First-principles investigation of CZTS Raman spectra

Abstract

${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$ (CZTS) is an earth-abundant photovoltaic absorber material predicted to provide a sustainable solution for commercial solar applications. However, the efficiency of such solar cells is rather limited, cation disorder being often designated as the culprit. Raman spectroscopy has been widely used to characterize CZTS. Nonetheless, the interpretation of the spectra in terms of the atomic-scale disorder is precluded by the lack of consensus between theoretical and experimental results. In particular, there is a strong discrepancy in the relative intensities of the two prominent $A$ phonon peaks of the spectra. In the present study, we demonstrate that the internal parameters characterizing the position of the S atoms strongly influence these intensities. We show that agreement with experiments can be completely recovered when adopting the geometry computed using a hybrid exchange-correlation functional. Finally, using special quasirandom structures, we demonstrate that the disorder only leads to a change of the shape of the Raman peaks (tailing or leading edges, shouldering and splitting). This could be exploited to assess the quality of the sample in terms of how ordered they are.