We have prepared meso-bis(diarylamino)-substituted porphyrins (cis-ZnP and trans-ZnP), meso-diarylaminosubstituted 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 TiCl 4 -treated, TiO 2 double layers was used. The power conversion efficiency (η) was in the order of the trans-ZnP (3.8%) < ZnP (4.4%) < cis-ZnP (5.5%) < mono-ZnP (6.5%) sensitized TiO 2 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 TiO 2 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 TiO 2 cells are lower than those of the mono-ZnP and ZnP-sensitized TiO 2 cells. Overall, the difference in the J SC and V OC values of the four TiO 2 cells leads to that in the η values. The basic information obtained here will be useful to design molecular structures of porphyrins for highly efficient dye-sensitized solar cells.
Novel benzo[f]quinoxalino [2,3-β]porphyrin carboxylic acid (ZnBQA) and cyanoquinoxalino [6,7-β]porphyrin carboxylic acid (ZnQCA) have been synthesized to evaluate the effects of the π-elongation and the fused position of quinoxaline-fused porphyrins on the optical, electrochemical, and photovoltaic properties. ZnBQA showed a split, red-shifted Soret band relative to that of quinoxalino[2,3-β]porphyrin acid (ZnQMA), while the Q bands are rather blue-shifted. On the other hand, both Soret and Q-bands of ZnQCA are red-shifted compared to those of ZnQMA. The optical HOMO-LUMO gaps are consistent with those estimated by density functional theory calculations. The photovoltaic properties were compared under the optimized conditions in which a sealed device structure with TiCl 4 -treated, TiO 2 double layers was used. The ZnBQA cell exhibited a relatively high power conversion efficiency (η) of 5.1%, while the ZnQCA cell yielded a low η value of 0.80%. Both of the η values are smaller than those of reference cells under the optimized conditions (η ) 6.3% for ZnQMA; η ) 8.4% for N719). The weak electronic coupling between the LUMO of ZnBQA and a conduction band (CB) of the TiO 2 may result in the low electron injection efficiency as well as the low incident photon-to-current efficiency (IPCE) for the ZnBQA cell (maximum IPCE ) 60%) relative to the ZnQMA cell (maximum IPCE ) 75%), leading to the lower η value of the ZnBQA cell than that of the ZnQMA cell. The ZnQCA cell exhibited the further low IPCE value up to 10% due to the short fluorescence lifetimes (0.2, 5 ps) that are comparable to the typical time scale (0.1-10 ps) of electron injection processes from a porphyrin excited singlet state to a TiO 2 surface. In addition, the open circuit potential of the ZnQCA cell also significantly decreased by the effect of the charge recombination from the injected electrons in the CB of the TiO 2 to the I 3 derived from the loose packing of ZnQCA molecules on the TiO 2 surface.
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