Solar Cells

Materials, Manufacture and Operation

Augustin McEvoy, L. Castaner, Tom Markvart

• Publisher: Academic Press
• Year: 2012
• Edition: 2
• eBook ISBN:
  9780080993799
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Enormous leaps forward in the efficiency and the economy of solar cells are being made at a furious pace. New materials and manufacturing processes have opened up new realms of possibility for the application of solar cells. Crystalline silicon cells are increasingly making way for thin film cells, which are spawning experimentation with third-generation high-efficiency multijunction cells, carbon-nanotube based cells, UV light for voltage enhancement, and the use of the infrared spectrum for night-time operation, to name only a few recent advances. This thoroughly updated new edition of Markvart and Castaner's Solar Cells, extracted from their industry standard Practical Handbook of Photovoltaics, is the definitive reference covering the science and operation, materials and manufacture of solar cells. It is essential reading for engineers, installers, designers, and policy-makers who need to understand the science behind the solar cells of today, and tomorrow, in order to take solar energy to the next level. - A thorough update to the definitive reference to solar cells, created by a cast of international experts from industry and academia to ensure the highest quality information from multiple perspectives - Covers the whole spectrum of solar cell information, from basic scientific background, to the latest advances in materials, to manufacturing issues, to testing and calibration. - Case studies, practical examples and reports on the latest advances take the new edition of this amazing resource beyond a simple amalgamation of a vast amount of knowledge, into the realm of real world applications

Part IC

Thin Film Technologies

Chapter IC-1 Thin-Film Silicon Solar Cells1

Chapter IC-2 CdTe Thin-Film PV Modules

Chapter IC-3 Cu(In,Ga)Se2 Thin-Film Solar Cells

Chapter IC-4 Progress in Chalcopyrite Compound Semiconductor Research for Photovoltaic Applications and Transfer of Results into Actual Solar Cell Production

Chapter IC-5 High-Efficiency Back-Contact Silicon Solar Cells for One-Sun and Concentrator Applications

Chapter IC-6 High Efficiency III–V Multijunction Solar Cells

Chapter IC-1

Thin-Film Silicon Solar Cells¹

Arvind Shah

with the collaboration of Horst Schade

Chapter Outline

1. Introduction

1.1. Tandem and Multijunction Solar Cells

2. Hydrogenated Amorphous Silicon (a-Si:H) Layers

2.1. Structure of Amorphous Silicon

2.2. Gap States in Amorphous Silicon: Mobility Gap and Optical Gap

2.3. Conductivity and Doping of Amorphous Silicon

2.3.1. Conductivities

2.3.2. Doping

3. Hydrogenated Microcrystalline Silicon (μc-Si:H) Layers

3.1. Structure of Microcrystalline Silicon

3.2. Optical Absorption, Gap States, and Defects in Microcrystalline Silicon

3.3. Conductivities, Doping, Impurities, and Ageing in Microcrystalline Silicon

3.3.1. Conductivities

3.3.2. Doping

3.3.3. Impurities

3.3.4. Ageing

4. Functioning of Thin-Film Silicon Solar Cells with p–i–n and n–i–p Structures

4.1. Role of the Internal Electric Field

4.1.1. Formation of the Internal Electric Field in the i Layer

4.1.2. Reduction and Deformation of the Internal Electric Field in the i Layer

4.2. Recombination and Collection

4.3. Shunts

4.4. Series Resistance Problems

4.5. Light Trapping

5. Tandem and Multijunction Solar Cells

5.1. General Principles

5.2. a-Si:H/a-Si:H Tandems

5.3. Triple-Junction Amorphous Cells with Silicon–Germanium Alloys

5.4. Microcrystalline–Amorphous or “Micromorph” Tandems

6. Module Production and Performance

6.1. Deposition of the Thin-Film Silicon Layers

6.2. Substrate Materials and Transparent Contacts

6.3. Laser Scribing and Cell Interconnection

6.4. Module Encapsulation

6.5. Module Performance

6.6. Field Experience

7. Conclusions

References

1 Introduction

Silicon thin films for solar cells are at present predominantly deposited by plasma-enhanced chemical vapour deposition (PECVD) either from silane (SiH₄) or preferably from a mixture of silane and hydrogen. They are either amorphous or microcrystalline. They contain about 5% to 15% of hydrogen atoms. The hydrogen atoms are essential, as they passivate a large part of the inherent defects in these semiconductor films.

Amorphous silicon thin films were first deposited by PECVD by R.C. Chittick et al. [2]; this work was continued in a systematic manner by Walter Spear and Peter Le Comber and their research group at the University of Dundee in the 1970s. In a landmark paper published in 1975 [3] (see also [4]), they demonstrated that amorphous silicon layers deposited from silane by PECVD could be doped by adding to the plasma discharge either phosphine (PH₃) to form n-type layers or diborane (B₂H₆) to form p-type layers: They showed that the conductivity of these thin amorphous silicon layers (which contain about 10% to 15% hydrogen) could be increased by several orders of magnitude. Their pioneering work made it possible to use hydrogenated amorphous silicon (a-Si:H) to fabricate diodes and thin-film transistors, which can be used for the active addressing matrix in liquid crystal displays. It was Dave Carlson and Chris Wronski who fabricated the first amorphous silicon solar cells at the RCA Laboratories; the first publication in 1976 described cells with an efficiency of 2% [5], this value being increased to 5% within the same year [6]. A year later, Staebler and Wronski reported [7] on a reversible photodegradation process that occurs within amorphous silicon solar cells when the latter are exposed to light during long periods (tens to hundreds of hours). This effect is called the Staebler–Wronski effect (SWE), and it is a major limitation of amorphous silicon for solar cell technology. It is due to an increase of midgap defects, which act as recombination centres. It is a reversible effect: the initial, nondegraded state can be restored by annealing at 150°C for several hours. By affecting the quality of the photoactive layer within the cell, the SWE causes the efficiency of amorphous silicon solar cells to decrease during the first months of operation. After about a thousand hours of operation, the efficiency more or less stabilizes at a lower value. This is why it is important to always specify stabilized efficiencies for amorphous silicon solar cells. In the initial phase of amorphous silicon solar cell development, it was hoped to overcome this degradation effect. So far, nobody has succeeded in fabricating amorphous silicon layers that do not show any photodegradation. However, by adding hydrogen to silane during the plasma deposition of the layers, and by increasing the deposition temperature, the photodegradation can be somewhat reduced. Furthermore, by keeping the solar cells very thin (i-layer thickness below 300 nm), one can reduce the impact of the Staebler–Wronski effect on the cell’s efficiency. An important feature of amorphous silicon solar cells, introduced also by Carlson and Wronski, is that one does not use the classical structure of a p–n diode, as in almost all other solar cells, but one uses a p–i–n diode, keeping the doped layers (p- and n-type layers) very thin and employing the i layer (i.e., an intrinsic or undoped layer) as the photogeneration layer, where the light is mainly absorbed and its energy transferred to the charge carriers (holes and electrons). There are two reasons for this: (1) the electronic quality of doped amorphous layers is very poor; they have a very high density of midgap defects or recombination centres, so that practically all carriers, which are photogenerated within the doped layers are lost through recombination; (2) within the whole i layer of a p–i–n diode an internal electric field is created that separates the photogenerated electrons and holes and helps in collecting them in the n and p layers, respectively. The internal electric field is absolutely essential for the functioning of an amorphous silicon solar cell—without this field most of the photogenerated carriers would not be collected, and, thus, the cell’s performance would be totally unsatisfactory. The theory of p–i–n diodes has not been studied to the same extent as that of classical p–n diodes, and further work is clearly called for.

Amorphous silicon solar cells at first found only “niche” applications, especially as the power source for electronic calculators. For 15 years or so, they have been increasingly used for electricity generation: they seem particularly well suited for wide applications in building-integrated photovoltaics (BIPV). One of their main advantages is that they are available in the form of monolithically integrated large-area modules (and even as flexible modules based on stainless steel or polymer substrates). Another significant advantage is that their temperature coefficient is only –0.2%/°C—i.e., less than half of that prevailing in wafer-based crystalline silicon solar cells. At present, single-junction amorphous silicon solar cells attain in the laboratory stabilized efficiencies of more than 10% [8], whereas single-junction commercial modules have stabilized total-area efficiencies between 6% and 7%.

Microcrystalline silicon thin films containing hydrogen (μc-Si:H films) were first described in detail by S. Veprek and co-workers [9], who used a chemical transport technique to fabricate them. The first report of depositing μc-Si:H films with PECVD, from a plasma of silane strongly diluted with hydrogen, was published by Usui and Kikuchi in 1979 [10]. The plasma-deposition techniques for microcrystalline...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Part IA: Solar Cells
• Part IB: Crystalline Silicon Solar Cells
• Part IC: Thin Film Technologies
• Part ID: Dye-Sensitized and Organic Solar Cells
• Part II: Testing, Industry and Environment
• Part IIB: Environment
• Part IIC: Industry
• Appendix A: Constants, Physical Quantities, and Conversion Factors
• Appendix B: List of Principal Symbols
• Appendix C: Abbreviations and Acronyms
• Appendix D: Useful Web Sites and Journals
• Appendix E: International Standards with Relevance to Photovoltaics
• Appendix F: Books About Solar Cells, Photovoltaic Systems, and Applications
• Index