Bottom-up growth of hierarchical electrodes for highly efficient dye-sensitized solar cells

The nonconventional bottom-up growth of TiO2 was first demonstrated in the preparation of hierarchical TiO2 electrodes for use in highly efficient dye-sensitized solar cells. The simple immersion of a substrate in a precursor solution enabled the growth of TiO2 particulate films. Here, we have implemented a hierarchical growth strategy in which two stages of controlled growth yielded first macroscale TiO2 particles, followed by mesoscale TiO2 particles. We successfully fabricated electrode films up to 20 μm thick via a growth rate of 0.3 μm/min. The specific area of the electrodes was controlled via the deposition of mesoscale TiO2 particles. The deposited particles displayed a rutile phase with an average size of several tens of nanometers in diameter, as confirmed by XRD and high-resolution TEM imaging. After depositing the second layer of mesoscale TiO2 particles, the photocurrent density increased by a factor of 3. A maximum efficiency of 6.84% was obtained for the hierarchically structured TiO2 electrodes under 1 sun illumination. The hierarchical TiO2 electrodes were compared with macroporous TiO2 electrodes, revealing that the higher photocurrent density could be attributed to a longer electron recombination lifetime and a high specific area. The longer recombination lifetime was supported by the presence of fewer defective TiO2 surfaces, as confirmed by the XPS spectrum.