Anomalous temperature dependence of the open‐circuit voltage of InxSy‐buffered Cu(In,Ga)(Se,S)2 solar cells simulated in broad temperature range

We investigated an open-circuit voltage phenomenon of Cu(In,Ga)(Se,S)(2) thin-film solar cells observed by device simulations which is manifested in an anomalous temperature dependence (meaning deviating from the linear dependence caused by the temperature dependence of the saturation current) induced by a variation of the buffer electron affinity. In order to study the origin of this effect, we performed simulations of temperature-dependent current-voltage characteristics while we separately consider thermionic emission and the classical drift-diffusion model as the decisive charge carrier transport across the heterointerfaces. The anomalous open-circuit voltage phenomenon is in general an electrostatic effect caused by accumulated negative charge at the window/buffer interface created by photo-generated electrons within the buffer, which partially reach the i-ZnO layer. This results from a suppress electron transport from the window to the buffer layer. This electron accumulation induces an additional electric field which hinders the elevation of the hole quasi-Fermi energy within buffer and absorber layer and therefore increases the quasi-Fermi level splitting which finally leads to an increased open-circuit voltage. In total, we identified three different temperature regimes for the open-circuit voltage and will explain its origin in this work. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim