Correlating Structural Properties and Performance in Thiophene-Bridged Benzodithiophene Donors for Bandgap Narrowing and Efficient Charge Transport in Organic Photovoltaics Cells (OPVCs): Insights from DFT/TD-DFT

Studying correlating structural properties and performance is critical for the rational advancement of donor materials in organic photovoltaics cells (OPVCs). In this study, thiophene-bridged benzodithiophene (BDT) donors were strategically engineered to probe the impact of π-bridge modulation on electronic structure, optical transitions, and photovoltaic-relevant descriptors using density functional theory (DFT) and time-dependent DFT (TD-DFT). Systematic extension of π-conjugation induces substantial bandgap narrowing from 1.68 to 2.01 eV through enhanced frontier orbital delocalization and reduced HOMO-LUMO separation. This structural modulation produces a pronounced bathochromic shift in absorption maxima up to 815 nm, enabling near-infrared light harvesting. Frontier orbital and density of states analyses reveal intensified intramolecular charge transfer from the electron-rich BDT core to electron-deficient terminals, promoting effective charge separation. Increased dipole moments and oscillator strengths further support improved excitonic dissociation propensity. Theoretical evaluation of open-circuit voltage (Voc) and exciton binding energy (Eb) derived from calculated energy levels suggests favorable donor-acceptor energetic alignment and reduced Coulombic binding. These findings establish π-bridge engineering as a decisive molecular strategy for governing bandgap modulation and charge transport potential in next-generation BDT-based OPVCs systems.