Light Absorption and Emission in Semiconductors with Band Gap Fluctuations—a Study on Cu(In,Ga)Se2 Thin Films

This work investigates the influence of lateral fluctuations of the fundamental band gap on the macroscopic light absorptance and emission spectra of spatially inhomogeneous semiconductors. A model assuming a Gaussian distribution for the local band gaps yields closed-form expressions for the spectral absorptance and emission. Band gap fluctuations broaden the absorption edge of the fundamental band gap, as well as the associated emission peak. The spectral position of the photoluminescence emission peak depends on the length scale of the fluctuations in relation to the characteristic charge carrier transport length. We apply the model to experimental results from Cu(In1−x,Gax)Se2 thin films routinely used as photovoltaic absorbers in thin-film ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. The films feature band gap fluctuations with standard deviations between 15 and 65 meV which would lead to losses in the range of 5–80 mV for the open circuit voltage of solar cells made from these films. The pure ternary compounds CuInSe2 and CuGaSe2 exhibit smaller standard deviations than their quaternary alloys. This experimental finding indicates alloy disorder as one possible source of band gap inhomogeneities. The length scale of the observed fluctuations turns out to be much smaller than the minority carrier diffusion length. Hence, the band gap fluctuations occur on a length scale below 100 nm.