Copper indium disulfide solar cell absorbers prepared in a one-step process by reactive magnetron sputtering from copper and indium targets

Abstract

Reactive magnetron sputtering from copper and indium targets in argon-hydrogen sulfide mixtures has been used to prepare CuInS 2 absorbers for thin film solar cells. Compared with the sequential process, where sputtered metal films are sulfurized, this technique offers the advantage of a one-step process. The influence of substrate temperature, total sputtering pressure and the copper-to-indium ratio on structure and morphology was investigated. The argon-to-hydrogen sulfide ratio was fixed at 1:1. Lower sputtering pressure and higher substrate temperature lead to larger crystal grains. The films were prepared copper-rich and exhibited only the CuInS 2 (roquesite) and the CuS (covellite) phases. Due to the phase separation, known from the growth of copper-rich CuInS 2 films, the metallic CuS phase on top of the as grown films could be removed by wet-chemical etching. As in the case of the sequential process or of coevaporated films, the copper-to-indium ratio has a strong influence on grain size and electronic properties. First solar cells prepared with these absorber films exhibited a maximum efficiency of 6.4% at AM1.5 illumination. The diffusion length in the first cells prepared from reactive magnetron sputtered absorbers are three to four times lower than that of coevaporated CuInS 2 films. However, this ion assisted deposition process offers the possibility to prepare absorber films at lower temperatures compared with a thermally activated process, like sulfurization or coevaporation. Furthermore, the presented results are a further step towards a continuous vacuum process for CuInS 2–thin film solar cell preparation.