Effects of π-Elongation and the Fused Position of Quinoxaline-Fused Porphyrins as Sensitizers in Dye-Sensitized Solar Cells on Optical, Electrochemical, and Photovoltaic Properties

Novel benzo[f]quinoxalino[2,3-beta]porphyrin carboxylic acid (ZnBQA) and cyanoquinoxalino[6,7-beta]porphyrin carboxylic acid (ZnQCA) have been synthesized to evaluate the effects of the pi-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-beta]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 TiCl4-treated, TiO2 double layers was used. The ZnBQA cell exhibited a relatively high power conversion efficiency (eta) of 5.1%, while the ZnQCA cell yielded a low eta value of 0.80%. Both of the eta values are smaller than those of reference cells under the optimized conditions (eta = 6.3% for ZnQMA; eta = 8.4% for N719). The weak electronic coupling between the LUMO of ZnBQA and a conduction band (CB) of the TiO2 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 eta 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 TiO2 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 TiO2 to the I-3(-) derived from the loose packing of ZnQCA molecules on the TiO2 surface.