Design of Fluorene-Based D-π-A′-π-A Dyes: Fine-Tuning Mechanism of Auxiliary Acceptors and π-Bridges on Photovoltaic Performance

1. Research Hypothesis Our central hypothesis is that strategic engineering of FTCN dyes through three modifications will enhance photovoltaic performance: replacing thiophene with selenophene in the π-bridge, introducing an auxiliary acceptor, and adding cyano and nitro groups. First, different π-bridges influence planarity and charge transfer. Second, electron-withdrawing groups modulate LUMO levels, with nitro inducing stronger effects. Third, combining modifications produces synergistic effects exceeding the sum of individual changes.
2. Data Collection MethodologyCalculations used Gaussian 16 and Multiwfn 3.8. Ground-state optimizations used B3LYP/6-31G+(d,p). Excited-state calculations used CAM-B3LYP/6-31G+(d,p) with IEFPCM solvent model. Parameters included geometries, energy levels, absorption, lifetimes, electron-hole distribution, and photovoltaic metrics. Nine dyes were designed with thiophene, selenophene, and acetylene bridges, combined with cyano and nitro substitutions.
3. Data Presentation and Notable Findings Thiophene Series: F-S, F-S-CN, F-S-NO₂ :F-S has an energy gap of 2.34 eV, with cyano and nitro producing minimal narrowing. Strong local excitation is observed. F-S absorbs at 522 nm, but cyano and nitro cause blue shifts to 509 nm and 503 nm. Light-harvesting efficiency is around 0.95, electron injection around -0.9 eV, regeneration driving force around 0.70-0.72 eV. The thiophene series shows limited response.Selenophene Series: F-Se, F-Se-CN, F-Se-NO₂:Selenophene alone reduces the gap by 0.02 eV to 2.32 eV, while with nitro narrows it to 2.21 eV. F-Se-NO₂ exhibits mixed charge transfer. F-Se produces a 7 nm red shift to 529 nm, but F-Se-NO₂ achieves a 310 nm red shift to 832 nm, with lifetime increasing to 4.699 ns. However, F-Se-NO₂ shows reduced light-harvesting efficiency of 0.28 and weak electron injection of -0.03 eV. Acetylene Series: F, F-CN, F-NO₂ :Acetylene induces highest planarity. Acetylene alone reduces the gap by 0.26 eV to 2.08 eV. Acetylene with nitro achieves a gap of 1.69 eV, a 0.65 eV reduction exceeding the sum of individual effects. F-CN exhibits complete charge transfer. F absorbs at 445 nm, but F-NO₂ achieves a 152 nm red shift to 597 nm. F achieves perfect light-harvesting efficiency of 1.00, electron injection of -1.66 eV, regeneration driving force of 0.33 eV, open-circuit voltage of 0.95 eV, and fill factor of 0.852.
4. Data Interpretation Energy gap below 2.2 eV favors good spectral coverage. Absorption above 550 nm indicates effective visible light utilization. Light-harvesting efficiency above 0.95 ensures effective photon capture. Electron injection below -1.0 eV indicates spontaneous injection. Regeneration driving force between 0.3 and 0.5 eV optimizes kinetics.The thiophene series shows limited response. The acetylene series demonstrates comprehensive enhancement, with acetylene-only achieving optimal balance across all parameters.