Revealing the Photophysical Mechanism of N,N′-Diphenyl-aniline Based Sensitizers with the D–D−π–A Framework: Theoretical Insights

The D–D−π–A framework based dyes are competent for lowering the aggregation and reducing the charge recombination inherently due to their 3D structures. Seven D–D−π–A-based N,N′-diphenyl-aniline (NNdPA) dyes are designed and investigated using cluster and periodic density functional theory (DFT) approaches to evaluate their prospect of application in the future dye-sensitized solar cells (DSSCs). The critical parameters related to short-circuit photocurrent density (JSC) and open-circuit voltage (VOC) of the considered dyes such as the driving force of electron injection (ΔGInject), the spontaneity of dye regeneration (ΔGReg), the exciton binding synergy (Eb), the charge transfer length (dCT), the reorganization energy (λTotal), the shift of the conduction band of TiO2 (ΔECB), the projected density of states (PDOS), and the chemical reactivity parameters were computed. Computed results implied that the fused π-conjugation bridge, along with the benzothiadiazole (BTD) unit, improves the absorption spectrum and charge separation. Also, incorporation of the benzene ring lowers λTotal with balancing its counterparts' reorganization energy. Considering the dye characteristics after electron injection, NNdPA04 with large Stokes shift would possess the most stable excited state due to longer excited state lifetime, τe, with the lowest driving force between excited state oxidation potential and conduction band minimum of TiO2. We found that the presence of the benzene ring in the fused π-conjugation unit increases the light harvesting by shifting the UV–vis spectrum to a longer wavelength. The values of ΔECB and ΔGReg suggest that the NNdPA04 and NNdPA10 would be able to suppress the charge recombination and thus enhance VOC of NNdPA-based dyes. The outcomes inferred that the designed NNdPA04 dye could be the lead candidate for the photoefficient NNdPA-based DSSCs. Our work also provides a rational insight into designing the D–D−π–A dyes with a fused π-conjugation.