Composite thin films prepared by electrophoresis using two types of TiO<sub>2</sub> nanoparticles

Abstract

Anatase titanium dioxide (TiO2) films have attracted interest as a cathode material for dye-sensitized solar cells (DSSCs). In order to improve the power conversion efficiency (PCE) of DSSCs, it is necessary to create long electron conduction paths in the film and increase both the specific surface area and light harvesting efficiency (LHE) for incident light. To achieve these, the deposition of TiO2 composite thin films by constant current electrophoresis was attempted using colloid mixtures of two types of TiO2 nanoparticles with different sizes and surface properties: ca. 5 nm TiO2 nanoparticles synthesized in this study (TNPs) and commercial TiO2 nanoparticles of ca. 20 nm (P25) size. The size of TNPs is too small for visible light wavelengths and it is difficult to deposit thick films with TNPs, so the LHE of these films is low. Various TNP:P25 ratios were investigated to enhance the LHE and PCE for DSSCs. The P25 particles form a matrix structure in the thin film, and the TNPs are incorporated into this structure and fill the spaces between adjacent P25 nanoparticles. The upper limit of film thickness achieved without the film separating from the substrate increased with the mass content of P25. The LHE of the TiO2 composite thin films was improved and could easily be controlled by adjusting the mass ratio of TNP:P25. The optimal PCE for DSSCs was obtained using TiO2 composite thin films deposited using a TiO2 colloid mixture with a TNP:P25 mass ratio of 2:7.