Structural and vibrational properties of cubic and orthorhombic SnS polymorphs for photovoltaic applications

SnS has attracted the attention of the PV community due to the combination of its desirable optical properties (direct band gap of 1.37 eV, and absorption coefficient of >105 cm-1) with binary and abundant elemental composition, which in theory simplifies its synthesis. However, currently the best SnS based PV device efficiency is 4.36 %. Limited performance is attributed to band gap alignment issues, doping content (controlled by Sn/S ratio) and poor film morphologies (due to the anisotropic structure). In this context Raman spectroscopy (RS) analysis could be useful, as it facilitates the accurate evaluation of material properties, as demonstrated in defect studies of other chalcogenide materials. We present an RS study, supported by XRD and XPS analyses, of cubic and α-SnS films grown by CVD single source precursor. In particular a complete analysis of SnS vibrational properties has been made using 7 excitation wavelengths (from UV to IR). The study describes, with high accuracy, the clear differences in Raman modes of both polymorphs: 7 or 4 peaks for cubic or α-SnS structures, respectively. Depending on the chosen excitation wavelength, pre-resonant behaviors are observed. Additionally the impact of various Sn/S ratios on Raman shift and band broadening has been evaluated and discussed. A clear RS feature dependence with composition has been observed. A methodology for accurate Sn/S ratio evaluation by RS is presented which will be relevant for SnS solar cell improvement.