ZnO/InxSy/Cu(In,Ga)Se2 solar cells fabricated by coherent heterojunction formation

This paper reports on physical vapour deposition (PVD) of indium sulphide buffer layers in polycrystalline ZnO-window/buffer/Cu(In,Ga)Se2-absorber thin film solar cells. The coevaporation of In and S and the compound-evaporation of In2S3-powder are compared. The production of highly efficient solar cells is only possible under well-defined indium-sulphide growth conditions resulting in high structural order at the buffer/absorber interface and in good process homogeneity and reproducibility. In the present work, these conditions could only be guaranteed for the compound-evaporation technique. Post-annealing of the whole cells in air or vacuum affects mainly the window/buffer interface and leads to a significant increase of open-circuit voltage, fill factor, and efficiency of the compound-evaporated cells. The highest efficiency obtained so far is 14.8% without antireflective coating, a value comparable to our CdS-buffer-based solar cells. This way, physical vapour deposition of indium sulphide has demonstrated its potential for being integrated in coherent in-line fabrication of Cu(In,Ga)Se2-based solar cells.