Electron Transport Dynamics in TiO₂ Films Deposited on Ti Foils for Back-Illuminated Dye-Sensitized Solar Cells

In this study, we examine the electron transport dynamics in TiO2 films of back-illuminated dye-sensitized solar cells. The TiO2 films are fabricated using electrophoretic deposition (EPD) and the conventional paste-coating (PC) of TiO2 nanoparticles on Ti-foil substrates. Intensity-modulated photocurrent spectroscopy reveals that red-light irradiation is more efficient than blue-light irradiation for generating photocurrents for back-illuminated cells. A single trapping-detrapping diffusion mode, without trap-free diffusion, reveals the electron transport dynamics involved in the backside illumination. The closely-packed EPD films exhibit a shorter electron transit time than does the loosely packed PC films. The porosity dependence of the electron diffusion rate is consistent with the 3D percolation model for metallic solid spheres. The EPD films possess longer electron lifetimes because of their smaller void fraction, which suppresses recombination with electrolytes. The EPD cells, which feature rapid electron transport and suppressed recombination in the TiO2 films, exhibit a maximum power conversion efficiency of 7.1%, which is higher than that of PC cells (6.0%). Because the distance between electron injection and collection is close to the film thickness and the transport lacks trap-free diffusion, the performance of back-illuminated cells is more sensitive to TiO2 film thickness and porosity than the performance of the front-illuminated cells. This study demonstrates the advantages of EPD-film architecture in promoting charge collection for high power conversion.