A Study of Increased Resistivity of FTO Back Contact for CZTS Based Absorber Material Grown by Electrodeposition-Anneali
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A Study of Increased Resistivity of FTO Back Contact for CZTS Based Absorber Material Grown by Electrodeposition-Annealing Route Prashant K. Sarswat1, Michael L. Free1 and Ashutosh Tiwari2 1
Metallurgical Engineering, University of Utah, Salt Lake City, UT, United States.
2
Materials science and engineering, University of Utah, Salt Lake City, UT, United States.
ABSTRACT CZTS (Copper zinc tin sulfide) thin films have been synthesized on transparent conducting oxide (TCO) back contacts on a glass substrate, allowing sun light to pass through the entire solar cell. Aqueous solution based co-electrodeposition followed by elevated temperature sulfurization, was used to grow CZTS on transparent fluorinated tin oxide. Loss in conductivity of FTO is observed after sulfurization, causing reduced device efficiency. Increased resistivity of the FTO is likely due to outdiffusion process. A systematic study of resistivity of back contact at various sulfurization temperatures and times is discussed. Various remedial measures for improved conductivity of back contact were proposed and conducted. INTRODUCTION CZTS is a direct band gap material with a band gap of approximately 1.5 eV, and it has a very high absorption coefficient [1, 2]. In most thin film CZTS solar cells, molybdenum is used as back contact [1, 2]. Molybdenum has several advantages over other materials, but it is not transparent. Thus, it is not very useful for bifacial and tandem solar cell applications, which require light transmittance through the solar cell [3]. On the other hand, use of FTO allows transmission that facilitates bifacial and tandem solar cells. Thus, synthesis of CZTS on FTO is an alternative methodology to fabricate semitransparent solar cells as well as monofacial devices [3]. Fluorinated tin oxide (FTO) is one of the ceramic materials used in various electronic and optical applications including solar cells [4]. FTO is very useful because of high transparency and conductivity and can be prepared by various techniques such as reactive thermal evaporation, spray pyrolysis, chemical vapor deposition and sputtering [4-5]. Among the available TCOs, FTO is the leading material candidate because it is relatively chemically inert, mechanically hard, and high temperature resistant [3-7]. A device using FTO as a back contact and CZTS as an absorber material can be fabricated, so that semitransparent as well as bifacial solar cells can be made. Though the proposed TCO backed structure has also been reported for CIGS based cells, it can also be used for CZTS based solar cells [8]. However, despite several advantages of the proposed TCO/CZTS solar cell, elevated temperature sulfurization causes enormous loss of conductivity of the FTO back contact, which results in reduced performance [8]. The loss of conductivity was systematically studied, and processing parameters were optimized in this study. The optimization includes temperature, time of sulfurization, and geometry of sulfurization. Remedial measures to enhance conductivity included indirect and
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