ZnO Thin Film Deposition for TCO Application in Solar Cell

ZnO is a well-known suitable candidate for the Transparent Conducting Oxide (TCO) layer of thin film compound solar cells. In this paper we have discussed the deposition of ZnO thin film on glass substrate by reactive DC magnetron sputtering using oxygen as a reactive gas. Samples are prepared by varying oxygen flow rates during the deposition process. After deposition, samples are annealed at 300°C for 2 hours in vacuum environment. All the properties of the film are measured before and after annealing. All the samples are tested for the optical transparency, band gap, and electrical resistivity before and after annealing. Band gap of film is observed to be 3.2？eV. XRD and SEM measurements of the samples show the variation in the crystal structure and surface morphology of the film with varying oxygen flow rate and annealing also. Around 600？nm thick ZnO film with ？？·cm resistivity and 80% transparency without any doping is achieved. 1. Introduction ZnO has attracted attention as Transparent Conducting Oxide (TCO) because of its large band gap (3.3？eV), low resistivity ( ？？·cm), abundance, and nontoxicity. Optoelectric properties of ZnO depend on deposition and postdeposition treatment conditions as these properties change significantly with the conditions like nature of chosen doping element, the adsorption of oxygen during the deposition process, film deposition temperature, and postdeposition annealing atmosphere [1]. Singh et al. [1] has shown that the lowest resistivity was found at 300°C deposition in oxygen environment. Our aim is to show that if the film is annealed at 300°C after deposition in reducing environment, it would show much lower resistivity (~10？3？？·cm). Magnetron sputtering is a well-known technique of depositing ZnO thin film [2]. In the present study, we have investigated the effect of oxygen partial pressure during film deposition by DC magnetron sputtering technique and annealing in reducing environment after deposition. The purpose of this work is to determine the deposition and treatment conditions that can produce highly conducting and transparent ZnO film for thin film solar cell application. 2. Experimental Figure 1 shows the schematic of the experimental setup for magnetron sputtering deposition. Distance between the target and substrate was ~15？cm. Base pressure of the system was ？mBar. Total operating pressure during deposition was ？mBar. Deposition is done in a mixture of argon and oxygen gases. Zn target of 3′′ diameter, 5？mm thick, and 99.999% purity is used for sputtering. Deposition power was around ~275？W.