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 (π)-acceptor (A) type β-functionalized imidazole-fused porphyrins, 4 a-4 c by grafting the acceptor such as cyanoacrylic acid/carboxylic acid at 2^nd position of imidazole fused to the β,β′ edge of porphyrin donor via π-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, 4 a and 4 b possessing cyanoacrylic acid acceptor exhibited broadened Soret bands compared with that of dye 4 c containing carboxylic acid acceptor, which led to good light-harvesting properties and thereby broader IPCE curves. The DSSCs of the dye, 4 a possessing thiophene linker and cyanoacrylic acid acceptor exhibit higher power conversion efficiency than the analogous dyes, 4 b and 4 c with phenylene π-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 π-linker and acceptor of the dyes.