Naphthyl-Fused π-Elongated Porphyrins for Dye-Sensitized TiO2 Cells

Novel unsymmetrically π-elongated porphyrins, in which the naphthyl moiety is fused to the porphyrin core at the naphthyl bridge with a carboxyl group ( fused-Zn-1 ) or at the opposite side of the phenyl bridge with a carboxyl group ( fused-Zn-2 ), have been synthesized to improve the light-harvesting abilities in porphyrin-sensitized solar cells. As the results of π-elongation with low symmetry, Soret and Q bands of fused-Zn-1 and fused-Zn-2 were red-shifted and broadened, and the intensity of Q-band relative to that of Soret band was enhanced. The fused-Zn-1 and fused-Zn-2 -sensitized TiO 2 solar cells showed the power conversion efficiencies (η) of 4.1% and 1.1%, respectively, under standard AM 1.5 conditions. The η value of the fused-Zn-1 cell was improved by 50% compared to the reference cell using unfused porphyrin ( Zn-1 ). The fused-Zn-1 -sensitized cell revealed high IPCE (incident photon-to-current efficiency) values of up to 55%, extending the response of photocurrent generation close to 800 nm. Thus, the improved photocurrent generation of the fused-Zn-1 -sensitized cell relative to the Zn-1 -sensitized reference cell is responsible for the remarkable difference in the η values. The η value of the fused-Zn-2 cell was much lower than that of the fused-Zn-1 cell. DFT calculations disclosed that there are significant electron densities on the carboxyl group in the LUMO of fused-Zn-1, whereas there are little electron densities on the carboxyl group in the LUMO of fused-Zn-2 . Accordingly, the larger electronic coupling between the porphyrin and the TiO 2 surface in the fused-Zn-1 -sensitized cell may be responsible for the high cell performance, due to the efficient electron injection from the porphyrin excited singlet state to the conduction band of the TiO 2 electrodes. To further improve the cell performance, 5-(4-carboxylphenyl)-10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphyrinatozinc(II) ( Zn-3 ), possessing different light-harvesting properties, was coadsorbed with fused-Zn-1 onto an TiO 2 electrode. Under the optimized conditions, the cosensitized cell yielded maximal IPCE value of 86%, short circuit photocurrent density of 11.7 mA cm −2, open-circuit voltage of 0.67 V, fill factor of 0.64, and η of 5.0% under standard AM 1.5 conditions.