Local fluctuations of absorber properties of Cu(In,Ga)Se 2 by sub-micron resolved PL towards “real life” conditions

Abstract

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 × 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 (∑ Y PL, xi ) and their respective quasi-Fermi level splitting (( E Fn − E Fp)~ ln(∑ Y PL, xi )), while the average of the ( E Fn − E Fp) from laterally resolved measurements reads (∑ln( Y PL, 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.