Energy level tuning of push-pull porphyrin sensitizer by trifluoromethyl group for dye-sensitized solar cells

We have introduced trifluoromethyl groups into meta-positions of two meso-phenyl rings of a push-pull-type porphyrin dye (ZnP-CF3) to modulate the energy levels of the porphyrin dye for dye-sensitized solar cells (DSSCs). The light-harvesting ability of ZnP-CF3 is almost comparable to those of reference porphyrins where the trifluoromethyl groups are replaced with methyl (ZnP-CH3) or tert-butyl groups (YD2). We revealed that the introduction of the electron-withdrawing trifluoromethyl groups is effective to lower the excited-state oxidation potential of the porphyrin (-0.80 V vs NHE), which is consistent with the theoretical calculation. Meanwhile, DSSCs with ZnP-CF3 exhibited a lower power conversion efficiency (eta) of 5.95% than DSSCs with ZnP-CH3 (eta = 7.33%) and YD2 (eta = 8.97%) because of the severe aggregation tendency of ZnP-CF3 arising from the trifluoromethyl groups. In addition, the insufficient steric bulkiness of the trifluoromethyl and methyl groups relative to tert-butyl groups would result in the lower short circuit current and open circuit voltage for ZnP-CF3 and ZnP-CH3 due to fast charge recombination between electrons in the conduction band of TiO2 and I-3-in the electrolyte solution. Overall, introducing both trifluoromethyl groups and bulky substituents into a porphyrin core would be necessary to boost cell performance.