Effects of meso-Diarylamino Group of Porphyrins as Sensitizers in Dye-Sensitized Solar Cells on Optical, Electrochemical, and Photovoltaic Properties

We have prepared meso-bis(diarylamino)-substituted porphyrins (cis-ZnP and trans-ZnP), meso-diarylamino-substituted porphyrin (mono-ZnP), and porphyrin reference without the meso-diarylamino group (ZnP) to evaluate the effects of substituent number and position of the diarylamino groups on the optical, electrochemical, and photovoltaic properties of the porphyrins for the first time. With increasing the number of the diarylamino groups, the light-harvesting properties were improved in the visible region. The optical and electrochemical HOMO-LUMO gaps were parallel to those estimated by DFT calculations. The photovoltaic properties were compared under the optimized conditions in which a sealed device structure with TiCl4-treated, TiO2 double layers was used. The power conversion efficiency (eta) was in the order of the trans-ZnP (3.8%) < ZnP (4.4%) < cis-ZnP (5.5%) < mono-ZnP (6.5%) sensitized TiO2 cells. The low light-harvesting efficiency (LHE) rather than the high adsorbed photon-to-current efficiency (APCE) in the ZnP-sensitized solar cell leads to the moderate short circuit current (J(SC)), whereas both the high APCE value and the moderate LHE value in the mono-ZnP-sensitized TiO2 cell result in the highest J(SC) value. Considering the similarity in the LHE values of the trans-ZnP and cis-ZnP-sensitized cells, the APCE values correlate the J(SC) values. The open circuit potentials (V-OC) of the trans-ZnP and cis-ZnP-sensitized TiO2 cells are lower than those of the mono-ZnP and ZnP-sensitized TiO2 cells. Overall, the difference in the J(SC) and V-OC values of the four TiO2 cells leads to that in the eta values. The basic information obtained here will be useful to design molecular structures of porphyrins for highly efficient dye-sensitized solar cells.