Insight into the positional effect of bulky rigid substituents in organic sensitizers on the performance of dye-sensitized solar cells

Bulky rigid substituents, featuring distinctive steric configuration and interesting electronic properties, play an important role in advancing performance of metal-free sensitizers toward dye-sensitized solar cells (DSSCs). However, the question about how to choose the relative position of bulky substituent for optimum device performance remains unclear, which necessitates understanding the structure-property relationship of organic sensitizers. In this work, two new isomeric dyes, W09 and W10, are designed to study the role of bulky rigid truxene unit which locates at the pi-bridge and electron-donor, respectively. The comparative effects of truxene position on the optical properties, interfacial kinetic parameters and their synergistic contribution to the photovoltaic performance have been systematically investigated. A significantly improved molar absorption coefficient accompanied with a larger driving force for dye regeneration is obtained for W10 relative to those of W09, manifesting the effect of changing the truxene location. Combined with the slower charge recombination kinetics at the TiO2/dye/electrolyte interface, W10 sensitized solar cells exhibit the highest conversion efficiency of 7.62%, which is around 20% higher than that of W09. This work has demonstrated the feasibility of judiciously locating bulky rigid substituents into the right position to construct highly-efficiency metal-free sensitizers.