The tetrapod-like ZnO nanopowders are employed to construct an efficient electron transport network as the photoanode of the dye-sensitized solar cells (DSCs). Due to the high extinction coefficient of the metal-free D149 dye offering a high photocurrent through the ZnO network, the best performance of DSCs based on a 42 µm terapod-like ZnO film showed a high energy conversion efficiency of 4.9% with a high short-circuit photocurrent density of 12.3 mA cm⁻², an open-circuit photovoltage of 0.6 V, and a fill factor of 0.65 under AM 1.5 irradiation. Highly efficient electron transport may also be ascribed by a long effective electron diffusion length of 46 µm determined from the electrochemical impedance spectroscopy which is consistent with thickness dependent J_SC measurements.

The recombination reaction of injected electrons with triiodide ion in the electrolyte limits the efficiency of dye-sensitized solar cells (DSSCs). This study reports the preparation of a sponge-like and conformal TiO₂ underlayer by hydrogen peroxide oxidation of Ti foil. This underlayer serves as a charge recombination barrier layer at the nanocrystalline TiO₂/substrate interface, and suppresses the recombination reaction. This sponge-like TiO₂ underlayer increases the electrical contact area between the Ti substrate and nanocrystalline TiO₂, helping nanocrystalline TiO₂ attach to the Ti substrate. This study compares the performance of DSSCs that were subjected to different Ti surface treatments. Electrochemical impedance spectroscopy results confirm that the proposed sponge-like TiO₂ underlayer increased the open-current voltage (V_OC) and fill factor (FF) due to prolonged electron life time (τ_eff), and minimized resistance at the TiO₂/Ti interface (R_CT). With an optimal thickness of nanocrystalline TiO₂ and concentration of I₂, we achieved a conversion efficiency of 6.75% for a back-illuminated DSSC.