Fundamentals of Solar Cell Design
Description
Solar cells are semiconductor devices that convert light photons into electricity in photovoltaic energy conversion and can help to overcome the global energy crisis. Solar cells have many applications including remote area power systems, earth-orbiting satellites, wristwatches, water pumping, photodetectors and remote radiotelephones. Solar cell technology is economically feasible for commercial-scale power generation. While commercial solar cells exhibit good performance and stability, still researchers are looking at many ways to improve the performance and cost of solar cells via modulating the fundamental properties of semiconductors. Solar cell technology is the key to a clean energy future. Solar cells directly harvested energy from the sun's light radiation into electricity are in an ever-growing demand for future global energy production.
Solar cell-based energy harvesting has attracted worldwide attention for its notable features, such as cheap renewable technology, scalable, lightweight, flexibility, versatility, no greenhouse gas emission, and economy friendly and operational costs. Thus, solar cell technology is at the forefront of renewable energy technologies which are used in telecommunications, power plants, small devices to satellites. Large-scale implementation can be manipulated by various types used in solar cell design and exploration of new materials towards improving performance and reducing cost. Therefore, in-depth knowledge about solar cell design is fundamental for those who wish to apply this knowledge and understanding in industries and academics.
This book provides a comprehensive overview on solar cells and explores the history to evolution and present scenarios of solar cell design, classification, properties, various semiconductor materials, thin films, wafer-scale, transparent solar cells, and so on. It also includes solar cells' characterization, analytical tools, theoretical modeling, practices to enhance conversion efficiencies, applications and patents.
This outstanding new volume:
* Provides state-of-the-art information about solar cells
* Is a unique reference guide for researchers in solar energy
* Includes novel innovations in the field of solar cell technology
Audience: This book is a unique reference guide that can be used by faculty, students, researchers, engineers, device designers and industrialists who are working and learning in the fields of semiconductors, chemistry, physics, electronics, light science, material science, flexible energy conversion, industrial, and renewable energy sectors..
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Inamuddin, PhD, is an assistant professor at the Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India. He has extensive research experience in analytical chemistry, materials chemistry, electrochemistry, renewable energy, and environmental science. He has worked on different research projects funded by various government agencies and universities and is the recipient of multiple awards, including the Fast Track Young Scientist Award and the Young Researcher of the Year Award for 2020, from Aligarh Muslim University. He has published almost 200 research articles in various international scientific journals, 18 book chapters, and 120 edited books with multiple well-known publishers.
Mohd Imran Ahamed, PhD, is a research associate in the Department of Chemistry, Aligarh Muslim University, Aligarh, India. He has published several research and review articles in various international scientific journals and has co-edited multiple books. His research work includes ion-exchange chromatography, wastewater treatment, and analysis, bending actuator and electrospinning.
Rajender Boddula, PhD, is currently working for the Chinese Academy of Sciences President's International Fellowship Initiative (CAS-PIFI) at the National Center for Nanoscience and Technology (NCNST, Beijing). His academic honors include multiple fellowships and scholarships, and he has published many scientific articles in international peer-reviewed journals. He is also serving as an editorial board member and a referee for several reputed international peer-reviewed journals. He has published edited books with numerous publishers and has authored over twenty book chapters.
Mashallah Rezakazemi, PhD, received his doctorate from the University of Tehran (UT) in 2015. In his first appointment, he served as associate professor in the Faculty of Chemical and Materials Engineering at Shahrood University of Technology. He has co-authored in more than 140 highly cited journal publications, conference articles and book chapters. He has received numerous major awards and grants from various funding agencies in recognition of his research. Notable among these are Khwarizmi Youth Award from the Iranian Research Organization for Science and Technology (IROST), and the Outstanding Young Researcher Award in Chemical Engineering from the Academy of Sciences of Iran. He was named a top 1% most Highly Cited Researcher by Web of Science (ESI).
Content
Preface xv
1 Organic Solar Cells 1 Yadavalli Venkata Durga Nageswar and Vaidya Jayathirtha Rao
1.1 Introduction 1
1.2 Classification of Solar Cells 3
1.3 Solar Cell Structure 4
1.4 Photovoltaic Parameters or Terminology Used in BHJOSCs 5
1.4.1 Open-Circuit Voltage Voc 5
1.4.2 Short-Circuit Current Jsc 5
1.4.3 Incident-Photon-to-Current E¿ciency (IPCE) 5
1.4.4 Power Conversion E¿ciency ¿p (PCE) 6
1.4.5 Fill Factor (FF) 6
1.5 Some Basic Design Principles/Thumb Rules Associated With Organic Materials Required for BHJOSCs 6
1.6 Recent Research Advances in Small-Molecule Acceptor and Polymer Donor Types 7
1.7 Recent Research Advances in All Small-Molecule Acceptor and Donor Types 30
1.8 Conclusion 47
Acknowledgement 48
References 48
2 Plasmonic Solar Cells 55 T. Shiyani, S. K. Mahapatra and I. Banerjee
2.1 Introduction 56
2.1.1 Plasmonic Nanostructure 58
2.1.2 Classification of Plasmonic Nanostructures 59
2.2 Principles and Working Mechanism of Plasmonic Solar Cells 60
2.2.1 Working Principle 60
2.2.2 Mechanism of Plasmonic Solar Cells 61
2.3 Important Optical Properties 62
2.3.1 Trapping of Light 63
2.3.2 Scattering and Absorption of Sunlight 63
2.3.3 Multiple Energy Levels 63
2.4 Advancements in Plasmonic Solar Cells 64
2.4.1 Direct Plasmonic Solar Cells 65
2.4.2 Plasmonic-Enhanced Solar Cell 69
2.4.3 Plasmonic Thin Film Solar Cells 69
2.4.4 Plasmonic Dye-Sensitized Solar Cells (PDSSCs) 70
2.4.5 Plasmonic Photoelectrochemical Cells 71
2.4.6 Plasmonic Quantum Dot (QD) Solar Cells 71
2.4.7 Plasmonic Perovskite Solar Cells 72
2.4.8 Plasmonic Hybrid Solar Cells 72
2.5 Conclusion and Future Aspects 72
Acknowledgements 73
References 73
3 Tandem Solar Cell 83 Umesh Fegade
List of Abbreviations 83
3.1 Introduction 85
3.2 Review of Organic Tandem Solar Cell 86
3.3 Review of Inorganic Tandem Solar Cell 89
3.4 Conclusion 95
References 96
4 Thin-Film Solar Cells 103 Gobinath Velu Kaliyannan, Raja Gunasekaran, Santhosh Sivaraj, Saravanakumar Jaganathan and Rajasekar Rathanasamy
4.1 Introduction 104
4.2 Why Thin-Film Solar Cells? 105
4.3 Amorphous Silicon 105
4.4 Cadmium Telluride 108
4.5 Copper Indium Diselenide Solar Cells 111
4.6 Comparison Between Flexible a-Si:H, CdTe, and CIGS Cells and Applications 112
4.7 Conclusion 113
References 114
Contents vii
5 Biohybrid Solar Cells 117 Sapana Jadoun and Ufana Riaz
Abbreviations 117
5.1 Introduction 118
5.2 Photovoltaics 119
5.3 Solar Cells 119
5.3.1 First-Generation 120
5.3.2 Second-Generation 120
5.3.3 Third-Generation 120
5.3.4 Fourth-Generation 121
5.4 Biohybrid Solar Cells 121
5.5 Role of Photosynthesis 122
5.6 Plant-Based Biohybrid Devices 122
5.6.1 PS I-Based Biohybrid Devices 123
5.6.2 PS II-Based Biohybrid Devices 125
5.7 Dye-Sensitized Solar Cells 126
5.8 Polymer and Semiconductors-Based Biohybrid Solar Cells 126
5.9 Conclusion 129
References 129
6 Dye-Sensitized Solar Cells 137 Santhosh Sivaraj, Gobinath Velu Kaliyannan, Mohankumar Anandraj, Moganapriya Chinnasamy and Rajasekar Rathanasamy
6.1 Introduction 138
6.2 Cell Architecture and Working Mechanism 139
6.3 Fabrication of Simple DSSC in Lab Scale 142
6.4 Electrodes 144
6.5 Counter Electrode 145
6.6 Blocking Layer 146
6.7 Electrolytes Used 147
6.7.1 Liquid-Based Electrolytes 148
6.7.1.1 Electrical Additives 148
6.7.1.2 Organic Solvents 148
6.7.1.3 Ionic Liquids 149
6.7.1.4 Iodide/Triiodide-Free Mediator and Redox Couples 149
6.7.2 Quasi-Solid-State Electrolytes 149
6.7.2.1 Thermoplastic-Based Polymer Electrolytes 150
6.7.2.2 Thermosetting Polymer Electrolytes 150
6.7.3 Solid-State Transport Materials 150
6.7.3.1 Inorganic Hole Transport Materials 151
6.7.3.2 Organic Hole Transport Materials 151
6.7.3.3 Solid-State Ionic Conductors 151
6.8 Commonly Used Natural Dyes in DSSC 152
6.8.1 Chlorophyll 152
6.8.2 Flavonoids 152
6.8.3 Anthocyanins 153
6.8.4 Carotenoids 154
6.9 Calculations 154
6.9.1 Power Conversion Efficiency 154
6.9.2 Fill Factor 163
6.9.3 Open-Circuit Voltage 163
6.9.4 Short Circuit Current 163
6.9.5 Determination of Energy Gap of Electrode Material Adsorbed With Natural Dye 163
6.9.6 Absorption Coefficient 164
6.9.7 Dye Adsorption 164
6.10 Conclusion 164
References 165
7 Characterization and Theoretical Modeling of Solar Cells 169 Masoud Darvish Ganji, Mahyar Rezvani and Sepideh Tanreh
7.1 Introduction 170
7.2 Classification of SC 172
7.2.1 Inorganic Solar Cells 173
7.2.2 Organic Solar Cell 173
7.3 Working Principle of DSSC 175
7.4 Operation Principle of DSSC 176
7.5 Photovoltaic Parameters 177
7.6 Theoretical and Computational Methods 181
7.6.1 Density Functional Theory (DFT) 182
7.6.2 Basis Sets 183
7.6.3 TDDFT Method 183
7.6.4 Molecular Descriptors 184
7.6.5 Force Field Parameterization for MD Simulations 188
7.6.6 Excited States 189
7.6.7 UV-Vis Spectroscopy 190
7.6.8 Charge Transfer and Carrier Transport 192
7.6.9 Coarse-Grained (CG) Simulations 193
7.6.10 Kinetic Monte Carlo (KMC) Modeling 193
7.6.11 Car-Parrinello Method 195
7.6.12 Solvent Effects 196
7.6.13 Global Reactivity Descriptors 196
7.7 Conclusion 198
References 199
8 Efficient Performance Parameters for Solar Cells 217 Figen Balo and Lutfu S. Sua
8.1 Introduction 218
8.1.1 Potential, Production, and Climate of Ankara 225
8.2 Solar Radiation Intensity Calculation 225
8.2.1 Horizontal Superficies 225
8.2.1.1 On a Daily Basis Total Sun Irradiation 225
8.2.1.2 Daily Diffuse Sun Irradiation 227
8.2.1.3 Momentary Total Sun Irradiation 227
8.2.1.4 Direct and Diffuse Sun Radiation 228
8.2.2 On Inclined Superficies, Computing Sun Irradiation Intensity 228
8.2.2.1 Direct Momentary Sun Radiation 228
8.2.2.2 Diffuse Sun Radiation 228
8.2.2.3 Momentary Reflecting Radiation 229
8.2.2.4 Total Sun Radiation 229
8.3 Methodology 229
8.3.1 The Solar Radiation Assessments by Correlation Models With MATLAB Simulation Software 229
8.3.2 MATLAB Simulation Results and Findings 233
8.3.3 For Ankara Province, the Determinants of the Most Efficiency Solar Cell With AHP Methodology 233
8.4 Conclusions 238
References 240
9 Practices to Enhance Conversion Efficiencies in Solar Cell 247 Andreea Irina Barzic
9.1 Introduction 247
9.2 Basics on Conversion Efficiency 249
9.3 Approaches for Improving Conversion Efficiencies in Solar Cells 253
9.4 Conclusion 264
Acknowledgements 264
References 265
10 Solar Cell Efficiency Energy Materials 271 Zeeshan Abid, Faiza Wahad, Sughra Gulzar, Muhammad Faheem Ashiq, Muhammad Shahid Aslam, Munazza Shahid, Muhammad Altaf and Raja Shahid Ashraf
10.1 Introduction 272
10.2 Solar Cell Efficiency 274
10.3 Historical Development of Solar Cell Materials 275
10.4 Solar Cell Materials and Efficiencies 277
10.4.1 Crystalline Silicon 278
10.4.2 Silicon Thin-Film Alloys 282
10.4.3 III-V Semiconductors 284
10.4.4 Chalcogenide 287
10.4.4.1 Chalcopyrites 287
10.4.4.2 Cadmium Telluride (CdTe) 288
10.4.5 Organic Materials 289
10.4.6 Hybrid Organic-Inorganic Materials 293
10.4.6.1 Dye-Sensitized Solar Cell Materials 293
10.4.6.2 Perovskites 296
10.4.7 Quantum Dots 300
10.5 Conclusion and Prospects 302
References 303
11 Analytical Tools for Solar Cell 317 Mohamad Saufi Rosmi, Ong Suu Wan, Mohamad Azuwa Mohamed, Zul Adlan Mohd Hir and Wan Nur Aini Wan Mokhtar
11.1 Introduction 318
11.2 Transient Absorption Spectroscopy 319
11.2.1 Application of Transient Absorption Spectroscopy in Solar Cells 320
11.3 Electron Tomography 323
11.3.1 Application of Electron Tomography (ET) in Solar Cells 324
11.4 Conductive Atomic Force Microscopy (C-AFM) 327
11.4.1 Application of C-AFM in Solar Cells 329
11.5 Kelvin Probe Force Microscopy 330
11.5.1 Application of Scanning Kelvin Probe Force Microscopy for Solar Cells 334
11.6 Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy 335
11.6.1 Application of Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy in Solar Cell 338
11.7 Conclusion 340
References 340
12 Applications of Solar Cells 345 Mohd Imran Ahamed and Naushad Anwar
12.1 Introduction 345
12.2 An Overview on Photovoltaic Cell 348
12.2.1 History 348
12.2.2 Working Principle of Solar Cell 348
12.2.3 First-Generation Photovoltaic Cells: Crystalline Silicon Form 351
12.2.4 Second-Generation Photovoltaic Cells: Thin-Film Solar Cells 352
12.2.5 Third-Generation Photovoltaic Cells 353
12.3 Applications of Solar Cells 354
12.3.1 Perovskite Solar Cell 354
12.3.2 Dye-Sensitized Solar Cell 355
12.3.3 Nanostructured Inorganic-Organic Heterojunction Solar Cells (NSIOHSCs) 356
12.3.4 Polymer Solar Cells 357
12.3.5 Quantum Dot Solar Cell (QDCs) 358
12.3.6 Organic Solar Cells 360
12.4 Conclusion and Summary 362
References 362
13 Challenges of Stability in Perovskite Solar Cells 371 Mutayyab Afreen, Jazib Ali and Muhammad Bilal
13.1 Introduction 371
13.2 Degradation Phenomena and Stability Measures in Perovskite 373
13.2.1 Thermal Stability 373
13.2.2 Structural and Chemical Stability 375
13.2.3 Oxygen and Moisture 376
13.2.4 Visible and UV Light Exposure 378
13.3 Stability-Interface Interplay 379
13.3.1 Chemical Reaction at the Interface 379
13.3.2 Degradation on the Top Electrode 380
13.3.3 Hysteresis Phenomenon in PSC Devices 381
13.4 Effect of Selective Contacts on Stability 382
13.4.1 Electron-Transport Layers 382
13.4.2 Hole Transport Layers 384
13.4 Conclusion 387
References 387
14 State-of-the-Art and Prospective of Solar Cells 393 Zahra Pezeshki and Abdelhalim Zekry
Acronyms 393
14.1 Introduction 396
14.2 State-of-the-Art of Solar Cells 396
14.2.1 Production Volume 400
14.2.2 Cost Breakdown 400
14.2.3 Main Technologies 401
14.2.3.1 Si Solar Cell Arrays 401
14.2.3.2 DSSCs 403
14.2.3.3 Photoanodes 404
14.2.3.4 C/Si Heterojunctions 404
14.2.3.5 a-C/Si Heterojunctions 410
14.2.3.6 Non-Fullerene Acceptor Bulk Heterojunctions 410
14.2.3.7 a-Si 411
14.2.3.8 Perovskites 411
14.2.3.9 Metal-Halide-Based Perovskites 413
14.2.3.10 Sn-Based Perovskites 415
14.2.3.11 Heavily Doped Solar Cells 416
14.2.3.12 PV Building Substrates 416
14.2.3.13 Solar Tracking System 422
14.2.3.14 Solar Concentrators 425
14.2.3.15 Solar Power Satellite 426
14.2.3.16 Roof-Top Solar PV System 427
14.2.3.17 Short-Wavelength Solar-Blind Detectors 428
14.2.3.18 GCPVS 429
14.2.3.19 Microwave Heating in Si Solar Cell Fabrication 431
14.2.3.20 Refrigeration PV System 432
14.2.3.21 Solar Collectors and Receivers 433
14.2.3.22 Solar Drying System 435
14.2.3.23 Water Networks With Solar PV Energy 436
14.2.3.24 Wind and Solar Integrated to Smart Grid 437
14.2.3.25 Green Data Centers 440
14.3 Prospective of Solar Cells 443
14.4 Conclusion 445
References 447
15 Semitransparent Perovskite Solar Cells 461 Faiza Wahad, Zeeshan Abid, Sughra Gulzar, Muhammad Shahid Aslam, Saqib Rafique, Munazza Shahid, Muhammad Altaf and Raja Shahid Ashraf
15.1 Introduction 462
15.2 Device Architectures 464
15.2.1 Conventional n-i-p Device Structure 465
15.2.2 Inverted p-i-n Device Structure 465
15.3 Optical Assessment 466
15.3.1 Average Visible Transmittance 466
15.3.2 Corresponding Color Temperature 467
15.3.3 Color Rendering Index 468
15.3.4 Transparency Color Perception 468
15.3.5 Light Management 471
15.4 Materials 474
15.4.1 Photoactive Layer 474
15.4.2 Charge Transport Layers (ETL and HTL) 479
15.4.3 Transparent Electrode 481
15.5 Applications 484
15.5.1 Building-Integrated Photovoltaics 484
15.5.2 Tandem Devices 486
15.6 Conclusion 492
References 492
16 Flexible Solar Cells 505 Santosh Patil, Rushi Jani, Nisarg Purabiarao, Archan Desai, Ishan Desai and Kshitij Bhargava
16.1 Introduction 505
16.1.1 Need for Solar Energy Harnessing 505
16.1.2 Brief Overview of Generations of Solar Cells 506
16.1.3 Limitations of Solar Cells 508
16.1.4 What is Flexible Solar Cell (FSC)? 509
16.2 Materials for FSCs 510
16.2.1 Semiconductors 510
16.2.2 Substrates 512
16.2.3 Electrodes 513
16.2.4 Encapsulations 514
16.3 Thin-Film Deposition 514
16.3.1 R2R Processing 515
16.3.2 Chemical Bath Deposition 516
16.3.3 Chemical Vapor Deposition 517
16.3.4 Dip Coating 518
16.3.5 Spin Coating 520
16.3.6 Screen Printing 521
16.4 Characterizations for FSCs 522
16.4.1 Material Characterization 523
16.4.2 Device Characterization 529
16.5 Issues in FSCs 531
16.6 Performance Comparison of RSCs and FSCs 532
16.7 Applications of Flexible Solar Cell 532
16.8 Conclusion 533
References 534
Index 537
1
Organic Solar Cells
Yadavalli Venkata Durga Nageswar1* and Vaidya Jayathirtha Rao2
1CSIR - Indian Institute of Chemical Technology, Hyderabad, India
2Hetero Research Foundation, TSIE, Balanagar, Hyderabad, India
Abstract
Limitations faced in using fullerene as an acceptor molecule in BHJOSCs directed research toward non-fullerene-based acceptors in BHJOSCs. Polymer donor and small-molecule acceptor combination is successfully explored to develop higher performance BHJOSCs. Various novel small acceptor organic materials are synthesized and fabricated as sBHJOSCs in combination with suitable polymer donors available. Performances of organic solar cells improved to over 17%, and further, it may cross even 20%. Simultaneously, researchers explored fullerene all small molecules for BHJOSCs. All small-molecule BHJOSCs do not use polymer donor due to certain limitations. Progress achieved from these investigations is remarkable and the efficiency displayed is around 14%. Both the research lines are found to be exceptional and will provide further improvement in the solar cell efficiency. Various examples discussed in this chapter deal with the recent research results reported in the literature on both the research domains.
Keywords: UV-visible absorption, device architecture, film morphology, non-fullerene blends, all small organic molecules, optical band-gap, photovoltaic parameters, photo conversion efficiency
1.1 Introduction
Wind energy is renewable, land around turbine may be used for agriculture, and, further, newer technologies may provide better ways of converting wind energy. There are certain limitations in the wind energy conversion like: unreliable wind source, low production of electricity, higher capital cost, environmental damages, and higher level of noise production. Coal energy is also in practice in some countries and they may be continued, but there are disadvantages like carbon-dioxide production, environmental pollution due to coal burn waste accumulation, and problems associated with coal mining. Fossil fuels are also in practice for generating electricity in some countries, because they are inexpensive, portable, and easily burnt. Limitations to fossil fuels are nonrenewable source, emissions due to burning, and global warming. Geothermal energy is a base load energy source, safer than fossil fuels, and globally sustainable. Disadvantages of this geothermal energy are contamination of unwanted trace elements, localized depletion of energy, and energy imbalance leading to geological instability. Hydrothermal energy for the production of electricity is also in practice in some countries, which has advantages like controlled way of electricity production, ease of water pooling in dams, and utilization of water released after electricity production for irrigation/agriculture. Other disadvantages are higher capital costs, ecological and environmental disturbance, possible wreckage of dams leading to flooding, and hostilities arising due to improper sharing of water. Nuclear energy is another form of energy that can be utilized for generating electricity and is practiced in some countries. It is relatively a clean energy, most concentrated energy and does not require big places/areas. The disadvantage is potential accidental hazards due to control deficiencies, management failures, and possible leaks (nuclear fission process).
Energy from SUN is the most abundant form of renewable energy reaching the earth and is known as Solar Energy. Radiant light and heat emanating from SUN and reaching the earth can be utilized for various purposes. Solar energy is renewable, clean, and green and is a secured type of energy reaching earth 200,000 times more than the electrical energy generated in a day on the earth. Earth receives 174 peta Watts of sun radiation, in the form of 8% UV radiation, 46% visible light, and 46% infrared radiations, and is absorbed by earth atmosphere, oceans, and land mass. Because of the clean, green, and abundant renewable energy coming from sun is found to be more attractive for the researchers to work on the conversion of light energy in to electricity; presently, this has become a globally attractive and potential research domain to devise solar energy trapping units in to usable form of electricity, which can serve global energy requirements. Solar cell or photovoltaic (PV) cell is a device or unit which converts light in to electricity and globally research scientists are making all out efforts to prepare an efficient solar cell with an excellent photo-conversion efficiency, such that it can be developed as viable technology for society. Silicon solar cells are already in the hands of citizens/public having ~26% efficiency with some limitations, and this situation placed research on organic solar cells the most demanding and desirable field.
1.2 Classification of Solar Cells
The possible classification of solar cells is given in Figure 1.1. Among the many (Figure 1.1), organic solar cells attracted rigorous attention because of various advantages like simple preparation of organic solar materials, light weight (low density), low cost, flexibility of the PV modules, semitransparency, easy integration in to other products, low environmental impact, easy adoption of printing technology, and large area of fabrication.
Figure 1.1 Classification of solar cells.
Thin film solar cells are further put in to four categories. The two categories involving fullerenes have found limitations in due course of research investigations, although research was conducted on fullerene-based OSC over two decades. The other two categories, non-fullerene polymer smallmolecule and non-fullerene all small-molecule OSC, based on present scenario, are intensively investigated. Therefore, this chapter will be focused only on these two non-fullerene-based polymer-small-molecule and all small-molecule OSC.
1.3 Solar Cell Structure
Fundamental steps occurring in a schematic representation of a typical solar cell device and its functioning are schematically provided in Figures 1.2 and 1.3. (a) Typical OSC devices based on donor-acceptor in bulk hetero-junction configuration, another way it is the sandwich of active organic blend material in between anode and cathode electrodes with light absorbing property. (b) Donor-acceptor hetero-junction solar cells with basic steps involved: 1) Photo-excitation of the donor-acceptor blend to generate an exciton/excited state [radicalanion/electron-radicalcation/ hole pair bound by ionic and radical (Coulomb) interactions]. 2) Exciton/ excited state diffusion to the donor-acceptor interface. Excitons/excited states that do not reach the inter-face, they recombine and do not contribute to the photocurrent (longer diffusion length, LD). 3) Dissociation of bound excitons at the donor-acceptor interface to form a geminate radical-anion (electron)-radical-cation (hole) pair [increased interfacial charge separation requires optimal energy offset between LUMO (lowest unoccupied molecular orbital) of the donor and LUMO of the acceptor material]. 4) Free charge carrier transport and collection at the external electrodes (require high charge-carrier mobility). (c) Fundamental processes (light illumination, exciton formation, charge separation, charge migration, and charge collection) of bulk-heterojunction solar cells (p = donor material, n = acceptor material).
Figure 1.2 Typical solar cell.
Figure 1.3 Possible events present in BHJOSCs.
1.4 Photovoltaic Parameters or Terminology Used in BHJOSCs
1.4.1 Open-Circuit Voltage Voc
The voltage at which no current flows through a solar cell is called open circuit voltage Voc and it is the maximum voltage available from solar cell. Several studies have demonstrated a strong dependence of Voc on the energy difference ?E between the HOMO (highest occupied molecular orbital) of donor material and LUMO of acceptor material of an organic solar cell.
1.4.2 Short-Circuit Current Jsc
For V = 0, only the short-circuit current (Jsc) flows through the solar cell. Jsc represents the maximum current that could be obtained in a solar cell. This current depends on the number of absorbed photons, surface area of the photo active layer, device thickness, and charge transport properties of active material, which play important role.
1.4.3 Incident-Photon-to-Current Efficiency (IPCE)
The incident-photon-to-current efficiency is defined as the ratio of the number of incident photons Nphoton and the number of photo induced charge carriers Ncharge which can be extracted out of the solar cell.
1.4.4 Power Conversion Efficiency ?p (PCE)
It is a measure of the quality of the cell which provides evidence of how much power the cell will generate per incident photon. The efficiency ?p is the maximum electrical power Pmax per light input PL.
1.4.5 Fill Factor (FF)
The FF, which determines the quality of solar cell can be obtained from the ratio of the maximum power output to the product of its Voc and Jsc and is always < 1.
1.5 Some Basic Design Principles/Thumb Rules Associated With Organic Materials Required for BHJOSCs
The donor and acceptor molecules to be employed in BHJOSCs must have light absorption property matching the solar region, with high molar absorption coefficients and excellent width at half height of absorption spectrum. It would be best if...
