Molecular engineering, synthesis, and atomistic structure–property relationship of indoloquinoxaline-capped small donors for efficient organic solar cells

Abstract

The growing demand for high-performance organic photovoltaics has sparked great interest in small-molecule donor (SMD) materials that offer well-defined structures and superior batch-to-batch consistency. In this study, we report the molecular design, synthesis, and atomistic structure–property characterization of three indoloquinoxaline (IQ)-capped SMDs named DPP-Th-IQ, BT-Th-IQ, and TT-IQ for potential applications in all-small-molecule organic solar cells (ASM-OSCs). Each SMD features a distinct central core, including diketopyrrolopyrrole (DPP), benzothiadiazole (BT), or thieno[3,2-b]thiophene (TT) with thiophene as bridging units in the DPP and BT derivatives, to systematically tune electronic structures, optical profiles, and charge transport properties. Electrochemical analysis confirmed that all three SMDs possess well-aligned HOMO–LUMO levels conducive to pairing with the Y6 non-fullerene acceptor. Density functional theory (DFT) calculations revealed low hole/electron reorganization energies with extensive frontier-orbital delocalization, indicative of efficient charge transport. Photophysical experiments based on UV-vis, photoluminescence, and solvatochromic analysis and complementary computational characterization showed strong intramolecular charge transfer in SMDs. Electron density difference analysis explained that particularly the benzothiadiazole-based BT-Th-IQ donor exhibits the lowest exciton binding energy coupled with high charge transfer excitations, indicating efficient exciton dissociation. Donor–acceptor interfacial modeling further predicted robust face-on π–π stacking and favorable donor-Y6 orientations that support interfacial charge transfer. Importantly, all three SMDs demonstrated initial thin-film stability: films retained ≥90% of their initial absorbance after 30 hours of continuous AM 1.5G irradiation, and thermogravimetric analysis showed decomposition temperatures (5% weight loss) exceeding 250 °C. Overall, this study clarifies the interplay between molecular design, electronic structure, interfacial interactions, and stability, providing a strategic path toward next-generation high-efficiency ASM-OSCs based on IQ-capped donors.