Charge transport characteristics of dye-sensitized TiO₂ nanorods with different aspect ratios

Nanocrystalline TiO₂ spherical particle (NP) with a dimension of 5 × 5.5 nm and several nanorods (NR) with different aspect ratios (diameter × length: 5 × 8.5, 4 × 15, 4 × 18 and 3.5 × 22 nm) were selectively synthesized by a solvothermal process combined with non-hydrolytic sol-gel reaction. With varying the molar ratio of TTIP to oleic acid from 1:1 to 1:16, the NRs in the pure anatase phase were elongated to the c-axis direction. The prepared NP and NRs were applied for the formation of nanoporous TiO₂ layers in dye-sensitized solar cell (DSSC). Among them, NR2 (TiO₂ nanorod with 4 × 15 nm) exhibited the highest cell performance: Its photovoltaic conversion efficiency (η) of 6.07%, with Jsc of 13.473 mA/cm², Voc of 0.640 V, and FF of 70.32%, was 1.44 times that of NP with a size of 5 × 5.5 nm. It was observed from the transient photoelectron spectroscopy and the incident photon to current conversion efficiency (IPCE) spectra that the TiO₂ films derived from NR2 demonstrate the longest electron diffusion length (Le) and the highest external quantum efficiency (EQE).