Investigation of ZnO:Al doping level and deposition temperature effects on CIGS solar cell performance

Cu(In,Ga)Se-2 (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 At are typically similar to 20-25 cm 2 V(-1)s(-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 similar to 100 degrees C, though significant changes in device performance occur beyond 200 degrees 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 degrees to 360 degrees C using radio-frequency magnetron sputtering. For films grown in a 100% Ar ambient, optimal electrical and optical properties are achieved at similar to 150 degrees-200 degrees C. Controlled incorporation of H-2 in the Ar sputtering ambient at 200 degrees C increases mobility to 48 cm(2)V(-1)s(-1) for undoped ZnO and 36 cm(2)V(-1)s(-1) for 0.5 wt.%-doped ZnO:Al. Both values are considerably higher than the 25 cm(2)V(-1)s(-1) of 2 wt.%-doped ZnO:Al deposited at 200 degrees C in 100% At. We have explored I 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.