[Study on the modified surface layers of the CIGS thin films by Raman spectra]

In the present paper, the properties of Cu(In(1-x) Ga(x)) Se2 (CIGS) thin film absorber materials for the solar cells obtained by selenization of the precursors with In-rich or CuGa-rich surface layers were studied by XRD, SEM and Raman spectra. The photovoltaic devices based on the absorbers were measured and analyzed by illuminated J-V curve subsequently. The performance of the device constructed by the absorbers obtained by selenization of the precursors with CuGa-rich surface layer was improved greatly compared to that with In-rich surface layer. Through Raman spectra measurement, it was found that the Raman peak of the A1 mode was shifted for the CuGa-rich one, which is verified that the band gap of the surface layers was elevated. Moreover the value of increased Ga contents within the surface region of films were calculated by the relation between the Raman shifts and the Ga contents. As a result, the devices based on the thin films with the elevated surface energy band by selenizing the precursors with the CuGa-rich surface layer improved further the V(oc) and FF by about 74 mV and 8% respectively compared to that of corresponding to the one with In-rich surface layers, so that the conversion efficiency of the photovoltaic devices based on these thin films with CuGa-rich surface layer was improved by up to 9.4%. Meanwhile Raman scattering spectroscopy has proven to be a very powerful and useful technique for the surface analysis of such thin film solar cell semiconducuor materials.