Effects of 2 nd Phases, Stress, and Na at the Mo/Cu 2 ZnSnS 4 Interface
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Effects of 2nd phases, stress, and Na at the Mo/Cu2ZnSnS4 interface J.L. Johnson2, H. Nukala1, E.A. Lund3, W.M. Hlaing Oo1, A. Bhatia1, L.W. Rieth2, M.A. Scarpulla1,2 1 Materials Science and Engineering, University of Utah, Salt Lake City, UT 84112 2 Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112 3 Chemical Engineering, University of Utah, Salt Lake City, UT 84112 ABSTRACT Cu2ZnSnS4 (CZTS) is an alternative material to Cu(In,Ga)Se2 (CIGSe) for use in thin film photovoltaic absorber layers composed solely of commodity elements [1,2]. Thus, if similar material quality and performance can be realized, its use would allow scale-up of terrestrial thin film photovoltaic production unhindered by material price or supply constraints. Here we report on our research on the deposition of CZTS by RF sputtering from a single CZTS target and cosputtering from multiple binary sources on Mo-coated glass. We find some samples delaminate during post-sputtering furnace annealing in S vapor. Samples on borosilicate glass (BSG) delaminate much more frequently than those on soda-lime glass (SLG). We investigate the influences of the formation of frangible phases such as MoS2 at the CZTS/Mo interface and residual and thermal mismatch stress on delamination. We implicate fracture in a layer of MoS2 as the mechanism of delamination between the Mo and CZTS layers using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Wafer curvature measurements show significant (~400 MPa) deposition stress for minimally optimized Mo deposition; however nearly stress-free Mo layers with good adhesion can be deposited using a multi-step Mo deposition recipe. Co-sputtering CZTS adds 100 MPa of stress on both BSG and SLG, however delamination is nearly absent for samples deposited on low-stress Mo layers. We investigate metallic diffusion barrier layers to prevent the formation of MoS2 at the interface. Lastly we discuss the importance of removing Mo oxide by sputter etching before CZTS deposition and its effects on adhesion and series resistance. INTRODUCTION As is frequently mentioned, the elements in CZTS are earth abundant, low cost, non-toxic elements that allow the solar cell to scale up to large volume production. Here we report on the progress of our group in preparing CZTS films on Mo-coated glass from cosputtered precursor layers subsequently annealed in S vapor to form CZTS thin films. We concentrate herein on the deposition process and on the Mo/CZTS interface and their effects on resultant films. EXPERIMENTAL CZTS Single- and Multiple-Target Sputtering Most reports of CZTS thin film synthesis involve layers of Cu, Sn, and ZnS deposited serially followed by annealing in a sulfur atmosphere to cause reaction to form kesterite CZTS [1]. Cu is typically deposited first and diffuses very quickly through the stack during this interdiffusion process leaving Kirkendall voids at the CZTS/Mo interface [3]. It is also difficult to introduce stoichiometric amounts of S. Thus we chos
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