viable to use th is device architecture in large scale roll to roll ( R2R ) processing. To avoid the use of low work function metals employed in such devices, large scale R2R processed modules and laboratory scale model devices have utilized a device geometry in which the polarity of the solar cells is inverted 16 114 116 Figure 4 1 compares the typical solar cell geometry u sed for laboratory scale devices and the inverted solar cell geometry used both laboratory scale devices and large scale R2R processed modules for a P3HT:PCBM so lar cell As displayed in Figure 4 1 inverted cells have an oxide electron tran sporting layer ( ETL ) coated ITO as the bottom cathode and a screen printed Ag layer as the top anode. T o date this device architecture is prototypical for the fabrication of l arge scale R2R process ed modules as shown in Figure 4 2 16 114 116 Ultimately, there are two factors promoting the use of t he inverted device geomet ry for la rge scale R2R processing For one, the commercial availability of printable Ag formulations makes solution processing of the Ag electrode a possibility.
68 Second, the development of solution processed transition metal oxides such as TiO x and ZnO has lead to their use in large scale solar cell manufacturing as ETLs to reduce the ITO work function 117 118 In particular, Zn O colloidal nanoparticles are used for th e ETL because of their low work function, high electron mobility and optical transparency, as well as their ease of synthesis 119 120 However, the major challenges in using ZnO nanoparticle films as ETLs are the presence of defects with adsorbed oxygen and poor spatial distribution of the nanoparticles over a large area 121 125 For example the poor spatial distribution observed in ZnO nanoparticle films is attributed to the formation of aggregates which is highlighted in the AFM image shown in Figure 4 3 This AFM image of a knife over e d ge coated ZnO nanoparticle film shows d istinct particles with height ~ 60 nm and diameter of ~ 300 500 n m indicating that severe aggregation of the ZnO nanoparticles is typical for large scale processed films 115 Accordingly, there is a need to develop low defect and uniform ZnO films so as to realize high efficiency inverted polymer solar cells. Here, enhanced charge collection was demonstrated in inverted polymer BHJ solar cells using a ZnO poly(vinyl pyrrolidone) (PVP) composite sol gel film as the ETL leading to devices that operate with laboratory measured PCEs in excess of 8% and certified efficiencies of 7.4% under AM 1.5 G illumination at 10 0mW cm 2 The composite film, termed ZnO PVP nanocomposite for clarity, consists of ZnO nanoclusters whose growth is mediated by a PVP polymeric matrix 126 ZnO PVP nanocomposite films have been previously studied for chemical/bio sensing applications and have the following advantages over conventional ZnO sol gel films 127 130 First, t he ZnO nanocluster size and its concentration can be tuned by controlling the Zn 2+ /PVP ratio 127 128 Second, the
69 distribution of ZnO nanoclusters in the PVP polymer is uniform compared to the aggregation observed in ZnO sol gel films without PVP 127 128 We hypothesized that inverted solar cells using this composite would demonstrate enhanced devic e performance. Furthermore, because the sol gel processing for the ZnO PVP nanocomposite is performed in air, this approach to depositing the ZnO ETLs is compatible with large scale R2R processes. T he synthesis and BHJ solar cell perform ance of the polymer used in this study, a dithienogermole (DTG) containing alternating conjugated donor acceptor polymer with N octylthienopyrrolodione (TPD) was used as the acceptor has been previously reported 101 Inverted polydithienogermole thienopyrrolodione ( PDTG TPD ) :[ 6,6] phenyl C 71 butyric acid methyl ester (PC 71 BM) solar cells demonstrated a higher short circuit current density ( J sc ) and fill factor (FF) than devices with an analogous polydithienosilole containing polymer (PDTS TPD), leading to inv erted polymer solar cells with PCEs of 7.3% 131 To determine whether our ZnO PVP nanocomposite f ilms would enhance charge collection for inve rted solar cells, inverted PDTG TPD:PC 71 BM BHJ solar cells was fabricated using the nanocomposite as an ETL. The following inverted device geometry was used for our study: ITO/ZnO PVP nanocomposite/PDTG TPD:PC 71 BM/anode interfacial layer/Ag For the anode interfacial layer, molybdenum trioxide (MoO 3 ) was used which has a better charge extraction effi ciency than more commonly used poly(ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) 101 132 135 While both MoO 3 and Ag are thermally evaporated in our devices, these electrode materials are compatible with R2R processin g because MoO 3
70 can be coated from a nanoparticles suspension 136 and Ag paste can be screen printed 16 114 116 4. 2 Experimental Details 4.2.1 Device Fabrication The detailed synthesis, polymer characterization and device fabrication and testing for PDTG TPD:PC 71 BM has been reported elsewhere 110 PC 71 BM used for solar cell fabricati on was purchased from Solenne. Polym ers and PC 71 BM were dissolved in chlorobenzene with 1:1.5 (8 mg mL 1 :12 mg mL 1 ) weight ratio and 5% volume ratio of 1,8 diiodooctane (DIO) was added as a pro cessing additive prior to use. The ZnO PVP nanocomposite was prepared from a precursor, in which z inc acetate dihydrate (Zn(CH 3 COO) 2 2H 2 O, Aldrich, 99.9%, 11 mg ml 1 ) and polyvinylpyrollidone (PVP, 2.5 mg ml 1 ) were dissolved in ethanol. Ethanolamine was added to the precursor as a stabilizer in equal molar concentrat ion to zinc acetate dihydrate. Th e ZnO PVP precursor was spin coated on indium tin oxide (ITO) coated glass substrates, which were first cleaned with detergent, ultrasonicated in DI water, acetone, and isopropyl alcohol, and subsequently dried via N 2 The films were annealed at 200 o C for 40 minutes in air. After annealing and slow cooling to room temperature, the ZnO PVP composite films were UV ozone tre ated using a UV ozone cleaner. The film thicknesses for the ZnO PVP composite film before and after UV ozone treatment were 36 nm and 33 nm, respectively. The polymer fullerene solutions were then spin coated and the resulting film with thickness of 105 nm was annealed at 80 o C for 30 minutes. Finally, thin films of MoO 3 (10 nm) and Ag (100 nm) were deposited through shadow masks via therma l evaporation. The active area of the device was 4.6 mm 2
71 4.2.2 Device Characterization For PV measurements, a light mask with an area of 3.04 mm 2 was used to define the active area of the device. Device characterization was carried out in air after encapsulation using an Air Mass 1.5 Global (A.M. 1.5G) solar simulator with irradiation intensity of 100 mW cm 2 The beam spot used in EQE measurement was smaller than the device area to ensure accuracy of the measurement. 4.3 Results and Discussions 4. 3 .1 ZnO PVP Nanocomposite Film Characterization Surface Morphology: The enhan ced device performance with UV ozone treated ZnO PVP nanocomposite fi lms is believed to be attributable to the modified surface composition promoting charge collection. T he nanoscale surface morphologies of the as prepared and UV ozone treated ZnO PVP nanocomposite films were investigated by atomic force microscopy (AFM). Fig 4 4 a and b show s the three dimensional surface topograp hy images for the nanocomposite film before and after UV ozone treatment. The ZnO PVP nanocomposite film shows an increase in r.m.s. roughness from 7.07 nm to 9.18 nm following UV ozone treatment, sugge sting that PVP is removed with this treatment, leaving the ZnO nanoclusters exposed at the surface. This removal of the PVP is more clearly shown in the AFM phase images in Fig. 4 4 c and d. For the nanocomposite film with out UV ozone treatment, no nanoclusters can be observed indicating that the surface is c overed by a thin layer of PVP. However, the phase image for the UV ozone treated ZnO PVP nanocomposite film shows that the PVP domain siz e has been reduced to 50 100 nm and that the ZnO nanoclusters are now exposed on the surface. The PVP rich and ZnO nano cluster rich surfaces of the nanocomposite
72 films before and after UV ozone treatment, respectively, are shown schematically in Fig. 4 4 e,f. To determine whether the removal of PVP from the nanocomposite film surface altered the film thickness step height measurements were taken for the films before and after UV ozone treatment. The average thickness of the nanocomposite film shows negligible change after 10 min UV ozone treatment, which provides further evidence that PVP was removed only at the surface of the ZnO PVP nanocomposite film upon UV ozone treatment. Furthermore to illustrate the surface morphology of the ZnO PVP nanocomposite films before and after UV ozone treatment, 5 m scale phase images are shown i n Fig. 4 5 These images show that the remo val of PVP by UV ozone treatment significantly alters the surface morphology of the nanocomposite film. This change in surface morphology from being PVP rich before UV ozone treatment to ZnO rich after treatment supports our idea that the removal of PVP fr om the nanocomposite fil m by UV ozone treatment leads to improved charge collection in our inverted polymer solar cells due to better electronic coupling between the ZnO nanoclusters within the nanocomposite film and PC 71 BM in the active layer. Chemical Co mposition: To further confirm that the compositional changes determined from the AFM data were due to the removal of PVP, X ray photoemission spectroscopy (XPS) was performed on th e ZnO PVP nanocomposite films. Considering the period of UV ozone treatment requ ired to optimize the device performance, it is believed that some changes in the chemical composit ion of ZnO might be plausible. The core level XPS spectra for the C 1 s O 1 s and Zn 2 p were measured for the as p repared and 10 min UV ozone treat ed ZnO PVP nanocomposite films. The binding
73 energies were calibrated by taking the C 1 s p eak (284.6 eV) as a reference. The O 1 s XPS spectra for as prepared and UV ozone treated ZnO PVP nanocomposite films are shown in Fig. 4 6 a. UV ozone treatment incr eased the relative magnitude of the peak at 531.4 eV (corresponding to the oxygen atoms bonded to the zinc in the ZnO matrix 127 128 137 ) by ~37%. Thus the number of Zn O bonds in the wurtzite structure of ZnO at the surface of the film is increased UV ozone treatment also incr eased the relative magnitude of the peak at ~ 530.0 eV, which corresponds to O 2 ions present in the porous ZnO clusters, but not chemically bonded to zinc in the ZnO wurtzite structure Fig. 4 6 b shows the Zn 2 p 3/ 2 XPS spectra for the as prepared and UV ozone treated ZnO PVP nano composite films. The intensity of the peak at 1021.6 eV, which corresponds to the Zn O bonds 127 128 137 138 incre ases after UV ozone treatment. These results are in agreement with the result from the O 1 s XPS spectra Ba sed on the O 1 s and Zn 2 p XPS spectra, we conclude that the chemical composition of the ZnO nanoclusters on the surface of the nanocomposite film has become oxygen rich after UV ozone treatment. The atomic concentrations of carbon, oxygen, and zinc in the as prepared and 10 min UV ozone treated ZnO PVP nanocomposite films based on the C 1 s O 1 s and Zn 2 p XPS spec tra are summarized in Fig. 4 6 c. The atomic concentration of carbon from the PVP in the nanocomposite is significantly reduced by UV ozone treatm ent (from 38.2% to 15.7%). Conversely, the atomic concentrations of oxygen and zinc present in the nanocomposite film both increase from 28.5% and 33.3% for the untreated film to 31.6% and 52.6% respectively for the treated film The relatively smaller i ncrease in oxygen ato mic concentration compared to zinc is due to the competition between the
74 increases in oxygen content coming from UV Ozone treatment and the decrease in oxygen content coming from the removal of PVP. These results strongly support our a ssertion that UV ozone treatment removes PVP from the surface of the ZnO PVP nanocomposite film. Optical Tran s parency: Finally t he effect of UV ozone treatment on optical transmission for the as prepared and treated ZnO PVP nanocomposite films was studied UV vis NIR transmission spectra for these films are shown in Supplementary Fig 4 7. Following UV ozone treatment, a 6% to 10% increase in transmission acro ss the entire visible spectrum was observed in the nanocomposite film T his increase in optical tr ansmission was attributed to the changes in the effective index of refraction of the nanocomposite film upon UV ozone treatment. It should be note d that the increase in optical transparency is less than the enhancement observed in J sc for inverted solar ce lls with UV ozone treated ZnO P VP nanocomposite films. Therefore, although the increase in optical transparency contributes to the enhancement in J sc the improved charge collection due to enhanced electronic coupling between ZnO nanoclusters in the nanoco mposite film and PC 71 BM in the active layer is primarily responsible for this enhancement. 4. 3 .2 Inverted Polymer Photovoltaic Cell with ZnO PVP Nanocomposite ETL The photo J V characteristics for inverted PDTG TPD:PC 71 BM sol ar cells were measured unde r AM 1.5G solar illumination at 100 mW cm 2 The photovoltaic ( PV ) performance results for the inverted cells with ZnO PVP nanoc omposites are shown in Fig. 4 8 On initial light exposure the inverted solar cells had a low FF of 25.5% and J sc of 10.9 mA cm 2 Wi th continuous illumination, device performance was enhanced significantly over time 139 After ~10 min of light soaking an enhanced FF of 63.7% and
75 J sc of 12.9 mA cm 2 were obtained, resulting in an ave rage P CE of 7.0%. Previously we reported inverted PDTG TPD based polymer solar cells with FF of ~68% using colloidal ZnO NPs as ETL 101 We suspected that by using ZnO PVP composite as the ETL, the ZnO PVP surface would be compositionally rich in PVP creating a contact barrier between the ZnO nanoclusters and PC 71 BM lead to the lower FF of our present devices. T o ensure a good contact between the ZnO nanoclusters and PC 71 BM, we performed UV ozone treatment on the ZnO PVP nanocomposite films t o remove PVP from the surface. Previous work has shown that UV ozone treatment can remove PVP on colloidal nanoparticle film surfaces 140 The removal of PVP did not alter the size, shape, or distr ibution of the nanoclusters in the films 140 141 Based on these findings, we believed that the UV ozone treatment would improve electronic coupling between photo active layer and ZnO nanoclusters. The photo J V characteristics for the inverted PDTG TPD:PC 71 BM solar cells with UV ozone treated ZnO PVP nanoco mposites are shown in Fig. 4 9a All devices were tested under initial light exposure, and no additional light soaking was applied to the devices. The ZnO PVP nanocomposite films were UV ozone treat ed for 5, 10, 20, and 30 min leading to significant enhancements in J sc and FF for the inverted PDTG TPD:PC 71 BM solar cells compared to cells with as prepared nanocomposite films. Table 4 1 summarizes the device performance for inverted solar cells with treate d ZnO PVP nanocomposite films. UV ozone treating the ZnO PVP na nocomposite films for 10 min led to an optimal device with enhancements in both J sc and FF compared to the light soaked devices without UV ozone treatment, resulting in an ave rage PCE of 8.1%. This average PCE of 8.1 0.4% is based on measurement from 102 fabricated solar
76 cells Our best device had a J sc of 14.4 mA cm 2 V oc of 0.86 V, FF of 68.8% and PCE of 8.5 %. For devices with ZnO PVP nano composite films that had been UV ozone treated for less than or more than 10 min a reduction in FF was observed For the shorter treatment, we attribute this reduction in FF to incomplete removal of the PVP from the surface of the composite film For the longer treatment, excess oxygen is present on the ZnO film surface which reduces the electron extraction effici ency. Based on these findings, we conclude that removal of extra PVP from the ZnO PVP nanocomposite film surface by UV ozone treatment greatly enhances the charge collection efficiency of these devices. To confirm the accuracy of the photo J V measurements, the external quantum efficiency (EQE) spectra for the solar cells with as prepared and 10 min UV ozone treated ZnO PVP nanocomposite films were measured ; these are compared in Fig. 4 9b An enhanced efficiency is observed through out the full spectral range from 350 700 nm for cells with the UV ozone treated ZnO PVP nanocomposite films when compared to cel ls without UV ozone treatment. The maximum EQE for the optimized inverted PDTG TPD:PC 71 BM solar cell with UV ozone treated nanocomposite f ilms was 73.6% The J sc value was then calculated by inte grating the EQE data with the AM 1 .5G spectrum. The calculated J sc value of 14.5 mA cm 2 is in good agreement with the directly measured J sc value To study the difference in light soaking behavior f or inverted PDTG PC 71 BM solar cells with either as prepared or UV ozone treated ZnO PVP nanocomposite ETL, dark J V measurements were taken on the inverted devices before and after AM 1.5G solar illumination Fig. 4 10 shows the results for inverted devices measured with either no
77 light soaking or with light soaking using filters to block incident light belo w 600 nm or 350 nm wavelength. The devices were encapsulated and stored in the dark prior to measurements The dark J V curves for the device before light soaking and after light soaking using the 600 nm filter are identical, showing low current density values for the inverted device s. Therefore, long wavelength light soaking has no effect on device performance Howeve r, when the inverted device is light soaked using the 350 nm long pass filters, a significant enhancement in dar k current density is observed. The enhanced dark current density matches the values observed for typical dark J V measurements of these devices under forward bias Therefore, the inverted solar cells with UV ozone treated ZnO PVP shows immediate enhancement in conductivity upon illumination of short wavelength light. The fact that we see optimized performance during the initial J V scan for the UV ozone treated device indicates that the soaking time required is significantly less than that required for the untreated devices. Encapsulated devices with UV ozone treated ZnO PVP nanocomposite films were then sent to NEWPORT Corporation for certificatio n. The photo J V characteristics and the corresponding solar cell parameters are shown in Fi g. 4 11 A PCE of 7.4 +/ 0.2% was certified for the devices. Although this certified efficiency is 9 % less than that measured in our laboratory because of a reduct ion in J sc and FF in the certified device, we attribute the reduction in PCE in the certified cells to degradation because of a non op timized encapsulation process. The devices were retested in our laboratory after certification and we obtained a n efficien cy (7.2%) comparable to the certified results. We sought to further investigate the effect of UV ozone treatment on ZnO PVP composite films and the corresponding photovoltaic device stability. As shown in Fig. 4
78 12 after light soaking the performance of the device with untreated ZnO PVP nanocomposite ETL reduced significantly over time, showing that the enhancement in J sc and FF due to light soaking was only temporary 139 In contrast, the devi ce performance observed for solar cells with UV ozone treated ZnO PVP nanocomposite ETLs had much better stability In fact, we observed no measureable changes in J sc FF, or PCE over a period of one month provided t hat the encapsulated devices were s tored in a nitrogen glove box. Thus, UV ozone treatment of the ZnO PVP nanocomposite films provides enhanced stability in the inverted solar cells. 4. 3 Concluding Remarks To conclude, improved charge collection efficiency has been demonstrated in inverted PDTG TPD BHJ solar cells using a UV ozone treated ZnO PVP nanocomposite film as the ETL to obtain organic polymer solar cells with PCE s in excess of 8% under A.M.1.5 G illumination at 100 mW cm 2 The use of PVP as an organic capping molecule and polymeric m atri x for ZnO produced electron transporting nanocomposite films which had excellent film forming characteristics. We found that UV ozone treatment was required to remove PVP from the surface of the film, and consequently exposed the ZnO nan oclusters to the film surface. Using our UV ozone treated ZnO PVP nanocomposite ETL, inverted PDTG TPD solar cells with laboratory measured PCE higher than 8% have been realized due to improved charge collection by the na nocomposite film.
79 Table 4 1. Average solar cell performance for inverted PDTG TPD:PC 71 BM devices with 5, 10, 20, or 30 minute UV ozone treated ZnO PVP composite ETLs under initial AM 1.5G solar illumination 142 UV ozone treatment time J sc (mA cm 2 ) V oc (V) FF (%) Average PCE (%) Best PCE (%) 5 min 13.9 0.1 0.85 0.005 56.0 3.8 6.6 0.5 7.1 10 min 14.0 0.4 0.86 0 .003 67.3 1.5 8.1 0.4 8.5 20 min 14.1 0.2 0.86 0.003 64.8 0.8 7.8 0.2 8.0 30 min 14.0 0.1 0.86 0.003 61.9 1.6 7.5 0.3 7.6 Figure 4 1. The normal and inverted device geometries for P3HT:PCBM solar cells. A) The normal device geometry B) The inverted device geometry 116
80 Figure 4 2. An illustration of the process and the six processing steps employed during fabrication of the R2R coated modules. The modules utilize the inverted device geometry 116 Figure 4 3. AFM image (measuring 5 5 m 2 ) of a knife over edge coated ZnO nanoparticle film used in the fabrication of lar ge scale R2R processed modules 115
81 Figure 4 4. Tapping mode AFM images for as prepared and 10 min UV ozone treated ZnO PVP nanocomposite films Appropriate scales are shown on the right. A) 3 D topography images for the as pre pared composite film. B) 3 D topography images for the UV ozone treated composite fil ms C) Phase image for the as prepared composite film. D) Phase image for the UV ozone treated composite film. E) Schematic image for the as prepared composite film showing a PVP rich surface. F) Schematic image for the UV ozone treated composite film show ing a ZnO rich surface 142
82 Figure 4 5. Tapping mode AFM phase images for ZnO PVP nanocomposite films before and after UV ozone treatment (5 m scale size). A) As prepared composite films B) 10 minute UV ozone treated composite film 142
83 Figure 4 6 X ray photoemission spectra (XPS) for the as prepared and 10 minute UV ozone treated ZnO PVP nanocomposite films. A) O1s narrow scan XPS spectra. B) Zn 2p XPS narrow scan XPS spectra C) Atomic concentration of C, Zn, and O before and after UV ozone trea tment based on the corresponding XPS spectra 142
84 Figure 4 7. UV visible NIR transmission spectra for the as prepared and 10 minute UV ozone treated ZnO PVP nanocomposite films 142 Figure 4 8. Photo J V characteristics for inverted PDTG TPD:PC 71 BM solar cells highlighting the effec t of prolonged light soaking on device performance under AM 1.5G solar illumination at 100 mW cm 2 142
85 Figure 4 9 Photo J V characteristics and EQE spectra for inverted PDTG TPD:PC 71 BM solar cells with UV ozone treated ZnO PVP nanocomposite ETL for various treatment times. A) Photo J V characteristics for the devices. B) Corresponding EQE spectra for the device with 10 minute UV ozone treated ZnO PVP nanocomposite films. The EQE spectrum for the device with as prepared composite films is shown for compariso n 142
86 Figure 4 10. Dark J V characteristics for inverted PDTG TPD:PC 71 BM solar cells with UV ozone treated ZnO PVP nanocomposite ETL measured as is and after being light soaked using either a 600 nm or 350 nm band pass filter 142
87 Figure 4 11. Certified I V characteristics for an inverted PDTG TPD:PC 71 BM solar cell with 10 minute UV ozone treated ZnO PVP nanocomposite ETL. The device was certified by NEWPORT Corporation 142
88 Figure 4 1 2 Solar cell parameters versus time for encapsulated inverted PDTG TPD:PC 71 BM with as prepared and 10 minute UV ozone treated ZnO PVP nanocomposite ETLs. A) Short circuit current density ( J sc ) vs. time. B) Fill factor ( FF) vs. time. C) P ower c onversion efficiency (PCE) v s time 142
89 CHAPTER 5 LOSS MECHANISMS IN T HICK FILM LOW BANDAP POLYMER SOLAR CELLS 5.1 Introductory Remarks Polymer BHJ solar cells, which have the potential to provide low cost energy harvesting, are receiving much attention in research and development 109 114 116 143 In Chapter 4, inverted PDTG TPD:PC 71 BM solar cells with over 8% power conversion efficiency was demonst rated which is crucial for demonstrating the viability of this technology for large scale R2R processing 142 144 Despite t his performance for laboratory scale inverted devices, the cell geometry is not the only factor determining their compatibility for large scale manufacturing. Another key factor for large scale R2R processing of polymer solar cells is the active layer thic kness required to ensure high manufacturing yields in the PV modules 145 Most high eff iciency laboratory scale devices have an active layer that is too thin for large scale R2R proc essing. Krebs and coworkers have sh own that polymer:fullerene active layers deposited by slot die coating have a high surface roughness and non uniform film thickness due to the large difference between the wet film thickness and dry film thickness 145 This high surface roughness and non un iform film thickness for the active layer leads to a lower yield of serially connected polymer solar cells. To address this issue a larger solid concentration of polymer and fullerene is used in solution As a result, the active layer film thickness for t he R2R processed PV module is typically around 200 nm which is larger than the film thickness employed in optimized laboratory scale polymer solar cells Therefore, o btaining high efficiency devices with active layers thicker than 200 nm is critical for c ommercialization 115 145
90 In addition to the thickness requirem ent associated with large scale R2R processing, increased active layer thickness is also required for optimal light harvesting to produce high efficiency polymer solar cells. Presently, solar cells with enhanced light absorption have been demonstrated by e mploying low bandgap polymers with optical bandgap between 1.2 and 1.7 eV 95 101 113 146 147 To obtain a high FF in these high efficiency cells, with the exception of poly(3 hexylthiophe ne) (P3HT) BHJ solar cells, the active layer used is typically less than 100 nm. However, due to optical interference effects, a high EQE can only be obtained in these devices when the thickness exceeds 200 nm 148 Therefore, despite their high power conversion efficiencies (PCEs), low bandgap polymer solar cells do not take advantage of the true light harvesting potential o f the polymer used in such thin film devices. The average EQE is expected to increase for solar cells with a thicker active layer due to enhanced light absorption and optimum optical interference. Therefore, to meet the large scale R2R processing requireme nt and ultimately realize optimum light absorption, high efficiency polymer solar cells with a thicker active layer must be demonstrated. In the past, demonstrating high efficiency polymer solar cells with a thick active layer has been a challenge. Previous work has focused on obtaining high hole mobilities in low bandgap polymers when blended with fullerene. This has been achieved by developing novel polymer structural designs and by improving the nano scale morphology of polymer:fullerene BHJ films 149 150 The impr oved carrier transport helps alleviate the buildup of photogenerated carriers in devices with thicker active layer, which decrease s the probability of carrier recombination and, consequently, enhances the carrier collection efficiency 149 154 While this approach has been successful for
91 polymer solar cells such as poly(3 hexylthiophene): [6,6] phenyl C6 1 butyric acid methyl ester ( P3HT: PC 61 BM) solar cells employing low bandgap conjugated polymers wit h fused donor ac ceptor moieties show significant reductions in FF and PCE with increasing the BHJ film thickness despite showing record efficiencies for thin film devices 117 155 156 Minimizing thi s FF reduction and, consequently, achieving high efficiency in low bandgap polymer solar cells with thicker active layer is particularly important for the development of low cost, high efficiency polymer solar cell for commercial application. In this chapt er the thickness dependence of the efficiency in polymer solar cells employing the low bandgap polymer PDTG TPD is studied 101 Previous work reported that the PCEs of these cells can be as high as 8.1% with an active layer thickness of about 100 nm 142 144 I ncreasing the active layer thickness ( L ) up to 200 nm yielded devices with a PCE above 8.0 %, which shows the viability of the PDTG TPD: [6,6] phenyl C7 1 butyric acid methyl ester (PC 71 BM) system for large scale R2R processing. For devices with an active layer thickness > 200 nm, a marked reduction in FF and PCE were observed On the contrary, similar P3HT:PC 61 BM polymer solar cells showed almost no reduction in FF for devices with active layer thickness up to 453 nm. Interestingly, si milar space charge limited ( SCL ) hole mobility values were obtained for both polymer:fullerene BHJ films despite the difference in fill factor be havior with increasing active layer thickness. T hese results are interpreted as follows: the broader absorption band for the PDTG TPD:PC 71 BM system leads to increased charge carrier generation and increased space charge buildup in the photoactive layer com pared to P3HT:PC 61 BM. The onset of space charge accumulation coincides with increased
92 carrier recombination loss in the photoactive layer, which results in a significant FF reduction in devices with thicker active layer 150 A nalysis on the incident light intensity depen dence of the FF, as well as an an alysis of SCL photocurrent in PDTG TPD:PC 71 BM solar cells further suppo rt this hypothesis 157 In addition to space charge effects, the reduced FF observed in thick PDTG TPD:PC 71 BM solar cells is also attributed to a low carrier lifetime as compared to similar P3HT:PC 61 BM devices. Overall, t hese results indicate that recombination loss due to space charge effects and low carrier lifetime limits the performance of thick film low bandgap polymer solar cells. 5.2 Experiment al Details 5.2.1 Device Fabrication The detailed synthesis, purification, and polymer characterization for PDTG TPD has been reported in the work by Amb and coworkers 101 P3HT was pu rchased from Rieke Metals, Inc. Both PC 61 BM and PC 71 BM used for solar cell fabrication were p urchased from Solenne. For the inverted PDTG TPD based devices, the polymer and PC 7 1 BM were dissolved in chlorobenzene with 1:1.5 weight ratio and 5% volume ratio of 1,8 diiodooctane (DIO) was added as a processi ng additive to both solut ions. The ZnO PVP nanocomposite electron transporting layer, whose solution processing was discussed in Chapter 4 was spin cast on indium tin oxid e (ITO) coated glass substrates. The ITO substrates were first cleaned with detergent, ultrasonicated in DI water, acetone, and isopropyl alcohol, and subsequently dried via N 2 The polymer fullerene solutions were then spin coated at various spin speeds to control active layer thickness and the resulting film s were annealed at 80 o C for 30 minutes. Finally, thi n films of MoO 3 (4 nm) and Ag (100 nm) were deposited through shadow masks via thermal evaporation. The active area of the device was 4.6 mm 2 For the P3HT:PC 61 BM
93 solar cells, P3HT and PC 61 BM were dissolved in chlorobenzene with 1:1 (8 mg mL 1 :12 mg mL 1 ) weight ratio. Thermally evaporated films of MoO 3 (Sigma Aldrich, 4 nm) and bathocuproine (BCP, 5 nm) were used for the anode and cathode interlayers, respectively. The active layer thickness was controlled by varying the spin coating speed. Hole only devic es were fabricated to extract the space charge limited hole mobility. The device structures for the polymer solar cells and hole only devices fabricated in this chapter is shown in Figure 5 1. 5.2.2 Device Characterization Current density voltage measure ments were carried out in air after encapsulation using a Keithl e y 4200 semiconductor system. For light intensity dependent J V measurements, neutral density filters were used to control the incident light intensity from the AM 1.5G solar simulator For field dependent EQE measurements, a bias was applied to the device using a current amplifier. For all EQE measurements, t he beam spot used was smaller than the device area to en sure accuracy of the measurement 5.3 Results and Discussions 5.3.1 PDTG TPD:PC 71 BM Solar Cell Performance Figure 5 2 shows the photocurrent density voltage ( J V ) charac teristics and the corresponding external quantum efficiency (EQE) spectra for inverted PDTG TPD:PC 71 BM solar cells with 105 nm, 204 nm, and 258 nm thick active layers T he short circuit current density ( J sc ) for the solar cells increases with increasing active layer thickness due to enhanced light absorption, with the highest J sc of 16.1 mA cm 2 obtained for the device with an active layer thickness of 258 nm. The integrated current density from the EQE spectra, shown in Figure 5 2 is consistent with the measured J sc with only 5% deviation. The difference in the EQE spectra is due to the optical
94 interference effects between the incident light and light reflected from the Ag back electrode 148 For devices with L 200 nm, the interference effects no longer affect the photocurrent density of the device and the active layer absorbs almost all the incident light below 700 nm, resulting in EQEs above 70% from 400 nm to 700 nm. Table 5 1 summarizes the average solar ce ll parameters for the PDTG TPD:PC 71 BM devices wit h an active layer thickness varying from 90 nm to 409 nm The reduction in FF observed for PDTG TPD solar cells with increasing active layer thickness is the major factor limiting the device performance A p ower conversion efficiency (PCE) of 7.9% is obtained for the device with a 105 nm thick active layer, which is consistent with the results shown in Chapter 4 The efficiency remains constant for devices with L 204 nm, with an average PCE of 8.2% being obtained for devices with an active layer thickness of 204. Above 200 nm, the FF reduction becomes significant, from 69% in the 105 nm film to 42% in the 409 nm film. The performance for PDTG TPD:PC 71 BM solar cell s was then compared to similar devices based on P3HT:PC 61 BM which is a well studied material system for investigated the active layer thickness dependence of the efficiency for polymer solar cells 149 151 The photo J V and EQE curves for P3HT:PC 61 BM devices with active layer thickness from 100 nm to 453 nm are shown in Figure 5 3 Although the J sc increased in the devices due to enhanced light absorption in the P3H T:PC 61 BM BHJ film, the overall enhancement in J sc was only ~23 24% when the active layer thickness was increased from 100 nm to 453 nm. This small increase in J sc can be attributed to narrow band absorption of P3HT and, consequently, the limited fraction o f the solar spectrum absorbed by the P3HT:PC 61 BM solar cells. As shown in Table 5 2, t he fill factor for these devices remained above 65%
95 despite increasing the thickness from 100 nm to 453 nm. This is in stark contrast to the results obtained for the PDTG TPD:PC 71 BM solar cells. The high FF obtained for these devices is characteristic of the balanced e and h mobility in the active layer, as well as the ability to efficiently extract photogenerated c harge carriers. The low rate of hole generation in the thick film P3HT :PC 61 BM solar cells, as indicated by the small enhancement in J sc helps to prevent space charge from accumulating within the active layer. The average FF and average EQE versus active l ayer thickness for both PDTG TPD:PC 71 BM and P3HT:PC 61 BM solar cells is summarized in Figure 5 4. Here, wavelength range corresponding to the EQE spectra. For PDTG TPD solar cells, a high average FF can only be obtained for devices with L 204 was observed for P3HT:PC 61 BM solar cells with L EQE, only thick film PDTG TPD:PC 71 BM solar cells with L capable of reaching an average EQE of 65%. In previous work, the reduction in FF for thick film devices was attributed to polymer solar cells with low carrier mobility such as MDMO PPV:PC 61 BM 150 The low mobility increases the chances of carrier recombination and therefore reduces the FF. To confirm that low carrier mobility was not responsible for the reduction in FF, t he SCL hole mobility was extracted from J V measurements for PDTG TPD:PC 71 BM sing le carrier devices using the Mott Gurney Law outlined in Chapter 2 and compared those values with the mobility values for P3HT:PC 61 BM solar cells (see Figure 5 5) The hole mo bility values for the 105 nm and 409 nm thick PDTG TPD:PC 71 BM devices were 3.410 3 cm 2 V 1 s 1 and 2.310 3 cm 2 V 1 s 1 respectively. These values are comparable
96 with hole mobility values found for optimized P3HT:PC 61 BM solar cells reported in the litera ture 149 150 It suggests that the reduction in device performance in PDTG TPD:PC 71 BM solar cells is not due to the mobility since both PDTG TPD and P3HT have the same mobilities. T o determine the root cause for the reduction in FF observed in thick film PDTG TPD:PC 71 BM solar cells, the EQE spectra for the thin film and thick film devices were measured under different va lues of internal electric field 158 Figure 5 6 shows the field dependent EQE spectra for devices with 105 nm 204 nm, 258 nm, and 409 nm thick active layer, respectively. Here, the internal electric field ( E ) was approximated as For device s an active layer thickness 204 nm, inc reasing the applied field from 20 kV cm 1 to 7 0 kV cm 1 leads to a uniform enhancement in EQE across the entire spectral range. The increased applied field enhances the extra ction of photogenerated charges equally across the EQE spectrum. Interestingly, fo r devices with L > 204 nm a stronger field dependent enhancement in EQE is observed in the spectral range from 500 to 750 nm when the applied field is increased from 20 kV cm 1 to 7 0 kV cm 1 This wavelength range corresponds to the absorption spectrum fo r pristine PDTG TPD films 101 F or devices with thick active layer, the build up of carriers in PDTG TPD :PC 71 BM will hinder charge collection and contribute to the fill factor reduction in thick solar cell. As a result of the limited charge collection observ ed for thick film PDTG TPD solar cells measured at low field, we expect the photocurrent to be reduced in this field regime. Figure 5 7 shows t he normalized photocurrent density electric field ( J L E ) curves for PDTG TPD:PC 71 BM solar cells with increasing a ctive layer thickness For
97 inverted PDTG TPD:PC 71 BM solar cells with thickness 204 nm, the shapes of the J L E curves are identical. In this thickness regime, the internal electric field effectively extracts photogenerated charges from the active layer. For inverted devices with L > 204 nm, a change in the J L E profile is observed. In this thickness regime, a reduction in photocurrent is observed in the low field regime. T he internal electric field is no longer able to efficiently extract photogenerated c harges out of the active layer. Based on this result, the reduced fill factor observed in PDTG TPD:PC 71 BM solar cells with thicker active layer can be attributed to two factors: (1) the reduction in internal electric field due to increased active layer th ickness L and (2) the reduced photocurrent at low electric field due to limited charge collection The results in Figures 5 6 and 5 7 provide evidence that space charge accumulation contributes to the limited charge collection and reduced fill factor obse rved in thick film PDTG TPD:PC 71 BM solar cells. Furthermore, increased recombination loss in thick film devices is expected since carriers have to traverse a longer distance to be collected. To better understand the loss mechanisms in thick film low bandga p polymer solar cells, a detailed study on space charge limited (SCL) and recombination limited photocurrent is presented herein. 5.3.2 Loss Mechanisms in Thick Film PDTG TPD:PC 71 BM Solar Cells In comparing P3HT and PDTG TPD, a significant difference betw een these two polymers is the broader absorption spectra in PDTG TPD due to its lower optical bandgap (E gap 101 The UV visible NIR absorption spectra for P3HT:PC 61 BM and PDTG TPD:PC 71 BM solar cells alon g with the photon flux for an AM 1.5G solar spectrum are shown in Figure 5 8 From the solar spectrum, an integrated photon flux of 1.610 17 cm 2 s 1 is obtained for a wavelength range of 400 nm 800 nm. A larger fraction of this photon flux is absorbed by PDTG TPD compared to the P3HT due to its
98 broader absorption spectrum. Figure 5 8 also shows the percentage of the AM 1.5G solar spectrum absorbed for PDTG TPD and P3HT solar cells as a function of the active layer thickness. Here, these spectra were obtained from results of optical modeling using the n and k optical constants for each layer in the actual devices 159 Based on the results PDTG TPD:PC 71 BM solar cells have over 30% more absorption than that in P3HT:PC 61 BM solar cells. Due to its higher light absorption and higher rate of photogenerated charge carriers, it is obvious that thick film low bandgap polymer solar cells require more efficient charge collection to prevent space charge from accumulating in the active layer. The onset of space charge accumulation in polymer BHJ solar cells occurs when the rate of hole generation in the polymer is greater than the rate at which those carriers can be extracted out of the active layer 154 157 The resulting space charge effect screens the electric field in the active layer, resulting in reduction of electric field in the BHJ film 150 This reduction in electric field would negatively impact charge carrier transport and extraction in polymer solar cells The issue of SCL photocurrent in polymer solar cell s with thick active layer has been addressed with a model first proposed by Goodman and Rose 157 The space charge limit is reached when the effectiv e photocurrent ( J ph ) generated is equal or greater than the value shown in the following equation 157 160 : ( 5 1) wh ere q is the electr ic charge, is the dielectric permittivity, is the hole mobility and G is the generation rate The value of J ph is determined experimentally as J p h = J L
99 J D where J L and J D are the measured photocurrent density and dark current density respectively To study the role which space charge accumulation plays in PDTG TPD:PC 71 BM solar cells with a thick active layer, the SCL photocurrent model was used to confirm that the electrostatic space charge limit was reached in device s with thicker active layer. T he results for PDTG TPD:PC 71 BM solar cells w ere compared with similar device s based on P3HT:PC 61 BM, since P3HT solar cells serve as a model system for studying the space charge effects 1 49 151 The effective photocurrent J ph normalized to the saturation photocurrent J sat = qG max L was plotted on a double logarithmic scale against the effective voltage across the device, given by V eff = V 0 V Here V 0 is defined as the voltage where J ph = 0 and is slightly larger than V oc 154 analysis is a widely used tool for analyzing recombination loss processes in organic solar cells 161 163 Figure 5 9 shows the results for the PDTG TPD:PC 71 BM solar cells with 105 nm, 258 nm and 409 nm thick active layer. For the device with 105 nm thick active layer, two different voltage regimes can be observed. For V eff < 0. 30 V, J ph steadily increases with voltage due to the competition between diffusion and drift for photogenerated carrier transport at low field Above 0.30 V, the photocurrent satu rates with increasing voltage. In this saturation regime, the internal field is strong enough to efficiently extract photog enerated carriers and the high field is responsible for the dissociation of e h pairs. The voltage corresponding to the short circuit condition falls within the saturation regime, indicatin g that the hig h J sc and FF obtained for this device is due to efficient charge collection by the internal electric f ield. For the device with a 105 nm active layer, space charge effects were not obser ved base d on the data shown
100 in Figure 5 9 As the active layer thickness for PDTG TPD cells increased above 200 nm, a square root effective voltage dependence on J ph is observed. This J ph V 1/2 dependence corresponds to the onset of space charge limited p hotocurrent in thick PDTG TPD cells assuming a J ph G 3 /4 dependence is also observed 163 The solid lines in Figure 5 9 corresp ond to J ph V 1 /2 For the 409 nm thick device, the J ph V 1 /2 regime extends to the short circuit condition, which correlates well with the reduction in J sc and FF observed in this device. The normalized J ph V eff curve s for P3HT:PC 61 BM solar cells with 100 nm, 200 nm and 453 nm thick acti ve layers are shown in Figure 5 9 In contrast with the results obtained for the thick PDTG TPD:PC 71 BM devices, the curve s for all three P 3HT:PC 61 BM solar cells look very similar despite the increase in the active layer thickness. These results correlate well with the high FF of 65% to 68% obtained for the P3HT:PC 61 BM devices. Since the onset of the J ph V 1 /2 regime was not occur in the P3HT:PC 61 BM solar cells, the photo current in these devices are not space charge limited despite the increase in active layer thickness. To confirm that space charge accumulation was contributed to the reduction in efficiency in PDTG TPD:PC 71 BM solar cells with thick active layer, light intensity dependent J V measur ements were taken light for a thin film and thick film device Early studies of space charge effects in organic solar cells demonstrated the onset of SCL photocurrent through light intensity dependent measurements 157 In the present work the reduction in fill factor observed in PDTG TPD BHJ solar cells is attributed to space charge effects in the thick film device. Although the square root dependence of the effective voltage on J ph is an indication of SCL photocurrent, it may also correspond to
101 recombination limited photocurrent. In the case of recombination limited photocurrent the recombination of charge carriers becomes significant since the mean free electron or hole drift length becomes less than the active layer thickness 163 164 Since recombination limited photocurrent is give by: ( 5 2 ) where is the lifetime of free carriers, it is possible to determine whether space charge limited or recombination limited photocurrent is limiting the fill factor of the solar cell by analyzing the light intensity dependence of the photocurrent 163 164 While the SCL photocurrent scales with a 3/4 power depend ence on light intensity (Eq. 1), the photocurrent scales linearly in the absence of space charge effects (Eq. 2). The dependence of J ph and fill factor on incident light intensity ( P 0 ) was plotted for the 105 nm and 409 nm thick PDTG TPD:PC 71 BM solar cells (see Figure 5 10 ). Neutral density filters were used to control the incident light intensity, which was varied from 1 1.4 to 100 mW cm 2 The J ph P 0 data for the thin and thick PDTG TPD:PC 71 BM devices, shown in Figure 5 10, was extracted from the J ph V eff curves shown in the same figure. For the solar cell with a 105 nm thick active layer, J ph showed a linear dependence on light intensity with the slope of the linear fit to the data equal to 1.0 9 In contrast, a slope of 0. 77 is observed for the 409 nm thi ck PDTG TPD solar cell. The ~3/4 power dependence of J ph on the incident light intensity confirms the occurrence of SCL photocurrent in PDTG TPD:PC 71 BM solar cells at low bias. The dependence of the saturation of the saturation voltage ( V sat ) on incident l ight intensity provides further evidence, in which a slope of 0.50 is extracted from the V sat P 0 data 157 163 To f o r m a more clear physical picture, t he light intensity dependence of the fill factor was also
102 analyzed and plotted in Figure 5 10 The fill factor remained rel atively constant with decreasing incident light intensity for the 105 nm thick solar cell, which is expected since the device is not space charge limited at P 0 = 100 mW cm 2 and the thickness is sufficiently thin to ensure efficient charge carrier extraction For the 409 nm thick PDTG TPD solar cell, a 24% enhancement in fill factor was observed as the incident light intensity was decreased from 100 mW cm 2 to 1 1.4 mW cm 2 By lowering P 0 and, consequently, reducing the generation rate of charge carriers in the thick PDTG TPD:PC 71 BM active layer, space charge buildup was reduced As a result, enhanced charge carr ier collection and fill factor was observed in the solar cell Despite this enhancement, the fill factor of the 409 nm thick device at low light intensity does not reach the value obtained in the 105 nm device This result indicates that the reduced photocurrent observed for thick film devices, first shown in Figure 5 7, could not be completely recovered despite lowering the incident light intensity. There is still some degree of limited charge collection occurring in thick film PDTG TPD:PC 71 BM solar cells. To further understand the relationship between space charge effects and the increased losses observed in PDTG TPD:PC 71 BM solar cells with thick active layer, a first principles calculation is presented for the photocurrent loss relative to short ci rcuit current at low effective voltages. The short circuit current density J sc can be calculated from the following: ( 5 3 ) where S ( ) is the reference AM 1.5G power intensity and 1 and 2 are the wavelength limits of the absorption spectrum of the device. The external quantum efficiency is given
103 by where IQE is the internal quantum efficiency and the absorption efficiency. U sing the calculated from Figure 5 6 and assuming va rious values of IQE (from 50 to 100%), calculated values of J sc was obtained for PDTG TPD:PC 71 BM solar cells 164 Figure 5 1 1 shows the number of photons absorbed in the active layer and the corresponding J sc versus the thickness of this layer. The experimental results for PDTG TPD:PC 71 BM solar cells are shown for comparison. The calculated J sc strongly agrees well with the experimental results assuming an IQE of 90 100%, which is in agreement with previous results demonstrating that low bandgap polymer solar cells with IQE approaching 100% are obtainable 117 The photocurrent at low effective voltages can then be determined from the following equation 165 : ( 5 4 ) where k T is the temperature. In the low active layer thickness regime in which the calculated photocurrent is given by Equation 3. At some critical active layer thickness ( L c ), the solar cell becomes space charge limited (i.e. ). For devices with L L c the calculated photocurrent is given by Equation 1 since Using this formulism, the photocurrent loss relative to J sc for PDTG TPD:PC 71 BM solar cells, which is given as 1 J ph / J sc was calculated for a low effective voltage and plotted versus active layer thickness. This result is summarized in Figure 5 1 1 For this calculation, it was assumed that the number of photons absorbed in the a ctive layer was equal to the generation rate, G 159 166 Figure 5 11 shows the calculated photocurrent loss before the space charge limit was reached ( black line ), after the space
104 charge limit is reached ( gray line ), and the photocurrent loss extracted from the experimental data. For a given hole mobility, the calculated curves intersect when at the critical thickn ess L c (dotted line). For L L c the photocurrent loss in PDTG TPD:PC 71 BM solar cell significantly increases since J ph is space charge limited. Increasing the hole mobility in the active layer leads to a large r L c allowing for the active layer thickness to be increased without substantially increasing the photocurrent loss. These conclusions are in agreement with previous reports demonstrating that a hi gh hole mobility in the polymer: fullerene blend is required to maintain efficient operation in polymer s olar cells with thicker active layer 149 151 The experimental photocurrent loss was then extracted for the PDTG TPD:PC 71 BM solar cells and compared to the calculated results. The calculated photocurrent loss is comparable to the experimental values assuming a hole mobility of 1.510 3 cm 2 V 1 s 1 which is close to the hole mobility obtained experimentally. T his theoretical model was used to predict the valu e of L c for polymer solar cells with E g for the conjugated polymer ranging from 1.4 to 2.2 eV. To make this prediction, it was assumed that the absorption efficiency for the polymer fullerene blend scales linearly with the bandgap of the polymer. Furthermo re, the fraction of photons absorbed from AM1.5G versus active layer thickness calculated for PDTG TPD:PC 71 BM was used as a reference. Based on these assumptions, the critical thickness corresponding to the onset of SCL photocurrent was plotted versus band gap of the polymer. This result is shown in Figure 5 1 2 for polymer solar cells with increasing hole mobility. The critical thickness L c is linearly proportional to the bandgap of the polymer, and increases as the hole mobility of the polymer fullerene blend increases. Based on this result, it is clear
105 that low bandgap polymer solar cells with E g between 1.4 and 1.7 eV are severely limited in their active layer thickness due to the large carrier concentration generated in these device s. For example, a hole mobility 3 cm 2 V 1 s 1 would be required to ensure that L c > 200 nm. If this high hole mobility cannot be obtained in these devices, the solar cells would show significa nt reductions in FF with increasing active layer thickness 156 There are two important assumption s of the theoretical SCL photocurrent model employed in this work First, the SCL model assumes that since the accumulation of slow ca rriers in the active layer is responsible for the space charge limit being reached For polymer solar cells, SCL photocurrent was first observed in systems with very imb alanced carrier mobilities 150 In this work SCL photocurrent was observed for solar cells using a polymer showing high hole mobility and therefore having a smaller carrier mobility imbalance Neverthele ss, the experimental results clearly indicate that space charge accumulation contributed to the reduced fill and PCE observed in thick film PDTG TPD:PC 71 BM solar cells. Furthermore, by calculating the active layer thickness at which the model predicts that the space charge limit is reached when L 225 nm (see Figure 5 11). This prediction correlates well with the reduced photocurrent observed for devices with L > 200 nm. Second, the model assumes that the loss observed in thick fil m polymer solar cells is due to either SCL or recombination limited photocurrent. However, these two loss processes are interrelated 150 It is expected that SCL and recombination limited photocurrent will contribute to the losses observed in PDTG TPD solar cells with thick active layer.
106 5.4 Concluding Remarks To conclude, the loss mechanisms responsible for limited devic e performance in thick film PDTG TPD:PC 71 BM solar cells have been investigated. For polymer solar cells with thickness up to 200 nm, efficiencies in excess of 8.0 % we re obtained for devices under AM 1.5G illumination at 100 mW cm 2 These results show that the active layer thickness requirement inherent to large scale R2R processing can be satisfied while still maintaining the high power conversion efficiency, and highlights the potential of PDTG TPD:PC 71 BM polymer solar cells for large scale manufacturing. For L > 200 nm, space charge effects and reduced carrier lifetime limits charge collection in thick film device s This limited charge collection coincides with reduced fill factor and power conversion efficiency in thick devices. These results indicate that although high efficiencies can be obtained in solar cells with low bandgap conjugated donor acceptor poly mers, the high density of photo generated charge carriers severely limits the performance of solar cells with a thick active layer.
107 Table 5 1. Averaged solar cell performance for inverted PDTG TPD:PC 71 BM devices with various active layer thickness under initial AM 1.5G solar illumination. Active Layer Thickness J sc (mA cm 2 ) J sc (EQE) (mA cm 2 ) V oc (V) FF (%) PCE (%) 90 nm 12.5 +/ 0.1 12.3 0.88 +/ 0.007 68.5 +/ 0.01 7.5 +/ 0.001 105 nm 13. 3 +/ 0.1 1 3 0 0.8 7 +/ 0.00 6 68. 7 +/ 0.01 7 9 +/ 0.001 153 nm 1 3 5 +/ 0.1 1 3 5 0.86 +/ 0.007 6 8 1 +/ 0.01 8 0 +/ 0.001 204 nm 14. 9 +/ 0.2 14.7 0.8 6 +/ 0.009 64 5 +/ 0.01 8 2 +/ 0.002 258 nm 16.1 +/ 0.2 16.0 0.85 +/ 0.006 54 1 +/ 0.01 7 4 +/ 0.001 409 nm 15.2 +/ 0.1 14.9 0.82 +/ 0.005 41.6 +/ 0.01 5.2 +/ 0.001 Table 5 2. Averaged solar cell performance for inverted P3HT:PC 61 BM devices with various active layer thickness under initial AM 1.5G solar illumination. Active Layer Thickness J sc (mA/cm 2 ) J sc (EQE) (mA/cm 2 ) V oc (V) FF (%) PCE (%) 100 nm 6.1 +/ 0.23 6.4 0.62 70.4 +/ 0.1 2.7 +/ 0.12 150 nm 6.1 +/ 0.05 6.4 0.62 67.1 +/ 0.1 2.5 +/ 0.17 170 nm 7.0 +/ 0.43 7.7 0.62 72.0 +/ 0.1 3.1 +/ 0.25 200 nm 7.2 +/ 0.10 7.5 0.62 71.9 +/ 0.1 3.2 +/ 0.04 307 nm 7.6 +/ 0.25 6.4 0.58 68.7 +/ 1.1 3.0 +/ 0.10 340 nm 7.6 +/ 0.04 6.7 0.58 69.0 +/ 0.5 3.1 +/ 0.10 453 nm 7.4 +/ 0.29 6.5 0.56 65.5 +/ 2.2 2.7 +/ 0.20
108 Figure 5 1. Device structure for PDTG TPD:PC 71 BM solar cells, P3HT:PC 61 BM solar cells and corresponding single carrier devices studied in this work. A) Inverted PDTG TPD:PC 71 BM solar cell. B) PDTG TPD:PC 71 BM hole only device. C) P3HT:PC 61 BM solar cell. D) P3 HT:PC 61 BM hole only device.
109 Figure 5 2 Photo J V characteristics and EQE spectra for inverted PDTG TPD:PC 71 BM solar cells with increasing active layer thickness. A) Photo J V characteristics for PDTG TPD:PC 71 BM solar cells with 105 nm, 204 nm, and 258 nm thick ac tive layer. B) Correspondi ng EQE spectra for the devices
110 Figure 5 3. Photo J V characteristics and EQE spectra for inverted P3HT:PC 6 1 BM solar cells with increasing active layer thickness. A) Photo J V characteristics for P3HT:PC 6 1 BM solar cells with 100 nm, 200 nm, and 340 nm thick active layer. B) Corresponding EQE spectra for the devices.
111 Figure 5 4. Average fill factor and EQE versus active layer thickness for PDTG TPD:PC 71 BM and P3HT:PC 61 BM solar cells. A) Average fill factor vs. active layer thickness for the devices. B) Average EQE vs. active layer thickness for the devices
112 Figure 5 5. Current density times active layer thickness ( J L ) versus electric field curves for PDTG TPD:PC 71 BM hole only devices with 100 nm and 409 nm thick layers.
113 Figure 5 6. Field dependent EQE spectra for PDTG TPD:PC 71 BM solar cells with increasing active layer thickness. A) 105 nm thick active layer. B) 204 nm thick active layer. C) 258 nm thick active layer. D) 409 nm thick active layer. The EQE spectra were measured at 20 kV cm 1 and 70 kV cm 1
114 Figure 5 7. Normalized photocurrent density ( J L ) as a function of internal electric field for inverted PDTG TPD:PC 71 BM solar cells with increasing active layer thickness.
115 Figure 5 8 UV Vis NIR spectra and fraction of photons absorbed vs. active layer thickness for PDTG TPD:PC 71 BM and P3HT:PC 61 BM solar cells. A) UV Vis NIR absorption spectra for the devices. The photon flux for the AM 1 .5G solar spectrum is shown. B) The fraction of absorbed photons versus active layer thickness.
116 Figure 5 9 Normali z e d J ph V eff curves for under 100 mW cm 2 illumination for PDTG TPD:PC 71 BM and P3HT:PC 61 BM solar cells with increasing active layer thickness A) PDTG TPD:PC 71 BM cells with 105 nm, 258 nm, and 409 nm thick layer. B) P3HT:PC 61 BM cells with 100 nm, 200 nm, and 453 nm thick active layer. Dashed lines highlight the value of V eff corresponding the short circuit condit ion ( V eff = V 0 ). The solid lines correspond to J ph V 1/2 fits of the photocurrent in the SCL regime for PDTG TPD so lar cells.
117 Figure 5 10 Light intensity dependence of the photocurrent and fill factor for 105 nm thick and 409 m thick PDTG TPD:PC 71 BM solar cells. A) J ph V eff curves for the 105 nm thick device under various light intensities (from 10 to 100 mW cm 2 ). B) J ph V eff curves for the 409 nm thick device under the same light intensities. C) J ph versus light intensity for the same devices. D) Fill factor vs. light intensity for the same devices.
118 Figure 5 1 1 Calcul ated number of photons absorbed in the PDTG TPD:PC 71 BM layer under AM1.5G illumination and photocurrent loss as a function of layer thickness. A) Number of photons absorbed in the photoactive layer and the corresponding short circuit density J sc at various IQE values vs. active layer thickn ess. B) P hotocurrent loss relative to J sc (1 J ph / J sc ) versus layer thickness at various hole mobility values. For this calculation, V eff = 0.4 V and r = 4.78.
119 Figure 5 1 2 The critical active layer thickness corresponding to the onset of SCL photocurrent ( L c ) in low bandgap polymer solar cells versus bandgap of the polymer. The calculation was performed for various values of hole mobility, ranging from 1 10 4 cm 2 V 1 s 1 to 2 10 3 cm 2 V 1 s 1
120 CHAPTER 6 CONCLUSIONS By inverting the device geometry of o rganic electronic and optoelectronic devices it is possible to improve the device's stability and large scale R2R processing compatibility. However, it is important to also demonstrate enhanced device performance for orga nic electronic devices with this architecture. Early work on i nverted small mol ecule and polymer LEDs showed high drive voltage due to insufficient carrier injection. To address this problem, carrier injection for conventional and inverted organic single carrier devices was studied in Chapter 2. To facilitate carrier injection, electron acceptors were used as injection layer for devices with either geometry. It was found that inverted devices showed higher current density and injection efficiency co mpared to devices with a conventional architecture. The results were confirmed by cross referencing steady state J V and DI SCLC transient measurements. The enhanced carrier injection was attributed to a stronger degree of charge transfer between the elect ron accepting injection layer and the organic semiconductor in the inverted device. On that basis, the injection efficiency for devices with conventional structure was enhanced by sandwiching a p doped hole injection layer between the anode and the organic semiconductor Cont rolled doping of this inter layer allowed the injection efficiency to be tuned. Given this result, it was understood that a strong degree of charge transfer between the electron accepting injection layer and the organic semiconductor was required to obtain an ohmic contact. In future work, replacing the electron acceptors studied in this chapter with solution processed electron accepting interlayers is preferred for large scale processing. However, obtain ing strong charge transfer will be required to observe ohmic injection with these new novel materials.
121 Similar to reports on inverted OLEDs, early work on inverted OPV cells showed that the device performance was reduced when this geometry was employed. In Chapter 4, we showed that the re duced device performance of inverted polymer photovoltaic cells was due to limited carrier extraction by the bottom electron transporting layer. By introducing a UV ozone modified ZnO polymer composite layer the electronic coupling between this interlayer and the photoactive layer was improved. As a result, carrier extraction was improved and OPV cell s with power conversion eff iciencies over 8% were obtained. Thi s work illustrated the importance of carrier selective interlayers for OPVs. Much of the resear ch reported in the literature has focused on employing novel carrier selective interlayers for OPVs, such as the water/alcohol soluble polyfluorenes use d in the state of the art devices. This will continue to be an important research area for OPVs. Lastly, in Chapter 5 the thickness dependence of the efficiency was investigated for inverted organic photovoltaic cells. Demonstrating efficient OPV cell operation for devices with thicker active layer is important for large scale R2R processing. Since high carrier mobility v alues were obtained in the polymer photovoltaic cells studied, it was assum ed that the active layer thickness could be extended without sacrificing the device's performance. Power conversion efficiencies over 8 % was obtained for OPV cells with active layer thickness up to 2 00 nm. Increasing the active layer thickness further significantly reduced the cell's performance. This loss was attr ibuted to space charge effects in the thick photoactive layer. Future work on thick film OPV cells should focus on elimin ating space charge build up in the photoactive layer
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136 BIOGRAPHICAL SKETCH Cephas Small was born in 1979 in Brooklyn, NY as the youngest of four with two older sisters and an older brother He grew up in Uniondale, NY, a small suburban town on Long Island. Throughout elementary school, jun ior high school and high school, Cephas was heavily involved in activities involving creativity, self expression, and problem solving, such as music, math & science clubs, and sports. His first love was music, learning to play various styles of music on th e saxophone and participating in music competitions. After graduation from high school, he received a music scholarship from Northeastern U niversity, where he majored in c hemi cal e ngineering based on the advice of his favorite math teacher in high school. At Northeastern University, Cephas pursued his academic interests, continued playing the saxophone as part of his scholarship requirements, and served as a student leader with the National Society of Black Engineers (NSBE) on the chapter and regional leve ls. As he approached graduation, his mentors encouraged him to pursue graduate school since he excelled in coursework involving hands on design work. At the same time, through his involvements with NSBE, he met research faculty at Norfolk State Cephas was accepted to the histor ic black college and began his m degree thesis work after graduating from Northeastern. At Norfo lk State University, he learned how to conduct research under the tutelage During his time at Norfolk State, Cephas realized that he wanted to conduct device related research for his doctorate studies since the majority of his research work at that time was focused on materials characterization. After receiving his Master of Science he joined the Materials
137 Science and Engineering D epartment at the University of Florida in the fall of 2006. He was granted the opportunity to work with Professor Franky So in his Organic Electronic Materials and Device s Laboratory. There, he learned the operating principles for various organic electronic and optoelectronic devices. Under the direction of Dr. Franky So, he conducted high impact research work in t he area of organic electronics Cephas received his doctorate in the fall of 2012 He intends to continue his research work on the development of novel material and device design for organic elect ronics.