Photovoltaic Properties of Nano-particulate and Nanorod Array ZnO Electrodes for Dye-Sensitized Solar Cell

Gratzel-type dye-sensitized solar cell (DSSC) has been considered as a new candidate for the next solar cell device in the near future. Its distinct colorful and transparent features as well as the possibility of low-price production allow much extended applications of DSSC compared to conventional silicon solar cells. Nanocrystalline semiconducting electrode in DSSC, usually made of anatase TiO2, is the one of essential component of this device, which serves as a supporting transparent film for dye-molecules and then receives the photoelectrons injected from dye-molecules and furnishes them adequate diffusion path to the back-contact. Among various kinds of semiconducting materials with photovoltaic properties, anatase TiO2 has been known as the best due to good matching of conduction band (CB) edge with LUMO level of ruthenium dye-molecule. Considering that CB edge of ZnO has nearly the same energy level with that of TiO2, 7,8 ZnO should have shown a good photovoltaic performance like TiO2 or even more enhanced performance, in that ZnO has a wide conduction band originated from the strong covalent interaction of Zn-O through sp-hybridization while CB of TiO2 consists of narrow π3d-2p component. 9 Despite such good intrinsic electronic properties of ZnO, however, simple nanocrystalline ZnO electrodes were found to have very poor photovoltaic properties with solar energy conversion efficiency, η < 1% under 1 sun condition, and recently reported nanostructured ZnO electrode showed slightly improved values 1.5-3.9%. Such low photovoltaic properties of ZnO are mainly related with the unstable surface chemistry of ZnO to acidic dye molecule. In this regard, we prepared two kinds of ZnO electrodes, a sintered nano-pariculate electrode and an oriented nanorod electrode and investigated their photovoltaic properties.