Fill factor metastabilities in CIGSe-based solar cells investigated by means of photoluminescence techniques

In this paper we aim at a deeper understanding of fill factor (FF) deterioration under the absence of high energy photons in CIGSe-based solar cells (so-called “red kink” effect). In order to elucidate the mechanisms responsible for this phenomenon we take advantage of voltage dependent photoluminescence (PL-V) in combination with current voltage characteristics (I-V). Basing on a close correlation between PL-V and light I-V characteristics we show that the “red kink” effect is due to the redistribution of the electrical field in the absorber space charge region which in turn influences carrier collection. An important observation is that both, PL-V and light I-V, exhibit the same metastable behavior under light soaking (LS) and reverse bias treatment (REV). Since these metastabilities are caused by a defect charge state redistribution in the close-to-interface CIGSe layer (p+ layer), it allows us to locate the source of FF metastable behavior on the absorber side of the junction. Basing on numerical modeling of PL-V and light I-V characteristics we discuss presented results within a framework of existing models in which the redistribution of the negative charge in the window/buffer/absorber interface region has a direct influence on carrier collection. We show that the best agreement with experimental results can be achieved by a combination of a barrier at the window/buffer interface with the p+ layer model.