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

Semiconductor films prepd. by electrostatic layer-by-layer deposition can be used to fabricate dye-sensitized solar cells after treatment at 150°. However, the resulting photocurrent is less than when the film is sintered at 500°. The difference in short-circuit current is a factor of 2.2 with the Ru-based dye N719 and is 3.5 with the org. dye D5. The photocurrent at a given wavelength is proportional to the light-harvesting efficiency, charge injection efficiency and charge collection efficiency. Sintered films take up >60% more of either dye than un-sintered films and therefore absorb more photons. Electron injection is hindered in un-sintered films due to a conduction band edge potential 100 mV more neg. than in a sintered electrode. Addnl. injection effects could be due to adsorption of the dye to polymer rather than to TiO2 in un-sintered films, although the measurement results were inconclusive. Kinetic studies show electron transport times an order of magnitude faster then electron lifetimes in both sintered and un-sintered electrodes. Also, a Li+ insertion expt. shows that both films have good elec. connectivity between TiO2 nanoparticles. Un-sintered films thus exhibit efficient charge transport despite the presence of polymer and the lack of heat treatment to induce necking.