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Nanostructured organic solar cells defined by nanoimprint lithography

Dissertation
Author: Mukti Nath Aryal
Abstract:
Energy harvesting from sunlight via organic solar cells (OSCs) based on polymers as an electron donors and fullerenes as electron acceptors has been subject of intensive research due to the potential for low cost and large area devices with attractive market perspectives. One of the biggest challenges for OSCs is their low efficiency of power conversion, which is limited by quality of active layer morphology of donor-acceptor materials and interfaces between the components. Key reasons for this low efficiency include severe electron-hole recombination, which prevents charge pair propagation toward the electrodes and poor light absorptions due to thin polymer layer (∼100 nm). These problems can be dramatically alleviated if the charge-transfer polymers can be arranged as periodic nanostructures for active layer of ∼300 nm so that enough light absorption takes place and no phase overlap exists in the charge propagation path. This work reports the formation of ordered bi-continuous interdigitized active layer morphology, well defined interfaces for charge pair formation and propagation without recombination toward the electrodes. Such nanostructure arrays of poly(3-hexylthiophene) (P3HT) with well defined interfaces have been fabricated using nanoimprint lithography (NIL). The molds required for NIL are fabricated using innovative low cost and non-lithographic technique which is scalable to commercial use. Simultaneous control of nanostructured and 3-D chain alignment in P3HT nanostructures (nanowires and nanopillars) defined by NIL is revealed using out-of-plane and in-plane grazing incident X-ray diffraction measurements and enhancement in anisotropic charge carrier mobility favorable to solar cells and field effect transistors (FETs) is measured making FETs. Separate acceptor deposition is required for nanostructured solar cells which is challenging due to the limitation of solvent compatibility and self shadowing effect for thermal deposition. For this purpose, orthogonal solvent is investigated for spin processing, oblique angle deposition is used for thermal deposition and low glass transition temperature acceptor materials are researched for transfer imprinting process. The fabricated solar cells using the developed recipe, show improved performances as compared to bilayer devices.

TABLE OF CONTENTS Preface…………………………………………………………………………………………..…v Acknowledgements…………………………………………………………………………….…vi Lists of Figures…………………………………………………………………………………..xv Lists of Tables…………………………………………………………………………………...xix CHAPTER 1 MOTIVATION AND OUTLINES ……………………..…………….…………....1 1.1 Motivation ………………………………………………………………………….….1 1.2 Historical perspective……….…………………………………………………............4 1.3 Overview of work presented……….…………………………………………............5 CHAPTER 2 PHYSICS OF NANOSTRUCTURED SOLAR CELLS .………………………….9 2.1 Introduction… …………………………………………………………………….….9 2.2 Basic working principle ………………………………..…………………………...11 2.3 Polymer chain morphology in nanostructured solar cells……………………………13 2.4 Enhancement in charge mobility ……………………………………………………14 2.5 Exciton dynamics ………..……………………………………………………….....15 2.5.1 Exciton generation …………………………………………………..……15 2.5.2 Exciton diffusion………………………………………………………..…16 2.5.3 Exciton dissociation/charge separation and recombination dynamics …....17 2.5.4 Charge transfer and collection………………………………………….....18 2.6 Nanoscale donor acceptor morphology……………………………..………....…...19 2.7 Organic active layer-metal electrode interfaces …..…………….……………..…….21 2.8 Light absorption ………………………………….…………..……..…………..…...23 2.9 Dead zone …………………………..…………………………………………….....24 2.10 Independent control of donor and acceptor layer…………………….…..……….…25 xi

2.11 Characterization of solar cells …….……………………………….……….…....…25 2.12 Inter-relation of device parameters and power conversion efficiency…….………...28 2.12.1 Open circuit voltage (V oc )….………………………………...…..…….29 2.12.2 Short circuit current (I sc )……….………………………………….........29 2.12.3 Fill factor (FF )…………………………………………………………31 CHAPTER 3 NANOFABRICATION FOR ORGANIC SOLAR CELLS …….……….……....33 3.1 Introduction…………………………………………………………………………33 3.2 Fabrication of ordered nanoporous alumina membranes via electrochemical anodization ……………………………………………………………………..........34 3.2.1 Mechanical polishing ……………………………………........................34 3.2.2 Annealing………………………………………..……………………….35 3.2.3 Electrochemical polishing…………..……………………………….......35 3.2.4 Two steps anodization process and voltage reduction…….......................36 3.2.5 Detachment and handling of AAO membrane…………….......................38 3.3 Mold Fabrication …….…………………..………………………………………….41 3.2.6 Fabrication of nanoporous Si mold……….…….………………….........41 3.2.7 Fabrication of nanopillar Si mold……….…….………………………...44 3.2.8 Control of nanostructure dimensions………….………………….……..46 3.4 Fabrication of polymer nanostructures………...………………………………........46 3.5 Polymer nanostructures for solar cells ……...………………………………..….….48 3.6 Process development………………...…………………………………………........49 3.7 Standard operating procedure for the fabrication of nanostructure organic solar cells (OSCs)…………………………………………………………….…………………52 3.8 Summary………...…………………………………...………..…………………….54 CHAPTER 4 NANOIMPRINTING AND CHARACTERIZATION OF POLY(3- HEXYLTHIOPHENE) (P3HT) …………………………………………………………............55 4.1 Introduction ………………………………………………………………………...55 4.2 Experimental methods…….………………………………………………………..56 xii

4.2.1 Sample preparation…………..….……………………………………….57 4.2.2 Grazing incident X-Ray diffraction (GIXRD) measurement…...………..59 4.2.3 Mobility measurement …………………………...………………60 4.3 Results……………………………………………………………………..….……..61 4.3.1 GIXRD…………………………………..…………….….…..….……..61 4.3.2 Charge mobility…………….……………..……………………...……..68 4.4 Conclusions ………………………………….……………………………………..71 4.4.1 Simultaneous control of nanostructures and p3ht chain alignment…….71 4.4.2 Enhancement in hole mobility………………………………...........….72 CHAPTER 5 DEPOSITION OF ACCEPTOR MATERIAL FOR NANOSTRUCTURED SOLAR CELLS……………………………………………………………………...…………..73 5.1 Introduction……………………………………………………………..……….....73 5.2 Acceptor deposition methods for nanostructured solar cells…………………..…..75 5.3 Spin coating of acceptor material using orthogonal solvents ……….…………….76 5.3.1 Alternative material system……………………………….…………….80 5.4 Deposition of acceptor material using reversal nanoimprint lithography (RNIL)......……………………………………………………………….…………82 5.5 Thermal deposition…………………………………….……………...…………...84 5.6 Conclusions ………………………………………………………………………..87 CHAPTER 6 DEVICE FABRICATION AND CHARACTERIZATION……….…...………....89 6.1 Introduction………………..……………………………………………………....88 6.2 Substrate preparation, cleaning and spin coating .....................................................89 6.3 Interface enhancement factor …………………………………...……..………..…96 6.4 Acceptor deposition……………………………………...…………...…….….….98 6.5 Results……..…………………………………………………………..…..….…..99 6.5.1 Nanostructures vs. blend and bilayer devices…………………....101 6.5.2 Dependance of performance on nanostructures shape…………...102 xiii

xiv 6.5.3 Performance vs. deposition techniques………….……………….…….103 6.5.4 Performance vs. acceptor types………..……………………………….105 6.6 Conclusion….………………….……………………….……………………...106 CHAPTER 7 SUMMERY AND OUTLOOK………………..………..…..…………………...108 7.1 Introduction…………………………..………………..….……………………108 7.2 Summery……..….……………….….…………………………………………109 7.3 Outlook .……….………………...….…………………………………….........111 BIBLIOGRAPHY ………………………………..………….…………………………………113 VITA

LIST OF FIGURES Number Page

2.1 Solar cells device structures ……………………………………………………….……...…..9

2.2 Schematic of working principle of organic solar cells (OSCs)……………………….…..….12 2.3 Charge separation and recombination phenomenon in nanostructured and blend solar cells devices……………………………………………………………………………………….…...20

2.4 Device engineering for the enhancement of semiconductor-electrode interfaces quality………………………………………………………………………………………...….22

2.5 Schematic light absorption profile for blend and nanoimprinted devices…………….……..24

2.6 Equivalent circuit and I-V characteristics……………………………………………..……..26

2.7 Interrelation between various parameters of solar cells………………………………….......28

3.1 Representation of surface roughness with sine wave in electropolishing model……..……..35

3.2 A. 2”x4” AAO template, B. SEM top view of AAO pores, C. Side view of free standing AAO, and D. Pore diameter as a function of dissolution time in phosphoric acid………..……37 .. 3.3 Schematic of peeling of AAM from aluminum substrate………………………………......41

3.4 a) Schematic of transferring anodic alumina membrane into crystalline Si wafer as nanoimprint molds. b) SEM of the AAM with 50 nm nanopores; c) control of pore diameter by adjusting anodization time in phosphoric acid. d) Cross-sectional SEM image of anodic alumina membrane on Si after etching away the rough barrier layer; e) Top view and f) ……………….43

3.5 Schematic of fabrication of Si pillar mold using AAM mask from placing of AAM mask on PMMA coated Si to obtaining polymer nanopores by NIL using Si mold ..…………….…….44

3.6 SEM images of Si pillar mold using AAM mask: a) PMMA etched and Cr deposited b) Metal lift off c) 45 o tilt view of nanopillar Si mold after Cr wet etched and FDTS treatment, d) Nanoimprinted P3HT pores using the Si mold shown in c) ………………………….………45

3.7 Process flow of the thermal nanoimprint lithography.……………………………..………. 47 .. 3.8 SEM images of polymer nanostructures defined by NIL ………………………..…..…48 xv

4.1 A) Schematic of nanoimprinting process B) SEM top view of Si nanogratings molds of period 200 nm, width 65 nm and depth 200 nm insets show the cross section; C)SEM images of 45 o tilt view of P3HT nanogratings of period 200 nm, width 65 nm, height 200 nm and the residual layer of 20 nm, inset shows zoom in cross section; D) nanoporous mold with hexagonal pore array of diameter 80 nm and height 350 nm, separation of 30 nm insets show the cross section; E) hexagonal array of nanopillars of diameter 80 nm and height 200-250 nm, separation f 30 nm, with residual layer of 20 nm inset shows zoom in view. Scale bar 200 nm and 2 µm for arge scale………………………………………………………..……… o l ………………….…57   4.2 Schematic of thin film stage rotation for the XRD measurement:  is the rotation about z‐ axis  or    is  the  rotation  about  x‐axis    is  the  rotation  about y‐axis    is  the  incident  angle  with the sample surface…………………………………………………..…………………….59 .. 4.3 Schematic of P3HT nanowires OFET for the measurement of hole mobility in the direction parallel to the line gratings……………………………………………………...........................61

4.4 A) Schematic of GIXRD setup for out-of-plane and in-plane measurements. (B) Schematic of edge-on, face-on, and vertical orientation of P3HT chains on surface. Out-of-plane and in-plane measurement are used to detect the lattice constants along z- axis and x-axis, respectively. (C) Out-of-plane GIXRD measurement graphs for the nanogratings, nanopillars, and unpatterned thin film samples. (D) In-plane GIXRD measurement graphs for all samples, where Grating║ and Grating ┴ referred as measurements made with grating direction parallel and perpendicular to the direction of incident X-rays, respectively. The side figures show magnified views of the (100) peaks (top) and (010) peaks (bottom)……………………………………………………..62

4.5 Schematic of A) polymer flow during nanoimprinting and chain alignment due to P3HT side chain and side walls of FDTS treated hydrophobic mold, ideal structure of B) P3HT chain alignment in nanograting and, C) P3HT vertical chain alignment in nanopillar structures: cylindrical symmetry due to hydrophobic interaction from around the nanocavities of FDTS treated hydrophobic mold.…….…………………………………………………………….... 65

4.6 A) Schematic set up for preferred orientation measurement of nanograting sample. The detector is fixed at 2θ = 5.2 o and the angle between X-ray incident beam and line grating varies when the thin film stage rotates though an angle φ which becomes zero line gratings become parallel to the incident beam; B) diffraction intensity vs. φ. for the detector fixed at 2θ = 5.2 o . Only a scan ranges from -30 o to 30 o is shown……………………………….…….……………66

4.7 I DS vs V DS characteristics of nanograting FET along the grating (left) and perpendicular to the grating (right) with effective channel length of 30 μm, showing the accumulation mode operation when gate biases were applied from -5V to -35V with interval of -5V…...……. …..………...68

4.8 Schematic of simultaneous control of P3HT nanostructures and chain alignment in nanogratings for the better performance in solar cells and OFET via enhancement in mobility in vertical and along the grating respectively……………………………………………………..71

xvi

5.1 Schematic of acceptor deposition on a) P3HT nanostructures using b) spin processing c) Reversal nanoimprinting or transfer imprinting d) oblique angle thermal deposition……..……75

5.2 Extraction of low mol. wt. P3HT using Soxhilet extraction. ……………………………….77

5.3 the problem discovered during spin deposition of acceptor material: a) Dewetting, b) collapse c) partial dissolution d) defects ………………………………………………………………..78

5.4 Deposition of acceptor material showing better filling of P3HT nanostructure gratings. In this figure PCBM was used as an acceptor material. ………………………………...……………....79

5.5 C60 derivatives used in this work…………………………………………………………....81

5.6 Filling of acceptor materials on P3HT nanogratings using reversal NIL (RNIL): Higher Tg PCBM does not fill up the nanostructures while low T g PCB-C12 fill the gap. Some filled and some unfilled areas were observed due to non-uniform coating of PCB-C12 on PDMS substrate used in RNIL………………………………………………………………….………………....83

5.7 Oblique angle deposition of C60: a) schematic of deposition process; b) SEM images of 30 nm C60 deposition on P3HT nanogratings of w=100 nm, p=200 nm and h=60 nm from one direction where only one wall is covered (shown by arrow), c) from both direction for complete coverage. Scale bar is 100 nm…………………………………………………………………...86

6.1 Schematic flow of nanoimprinted solar cells fabrications: Patterning of ITO to define area, spin coating of PEDOT: PSS: d-sorbitol, spin coating of P3HT, nanoimprinting, demolding, acceptor deposition and the deposition of Al cathode for the complete fabrication……………………………………………………………………………….…….....89

6.2 P3HT thickness vs. Spin speed for given concentration………………………………..….. 91

6.3 Thickness measurement of P3HT film using Dektek profilometer a) video image shows film with scratch made for measurement purpose b) profile shows smooth P3HTfilm of 85 nm………………………………………………………………………………………………..92

6.4 Control of nanoimprint temperature, pressure over time. Regions a: raise of temperature state b: nanoimprint region, c: crystallization region, d cooling and e: demolding temperature, the final stage……………………………………………………………………………………………...95

6.5 IEF for nanogratings and nanopillar structures calculated using equation (20) and (21) for nanogratings and nanopillars respectively. Dotted lines are for pillar/pore nanostructures while solid lines are for gratings. 1:1 pattern to spacing ratio is used here………………………. …...97

6.6 I-V characteristics nanoimprinted P3HT/ PCBM solar cells devices as compare to blend and bilayer………………………………………………………………………………………..…101

xvii

xviii 6.7 I-V characteristics of nanograting and nanopillar nanostructured devices as compare to planar counterpart……………………………………………………………………………………..102

6.8 I-V characteristics of P3HT/PCB-C12 solar cells devices for various fabrication methods. …………………………………………………………………………………………………104

6.9 I-V characteristics of nanostructured P3HT/ solution processed fullerene derivatives…...105

LIST OF TABLES

Number Page

4.1 Device data for nanograting parallel, perpendicular and thin film transistors. ……….........69

4.2 Device data for thin film transistors with different thicknesses……...………………..……69

6.1 Nanoimprinting conditions………………………………………………..…………..……..95

6.2 Performance organic solar cells (OSCs) of various architecture and acceptor deposition…………………………………………………………………………………….....100

xix

CHAPTER 1 INTRODUCTION

1.1 MOTIVATION

Energy harvesting from solar radiation is going to be central issue for future global energy production. Limited energy supply from today’s resources such as natural gas, coal and nuclear power are the biggest challenges for the next few decades. The use of natural oil is the basic of prosperity in 20 th century, however due to the decline of global oil production as well as steady growth of world population, there is a big question mark for the sufficient energy supply in 21 st century unless a bold step is taken. 1-3 The long term detrimental effect of green house gases due to the use of fossil fuels and natural oil, the potential energy crisis due to unwise exploitation and their limited supply etc are forcing us for the research of the alternative environmentally friendly and low cost renewable energy resources. The world’s total energy uses ~14 Tera-joule/sec or 14 TW continuously. 4 The use needs to be doubled for growing population by 2050. Therefore without viable options for tremendous increase in energy production, the world’s technological, economic, and political horizons will be severely limited. Other than limited supply, today’s natural resources have a number of other issues. Coal which is costly, danger in mining, causes air pollutions and acid rain pollution which is detrimental to ecology. Natural gas is not only limited in supply but it also has high shipping cost; causes air pollution and still has growing demand. Nuclear power has high capital costs, unsafe and problematic in waste management. Hydropower takes heavy cost in construction and 1

2 limited to few sites. Most of energy resources except hydropower cause environmental and ecological damages. Wind, tidal wave, geothermal fuel cells, biomass and solar power are the alternative sources of energy which are environmentally sound and ecologically sustainable. Among them, geothermal energy and tidal energy can be obtained in small scale and limited to certain regions on Earth. Similarly wind energy is very unreliable and unpredictable and can only be produced in small scale. Fuel cells have more potential than other but require production and supply of hydrogen gas. Biomass is viable but requires of mixing with gasoline for power generation. Solar power is practical with advancing technology. It is free from most of the drawbacks of other energy resources. First, solar energy is available almost everywhere in the world and it is reliable. Place to place variations is not significant. For example, two extreme places of the U.S.:Desert Southwest gets only about 25% more sunlight annually than Kansas City and Buffalo receives only 25% less sunlight than Kansas City. The variation of PV cost is much less than the cost of conventional energy from hydropower, nuclear, natural gas, coal, or oil. 5 Second, it does not take much space as we might think. Instead of our sun’s energy falling on shingles, concrete, and under-used land, it would fall on PV providing us with clean energy while leaving our landscape largely untouched. PV can be put on roofs, on parking lots, along highway walls, on the sides of buildings, and in other dual-use scenarios- we wouldn’t have to use a single acre of new land to make PV our primary energy source. Third, it is environmentally friendly. Though environmental pollution due to PV energy is zero, one must consider the initial energy required for the production which may cause environmental pollution. Therefore energy pay back in terms of cost and environmental pollution is worth to consider. First consider the production cost. Payback calculations are based on paying back this electricity with PV

3 electricity produced by installed modules. Thus, energy payback is simply the ratio of energy used to make system (in kWh/unit area) to the energy produced by the system (in kWh/unit area- time) It is estimated that energy paybacks with the present technology varies from 1 to 4 years which can be reduced with advancing technology. Second, consider the pollutions from solar cells production. An average U.S. household uses 830 kWh of electricity per month. According to U. S. Department of Energy (DOE), on average, the production of 1,000 kWh of electricity with solar power reduces emissions nitrogen oxides ~5 pounds sulfur dioxide ~8 pounds of, and carbon dioxide ~ 1,400 pounds. According to DOE plan report in 2004, a rooftop system for an average household electricity can avoid emissions of~0.5 tons of sulfur dioxide, ~0.3 tons of nitrogen oxides, and ~ 100 tons of carbon dioxide in 28 years. 5 Thus, use of solar energy is a wise energy investment for environmental benefits as well. Therefore, the utilization of solar energy can be the best option to meet the growing energy demand and control of environmental pollution. With the utilization of 0.16 % area of earth surface and solar cells with 10% energy conversion efficiency, the energy ~20 TW (more than today’s need) can be produced. 4

On average 5 kW-hour solar energy/square yard can be received per day which in general means the average U.S. household can potentially generate 500-1000 kW-hours of electrical energy in one month. So if we could efficiently harness the sun’s energy there could be limitless energy for us to use. However, very small fraction of today energy supply is from solar cells mostly due to higher cost than the power from other means. 4 Solar power is viable with advancing technology however; the collection of solar radiation in a cost competitive way with conventional power generation is the major issue of this age.

4 1.2 HISTORICAL PERSPECTIVE A photovoltaic solar cell is defined as a device which can directly convert light energy into electrical energy. In 1839, Edmund Becquerel first reported a photocurrent silver coated platinum electrode when illuminated in aqueous solution. 6 Though photovoltaic devices were constructed long time back, the explanation came in 1905 by Albert Einstein for which he received novel prize in 1921. 7 40 years later, Russell Ohl patented junction semiconductor solar cells. 8 In 1954, three American researchers, Gerald Pearson, Calvin Fuller and Daryl Chapin, designed a silicon solar cell capable of 6% power conversion efficiency. 9 Soon after, solar cells were applied in telephone carrier system by Bell Laboratory at New York in 1955. 10 In 1977, the Solar Energy Research Institute (SERI) was opened in Golden Colorado, which later became the National Renewable Energy Laboratory (NREL). 11 Record efficiencies of various solar cells by today in laboratory measurement are as follows: Crystalline Si 24.7% 12 , CIGS (thin film) 19.4%, Dye-sensitized 11.2%, 13 Organic 7.9%. 14 At present more than 117 companies [Europe:48, China: 41, USA:25, Taiwan: 17, Japan: 9 and elsewhere:16] are producing photovoltaic (PV). According to PV status report the worldwide production would reach 38 GW at the end of 2010. 15 Though it seems a large number, it covers just about 0.3% of energy need and therefore today’s need is fast solar production with low cost. Today’s PV industries are dominated by Si solar cells. According to Richard M. Swanson report, 16 the rapid emergence of PV silicon demand has created a shortage of polysilicon that will constrain industry growth. For the past few years, the PV production is limited by tight supply and high market prices of polysilicon 17 This is due to the fact that the refining process is costly, though Si is abundant. PV industry uses more 50% of the total world production of high-purity polysilicon which was just a tiny portion a few years back. 15 And, therefore PV industry is going

5 to see the shortage of Si. Therefore, scientists are trying to search massive PV production possibility in low cost using alternative materials with low cost processing techniques. Thin film solar cells has low cost manufacturing potential because it is suitable for fully integrated processing, has high throughput and requires a small amount of active materials. 18

Currently, there are three major inorganic thin-film technologies: the polycrystalline semiconductors CdTe, amorphous/microcrystalline silicon (TFSi), and Cu(In, Ga)(S, Se)2 (CIGS). Among them CIGS technology exhibits highest cell efficiencies. 19 Yet again, there are number of challenges including materials costs, toxicity, etc. One of the greatest possibilities to lower the cost is the applications of polymeric solar cells because of low cost materials, easy processing techniques and environmentally friendly materials. Organic PV offers the perspective of low cost in terms of the cost of active layer organic semiconducting materials compared to inorganic semiconductors like Si, substrates, energy input and easy up scaling. These organic semiconducting plastics are very flexible. The solar cell can even printed out using an ink jet printer onto the plastic and rolled up during manufacture. Organic solar cells can be printed like printing papers in flexible substrate which can increase the throughput up to 1000 compared to other thin film technology.

1.3 OVERVIEW OF WORK PRESENTED Organic materials have several advantages as described above. The most attractive feature of organic solar cells (OSCs) is their ultralow cost. The potential roll to roll manufacturing, laminated packing, low cost material class are some of the reasons for this attraction in addition to easy integration in different devices and substantial ecological benefit.

6 However, the efficiency of OSCs are generally lower (7%) than inorganic counterpart (30%). 20

Although the efficiencies are steadily improving, the data are not satisfactory as compare to the research effort in last decades. 21 The challenges of developing OSCs are the following: 1) new materials, 2) new methods, 3) new device architectures, 4) new substrates 5) new encapsulation materials. The discovery of conducting polymer by Heeger et al. in 1977 was followed by the observation of electroluminescence in poly(p-phenylene vinylene) (PPV) in 1990 that initiated an exciting and rapidly expanding field of research into these materials. OSCs are made up of two types of materials called donor and acceptor. 22, 23

It is widely accepted that nanoscale morphology is the key parameter to obtain high performance OSCs. Intensive research has been done for the understanding of the nature of excitons, exciton interaction, increasing open circuit voltage in addition to some engineering and manufacturing issues to obtain high power conversion efficiency. However, achievement of perfect nanoscale morphology is becoming a major bottle neck for the existing popular OSCs fabrication techniques. Along with challenge of obtaining nanoscale morphology, obtaining well defined donor-anode interface and acceptor-cathode interface are also very important. Finally, the fabrication techniques must also be suitable for the device stability, scalable for mass production, applicable to potential new types of materials tailoring the properties of active layer to better match the solar spectrum. One way to achieve this is to incorporate quantum dots consisting of a low-band gap semiconductor to increase current via increase light absorption spectrum, 24 and incorporation nanostructures for the photonic and plasmonic effects. 25-27

This thesis presents a novel approach of nanoimprint lithography for the fabrication of OSCs of well defined interdigitized nanoscale active layer morphology with well defined donor- anode interface and acceptor-cathode interface. The presented techniques is scalable for mass

7 production, suitable for the deice stability and good for the enhancement of mobility via polymer crystallization and chain alignment, incorporation of quantum dots, photonic and plasmonic effects. The thesis is organized as follows: Chapter 1 describes motivation and outlines. Chapter 2 discusses physics involved in nanoimprinted solar cells. First, the physics of organic solar cells (OSCs) differs from inorganic counterparts. OSCs are also known as excitonic solar cells because light absorption in OSCs does not directly generate free charges rather it generates bound electron-hole pair called exciton. The dynamics of exciton involves generation, its diffusion, charge dissociation and transfer. The study of OSCs mostly centered to donor-acceptor blend type of devices and nanoimprinted OSCs discussed in this thesis is relatively new type of architecture. Therefore, Chapter two aims to explore and predict possible differences in physics of blend type of devices and nanoimprinted devices. Chapter 3 focused on the fabrication of nanoimprinted solar cells which will include the fabrication of molds and nanoimprinting protocol. The results for nanoimprinted polymer structure will also be presented in Chapter 3. Nanoimprinting process involves application of heat and pressure that causes polymer flow into nanocavities causing the change in polymer properties. Therefore change in polymer morphology i.e. chain ordering and crystallizations are studied which are presented in Chapter 4. Next step for nanoimprinted device fabrication is the deposition of acceptor material on nanoimprinted polymer structures which is challenging due to solvent compatibility. Various techniques are applied for this purpose such as modification of C60 to suit for the purpose the new techniques, investigation of new solvent and reversal nanoimprinting. In Chapter 5, various acceptor deposition techniques are discussed including the investigation of orthogonal solvents, use of alternative acceptor material for transfer nanoimprinting etc. The last step of device fabrication is

Full document contains 146 pages
Abstract: Energy harvesting from sunlight via organic solar cells (OSCs) based on polymers as an electron donors and fullerenes as electron acceptors has been subject of intensive research due to the potential for low cost and large area devices with attractive market perspectives. One of the biggest challenges for OSCs is their low efficiency of power conversion, which is limited by quality of active layer morphology of donor-acceptor materials and interfaces between the components. Key reasons for this low efficiency include severe electron-hole recombination, which prevents charge pair propagation toward the electrodes and poor light absorptions due to thin polymer layer (∼100 nm). These problems can be dramatically alleviated if the charge-transfer polymers can be arranged as periodic nanostructures for active layer of ∼300 nm so that enough light absorption takes place and no phase overlap exists in the charge propagation path. This work reports the formation of ordered bi-continuous interdigitized active layer morphology, well defined interfaces for charge pair formation and propagation without recombination toward the electrodes. Such nanostructure arrays of poly(3-hexylthiophene) (P3HT) with well defined interfaces have been fabricated using nanoimprint lithography (NIL). The molds required for NIL are fabricated using innovative low cost and non-lithographic technique which is scalable to commercial use. Simultaneous control of nanostructured and 3-D chain alignment in P3HT nanostructures (nanowires and nanopillars) defined by NIL is revealed using out-of-plane and in-plane grazing incident X-ray diffraction measurements and enhancement in anisotropic charge carrier mobility favorable to solar cells and field effect transistors (FETs) is measured making FETs. Separate acceptor deposition is required for nanostructured solar cells which is challenging due to the limitation of solvent compatibility and self shadowing effect for thermal deposition. For this purpose, orthogonal solvent is investigated for spin processing, oblique angle deposition is used for thermal deposition and low glass transition temperature acceptor materials are researched for transfer imprinting process. The fabricated solar cells using the developed recipe, show improved performances as compared to bilayer devices.