Atomic layer deposition of Zn(O,S) buffer layers for high efficiency Cu(In,Ga)Se/sub 2/ solar cells

Cd-free solar cells based on Cu(In,Ga)Se/sub 2/, with efficiencies of up to 16.0%, are achieved by replacing the (CBD)CdS layer with a Zn(O,S)/ZnO bilayer deposited by ALD. Problems with reproducibility of the device results are observed and are found to be correlated with thickness variations of the Zn(O,S) layer, probably induced by differences of the CIGS surface. An ultra-thin Zn(O,S) layer, possibly not completely covering the CIGS surface, is observed by XPS analysis for high efficiency devices. Degradation of fill factor is observed on some, but not all, Zn(O,S) devices after about 2 months of storage, but these devices recover after light-soaking at elevated temperature. In order to improve the reproducibility, a new Zn(O,S) process is developed that includes longer ALD pulses. Using this new process, a promising average result of 12.1% (without AR coating) for 10 consecutive ALD runs is obtained with a maximum spread of /spl plusmn/1% unit.