Local inversion of photocurrent in cadmium telluride solar cells

Abstract

Variable wavelength and voltage OBIC maps of different CdTe devices with intermediate lifetime (∼2 years) have evidenced the anomalous formation of negative photocurrent areas in forward bias conditions. These features have remarkable dimensions in the order of some millimetres, characteristic spectral distribution in different region of the spectrum and sub-bandgap generation tails. The maps were traced at wavelengths ranging from 350 to 1050 nm, with a spot size of approximately 300 μm, and a table resolution of 150 μm. An analysis of these phenomena is rather complicated, and only partially understood. However, the analysis of apparent quantum efficiency provides a qualitative picture for the formation of these reverse currents and deterioration of local and combined IV characteristics. There are two possible causes for this inversion, namely distributed series resistance and light-modulated barrier phenomena. In particular, the latter can be induced by diffusion of species within the device and related compensation effects, determining degradation and fundamental efficiency limits in CdTe solar cells.