Theoretical investigation and design of high-efficiency dithiafulvenyl-based sensitizers for dye-sensitized solar cells: the impacts of elongating π-spacers and rigidifying dithiophene.

DSSCs have been extensively investigated in the past decade, and the search for more efficient dyes for DSSCs remains challenging. In this work we discuss the influences of elongating π-spacers and rigidifying dithiophene on the performance of dithiafulvenyl (DTF)-based organic dyes using density functional theory (DFT) and time-dependent DFT methods. We show that systematically elongating the π-spacer of the DTF-2P dye by increasing the number of thiophene groups tends to red-shift the absorption peak and broaden the absorption range, thus improving the light-harvesting efficiency of DTF-2P-T and DTF-2P-2T. Furthermore, among the three dyes, DTF-2P-T would have the best performance because it performs nicely on the key parameters including the electron injection driving force (D), the light-harvesting efficiency (LHE), and the shift of the TiO2 conduction band (ΔEcb). In particular, DTF-2P-2T has a larger LHE despite the smaller D and ΔEcb compared with DTF-2P-T. Having realized the great merits of modification on π-spacers, afterwards, we designed a novel dye by rigidifying the dithiophene moiety of DTF-2P-2T. The resulting dye is proven to be very promising to challenge the conversion efficiency 8.29% of DTF-2P-T due to the improved ΔEcb and LHE. Our theoretical studies are expected to provide valuable insights into the molecular design of novel DTF-based dyes for the optimization of DSSC.