Chalcopyrite-based thin-film solar cells with the semiconductor Cu(In, Ga)(S, Se2) (CIGSe) as absorber material show the highest efficiencies among thin-film photovoltaics in the laboratory as well as in module production (see Part 1). This indium based thin-film technology has a huge potential for low-cost photovoltaic production, but the scarcity of indium (indium’s abundance in the continental crust is estimated to be approximately 0.05 part per million). Indium can be jointly refined from trace concentrations in leading ores as zinc, copper, and lead or the material comes from recycled scrap. Increasing prices of indium could easily limit the production growth. There is also an increased demand for indium for use in other technologies, for example items such as: flat panel displays, solders, thermal-
interface materials, batteries, compound semiconductors and light-emitting diodes. On the other hand, the worldwide reserve of viable indium is approximately 16 kton.[1] Nevertheless, the price of indium has varied dramatically in recent years from US$94/kg in 2002 to over US$1,000/kg in 2006. The price in 2012/2013 was about US$580/kg [35]. This reflects the scarcity of supply and the dependence on the small number of production facilities worldwide.

In order to secure the long term development of compound semiconductor based thin-film solar cells, the search for the replacement of indium is advisable. Semi­conductors suitable as absorber materials in thin-film solar cells should fulfil a number of requirements, the most important amongst them being a high absorption-coefficient (in the range of 105 cm 🙂 and a band-gap energy in the optimal range of about 1.4 eV. Here multinary compounds, like solid solutions between non-isotype binary II-VI and ternary I-III-VI2 compounds, are potential candidates. Moreover, there are a variety of chalcogenide minerals available as a source of novel, indium-free absorber materials. Replacing indium in CuIn(S, Se)2 by the abundant elements zinc and tin yields the quaternary compound Cu2ZnSn(S, Se)4, which is also a direct band-gap p-type semiconductor with an absorption coefficient higher than 104 cm-1 [36].