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 as 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, 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 reorganization energies for both holes and electrons, indicative of efficient charge transport. Photophysical investigations based on experimental UV-Vis absorption, photoluminescence, and solvatochromic analysis, and computational characterization showed strong intramolecular charge-transfer (ICT) characteristics in SMDs. Notably, DPP-Th-IQ exhibited pronounced solvent-induced shifts and BT-Th-IQ presented high electron–hole separation and lower exciton binding energy. Overall, the BT-Th-IQ exhibited extensive delocalization of frontier orbitals, largest charge-transfer fraction, and most efficient exciton dissociation. Moreover, donor–acceptor interfacial modeling demonstrated robust π–π stacking and favorable orientation with Y6 acceptor, enhancing exciton dissociation and electron–hole transport. These results identify BT-Th-IQ as particularly promising SMD, while DPP-Th-IQ and TT-IQ SMDs offer complementary strengths amenable to morphological or blend engineering. This work addresses the critical interplay between design, electronic structure, and interfacial interactions, providing a strategic approach for developing future high-efficiency ASM-OSCs based on IQ-capped small-molecule donors.