Impact of varied sulfur incorporation on the device performance of sequentially processed Cu(In,Ga)(Se,S)2 thin film solar cells

Abstract

In order to improve the performance of chalcopyrite Cu(In1 − x,Gax)(Se1 − ySy)2 solar cells, the implementation of a bandgap widening at the absorber/buffer interface via an increase of the [S]/[S + Se] ratio is investigated. In this work we examine industrially processed samples, which were fabricated via the deposition-reaction process with varied H2S pressure during rapid thermal processing (RTP). Precursors which were exposed to a crucial amount of sulfur during the RTP step resulted in samples with significantly improved device performance. The increase of the open circuit voltage by about 150 mV cannot sufficiently be explained by bandgap widening. Observation of a strongly reduced saturation current density and ideality factor in intensively sulfurized samples suggest subdued recombination via mid-gap defect states located in the space charge region. This hypothesis is supported by results of deep-level transient spectroscopy measurements, which show that in both samples a mid-gap minority defect is present albeit its concentration is about one magnitude larger in sulfur-poor samples. These results confirm that sulfur passivates recombination centers in the depletion layer and hence significantly increases the open circuit voltage and the overall device performance of the photovoltaic devices.