Combinatorial Synthesis and Characterization of Zinc-Tin-Oxide Transparent Conductors
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Combinatorial Synthesis and Characterization of Zinc-Tin-Oxide Transparent Conductors J.D. Perkins, J.A. del Cueto, J.L. Alleman, C. Warmsingh1, B.M. Keyes, L.M. Gedvilas, P.A. Parilla, B. To, D.W. Readey1 and D.S. Ginley. National Renewable Energy Laboratory, Golden, CO, 80401, U.S.A. 1 Colorado School of Mines, Golden, CO, 80401, U.S.A. ABSTRACT In this work, we discuss the combinatorial deposition and analysis of transparent conducting oxides along the ZnO-SnO2 composition tie line. Libraries were deposited by co-sputtering from ZnO and Sn-metal targets. The production and analysis of Zn-Sn-O libraries has already produced significant results, confirming the properties of the 2:1 (Zn2SnO4) region and revealing a second region of interest, the 1:1(ZnSnO3) composition. Subsequent film growth of these stochiometries by pulsed laser deposition has confirmed the potential for both of these stochiometries. Work is currently underway to optimize both Zn2SnO4 and ZnSnO3. Planned future work includes a combinatorial investigation of ternary metal systems such as Zn-In-Sn-O. INTRODUCTION In this work, we report on the current status of our combinatorial studies of the Zn-Sn-O system. First, we note that the reported conductivities for Zn-Sn-O transparent conducting oxides (TCOs) are quite low compared with well known simple TCOs such as In2O3:Sn (ITO) and ZnO:Al. In particular, for Zn2SnO4 σ ≈ 50 – 100 Ω-1-cm-1 [1,2] and for ZnSnO3 σ ≈ 250 Ω1 -cm-1 [3] whereas for ITO σ ≈ 104 Ω-1-cm-1 [4]. Nevertheless, our interest in Zn-Sn-O TCOs is three fold: First, Zn2SnO4 is an important interface layer in the CdS/CdTe world record polycrystalline solar cells (16.5% total-area efficiency) [5,6]; Second, ZnSnO3 has a very high electron work function of 5.3 eV which makes it attractive as a hole extracting electrode in polymer photovoltaics [1,7]; And third, Zn-Sn-O is a developmental step towards interesting multinary metal oxide systems such as Zn-Sn-In-O [8-11] and Zn-Sn-Cd-O [12]. COMBINATORIAL MATERIALS DEPOSITION AND CHARACTERIZATION The compositionally-graded combinatorial Zn-Sn-O TCO libraries were deposited using cosputtering from two targets (ZnO and Sn metal) onto glass substrates. The substrate size is either 2”x2” or 3”x3”. This large substrate size enables easy analysis of many different spots (i.e. compositions). The ZnO target is always RF sputtered but the metallic Sn target has been sputtered using either RF or DC modes. The substrate temperature is 250 °C during deposition and the libraries are generally post-deposition annealed ex-situ in flowing N2 at temperatures ranging from 400 to 700 °C. The Zn-Sn-O TCO libraries have been characterized using electron microprobe (EPMA) for metals stoichiometry, profilometry for thickness, 4-point IV for sheet resistance, UV/Vis/NIR optical transmission for optical properties and x-ray diffraction (XRD) for crystallinity and phase formation. S1.9.1
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