Single Phase Formation of SnS Competing with SnS₂ and Sn₂S₃ for Photovoltaic Applications: Optoelectronic Characteristics of Thin-Film Surfaces and Interfaces

Tin monosulfide (SnS) is one of the most promising binary compounds for thin-film solar cells owing to its suitable optical properties and abundance in nature. However, in solar cells it displays a low open circuit voltage and power conversion efficiency owing to multiphases in the absorber layers. In this study, we investigated approximately 1.2-μm-thick SnS thin films prepared via a two-step process involving (1) the deposition of metal precursor layers and (2) sulfurization at 400 °C. To investigate the phase variations inside the thin films we employed a dimpling method to get a vicinal cross-section of the sample. Kelvin probe force microscopy, conductive atomic force microscopy, and micro-Raman scattering spectroscopy were used to characterize the local electrical and optical properties of the sample. We studied the distribution of the Sn-S polytypes in the film and analyzed their electrical performances for solar cell applications. The work functions of SnS and SnS₂ were determined to be 4.3-4.9 and ∼5.3 eV, respectively. The local current transport properties were also measured; they displayed an interesting transition in the conduction mechanism, namely from Ohmic shunt current at low voltages to space-charge-limited current at high voltages.