Reactive magnetron sputtering of CuInS2 absorbers for thin film solar cells: Problems and prospects

We have investigated the reactive magnetron sputtering process from copper and indium targets in Ar/H2S sputtering atmospheres for the deposition of CuInS2 absorber films. This large area deposition method is in principle suited for the preparation of absorbers of good electronic quality. One of our best CuInS2 cells exhibits an efficiency of 11.4% and an open circuit voltage of 745 mV, the highest reported value for CuInS2 cells. However, it was found that the reproducibility of the photovoltaic layer properties is low. By analyzing the different defects in the films, it was found that the main defects are microscopic pin holes and crevices, which lead to short circuits. The intrinsic electronic quality of the magnetron sputtered CuInS2 films is excellent, proven by room temperature photoluminescence spectroscopy. The occurrence of short circuits depends decisively on the film morphology, which is determined by the copper-to-indium ratio during the deposition. Even for constant external deposition parameters this ratio changes over the life time of the targets, mainly caused by a significant decrease of the indium deposition rate. For a stable CuInS2 film deposition process an in situ control of the Cu/In ratio and the film morphology is advisable. (C) 2008 Elsevier B.V. All rights reserved.