DFT and TDDFT investigations of photovoltaic properties of triphenylamine based dyes: Impact of π-spacer tuning for DSSC with iodine and copper based redox shuttle

A series of dyes with a donor-π-acceptor (D-π-A) framework based on triphenylamine (TPA) were theoretically designed to enhance the photovoltaic performance of dye-sensitized solar cells (DSSCs). Based on an experimentally reported TPA dye containing a benzo[c][1,2,5] thiadiazole(Dye1) π-spacer, five new dyes were modelled by including benzo[c]thiophene(Dye2), 4,4′-bibenzo[c]thiophene(Dye3), 2H-benzo[d][1,2,3]triazole(Dye4), 2H,2′H-4,4′-bibenzo[d][1,2,3]triazole(Dye5), and 4-(benzo[c]thiophen-4-yl)-2H-benzo[d][1,2,3]triazole(Dye6) as π-spacers. Density Functional Theory and Time-Dependent Density Functional Theory were employed to study their electronic structures, charge transfer properties, and photovoltaic performance. All dyes showed negative Gibbs free energy for electron injection into TiO₂, signifying favourable charge transfer. The dye2 with benzo[c]thiophene spacer exhibited the highest open-circuit voltage, while those with dye5 and dye6 showed faster regeneration, particularly with copper-based redox shuttles. These candidates demonstrated high light-harvesting efficiency and ideal energy alignment, indicating their potential for high-efficiency DSSCs. These findings show that the π-spacer tuning is an effective strategy for optimizing dye performance in DSSCs.