Local fluctuations of absorber properties of Cu(In,Ga)Se2 by sub-micron resolved PL towards “real life” conditions

We analyze Cu(In,Ga)Se2 absorber layers for solar cells in a confocal microscope setup by photoluminescence (PL) experiments. We present results on lateral inhomogeneities of absorbers in terms of local fluctuations of the splitting of quasi-Fermi levels (EFn − EFp), which determines the local open circuit voltage (Voc) of the polycrystalline cell. These results can be extracted from spectrally resolved PL scans across several tens of microns. Excitation fluxes amount to 102 − 5 × 104 suns equivalent at 83-300 K. We analyze the statistical distribution of the occurring fluctuations of (EFn − EFp) which we plot in histograms, seemingly showing Gaussian-like shapes. The width of these - showing substantial dependence on excitation flux and temperature - has been extrapolated towards 1 sun equivalent light fluxes. Furthermore, we use these results to correct the absolute values (EFn − EFp) which can be derived from non-laterally resolved, calibrated PL-studies at 300 K and 1 sun equivalent on comparatively large areas (1 mm2). The latter ones provide access to the spatially averaged PL-yields (∑YPL,xi) and their respective quasi-Fermi level splitting ((EFn − EFp)~ ln(∑YPL,xi)), while the average of the (EFn − EFp) from laterally resolved measurements reads (∑ln(YPL,xi)). We show a comparison of the two magnitudes and thus strongly appeal for sufficient high spatial resolution for a consistent quantitative interpretation of luminescence experiments.