Molecular-level screening of triazole-linked C60-TPA derivatives as fullerene-based acceptor candidates for polymer solar cells: A DFT and TD-DFT study

The molecular design of fullerene-based acceptors remains an important route for tuning energetic alignment and optical response in polymer solar cells. Here, density functional theory (DFT) and time-dependent DFT (TD-DFT) were used to screen a series of triazole-linked fullerene-triphenylamine derivatives (C₆₀-TPA-R; R = H, Me, SMe, OMe, and N(Me)₂; A1-A5). Using a consistent conductor-like polarizable continuum model (CPCM) with chlorobenzene as the implicit solvent, para-substitution primarily modulates the donor-like manifold: the highest occupied molecular orbital (HOMO) shifts from -5.37 eV (A1) to -4.74 eV (A5), whereas the lowest unoccupied molecular orbital (LUMO) remains nearly invariant and C₆₀-centred (≈-3.57 to -3.55 eV), resulting in pronounced gap narrowing. Comparative donor-acceptor energetic descriptors referenced to a poly(3-hexylthiophene) (P3HT) donor model indicated preserved electron-transfer driving forces and modest variations in photovoltaic-related energetic trends, while HOMO-level alignment was substantially modified by para substitution, with A5 approaching a near-resonant alignment condition. Electron-affinity calculations showed nearly constant values across A1-A5 (≈3.50-3.52 eV), together with low electron reorganization energies (≈0.135-0.137 eV), comparable to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). TD-DFT and natural transition orbital (NTO) analyses further revealed substituent-dependent intramolecular electron-hole redistribution, while solar-weighted absorption analysis identified enhanced comparative light-harvesting response across the series. Swain-Lupton correlations provide physically interpretable structure-property relationships, showing that resonance-donating para substituents efficiently raise the donor-level energy while largely preserving the fullerene-centred acceptor manifold. Within this screening framework, A3 exhibits a balanced combination of electronic and charge-transfer descriptors among the investigated derivatives, whereas A5 displays a distinct optical and charge-transfer profile requiring further validation beyond the present molecular-level framework.