Anti-aggregation and intra-type Förster resonance energy transfer in bulky indoline sensitizers for dye-sensitized solar cells: A combined DFT/TDDFT and molecular dynamics study

We have performed a theoretical investigation using a combination of DFT/TDDFT and molecular dynamics simulations to explain the relationship between bulky donor groups and higher efficiencies for a recent indoline sensitizer YA422 derived from its counterparts IQ4 and YA421, which features an extended donor subunit for use in dye-sensitizer solar cells (DSSCs). Firstly, the absorption and fluorescence properties indicate that the Förster resonance energy transfer occurs only in YA422, where the λ_ems of the donor group D2 matches with the λ_abs of the whole molecule around the band at 533 nm. Secondly, the simulated heterogeneity between the sensitizer and (TiO₂)₁₂₄ before and after the co-adsorption of the additive CDCA shows that the nearest position of every monomer in YA422 is separated by a row of Ti atoms due to the steric hindrance introduced by the extended donor group, which not only forms the ordered alignment but also prevents aggregation. Meanwhile, the separated position decreases the self-decay which is shown by the complete intramolecular charge transfer in the aggregate structure of (TiO₂)₁₂₄. Furthermore, using a combined Newns-Anderson approach and the Marcus equation, a faster electron injection rate k_inject of YA422 (2.27 × 10¹⁵ s^-1) is obtained compared with those of IQ4 and YA421. We confirm that the higher conversion efficiency achieved by YA422 is caused by its bulky donor group which enhances the electron-donation and transfer. Finally, based on the above insights, we designed a novel sensitizer DW1 which is expected to be a promising candidate due to its enhanced absorption and larger k_inject compared with those of YA422.