Investigation of ZnO:Al Doping Level and Deposition Temperature Effects on CIGS Solar Cell Performance
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1012-Y01-08
Investigation of ZnO:Al Doping Level and Deposition Temperature Effects on CIGS Solar Cell Performance Joel N. Duenow1, Timothy A. Gessert2, David M. Wood1, Brian Egaas1, Rommel Noufi2, and Timothy J. Coutts2 1 Dept. of Physics, Colorado School of Mines, 1500 Illinois St., Golden, CO, 80401 2 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, 80401 ABSTRACT Cu(In,Ga)Se2 (CIGS) photovoltaic cells require a highly conducting and transparent top electrode for optimum device performance. ZnO thin films doped with 2 wt.% Al (ZnO:Al) are commonly used to ensure sufficient conductivity while providing acceptable transparency to the active absorber layers. Deposition of transparent conducting oxide (TCO) films on CIGS cells often is performed at room temperature in the manufacturing process because of production advantages. However, material properties and reproducibility may be poorer at room temperature than at higher temperatures. Maximum mobilities of 2 wt.%-doped ZnO:Al grown at room temperature in pure Ar are typically ~20-25 cm2V-1s-1. Relatively high carrier concentration is therefore required to achieve the desired conductivity. This high carrier concentration produces low infrared transmittance due to increased free-carrier absorption. World-record CIGS cells produced at the National Renewable Energy Laboratory (NREL) are known to tolerate photolithographic processing temperatures of ~100°C, though significant changes in device performance occur beyond 200°C. In this study, we investigate whether ZnO:Al films with superior material properties can be produced at the elevated temperatures consistent with CIGS processing parameters. We examine undoped ZnO and ZnO:Al with doping levels of 0.5, 1, and 2 wt.% Al2O3 for growth at substrate temperatures from 25° to 360°C using radio-frequency magnetron sputtering. For films grown in a 100% Ar ambient, optimal electrical and optical properties are achieved at ~150°-200°C. Controlled incorporation of H2 in the Ar sputtering ambient at 200°C increases mobility to 48 cm2V-1s-1 for undoped ZnO and 36 cm2V-1s-1 for 0.5 wt.%-doped ZnO:Al. Both values are considerably higher than the 25 cm2V-1s-1 of 2 wt.%-doped ZnO:Al deposited at 200°C in 100% Ar. We have explored whether these higher-mobility films can be exploited in the design of high-quality CIGS devices. Preliminary results show similar open-circuit voltage and only slightly lower short-circuit current compared to devices utilizing the standard 2 wt.%-doped ZnO:Al deposited at room temperature. This suggests that higher-performance devices may result once the TCO thickness is optimized for the higher mobility. INTRODUCTION Transparent conducting oxide (TCO) thin films are an integral part of photovoltaic (PV) cells, flat-panel displays, and electrochromic windows [1]. ZnO:Al is often used for Cu(In,Ga)Se2 (CIGS) cells because it offers acceptable electrical and optical properties when grown at the low temperatures preferred for the final steps of device processing. CIGS cells are known to degra
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