Development of double-layered transparent conducting oxides with thermal stability for dye-sensitized solar cells

Abstract

During the fabrication of a thin film photovoltaic device, the transparent conducting oxide (TCO), which constitutes an integral part of its basic structure, could be exposed at high temperature processes. Aluminum doped zinc oxide (AZO) is a well-known TCO with good properties in terms of light transparency and conductivity, however its electrical properties deteriorate after subsequent high temperature processes in air atmosphere. In this work, Al doped ZnO (AZO) films were prepared on glass substrate by RF magnetron multi-target sputtering. 2 wt% Al-doped ZnO target (purity 99.99%) was used for the deposition. Al:ZnO films with high transmission and low resistivity were obtained for the samples fabricated along the c-axis. AZO films which have good TCO properties were fabricated by adjusting and optimizing the target-to-substrate distances. Deposition conditions were optimized by adjusting target-to-substrate distances and as a result, the electrical properties of AZO films were improved. Firstly, Al:ZnO films were fabricated with various target-to-substrate distances at the substrate temperature of 450 oC. The optimal condition for deposition of Al:ZnO films with the lowest sheet resistance of 1.25 Ω/□ and a transmission toward 75 % in the visible range was obtained at 150 W and 3.5 cm. Then, the films were annealed at 500 oC in air atmosphere for 30 min in order to study their thermal stability. The sheet resistance increased drastically to 3.3 kΩ/□ after annealing due to oxygen vacancies annihilation. The SnO2/AZO double-layered transparent conducting oxide was presented in order to reach thermal stability and improve other characteristics. After the deposition of Al:ZnO, SnO2 films were consecutively deposited by RF magnetron multi-target sputtering at the optimized condition. (sputtering power: 150 W, the distance of target and substrate: 3.5 cm, Substrate temperature: 450 oC) SnO2 target (purity 99.99 %) was used for the deposition. The SnO2 has poo...