Impact of alkyl-substituted auxiliary acceptor positioning on dyes for DSSCs with Hagfeldt donor

The introduction of an auxiliary acceptor into donor/acceptor dye systems is one of the most widely used strategies to enhance the light-harvesting properties of dye-sensitized solar cells. However, our understanding of the effects of these auxiliary acceptors remains limited. In this study, we utilize two organic dyes that differ in the positioning of the auxiliary acceptor to explore their impacts on several important factors associated with short-circuit current density and open-circuit photovoltage. These parameters include light-harvesting ability, electron injection, conduction band energy shift and charge recombination. Contrary to common assumptions, we find that the positioning of the alkyl-substituted auxiliary acceptor affects not just the light-harvesting ability but also influences the charge recombination process by altering the dominant conformation of the Hagfeldt donor. Specifically, the auxiliary acceptor situated farther from the Hagfeldt donor is more effective in enhancing the dye's light-harvesting ability compared to those located near the donor end. Conversely, when the auxiliary acceptor is positioned near the Hagfeldt donor and introduces alkyl chains on one side, the two benzene rings substituted with alkoxy groups in the Hagfeldt donor tend to interact with the alkyl chain of the auxiliary acceptor. This interaction hinders the surface protection effect of the Hagfeldt donor and reduces its inhibitory effect on the electron recombination process. These new insights into the effects of alkyl-substituted auxiliary acceptors on inhibiting charge recombination through their influence on the dominant conformation of the Hagfeldt donor opens avenues for the systematic design of high-performance sensitizers.