ZnO–TiO2 Nanocomposite Films for High Light Harvesting Efficiency and Fast Electron Transport in Dye-Sensitized Solar Cells

Abstract

Electron transport and recombination are the essential processes that determine the charge collection efficiency in dye-sensitized solar cells (DSSC). While nearly 100% of charges are collected in well-built ordinary DSSCs, this value can be sharply reduced by the use of redox couples other than iodide/triiodide due to fast electron recombination. To compensate, structures capable of fast electron transport are needed. Nanorod arrays that have this attribute tend to suffer from low surface area, resulting in low dye loading and reduced light harvesting. We have therefore developed a novel nanocomposite structure consisting of zinc oxide (ZnO) nanorods coated with titanium dioxide (TiO2) nanoparticles using an electrostatic layer-by-layer (LbL) deposition technique. The titanium dioxide nanoparticle coating can add an order of magnitude of surface area and is compatible with known high-performance dyes. This composite nanostructure has been designed to take advantage of the improved electron transport along the nanorods and surface area provided by the nanoparticles, yielding good charge collection and light harvesting. Transient measurements indicate that the composite film can transport electrons at least 100 times faster than a nanoparticulate TiO2 film. In tests using ferrocene/ferrocenium as a model alternative redox couple with fast recombination, current–voltage measurements indicate that the ZnO–TiO2 hybrid films generate much higher currents than conventional TiO2 nanoparticulate films. However, not all charges successfully transfer from TiO2 to ZnO due to an energy barrier between the materials.