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-mu m-thick SnS thin films prepared via a two-step process involving (I) the deposition of metal precursor layers and (2) sulfurization at 400 degrees 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 similar to 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.