Ionic Liquid Electrochemical Synthesis of Earth Abundant Monophase Chalcostibite p-CuSbS 2 Photo-absorber Thin films for
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Ionic Liquid Electrochemical Synthesis of Earth Abundant Monophase Chalcostibite p-CuSbS2 Photo-absorber Thin films for Heterojunction Solar Cells With n-ZnO Nishanth R. Janardhana1,2, Navjot Kaur Sidhu1,2, Ratheesh R. Thankalekshmi,2, Alok C. Rastogi1,2 1
Electrical and Computer Engineering Department, Center for Autonomous Solar Power (CASP), Binghamton University, State University of New York, Binghamton, NY 13902, U.S.A 2
ABSTRACT Single step synthesis of monophase CuSbS2 thin films by electro-deposition in ionic liquid electrolyte based on choline chloride and urea (ChCl:U) eutectic mixture is described. The formation of binary CuxS and SbxSy film phases using CuCl2 and SbCl3 precursors along with Na2S2O3 as sulfur source in ChCl:U are established as -0.59 V and -0.36 V vs. Pt, respectively by cyclic voltammetry and used to optimize CuSbS2 thin films growth potential and precursor composition. CuSbS2 films deposited at -0.65 V vs Pt with 1:1 Cu to Sb precursor ratio at 80⁰C are highly crystalline in chalcostibite orthorhombic structure. Deviant Cu/Sb ratio at 1:0.71 and 1:1.4 reveal inclusion of Cu3SbS3 and Sb2S3, respectively. Direct 1.65 eV band gap for single phase CuSbS2 film and with inclusive secondary phases at 1.73±0.1 eV and 2.13 eV is observed. As-deposited CuSbS2 films are p-type and n-p hetero-junction device in the n-ZnO/p-CuSbS2/Ag structure shows rectifying I-V curves and dependence on the CuSbS2 film growth conditions. INTRODUCTION The CuSbS2 belonging to simple ternary Cu-Sb-S system is a potential semiconductor material among the earth abundant and non-toxic photo-absorbers for low-cost thin film solar cells [1]. The crystalline structure and optoelectronic properties of CuSbS2 based on theoretical and single crystal studies show that CuSbS2 has a direct band gap 1.38-1.56 eV and high absorption coefficient, 1-5x104 cm-1 both relevant for fabrication of efficient photovoltaic device [1]. Due to sensitivity of the electronic and optical properties including the band gap to the CuSbS2 thin film preparatory methods, its application as photo-absorber in thin film solar cells has not been fully explored. The lack of unanimity on the nature of optical band-gap and p-type doping is due to the variation in the film structure and composition, especially secondary phase CuS, Sb2S3 and Cu3SbS4 inclusions in CuSbS2 films synthesized by direct methods, chemical spray or vacuum deposition and indirect methods, sulfurization of Cu and Sb layers in S or H2S vapor [2, 3]. In chemical spray deposition, precursor composition, additives and spray temperature are critical to the exclusion of secondary phases. CuSbS2 synthesis by thermal evaporation produces mixed CuSbS2 and Sb2S3 phases and low crystallinity reflecting on the optical band-gap. CuSbS2 films formed by chalcogenization of Cu/Sb stacks and Cu-Sb alloyed film in the presence of S, Se vapor [4], require annealing rate control and individual layer thickness to avoid binary phase separation.
In this work, we report on a single step synthesis CuSbS2 fi
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