Quantitative measurements of electronic nonuniformities in thin film photovoltaics and their impact on device performance

Thin film photovoltaics, such as those based on CdTe and CIGS, exhibit both intentional and unintentional spatial variations in electronic properties which can have a profound impact on device performance. Here, we expand upon the techniques of electroluminescence (EL) and electron beam induced current (EBIC) to quantify these electronic nonuniformities. We experimentally observe that EL images of CIGS solar cells grown by co-evaporation at the National Renewable Energy Laboratory (NREL) can be strongly affected by secondary diodes if the cells are imaged in the dark. However, by using ultraviolet (UV) light during the measurement, it is possible to minimize the effect of these diodes and image electronic nonuniformities present under operating conditions. Using a UV offset, we find that variations in EL intensity across the cells can be attributed to a laterally nonuniform bandgap and carrier collection length. We show these two effects can be separated and quantified in some cases using EBIC. However, our experiments also show that for the set of CIGS samples studied, the minority carrier diffusion length in the quasi-neutral bulk is not correlated with the open circuit voltage of the device indicating separate recombination processes dominate carrier collection and forward current in these cells. Based on these results, we highlight important differences between the interpretation of EL images of CIGS solar cells compared to silicon cells and identify areas for further research.