Low cost processing of CIGS thin film solar cells

doi:10.1016/j.solener.2004.08.015

Solar Energy

Volume 77, Issue 6, December 2004, Pages 749-756

https://doi.org/10.1016/j.solener.2004.08.015 Get rights and content

Abstract

A set of low cost techniques with realistic potential for direct manufacturing costs reduction were developed in the last five years while the industrial Cu(In,Ga)Se₂ (CIGS) solar cell production is based on vacuum processes, which require high initial investment into production machines. The common properties of these low cost techniques are the use of simple and fast non-vacuum deposition methods and the prefixing of the film-composition on a molecular level in a precursor layer, which is chemically and thermally treated to form a high quality CIGS film. The paste coating approaches use premixed inks which are applied by doctor-blade coating to yield solar cell efficiencies of 13.6%, with the potential to reach 15% and more in the next years. The choice of the precursor material has to be made with respect to the used selenization conditions to avoid detrimental impurity phases. A new precursor material is discussed, which allows fast conversion in selenium atmosphere and was used to produce solar cells with 6.7% efficiency. The CIGS film thickness has to be increased for complete absorption of the incident light.

Introduction

The direct conversion of sunlight into electrical energy is a preferred method for clean and safe energy production in future. As long as cheap traditional energy resources are still abundant, a breakthrough of photovoltaics is only possible with low production costs. Niche markets such as space and off-grid applications have always been the motor for research and development in photovoltaics. Nevertheless in recent years, the photovoltaic market with its fast annual growth rates of over 35% during 1996–2003 has started to attract the attention of investors, looking for new promising investment fields. Thin-film solar cell technologies have raised the expectation that photovoltaics will be able to compete with traditional energy production methods due to expected lower processing costs. But so far with none of the used technologies the promised potential has been achieved and the global production volume is still below 750 MW/Year. One reason is the high investment required for machines, another is the shortage of feed stock for silicon-wafer technology, which is still dominating the market. More research and development is necessary for the breakthrough in commercial manufacturing technologies for thin film photovoltaics.

Among the investigated thin film absorber materials polycrystalline Cu(In_x,Ga_1−x)(S_ySe_1−y)₂, (CIGS) is one of the favourites, since it has a high optical absorption coefficient, a tuneable band gap and somewhat benign grain boundaries (Schock and Noufi, 2000). The conventional CIGS solar cell is a layer stack consisting of a molybdenum back contact, the CIGS absorber, a thin CdS or ZnS or In₂S₃ buffer layer, and a bilayer of intrinsic and aluminium doped zinc oxide as transparent conductive oxide (TCO) front contact. Traditionally, this layer stack is deposited on a soda-lime glass substrate, but the recent trend includes flexible substrates such as polymer or metal-foils. These substrate materials are better suited for many applications including photovoltaic integrated buildings and space power generation and they allow roll-to-roll manufacturing, leading to substantial cost reductions.

This paper focuses on techniques with low initial investment for the production equipment. Therefore, simple deposition techniques such as spraying and paste coating are reviewed. Electrodeposition is not covered in detail since another article in this journal issue focuses thereon.

Section snippets

Conventional CIGS deposition processes

Sophisticated vacuum co-evaporation techniques (Fig. 1) give the best control of composition and compositional grading through the films. A maximum CIGS cell efficiency of 19.3% has been obtained by a three-stage co-evaporation process (Ramanathan et al., 2003). The three-stage process has not been implemented in industrial large area module production. However, other co-evaporation process variants are used for module production as large as 1.2 × 0.6 m² at Global Solar and Würth Solar (Powalla

Low cost processes

There are many criteria for characterizing low cost processes. CIGS layer deposition remains the challenging step for low cost cell processing, since it is the most important and complex layer of the cell. High material utilization efficiencies close to 100% are important in order to keep material costs and processing waste as small as possible. These low cost techniques generally follow the idea of the sequential techniques since a low temperature precursor deposition step precedes a chemical

Non-vacuum absorber formation techniques

Non-vacuum technologies use simple and low cost equipment and enable fast processing speed. But the lack of a high purity vacuum environment during processing has to be compensated with a careful choice of precursor materials and additives to avoid undesired contamination. As shown in Fig. 2, two different approaches evolved: In one the precursor materials decompose during deposition on the substrate and form the CIGS compound directly (spray pyrolysis, electrodeposition of compound layers)

Chemical spray pyrolysis

Although the spray pyrolysis technique is one of the best-investigated non-vacuum deposition processes (mainly for CuInS₂), only few results about cell performance were reported. To the best of our knowledge, the efficiency record dates back to 1989 (Duchemin et al., 1989) and is ∼5% for a small area cell. The spray technique to disperse and deposit a precursor solution is very well suited for uniform large area coating. The process consists of the decomposition and reaction of premixed

Electrodeposition

Electrodeposition has been successfully used to deposit quaternary CIGS films with a small area efficiency of over 10% after a subsequent thermal annealing (Guimard et al., 2003). By applying additional In and Ga and a high temperature annealing in vacuum, an efficiency of 15.4% was obtained (Bhattacharya et al., 2000). The stability of the chemical solution, large area uniformity and high deposition rates are still a challenge. Electrodeposition of an elemental-layer stack which was subjected

Paste coating

Typical paste coating techniques are screen printing, doctor-blade coating and curtain coating. Paste coating is a fast process and can be applied in continuous roll-to-roll deposition. The paste is prepared with the precursor materials (e.g. nanoparticles) in the desired stoichiometry and a liquid binder, used as a transfer media. Paste rheology can be tailored with material load and additives, which affect the final film thickness and homogeneity (Kapur et al., 2003a). In contrast to liquid

Conclusions and future prospects

The promising results from paste coating and selenization methods as well as electrodeposition have shown that non-vacuum deposition processes can be used for high quality CIGS absorber formation. After unsuccessful initial attempts with paste coating, the breakthrough followed only five years ago when non-vacuum processes with efficiencies above 10% were announced. In the mean time the maximum efficiency on soda-lime glass was increased to 13.6% and the process was successfully applied to

Acknowledgments

The funding by the Swiss Federal Commission for technology and Innovation is gratefully acknowledged. Further, the authors would like to thank Dr. V.K. Kapur from ISET for valuable discussions and also to Dr. Banger and Dr. Hepp from the NASA Glenn Research Center for allowing to use their illustration material in this paper.

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¹: Also at: Department of Electronic and Electrical Engineering, Loughborough University, UK.

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Low cost processing of CIGS thin film solar cells

Abstract

Introduction

Section snippets

Conventional CIGS deposition processes

Low cost processes

Non-vacuum absorber formation techniques

Chemical spray pyrolysis

Electrodeposition

Paste coating

Conclusions and future prospects

Acknowledgments

Thin Solid Films

Thin Solid Films

Thin Solid Films

J. Cryst. Growth

Thin Solid Films

Thin Solid Films

Solar Cells

Thin Solid Films

Thin Solid Films

Solar Energ. Mat. Solar Cells

Thin Solid Films.

Thin Solid Films

Solar Energ. Mat. Solar Cells

Comparison of material properties of CuInSe2 films produced by reaction of metallic alloys to H2Se/Ar and elemental Se vapour

Jpn. J. Appl. Phys.

Comparison of material properties of CuInSe₂ films produced by reaction of metallic alloys to H₂Se/Ar and elemental Se vapour