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Single-step organic vapor phase sulfurization synthesis of p-SnS photo-absorber for graded band-gap thin film heterojunction solar cells with n-ZnO 1-x S x

Tin sulfide has emerged as a promising solar absorber among the IV-VI binary compound which is earth-abundant and non-toxic. This research provides a new perspective on synthesis of photosensitive monophasic SnS films by organic chemical vapor sulfurization of Sn thin film. S-radicals formed by closed space pyrolysis of di-tert-butyl disulfide (TBDS) diffusively react with Sn to produce SnS film. SnS being an amphoteric semiconductor converts to n-type by trivalent Sb and Bi dopants. The organic vapor sulfurization method described in this research facilitates single-step synthesis of buried junction structures and thus SnS solar cells in a p-n homojunction or p-i-n structures. In this work, vacuum evaporated Sn thin film with a thickness of 100 nm, was converted to SnS by sulfurization under 100 sccm flow of TBDS vapor preheated to 100°C and structural phase evolution and film growth kinetics were investigated for sulfurization at 200°C, 300°C and 400°C for a periods 90 min. X-ray diffraction studies establish single phase highly crystalline film in orthorhombic crystal structure forms at 200°C. Raman scattering results confirm SnS formation with the identification of 2Ag, 2B2g optical phonons modes. Optical bandgap studies confirm a low energy 1.1-1.4 eV indirect bandgap and a strong absorption threshold between 1.4 to 1.6 eV direct band gap depending on the sulfurization conditions correlating with intrinsic defects and phase structure of the film.