The Effect of Substrate Wettability on Hydrazine-Processed CZTS Thin Films
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The Effect of Substrate Wettability on Hydrazine-Processed CZTS Thin Films Zhaoning Song, Adam B. Phillips, Yao Xie, Rajendra R. Khanal, Jon M. Stone, and Michael J. Heben Wright Center for Photovoltaics Innovation and Commercialization, School for Solar and Advanced Renewable Energy, Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio 43606, U.S.A. ABSTRACT We report a study on the wetting and spreading of hydrazine-CZTS solution on a series of solid surfaces. The work of adhesion between a hydrazine solution and soda-lime glass, Si, graphite, ITO, SnO2, ZnO, CdS, In2S3, Cu, Au, Ag, Al, Ni, Mo, and carbon single-walled nanotubes was calculated using observed contact angles and the areas of the interface. The surface roughness of drop-casted CZTS precursor films was lower on surfaces with better hydrazine wettability. This suggests that the surface roughness of solution-processed films can be controlled by altering the wetting behavior of the solution on the substrate. INTRODUCTION The hydrazine-based spin coating technique has proven to be an effective method for fabricating Cu chalcogenide thin films and devices. Efficiencies of 15% for CuInGaSe2 (CIGS) [1] and 11% for Cu2ZnSn(S,Se)4 (CZTS) [2] solar cells have been reported. During the solutionbased process, drops of hydrazine solution interact with the substrate, wet, spread, and form a coherent film as the solvent evaporates. One of the crucial factors that affect the film’s quality and device performance is the wettability of the hydrazine-CZTS solution on the substrate. For instance, coating a Cu chalcogenide film on a non-wetted soda-lime glass surface has been proven to be much harder than that on a completely wetted Mo surface. In order to study the wettability behavior of hydrazine-CZTS solution, and develop an approach to deposit smooth and continuous film on a glass substrate, the wetting behavior of hydrazine solution on a series of surfaces and the subsequent film morphology were investigated. All of the surfaces we selected are of importance to the thin film solar cell industry and the study of the wetting behavior and adhesion of hydrazine solution on those surfaces is critical to guide the innovation of Cu chalcogenide thin film solar cells. EXPERIMENTAL DETAILS To characterize the wettability and adhesion of hydrazine-CZTS solution on various substrates, the work of adhesion between the liquid and the solid surface was measured using a static contact angle method in a nitrogen-filled glove box. In this research, a 0.4 M hydrazineCZTS solution was prepared by dissolving 1.2 M Cu2S (99.99%, Sigma Aldrich), 0.57 M SnSe (99.999%, Alfa Aesar) and 0.70 M Zn (99.99%, Sigma Aldrich) in anhydrous hydrazine (98%, Sigma Aldrich) following the procedures reported by others [2, 3]. Excessive chalcogen was added to guarantee the full dissolution of Cu and Sn constituents, and to facilitate the formation of dispersed ZnSe(N2H4) nanoparticles [4]. As a result, the hydrazine-CZTS precursor solution contains both dispersed nanoparticles and
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