Band gap fluctuations in Cu(In,Ga)Se_2 thin films

摘要

A simple statistical model describes measured absorption and photoluminescence data of Cu(In_(1-x), Ga_x)Se_2 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 σ_(Eg) reaches a maximum of σ_(Eg) = 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 ΔV_(OC) of the maximum possible open-circuit voltage V_(OC) of almost 150 mV. However, the experimentally measured, low V_(OC) 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(In_(1-x), Ga_x)Se_2 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.