Electron transport patterns in TiO₂ nanocrystalline films of dye-sensitized solar cells

This study reports synthesis and characterization of nanoparticles for fabricating the TiO₂ nanocrystalline films used in dye-sensitized solar cells: phase-pure anatase nanoparticles from a titanate-directed route, and brookite (27%) and rutile (1.2%)-containing anatase nanoparticles from a sol-gel route. After nanoparticle-necking into films, X-ray diffraction pattern simulation shows that the defect density of the anatase (AN) films is less than that of the brookite/rutile-containing anatase (AN-br) films. The defect states in the AN-br films lower the short circuit current and conversion efficiency of the resulting solar cells. Intensity-modulated photocurrent/photovoltage spectroscopic analysis demonstrates electron transport in trap-free and trap-limited diffusion modes and shows that the defects serve as electron trap state to retard electron transport for collection and increase the traveling time prior to recombination. Impedance analysis shows that the trap-free mode extends the electron diffusion length in TiO₂ films and its contribution magnitude governs the electron collecting efficiency.