Strategic Core Engineering of Benzo[c][1,2,5]thiadiazole–4-Alkoxythiazole Donors: A Pathway to Low Band Gap Photovoltaic Materials

Five new D–A–D type donor molecules (M1–M5) incorporating a benzo [c][1,2,5]thiadiazole (BTD) core linked by 4-alkoxythiazole units were theoretically investigated for prospective organic solar cell (OSC) applications. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations reveal that all designed molecules exhibit low band gaps between 1.72 and 1.89 eV, surpassing the unsubstituted BTD reference (2.92 eV), and display strong visible absorption maxima ranging 557–576 nm. When paired with fullerene acceptors (PC61BM/PC71BM), open-circuit voltages (Voc) extend from 0.63 to 0.86 V, highlighting the sensitivity of frontier orbitals to strategic donor substitution. Reorganization energy analysis further indicates sub-0.01 eV electron and hole transport barriers, suggesting excellent carrier mobility. In comparison with previously studied BTD-based donors, these new materials achieve broader spectral coverage and improved exciton splitting, underscored by exciton binding energies reduced from 2.13 eV (core) to around 1.50–1.70 eV (M4, M5). Such enhancements, combined with readily accessible synthetic building blocks (e.g., thiophene, carbazole), demonstrate the practical feasibility of scaling up production for next-generation OSC devices. Overall, these results affirm that precise core engineering of BTD–4-alkoxythiazole scaffolds can yield donor molecules with balanced bandgap narrowing, favorable Voc, and efficient charge transport, thus offering strong potential for industrial development of high-performance organic photovoltaics.