Enlightening the synergic effect of anatase/rutile mixtures in solar cells

The synergic effect, observed in dye sensitized solar cells assembled with mesoporous electrodes made of mixtures of anatase and rutile nanoparticles, was revealed to depend on a delicate balance of competing contributions from chemical capacitance and resistance to electron diffusion instead of a longer electron lifetime in TiO2 conduction band, as consequence of a slower electron recombination rate at the electrode/electrolyte interface. The parameters influencing the charge diffusion in actually running dye-sensitized solar cells (DSCs) were carefully evaluated by impedance spectra simulation and correlated with the percentage of rutile and anatase in the respective mesoporous electrode. Cells were assembled with mesoporous titanium dioxide (TiO2) electrodes fabricated with sixteen different anatase/rutile mixtures, ranging from pure anatase to pure rutile. All measurements were carried out in triplicate in order to give statistically validated results. The enhanced charge diffusion at 15% of rutile in the mixed film cells was found to be associated with the presence of isolated rutile islands embedded in a more conducting anatase phase, as confirmed by confocal Raman microscopy.