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 degrees C). However, the resulting photocurrent is much less than when the film is sintered at 500 degrees 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 TiO2 in unsintered films, although our measurements were inconclusive on this point. Kinetic studies show electron transport times (tau(tr)) an order of magnitude faster then electron lifetimes (tau(e)) in both sintered and unsintered electrodes. Furthermore, a Li+ insertion experiment shows that both films have good electrical connectivity between TiO2 nanoparticles. Unsintered films thus exhibit efficient charge transport despite the presence of polymer and the lack of heat treatment to induce necking.