Investigation of ZnO:Al Doping Level and Deposition Temperature Effects on CIGS Solar Cell Performance

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 produced at NREL. 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.