Energetic factors governing injection, regeneration and recombination in dye solar cells with phthalocyanine sensitizers

Phthalocyanines (Pcs) are promising candidates for photon collectors for the near-infrared region of the solar spectrum in dye solar cells (DSC). Using two new Pc sensitizers, differing only in their Zn or metal-free center, we discuss the behaviour of the Pc dyes as sensitizers in DSC, and their influence on the solar cell performance, in comparison with standard N719 dye, at identical electrolyte conditions. Based on the separate identification of recombination resistance and the energy levels of titania by impedance spectroscopy, we determine the recombination rates and we also map the energies of the relevant components of the solar cells. An integration of the absorbance of the three sensitizers with the solar spectral distribution shows that electron injection should be similar for all the dyes (although 20% less in the case of the Pc with Zn center). However, since the LUMO level of these Pcs is low on the energy scale, it is necessary to use an electrolyte that keeps the conduction band of titania down to obtain some injection from the Pcs. Hence, the photovoltage is limited because the conduction band of titania is at a much lower position than in normal DSCs. We find that the conduction band of titania with the metal-free Pc is located on a similar energy level as for N719, and the recombination rate is not significantly different from the N719 cell. The main reason for the lower performance of this Pc is the lower injection than the ruthenium complex. In the case of the ZnPc, the conduction band is at higher energy than for N719. This allows that the V-oc obtained in these two samples becomes nearly the same despite the lower electron injection in the Zn phthalocyanines.