Subwavelength inhomogeneities in Cu(In,Ga)Se2 thin films revealed by near-field scanning optical microscopy

We have analyzed Cu(InGa)Se-2 thin films, fabricated by standard deposition technique for solar cells, by near-field scanning optical microscopy (NSOM) which allows for simultaneous detection of photoluminescence (PL) and AFM-like topography. In comparison with our former confocal microscopy studies this approach yields a lateral optical resolution of approximate to 200 nm which is substantially below the diffraction limit. The sample is investigated by a fiber probe which guides the laser excitation to the sample surface and collects the PL signal. The recorded scans show significant lateral variations of the PL yield on length scales of 0.2-1.5 mu m. By applying Planck's generalized law we extract lateral variations of the splitting of quasi-Fermi levels Delta(E-Fn-E-Fp) and optical threshold energies Delta E-op,E-th (band gap) in the absorber. We find substantial lateral variations of these properties with Delta(FWHM)(E-Fn-E-Fp) approximate to 17 meV (FWHM: full width half maximum), Delta(FWHM)(E-op,E-th) approximate to 18 meV and Delta(FWHM)(thickness) approximate to 220 nm. While no significant correlation between local surface topography and PL yield or Delta(E-Fn-E-Fp) is observed, we find a clear anticorrelation between the optical threshold energies and the recorded height signal. Our results extend previous studies of inhomogeneity effects on solar cell absorbers towards the 100 nm regime. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim