Improvement of electrical properties of metal oxides for application to solar cells

Abstract

It is necessary for transparent conducting electrodes used in dye-sensitized or perovskite solar cells to have high thermal stability which is required when $TiO_2$ is coated on the electrode. AZO films with their low-cost and good TCO properties are unfortunately unstable above 300 ℃ in air because of adsorbed oxygen. In this paper, the thermal stability of AZO films is enhanced by depositing an oxygen barrier on AZO films to block the oxygen. As the barrier material, $SnO_2$ is used due to its high heat stability, electrical conductivity, and transmittance. Moreover, when the $SnO_2$ is grown as amorphous phase, the protective effect become greater than the crystalline phase. The thermal stability of the amorphous-$SnO_2/AZO$ films varies depending on the thickness of the amorphous $SnO_2$ layer. Because of the outstanding oxygen blocking properties of amorphous $SnO_2$, its optimal thickness is very thin and it results in only a slight decrease in transmittance. The sheet resistance of the amorphous-$SnO_2/AZO$ film is 5.4 Ω $sq^{-1}$ after heat treatment at 500℃ for 30 min in air and the average transmittance in the visible region is 83.4%. The results show that the amorphous-$SnO_2/AZO$ films have thermal stability with excellent electrical and optical properties. Sputter-deposited $SnO_2$ films were adopted as the electron transporting layer for planar-heterojunction perovskite solar cells, and 18.8% power conversion efficiency was obtained under standard conditions $(AM 1.5 G radiation, 100 mW cm^{-2})$. Photovoltaic performance of devices varied with the sputtering conditions of $SnO_2$ ETLs. The open circuit voltage of devices increased as the oxygen proportion increased during the sputtering of $SnO_2$ ETLs due to the reduce oxygen related defects in $SnO_2$ films. The fill factor of devices increased as substrate temperature increased during $SnO_2$ sputtering due to the improved electron transport in $SnO_2$ films. Our study showed the possibility of obtaining highly efficient large area perovskite solar cells with sputter-deposited $SnO_2$ ETL. The efficiency of the planar structure could be further improved in the future through further passivation of the electron transporting layer by doping. Large-area devices $(substrate 25 cm^2, active area 15 cm^2)$ were fabricated using the sputter-deposited $SnO_2$ as ETLs and PCE of 9.1% was obtained. Further improvement of device efficiency could be achieved by changing deposition method of perovskite layer and HTM layer from spin-coating to other method which can fabricate highly uniform and pinhole-free films.