Low-temperature TiO₂ Films for Dye-sensitized Solar Cells: Factors Affecting Energy Conversion Efficiency

Semiconductor films prepared by electrostatic layer-by-layer deposition can be used to fabricate dye-sensitized solar cells after low-temperature treatment (150 °C). However, the resulting photocurrent is much less than when the film is sintered at 500 °C. The difference in short-circuit current is a factor of 2.2 with the Ru-based dye N719 and is 3.5 with the organic dye D5. The photocurrent at a given wavelength is proportional to the light-harvesting efficiency, charge injection effciency, and charge collection efficiency. Sintered films take up more than 60% more of either dye than unsintered films and therefore absorb more photons. Electron injection is hindered in unsintered films due to a conduction band edge potential 100 mV more negative than in a sintered electrode. Additional injection effects could be due to adsorption of the dye to polymer rather than to TiO 2 in unsintered films, although our measurements were inconclusive on this point. Kinetic studies show electron transport times (τ tr ) an order of magnitude faster then electron lifetimes (τ e ) in both sintered and unsintered electrodes. Furthermore, a Li + insertion experiment shows that both films have good electrical connectivity between TiO 2 nanoparticles. Unsintered films thus exhibit efficient charge transport despite the presence of polymer and the lack of heat treatment to induce necking.