Band gap fluctuations in Cu(In,Ga)Se2 thin films

Abstract

AbstractA simple statistical model describes measured absorption and photoluminescence data of Cu(In1-x, Gax)Se2 thin films. The broadening of the transition peak in the absorption spectra stems from band gap fluctuations. The extent of the spatial inhomogeneities as expressed in the standard deviation Eg μ reaches a maximum of Eg μ = 90 meV for films with equal amounts of indium and gallium, indicating alloy disorder as one possible source of the band gap fluctuations. The fluctuations observed lead to a decrease δVOC of the maximum possible open-circuit voltage VOC of almost 150 mV. However, the experimentally measured, low VOC of solar cells with high gallium content cannot be explained by band gap fluctuations alone. Consequently, our analysis suggests that the dominant recombination process in Cu(In1-x, Gax)Se2 thin film solar cells with high gallium content is not governed by the band gap energy, but is more likely due to deep levels within the forbidden gap.