Photoluminescence and Raman spectroscopy on doped CuInS2 thin films for solar cells

Doping with group II elements in particular Zn and Mg leads to open circuit voltages in CuInS2 based solar cells of around 800 mV which is nearly 100 mV higher than the values found for the undoped cells from the same device process, while maintaining similar efficiency. To investigate the effect of dopants on the defect chemistry and crystal structure we applied photoluminescence PL and Raman spectroscopy. From PL spectroscopy we find a new, broad emission band at E 1.35 eV for CuInS2 films doped with Zn or Mg. Intensity and temperature dependent PL measurements indicate that this emission is strongly influenced by potential fluctuations in contrast to the commonly found donor acceptor pair recombination in undoped CuInS2 samples. From the Raman spectroscopy study we find a change in the structural properties originating from the dopants for the same set of samples. While E1 LO B1 2 LO modes cation anion vibrations are strongly increased with doping, it appears that the symmetric A1 mode pure anion vibrations of the sulphur lattice is not visibly influenced by the Zn incorporation. We also investigated the influence of Zn incorporation on Cu poor absorber layers. We find that doping of In rich films reduces significantly the defect ordering in the lattic