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

We analyze Cu(In,Ga)Se-2 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 (E-Fn - E-Fp). which determines the local open circuit voltage (V-oc) of the polycrystalline cell. These results can be extracted from spectrally resolved PL scans across several tens of microns. Excitation fluxes amount to 10(2) - 5 x 10(4) suns equivalent at 83-300 K. We analyze the statistical distribution of the occurring fluctuations of (E-Fn - E-Fp) 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 (E-Fn - E-Fp) which can be derived from non-laterally resolved, calibrated PL-studies at 300 K and 1 sun equivalent on comparatively large areas (1 mm(2)). The latter ones provide access to the spatially averaged PL-yields (Sigma Y-PLxi) and their respective quasi-Fermi level splitting ((E-Fn - E-Fp)similar to In(Sigma Y-PLxi)), while the average of the (E-Fn - E-Fp) from laterally resolved measurements reads (Sigma ln(Y-PLxi)). 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. (C) 2008 Elsevier B.V. All rights reserved.