Evidence of Limiting Effects of Fluctuating Potentials on VOC of Cu(In, Ga)Se2 Thin-Film Solar Cells

In this paper we present a consistent theoretical approach and an extensive experimental study of Cu(In, Ga)Se-2- (CIGS-)based solar cells to investigate the influence of fluctuating potentials on the limitations of solar-cell performance. The absorptance is calculated for extensions to the Shockley-Queisser model involving the description of tail states under the Urbach-rule, optimal-fluctuation-theory, and bandgap-fluctuation models, as well as the expected values for the saturation current density, short-circuit current density, and open-circuit voltage (V-OC). Three CIGS-based solar cells with [Cu]/([Ga]+[In]) ratios of 0.53, 0.71, and 0.84 are grown to intentionally have sufficiently different amplitudes of fluctuating potentials. We show both theoretically and experimentally the role played by fluctuating potentials, in particular in the V-OC losses. We provide evidence for a higher degree of correlation of electrostatic fluctuating potentials with V-OC losses in comparison with band-gap fluctuations. Additionally, our results show the influence of fluctuating potentials not just at low temperature but also at room temperature.