Brookite-Based Dye-Sensitized Solar Cells: Influence of Morphology and Surface Chemistry on Cell Performance

The transport and recombination properties of dye sensitized solar cells based on phase-pure anatase and brookite nanomaterials are compared as a function of the surface chemistry and morphology. Phase-pure brookite has been synthesized from amorphous TiO2 using two different solutions at low and high pH, resulting in different size and morphology of brookite nanoparticles. The smaller short-circuit current density (J(SC) = 6.6 mA cm(-2)) for acidic brookite compared to anatase (9.8 mA cm(-2)) was related to the light harvesting efficiency because of the lower amount of dye adsorbed. However, a larger open-circuit voltage for acidic brookite indicates the promise of the material. The basic brookite-based solar cells gave a very low J(SC) (0.10 mA cm(-2)), which increased dramatically by a factor of about 30 after an acid treatment of the films, illustrating the effect of surface chemistry. A combination of experiments shows that the improvement is related to an increase in injection efficiency. Electrochemical impedance and intensity-modulated photocurrent and photovoltage spectroscopies show that electron transport is faster in the acid-treated basic brookite nanomaterial, related to the larger feature sizes. However, the recombination kinetics is also significantly faster, with as net result a smaller diffusion length and hence smaller collection efficiency.