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

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• 作者：Thomas J. Whittles ; Lee A. Burton ; Jonathan M. Skelton ; Aron Walsh ; Tim D. Veal ; Vin R. Dhanak
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：June 14, 2016
• 年：2016
• 卷：28
• 期：11
• 页码：3718-3726
• 全文大小：526K
• 年卷期：0
• ISSN：1520-5002

文摘

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 3d_5/2 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.