Non-uniformities of opto-electronic properties in Cu(In,Ga)Se2 thin films and their influence on cell performance studied with confocal photoluminescence

We have studied a series of high quality Cu(In,Ga)Se₂ thin film solar cells (efficiencies 15% - 18%) with spectrally resolved confocal photoluminescence (PL) which allows for a spatial resolution of ¿ 0.8 ¿m. The recorded PL data were analyzed by application of Planck´s generalized radiation law, which allows for quantification of values for the local splitting of quasi-Fermi levels and of the local threshold energy of optical absorption for each scan-pixel, corresponding to the local open circuit voltage (V_oc) in a solar cell and the optical band gap energy of the absorber, respectively. We quantify representative values for spatial variations of these parameters for each cell and discuss qualitatively as well as quantitatively the influence of local inhomogeneities on the resulting total cell quality in terms of V_oc and efficiency.