Ga-doped ZnO transparent electrodes with TiO 2 blocking layer/nanoparticles for dye-sensitized solar cells
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NANO EXPRESS
Open Access
Ga-doped ZnO transparent electrodes with TiO2 blocking layer/nanoparticles for dye-sensitized solar cells Ji-Hong Kim, Kyung-Ju Lee, Ji-Hyung Roh, Sang-Woo Song, Jae-Ho Park, In-Hyung Yer and Byung-Moo Moon*
Abstract Ga-doped ZnO [GZO] thin films were employed for the transparent electrodes in dye-sensitized solar cells [DSSCs]. The electrical property of the deposited GZO films was as good as that of commercially used fluorine-doped tin oxide [FTO]. In order to protect the GZO and enhance the photovoltaic properties, a TiO2 blocking layer was deposited on the GZO surface. Then, TiO2 nanoparticles were coated on the blocking layer, and dye was attached for the fabrication of DSSCs. The fabricated DSSCs with the GZO/TiO2 glasses showed an enhanced conversion efficiency of 4.02% compared to the devices with the normal GZO glasses (3.36%). Furthermore, they showed better characteristics even than those using the FTO glasses, which can be attributed to the reduced charge recombination and series resistance. Introduction Dye-sensitized solar cells [DSSCs] have been recognized as an alternative to the conventional p-n junction solar cells because of their simple fabrication process, low production cost, and transparency. A typical DSSC consists of a transparent conductive oxide [TCO] electrode, a dye-sensitized oxide semiconductor nanoparticle layer, a liquid redox electrolyte, and a Pt counter electrode [1,2]. Generally, fluorine-doped tin oxide [FTO] is commonly used for DSSCs as TCO due to its good thermal stability. However, FTO films have some drawbacks including high cost, insufficient conductivity, and low optical transmittance. Therefore, new TCO materials are required to replace FTO glasses [3]. ZnO-based materials have emerged as a promising material for transparent electrodes in solar cell applications. Since undoped ZnO shows high resistivity owing to low carrier concentration, group-III elements are doped into ZnO. Among them, Ga-doped ZnO [GZO] has several advantages such as higher resistance to oxidation and less lattice deformation compared to the other materials [4,5]. Nevertheless, little effort has been spent on attempts to use GZO as TCO for DSSCs since the surface structure * Correspondence: [email protected] Department of Electrical Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul, 136-713, South Korea
of the ZnO-based materials may be destroyed when they are immersed in the acidic dye solution containing a Ru complex for a long time. Besides, the electrical conductivity of GZO films can deteriorate after thermal annealing at high temperature which is required to form the TiO 2 semiconductor nanoparticle layer [6]. In this paper, we suggest the use of GZO transparent electrodes with a TiO2 blocking layer for DSSCs. The TiO 2 blocking layer can protect the GZO electrodes from the acidic dye solution and the oxidation at high temperature. The use of a thin TiO2 blocking layer can also reduce the recombination of electrons at the electrode/electrolyte int
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