Copper indium gallium selenide (CIGS) based solar cells are receiving worldwide attention for solar power generation. They are efficient thin film solar cells that have achieved 22.8% efficiency comparable to crystalline silicon (c-Si) wafer based solar cells. For a production capacity of 1000 MW y−1 with 15% module efficiency, the CIGS module production cost is expected to be $0.34 W−1. For CIGS cells over glass, a graded bandgap high temperature deposition process has been established, however, this process has not been established for CIGS over flexible polymer substrates which is a low temperature process. For small area devices, the main focus is precise control over CIGS film stoichiometry and efficiency. For industrial production, apart from stoichiometry and efficiency, low-cost, reproducibility, high-throughput and process tolerance are of much importance in commercializing the technology. Due to process complexity, CIGS module production is lagging behind that of cadmium telluride (CdTe) modules. In this review article, the working mechanism of CIGS solar cells with a back surface field, the importance of developing CIGS solar cells, and the limitations for their commercialization are discussed. CIGS solar cells are compared with c-Si solar cells. After briefly reviewing the history of the chalcopyrite alloy system, graded bandgaps, effects of sodium distribution in CIGS layers, growth of CIGS layers using various techniques, role of buffer layer/transparent conducting oxides, CdS free buffer layers, concerns related to flexible solar cells, and factors affecting the cell efficiency are reviewed. Further efficiency improvement options are discussed. Cell stability, challenges, solutions and future prospects of CIGS solar cells are outlined.

Copper indium gallium selenide based solar cells – a review

Electrodeposition of semiconductors

Thin film heterojunction solar cells based on chalcopyrites such as Cu(In,Ga)Se2 have achieved impressive efficiencies. However concern about the long term sustainability of photovoltaics based on scarce or expensive raw materials has prompted the search for alternative absorber materials. In this work, films of the p-type absorber Cu2ZnSnS4 (CZTS) were prepared by electroplating metallic precursors sequentially onto a molybdenum-coated glass substrate followed by an nealing in a sulfur atmosphere. The polycrystalline CZTS films were characterized by photoelectrochemical methods, which showed films were p-type with doping densities of the order of 1016 cm–3 and a band gap of 1.49 eV, close to the optimum value for terrestrial solar energy conversion. Preliminary results obtained for solar cells fabricated with this material are promising. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

New routes to sustainable photovoltaics: evaluation of Cu2ZnSnS4 as an alternative absorber material

Polycrystalline thin films of copper indium diselenide and its alloys with gallium and sulphur (CIGS) have proven to be suitable for use as absorbers in high-efficiency solar cells. Record efficiency devices of 20% power conversion efficiency have been produced by co-evaporation of the elements under high vacuum. However, non-vacuum methods for absorber deposition promise significantly lower capital expenditure and reduced materials costs, and have been used to produce devices with efficiencies of up to 14%. Such efficiencies are already high enough for commercial up-scaling to be considered and several companies are now trying to develop products based on non-vacuum deposited CIGS absorbers. This article will review the wide range of non-vacuum techniques that have been used to deposit CIGS thin films, highlighting the state of the art and efforts towards commercialization.

Non‐vacuum methods for formation of Cu(In, Ga)(Se, S)2 thin film photovoltaic absorbers

In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.

A. Chaumont

Papers

Chalcopyrite thin film solar cells by electrodeposition