Theoretical Study on the Optoelectronics and Charge Transport Properties of Difluoro Benzothiadiazole-based Hole...

Fluorination is an effective strategy to fine-tune the electronic structure and optoelectronic response of organic semiconductors by modulating frontier orbital energies, intramolecular charge transfer, and solid-state/solution interactions. Herein, four fluorinated derivatives (DFBT1-DFBT4) were rationally designed by introducing fluorine substituents on the central core of a reference molecule (DFBT-PMTP), and their properties were evaluated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Relative to the DFBT-PMTP (HOMO = -4.91 eV; LUMO = -1.87 eV), fluorination systematically stabilizes the LUMO (down to -2.10 eV) and increases electron affinity (0.94 to 1.04 eV), while moderately tuning the HOMO (-4.90 to -5.01 eV) and narrowing the bandgap (2.83 to 2.98 eV). TD-DFT predicts a modest red-shift in absorption, with [Formula: see text] spanning 389-409 nm in gas phase and 399-419 nm in the chlorobenzene, accompanied by intense electronic transitions (high oscillator strengths). Solvation free energies indicate favorable stabilization in chlorobenzene and dimethyl sulfoxide ([Formula: see text] ≈ -8.18 to -8.86 and - 10.87 to -11.66, respectively), consistent with moderate hydrophobicity (LogP ≈ 2.72-2.93). Charge-transport descriptors improve upon fluorination, with reduced internal reorganization energies ([Formula: see text] down to 0.5308 eV for DFBT4; [Formula: see text] down to 0.4756 eV for DFBT3), and enhanced Marcus-type hole transfer rates relative to DFBT-PMTP (up to 3.85 × 10¹² s^- 1 for DFBT2). Overall, fluorination offers a practical handle to balance energy-level tuning, optical response, solubility tendency, and charge-transport propensity; notably, DFBT2 is highlighted for the highest predicted hole-transfer kinetics, while DFBT4 (and DFBT3) stand out for minimized reorganization losses, together identifying the most promising candidates for hole-transport layers in perovskite solar cells.