β-Functionalized Imidazole-Fused Porphyrin-Donor-Based Dyes: Effect of π-Linker and Acceptor on Optoelectronic and Photovoltaic Properties

We have designed and synthesized three new donor (D)-linker (pi)-acceptor (A) type beta-functionalized imidazole-fused porphyrins, 4a-4c by grafting the acceptor such as cyanoacrylic acid/carboxylic acid at 2nd position of imidazole fused to the beta,beta' edge of porphyrin donor via pi-linker such as thiophene/phenylene and examined them as sensitizers for dye-sensitized solar cells (DSSCs). The structure-activity relationships of the dyes are systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. Dyes, 4a and 4b possessing cyanoacrylic acid acceptor exhibited broadened Soret bands compared with that of dye 4c containing carboxylic acid acceptor, which led to good light-harvesting properties and thereby broader IPCE curves. The DSSCs of the dye, 4a possessing thiophene linker and cyanoacrylic acid acceptor exhibit higher power conversion efficiency than the analogous dyes, 4b and 4c with phenylene pi-linker and cyanoacrylic acid or carboxylic acid acceptor. The hike in the efficiency comes from the increased short-circuit current and open-circuit voltage due to reduced dye aggregation. Electrochemical impedance spectroscopy studies reveal that the life-time of the injected electrons in the conduction band of TiO2 and charge recombination rate at the TiO2/dye/electrolyte interface mainly dependent on nature of pi-linker and acceptor of the dyes.