The role of the π-bridge length in the performance of diketopyrrolopyrrole-based organic dyes for dye-sensitized solar cells

Three novel diketopyrrolopyrrole-based metal-free organic dyes (TPh, BPh and SPh) with different π-bridge lengths were designed and synthesized for dye-sensitized solar cells (DSSCs). The phenyl units were employed as the additional π-bridge for the three dyes, which located between the diketopyrrolopyrrole core and cyanoacrylic acid unit. Interestingly, it is the first time to report a diketopyrrolopyrrole-based dye with only one additional π-bridge unit (SPh). The effects of π-bridge length on photophysical, molecular planarity, energy level and photovoltaic properties of the dyes were systematically investigated. The results revealed that the prolongation of π-bridge length by introducing an additional phenyl unit results in an extra torsion in the structural framework and reduces the molecule planarity, which has a negative impact on improving the light-harvesting ability, while it is beneficial in lowering the tendency of aggregation and enhancing the excited electron injection efficiency. Finally, with a moderate π-bridge length (BPh), the best balance of overall performances is achieved, resulting in the highest power conversion efficiency (6.57%) without chenodeoxycholic acid.