Band Alignments, Valence Bands, and Core Levels in the Tin Sulfides SnS, SnS2, and Sn2S3: Experiment and Theory

Abstract

Tin sulfide solar cells show relatively poor efficiencies despite attractive photovoltaic properties, and there is difficulty in identifying separate phases, which are also known to form during Cu2ZnSnS4 depositions. We present X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy measurements of single crystal SnS, SnS2, and Sn2S3, with electronic-structure calculations from density functional theory (DFT). Differences in the XPS spectra of the three phases, including a large 0.9 eV shift between the 3d5/2 peak for SnS and SnS2, make this technique useful when identifying phase-pure or mixed-phase systems. Comparison of the valence band spectra from XPS and DFT reveals extra states at the top of the valence bands of SnS and Sn2S3, arising from the hybridization of lone pair electrons in Sn(II), which are not present for Sn(IV), as found in SnS2. This results in relatively low ionization potentials for SnS (4.71 eV) and Sn2S3 (4.66 eV), giving a more comprehensive explanation as to the origin of the poor efficiencies. We also demonstrate, by means of a band alignment, the large band offsets of SnS and Sn2S3 from other photovoltaic materials and highlight the detrimental effect on cell performance of secondary tin sulfide phase formation in SnS and CZTS films.