Influence of copper excess on the absorber quality of CuInSe2

The compositional dependence of the optoelectronic quality of CuInSe2 thin film absorbers is investigated on single-and poly-crystalline films with varying [Cu]/[In]-ratios. We quantify the quality of the absorbers by the splitting of quasi-Fermi levels, determined by spectral photoluminescence. This quantity determines the maximum achievable open circuit voltage by an absorber. Our results indicate a significant increase of this value for growth under Cu-excess, indicating a decrease of recombination losses. By comparison of the predicted achievable open circuit voltage and the actually measured ones of finished solar cells, we find a huge un-utilized potential for the Cu-rich devices. (C) 2011 American Institute of Physics [doi:10.1063/1.3648111]