Terminal acceptor engineering for reduced energy dissipation and enhanced charge transport in benzodithiophene-core based donor molecules: a computational route to efficient organic solar cells

Abstract

The development of donor materials with the finely tuned frontier orbital alignment, broad optical absorption and efficient charge transport is still a key challenging issue for next generation high efficiency organic solar cells (OSCs). In this study, benzodithiophene-centered small donor molecules were systematically designed via terminal acceptor modification to optimize optical, electronic, and photovoltaic properties. Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations at the B3LYP/6-31G(d,p) level reveal that strategic substitution with strongly electron withdrawing terminals induces substantial intramolecular charge transfer (ICT), narrows the bandgap (E_g) and red shifts the absorption maxima up to 903 nm in solvent. Density of states and transition density matrix calculations reveal efficient charge delocalization from donor to acceptor, especially for SM-6 and SM-8, which have larger off-diagonal electron–hole coupling and planarized backbones that are favorable for π–π stacking. The exciton binding energies (E_b) decrease from 0.26 to 0.14 eV, whereas the electron reorganization energy (λ_e) goes down from 0.178 to 0.091 eV, indicating improved charge mobility and reduced energetic disorder. Theoretical device level parameters show enhanced open circuit voltages (V_oc) up to 1.39 eV, high fill factors (FF) approaching 0.91, and minimized energy losses (E_loss) down to 0.42 eV for the optimized donors. Collectively, these results demonstrate that terminal group engineering in BDT-based donors effectively tailors energy level alignment, light harvesting, and charge transport, establishing a rational design pathway toward small molecule donor systems capable of achieving high photovoltaic efficiency in OSCs, particularly through the simultaneous reduction of electron reorganization energy and energy loss.