Fabrication and Characterization of Low-Cost, Large-Area Spray Deposited Cu 2 ZnSnS 4 Thin Films for Heterojunction Sola
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Fabrication and Characterization of Low-Cost, Large-Area Spray Deposited Cu2ZnSnS4 Thin Films for Heterojunction Solar Cells Sandip Das, Kelvin J. Zavalla, M. A. Mannan, and Krishna C Mandal* Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA.
ABSTRACT Large-area Cu2ZnSnS4 (CZTS) thin films were deposited by low-cost spray pyrolysis technique on Mo-coated soda-lime glass (SLG) substrates at varied substrate temperatures of 563-703°K. Deposition conditions were optimized to obtain best quality films and effect of post deposition thermal processing of the as-deposited films under H2S ambient were investigated. Structural, morphological, and compositional characterization of as-deposited and H2S treated CZTS absorber layers were carried out by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw, and Hall-effect measurements. Films grown at ~360°C substrate temperature showed superior optoelectronic properties, improved stoichiometry and smoother morphology compared to films grown at much higher or lower temperatures. Film properties were significantly improved after the H2S processing. Our results show that large area high quality CZTS films can be fabricated by lowcost spray pyrolysis technique for high throughput commercial production of CZTS based heterojunction solar cells. INTRODUCTION High efficiency, economically sustainable solar cells using earth-abundant and non-toxic materials is necessary to meet TW-scale photovoltaic power generation predicted by 2030 [1]. Current state-of-the-art thin film solar cell technologies based on CuInGaSe2 (CIGS) and CdTe absorbers have reached record efficiencies of 20.3% and 17.3% [2-3]. However, use of toxic Cd in CdTe solar cells threatens the environmental health hindering mass production and supply limitations of scarce and expensive In, Ga in CIGS cells are expected to limit the production capacity of these chalcogen-based solar cells in near future. Therefore, it is necessary to explore new solar absorber materials consisting of earth-abundant, environment-friendly and cheaper constituent elements. Recently, Cu2ZnSnS4 (CZTS) has emerged as a potential candidate as an alternative to existing CIGS and CdTe absorbers in thin film solar cells [4-6]. All constituent elements in CZTS are abundant in earth’s crust (Cu: 50 ppm, Zn: 75 ppm, Sn: 2.2 ppm, S: 260 ppm) compared to CIGS/CdTe (Cd: 0.11 ppm, In: 0.049 ppm, Ga: 18 ppm; Te: 0.005 ppm), much cheaper and are non-toxic. CZTS is an excellent absorber material with an ideal direct bandgap (Eg ~1.5 eV at 300K), large optical absorption co-efficient ( >104 cm-1) and has recently achieved a record efficiency of 11.1% for Se enriched CZTS [6]. Ito and Nakazawa first observed the photovoltaic effect in a CZTS/CTO heterodiode with an open-circuit voltage (VOC) of 165 mV [7]. They prepared the CZTS thin film by atom beam
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