One-Step, Surfactant-Free Hydrothermal Method for Syntheses of Mesoporous TiO2 Nanoparticle Aggregates and Their Applications in High Efficiency Dye-Sensitized Solar Cells

A facile, one-step, template-less, surfactant-free hydrothermal process, using a metal salt as the precursor, is developed to prepare submicrometer sized mesoporous TiO2 nanoparticle aggregates (NPGs). The as-prepared TiO2 NPGs are crystalline of the anatase phase, with a high specific surface area of 166 m(2)/g, an average pore size of 8.9 nm, and an average NPG size of 840 nm. With these NPGs, a new form of composite photoanode, consisting of the mesoporous TiO2 NPGs and xerogels, is proposed for high efficiency dye-sensitized solar cells (DSSCs). TiO2 xerogels are incorporated into the TiO2 NPGs layer with an impregnation process to form the TiO2 NPGs/xerogels composite. A high power conversion efficiency of 8.41% is achieved for DSSCs based on the TiO2 NPGs/xerogels composite photoanode, representing a 38% efficiency boost over the efficiency Of 6.11% achieved with a P25 TiO2 based cell. The success of the present composite TiO2 nanostructure can be attributed to the effective utilization of the inter-NPG space with the infiltration of the TiO2 xerogels, the excellent structural connectivity within and across the NPG and xerogel domains for fast electron transport, the high specific surface areas of both the NPGs and xerogels for providing abundant dye adsorption for generation of photoinduced electrons, the formation of a TiO2 xerogel blocking layer on top of the photoanode substrate, and the submicrometer size of the NPGs for much improved light harvesting efficiency. This new type of composite photoanode, different from the 0D/1D nanostructure based ones, proves effective by taking structural advantages from both constituent nanostructures, the mesoprous NPGs and xerogels, and opens up a new way of thinking in the structural design of the photoanodes.