Theoretical insights into the cooperative effects of rigid π-bridges and auxiliary acceptors on the photovoltaic properties of D-π-A′-π-A sensitizers

Abstract

Context

The photovoltaic performance of dye-sensitized solar cells (DSSCs) is critically governed by the energy-level alignment, excited-state charge-transfer dynamics, and dye–TiO₂ interfacial interaction of the sensitizer. In this work, nine D-π-A′-π-A organic dyes based on an aminofluorene donor, 2-(2,4-difluorophenyl)pyridine as the auxiliary acceptor (A′), and cyanoacrylic acid as the terminal acceptor, with thiophene, selenophene, or acetylene as the π-bridge, were theoretically investigated to elucidate the cooperative effects of π-bridge engineering and substituent modification on photovoltaic performance. The computed results show that the acetylene-bridged nitro-substituted dye (F-NO₂) exhibits the narrowest HOMO–LUMO gap (1.69 eV) with an absorption maximum at 597 nm, while the selenophene-bridged nitro dye (F-Se-NO₂) reaches the longest absorption wavelength (832 nm) but with a low oscillator strength (0.14) and marginal electron injection driving force (−0.03 eV). In contrast, the unsubstituted acetylene dye (F) shows the most balanced performance: highest light-harvesting efficiency (1.00), strongest injection driving force (−1.66 eV), and highest open-circuit voltage (0.95 V). Adsorption energy calculations reveal that selenophene-based dyes exhibit significantly more negative Eads values (ranging from −7.89 to −19.05 eV) compared with thiophene-based (−0.54 to −1.36 eV) and acetylene-based (−0.82 to −3.54 eV) counterparts, indicating stronger interfacial binding to the TiO₂ surface. These findings indicate that π-bridge engineering and substituent modification cooperatively regulate molecular properties, yet enhanced charge separation and spectral broadening are often accompanied by reduced injection driving force or voltage output, highlighting the need for balanced optimization. This study provides quantitative structure–property relationships for the rational design of D-π-A′-π-A sensitizers.

Methods

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed using Gaussian 16. Ground-state geometries were optimized at the B3LYP/6-31+G(d,p) level with Grimme’s D3 dispersion correction, and excited-state properties were calculated at the CAM-B3LYP/6-31+G(d,p) level in solvent with the IEFPCM solvation model. A (TiO₂)₉ cluster was employed to model the TiO₂ surface, and dye@TiO₂ complexes were optimized at the B3LYP/def2-SVP level with D3 correction. Molecular geometries, frontier molecular orbitals, absorption spectra, light-harvesting efficiency, electron injection driving force, open-circuit voltage, and adsorption energies were systematically calculated. Molecular visualizations were generated using Multiwfn 3.8.