Advances in Thin-Film Solar Cells

I. M. Dharmadasa

• Publisher: Jenny Stanford Publishing
• Year: 2018
• eBook ISBN: 9780429668388
• Topic: Physical Sciences
• Subtopic: Biology

Solar energy conversion plays a very important role in the rapid introduction of renewable energy, which is essential to meet future energy demands without further polluting the environment, but current solar panels based on silicon are expensive due to the cost of raw materials and high energy consumption during production. 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), and dye-sensitised and organic materials. However, progress has been slow due to a lack of proper understanding of the physics behind these devices.

This book concentrates on the latest developments and attempts to improve our understanding of solid-state device physics. The material presented is mainly experimental and based on CdTe thin-film solar cells. The author extends these new findings to CIGS thin-film solar cells and presents a new device design based on graded bandgap multi-layer solar cells. This design has been experimentally tested using the well-researched GaAs/AlGaAs system, and initial devices have shown impressive device parameters. These devices are capable of absorbing all radiation (UV, visible and infra-red) within the solar spectrum and combine "impact ionisation" and "impurity photovoltaic" effects.

The improved device understanding presented in this book should impact and guide future photovoltaic device development and low-cost thin-film solar panel manufacture. This new edition features an additional chapter besides exercises and their solutions, which will be useful for academics teaching in this field.

Contents

Preface to the Second Edition

List of Symbols and Abbreviations

1 Photovoltaic Solar Energy Conversion

1.1 Introduction

1.2 Photovoltaic Effect

1.3 Solar Energy Materials

1.4 Electronic Devices Used for Solar Energy Conversion

1.4.1 p-n Junctions

1.4.2 p-i-n Junctions

1.4.3 Hetero-junctions

1.4.4 n-n and p-p Junctions

1.4.5 Metal/Semiconductor (or Schottky) Contacts

1.4.6 Metal/Insulator/Semiconductor Interfaces

1.5 Characteristics of a Solar Cell

1.5.1 I-V Characteristics of a Solar Cell Under Dark Conditions

1.5.2 I-V Characteristics of a Solar Cell Under Illuminated Conditions

1.5.3 How to Maximise V_oc

1.5.4 How to Maximise J_sc

1.5.5 How to Maximise FF

1.6 Summary

Exercises

2 Status Report on Solar Energy Technologies

2.1 Introduction

2.2 Si Solar Cell Technology

2.4 PV Technology Based on III-V Compounds

2.5 Disruptive Technology for PV Development

2.6 Emerging Low-Cost Thin-Film Technologies

2.7 Next-Generation Solar Cells

2.8 Summary

Exercise

3 Electrochemical Deposition of Solar Energy Materials

3.1 Introduction

3.2 Electrodeposition of Semiconductors

3.3 Strengths and Advantages of Electrodeposition

3.3.1 Simplicity, Low-Cost, Scalability, and Manufacturability

3.3.2 Self-Purification and Built-In Hydrogen Passivation

3.3.3 Extrinsic and Intrinsic Doping

3.3.4 Ability in Bandgap Engineering

3.3.5 Other Advantages of Electrodeposition

3.4 Experimental Evidence

3.4.1 Observations in XRD

3.4.2 Observations in XRF

3.4.3 Observations in PEC Cell Measurements

3.4.4 Observations in Optical Absorption Measurements

3.4.5 Observations in Photoluminescence

3.4.6 Impurity Control in Semiconductors

3.5 Issues in Electrodeposition of Semiconductors

3.6 Summary of Electroplated Materials to Date

3.7 Applications in PV Devices

3.8 Summary

Exercises

4 Background of the CdTe Solar Cell and the New Device Concept

4.1 Introduction

4.2 The Previous Model for a Glass/Conducting Glass/CdS/CdTe/Metal Solar Cell

4.3 Key Observations That Led to the Formulation of a New Model

4.3.1 Surface Modification of CdTe

4.3.2 Effects of Surface Modification on Defect Levels

4.3.3 Effects of Defect Levels on Electronic Devices

4.3.4 Similar Observations on Thin-Film CdS/CdTe Solar Cells

4.4 New Concept for CdS/CdTe Solar Cell

4.5 Description of Experimental Results Using the Two Models

4.5.1 Current-Voltage (I-V) Characteristics

4.5.2 Capacitance-Voltage (C-V) Characteristics

4.5.3 Electron Beam-Induced Current Measurements

4.5.4 Observation of Discrete Barrier Heights and V_oc Values

4.5.5 A Thin-Film CdTe Solar Cell Device Without a CdS Layer

4.5.6 Results from Electrical Contacting Work

4.5.7 Doping of CdS and CdTe Layers

4.5.8 Further Experimental Evidence to Confirm the True Structure of the Device

4.6 Predictions for the Further Development of CdS/CdTe Solar Cells and Latest Observations

4.6.1 Doping of Window and Absorber Materials with n-Dopants

4.6.2 Improvements to Back Contact Using MIS-Type Structures

4.6.3 A Multi-layer Graded Bandgap Approach

4.6.4 Dealing with Defects

4.6.5 Progress during the Period 2011-2016

4.7 Summary

Exercises

5 Extension of the New Model to CIGS Thin-Film Solar Cells

5.1 Introduction

5.2 Summary of Accumulated Knowledge on CIGS-Based Materials

5.2.1 Different Growth Techniques

5.2.2 Structural, Optical, and Electrical Properties

5.2.3 Ordered Defect Compound Layer

5.2.4 Latest Developments in Materials Growth

5.3 Summary of Accumulated Knowledge on CIGS-Based Solar Cells

5.3.1 Conventional Device Structure

5.3.2 Frequently Used Energy Band Diagram

5.4 Current Views of the Physics Behind CIGS Solar Cells

5.4.1 p-CIGS/n-CdS Hetero-junction

5.4.2 p-CIGS/n-CIGS Homo-junction

5.4.3 p-CIGS/n-ODC Hetero-junction

5.5 Reported Device Performance

5.6 Recent Work on Metal/p-CIGS Interfaces

5.7 Deeper Understanding of Mo/CIGS/CdS/ i-ZnO/n-ZnO:Al/Metal-Grid Solar Cells

5.7.1 Type I CIGS-Based Solar Cell

5.7.2 Type II CIGS-Based Solar Cell

5.8 Discussion on Further Improvements of CIGS Solar Cells

5.8.1 Optimisation of Growth, Doping, and Bandgap Engineering

5.8.2 Defect Level Identification and Engineering

5.8.3 Growth of CIGS with Controlled Orientation

5.8.4 Replacement of Mo Using TCO for Tandem and Double-Faced Solar Cells

5.8.5 Further Improvements of the Device Structure

5.9 Conclusions

5.10 Summary

6 Effective Harvesting of Photons

6.1 Introduction

6.2 Tandem Solar Cells

6.2.1 Connection in Series

6.2.2 Connection in Parallel

6.3 Comparison of the Two Connecting Methods

6.3.1 Disadvantages of Series Connections

6.3.2 Advantages of Parallel Connections

6.4 Conclusions

6.5 Summary

Exercise

7 Multi-layer Graded Bandgap Solar Cells

7.1 Introduction

7.1.1 Incorporation of the Impurity PV Effect

7.1.2 Incorporation of Impact Ionisation

7.2 Summary of Growth and Process Details of the Device Structure

7.3 Experimental Results of Fully Processed Devices

7.3.1 Electrical Properties Under Dark Conditions

7.3.2 Electrical Properties Under AM1.5 Illumination

7.3.3 IPCE Measurements

7.3.4 EBIC Measurements

7.3.5 SIMS Profiling

7.3.6 Optimisation of Si Doping Concentration

7.4 Discussions

7.5 Summary

Exercise

8 Solar Cells Active in Complete Darkness

8.1 Introduction

8.2 ...

Table of contents

• Cover
• Halftitle Page
• Title Page
• Copyright
• Table of Contents