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



We have analyzed Cu(InGa)Se2 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 ≈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 μm. By applying Planck's generalized law we extract lateral variations of the splitting of quasi-Fermi levels Δ(EFnEFp) and optical threshold energies ΔEop,th (band gap) in the absorber. We find substantial lateral variations of these properties with ΔFWHM(EFnEFp) ≈ 17 meV (FWHM: full width half maximum), ΔFWHM(Eop,th) ≈ 18 meV and ΔFWHM(thickness) ≈ 220 nm. While no significant correlation between local surface topography and PL yield or Δ(EFnEFp) 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. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)