Cu(In,Ga)Se2 solar cells with a ZnSe buffer layer: interface characterization by quantum efficiency measurements

Abstract

We investigate Cu(In,Ga)Se 2 -based solar cells with a new ZnSe buffer layer deposited by metal-organic vapour deposition and compare their electronic properties to reference cells using a standard CdS buffer layer. The best solar cell with a ZnSe buffer layer achieves an efficiency of 11.6%. We further investigate a large series of solar cells with varied thickness of both types of buffer layers by means of quantum efficiency measurements in equilibrium and under light and voltage bias. The characterization of the devices concentrates on the analysis of the collection of photogenerated holes from the buffer layer. We introduce a new method to determine the recombination probability of holes at the buffer/absorber interface. We find a similar interface recombination probability of about 40% for both devices, those with a ZnSe buffer layer and those with a CdS buffer layer. An anomalous enhancement of the quantum efficiency measured under current bias is ascribed to a barrier modulation effect which is caused by light absorbed in the buffer layer.