Phase Transformations and Crystalline Quality of CuInS2 Thin Films

Eveline Rudigier, Fuhs, Walther (Prof. Dr.), Physik
In recent years, due to the growing interest in alternative energies, photovoltaics has become an active field for research. Economically, thin film solar cells offer an interesting alternative. Cu-chalcopyrite semiconductor materials are used as absorber layers in heterojunction solar cell devices. These materials exhibit a direct band gap and absorb the sunlight within a layer of a few microns thickness due to their high absorption coefficients. Compared to the knowledge accumulated on
more » ... cumulated on classical semiconductors such as silicon, the understanding of the physical properties of chalcopyrite materials is rather limited. The chalcopyrite semiconductor CuInS2 (CIS) is particularly promising for photovoltaic applications as it is Se-free and its band gap (1.5 eV) is well adapted to the solar spectrum. Conversion efficiencies above 12 % on a laboratory scale have been demonstrated. In CuInS2-based thin film solar cells the photo collection and charge carrier generation mainly occurs in the p-absorber (CuInS2), whereas the n-type layer is transparent and only needed to form the pn-junction. The performance of CuInS2-based photovoltaic devices is currently limited by the open circuit voltage Voc. This value is significantly below the theoretical limit calculated for an ideal solar cell with a direct band gap of 1.5 eV. In order to overcome this limitation recent research has focused on modifications, such as the preparation of CuInS2 thin films with different Cu/In-ratios and with different doping elements. In the course of these investigations it has been found that both, different Cu/In-ratios as well as the presence of doping elements, have an influence on the structural and electronic properties. Theoretical calculations on polytypes and defect structures of chalcopyrites motivated the investigation of those crystalline phases in order to explain the dependence of the device performance on the composition. Previously, it has been found that in Cu-poor prepared CuInS2 films the so-called CuAu-phase is highly [...]
doi:10.17192/z2004.0638 fatcat:2bhzyjhbnrhqnlztyd3nov5t5i