Rapid introduction of renewable energy is essential in order to meet future energy demands without further polluting the environment. Solar energy conversion Dharmadasa plays a very important role in this, but current solar panels based on silicon are expensive because of the high cost of processing crystalline silicon, a technology I. M. Dharmadasa that demands high energy consumption. The way forward is to move towards thin- film solar cells using alternative materials and low-cost manufacturing methods. The photovoltaic community is actively researching thin-film solar cells based on amorphous silicon, cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), dye-sensitised materials, and organic semiconductors/polymers. However, progress has been slow owing to the complications of the physics behind these devices. This book concentrates on the latest developments in and understanding of THIN-FILM device physics underlying thin-film solar cells. The material presented is mainly experimental and based on CdTe thin-film solar cells. The author extends these new findings to CIGS- and GaAs-based thin-film solar cells and presents a new device design based on graded bandgap multilayer solar cells. This design has been experimentally tested using the well-researched GaAs/AlGaAs system, and initial ≈ ≈ Advances in devices have shown impressive device parameters (Voc 1175 mV, FF 0.85, and Jsc ≈ –2 12 mAcm ). In particular, the Voc represents the highest recorded value together with the highest possible FF values to date for a single PV device, indicating the right approach for PV solar cell development. This device is capable of absorbing all radiation (ultraviolet, visible and infrared) within the solar spectrum as well as SOLAR CELLS heat energy from the surroundings and combines these with “impact ionisation” and “impurity photovoltaic” effects. The conversion efficiency of graded bandgap device has improved to ~20% using only two growth attempts. The improved device understanding presented in this book should impact and guide future device design and low-cost thin-film solar panel development and manufacture. Advances in I. M. Dharmadasa is professor of applied physics and head of the Electronic Materials and Sensors Group at Materials and Engineering Research Institute at Sheffield Hallam University, THIN-FILM UK. He has worked extensively in semiconductor research since becoming a PhD student at Durham University as a Commonwealth Scholar, in 1977. He has over 200 publications and presentations on both research and development and applications of solar energy and 6 patents on SOLAR CELLS thin-film solar cells and has successfully supervised 14 PhD programmes to date. Winner of several scholarships and awards, including the 2001 Eurosolar UK Prize for inspiring renewable energy applications, Prof. Dharmadasa is also passionate about promoting the use of renewable energy for economic development and poverty reduction and is active in setting up solar-powered energy hubs in developing countries to empower rural communities through education and commerce. He blogs at www.apsl.org.uk. V213 ISBN-13 978-981-4316-07-1 This page intentionally left blank CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2012 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20120829 International Standard Book Number-13: 978-9-81436-412-6 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. 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Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com August 3, 2012 16:23 PSP Book - 9in x 6in 00-Dharmadasa–prelims Contents Preface xiii List of Symbols and Abbreviations Used in the Book xvii 1 Photovoltaic Solar Energy Conversion 1 1.1 Introduction 1 1.2 Photovoltaic Effect 2 1.3 Solar Energy Materials 3 1.4 Electronic Devices Used for Solar Energy Conversion 4 1.4.1 p-n Junctions 5 1.4.2 p-i-n Junctions 6 1.4.3 Hetero-Junctions 7 1.4.4 n-n and p-p Junctions 8 1.4.5 Metal/Semiconductor (or Schottky) Contacts 8 1.4.6 Metal-Insulator–Semiconductor Interfaces 11 1.5 Characteristics of a Solar Cell 13 1.5.1 I-V Characteristics of a Solar Cell Under Dark Conditions 13 1.5.2 I-V Characteristics of a Solar Cell Under Illuminated Conditions 16 1.5.3 How to Maximise Voc 19 1.5.4 How to Maximise J sc 20 1.5.5 How to Maximise FF 20 1.6 Next-Generation Solar Cells 21 1.7 Summary 22 2 Status Report on Solar Energy Technologies 25 2.1 Introduction 25 2.2 Si Solar Cell Technology 27 2.3 PV-Manufacturing Cost Based on Si Technology 30 2.4 PV Technology Based on III-V Compounds 31 August 3, 2012 16:23 PSP Book - 9in x 6in 00-Dharmadasa–prelims vi Contents 2.5 New Technology for PV and Nano-Divices 32 2.6 Emerging Low-Cost Thin-Film Technologies 33 2.7 Summary 35 3 Electrochemical Deposition of Solar Energy Materials 37 3.1 Introduction 37 3.2 Electrodeposition of Semiconductors 38 3.3 Strengths and Advantages of Electrodeposition 40 3.3.1 Simplicity, Low-Cost, Scalability, and Manufacturability 40 3.3.2 Self-Purification and Built-in Hydrogen Passivation 41 3.3.3 Extrinsic and Intrinsic Doping 42 3.3.4 Ability in Bandgap Engineering 43 3.3.5 Other Advantages of Electrodeposition 43 3.4 Experimental Evidence 44 3.4.1 Observations in XRD 44 3.4.2 Observations in XRF 44 3.4.3 Observations in PEC Cell Measurements 46 3.4.4 Observations in Optical Absorption Measurements 49 3.4.5 Observations in Photoluminescence 49 3.4.6 Impurity Control in Semiconductors 51 3.5 Issues in Electrodeposition of Semiconductors 51 3.6 Current Work and Future Prospects 53 3.7 Summary 56 4 Background of the CdTe Solar Cell and the New Device Concept 59 4.1 Introduction 59 4.2 The Conventional Model for a Glass/Conducting Glass/CdS/CdTe/Metal Solar Cell 60 4.3 Key Observations That Led to the Formulation of aNewModel 63 4.3.1 Surface Modification of CdTe 63 4.3.2 Effects of Surface Modification on Defect Levels 64 4.3.3 Effects of Defect Levels on Electronic Devices 65 August 3, 2012 16:23 PSP Book - 9in x 6in 00-Dharmadasa–prelims Contents vii 4.3.4 Similar Observations on Thin-Film CdS/CdTe Solar Cells 66 4.4 New Concept for CdS/CdTe Solar Cell 68 4.5 Description of Experimental Results Using the Two Models 71 4.5.1 Current-Voltage (I-V) Characteristics 72 4.5.2 Capacitance-Voltage (C-V) Characteristics 73 4.5.3 Electron Beam–Induced Current Measurements 73 4.5.4 Observation of Discrete Barrier Heights and Voc Values 74 4.5.5 A Thin-Film CdTe Solar Cell Device Without aCdSLayer 74 4.5.6 Results From Electrical Contacting Work 75 4.5.7 Doping of CdS and CdTe Layers 76 4.5.8 Further Experimental Evidence to Confirm the True Structure of the Device 78 4.6 Predictions for Further Development of CdS/CdTe Solar Cells and Latest Observations 80 4.6.1 Doping of Window and Absorber Materials with n-Dopants 80 4.6.2 Improvements to Back Contact Using MIS-Type Structures 86 4.6.3 A Multi-Layer Graded Bandgap Approach 88 4.6.4 Dealing with Defects 89 4.7 Summary 91 5 Extension of the New Model to CIGS Thin-Film Solar Cells 95 5.1 Introduction 95 5.2 Summary of Accumulated Knowledge on CIGS-Based Materials 96 5.2.1 Different Growth Techniques 96 5.2.2 Structural, Optical, and Electronic Properties 96 5.2.3 Ordered Defect Compound Layer 97 5.2.4 Latest Developments in Materials Growth 97 5.3 Summary of Accumulated Knowledge on CIGS-Based Solar Cells 98 August 3, 2012 16:23 PSP Book - 9in x 6in 00-Dharmadasa–prelims viii Contents 5.3.1 Conventional Device Structure 98 5.3.2 Frequently Used Energy Band Diagram 99 5.4 Current Views of the Physics Behind CIGS Solar Cells 100 5.4.1 p-CIGS/n-CdS Hetero-Junction 101 5.4.2 p-CIGS/n-CIGS Homo-Junction 101 5.4.3 p-CIGS/n-ODC Hetero-Junction 101 5.5 Reported Device Performance 102 5.6 Recent Work on Metal/p-CIGS Interfaces 104 5.7 Deeper Understanding of Mo/CIGS/CdS/i-ZnO/ n-ZnO:Al/Metal-Grid