Single-step organic vapor phase sulfurization synthesis of p-SnS photo-absorber for graded band-gap thin film heterojunc
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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-ZnO1-x Sx Faruk Ballipinar1,2, Alok C. Rastogi1,2 1 Electrical and Computer Engineering, Binghamton University, SUNY, Binghamton, New York 13902 USA 2 Center for Autonomous Solar Power (CASP), Binghamton University, SUNY, Binghamton, New York 13902 USA ABSTRACT 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. INTRODUCTION The quest for low cost, non-toxic and earth abundant photo-absorbers as an alternative to toxic Cd in cadmium telluride (CdTe) or scarce In and Ga in copper indium gallium (di)selenide (CIGS) have spurted research in new materials systems for thin film solar cells. Current favorite is Cu2ZnSnS4 (CZTS), but due to composition dependent narrow stability range for photovoltaic properties, simple binary chalcogenide semiconductors have generated renewed interest. Tin sulfide (SnS), a potential photo-absorber among others FeS2 and ZnSnP2, is being actively investigated. SnS is a p-type semiconductor (p~1015 -1017 cm-3), has a direct band gap of 1.4-1.8 eV and an indirect band gap of 1.0-1.3 eV with high absorption coefficient (>104 cm-1) utilizing near optimum solar radiation with thin absorber in solar cell [1]. Commonly, SnS films are deposited by direct methods like electrodeposition, atomic layer deposition, chemical bath deposition, vacuum evaporation, sputtering and chemical spray pyrolysis [2]. Tin sulfide compounds exist in different phases such as SnS2 and Sn2S3 however all of these compounds are semiconductor materials and reveal p or n type conductivit
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