Asymmetric Small Molecule Acceptors for Organic Photovoltaics: Insights from Multiple Perspectives
The rapid development of small molecule acceptors (SMAs) has enabled organic photovoltaics (OPVs) to achieve power conversion efficiencies exceeding 21%. Structural asymmetry has emerged as a particularly effective approach for boosting acceptor performance. This review provides a comprehensive overview of asymmetric SMAs from the molecular scale to the nanoscale and macroscale. The main text is organized into four sections: molecular design strategies for structural asymmetry, crystal structure evolution from symmetry breaking, morphological characteristics revealed by advanced characterization techniques, and energy loss mechanisms involving asymmetric SMAs. At the molecular scale, asymmetry enables precise modulation of dipole moments and intermolecular interactions, directly affecting crystalline packing and charge-transport networks. At the nanoscale, it further regulates domain purity, phase continuity, and molecular orientation. Asymmetric designs can also help mitigate non-radiative voltage loss through modulation of charge-transfer state energetics. Overall, asymmetric molecular design introduces additional structural and electronic tunability, offering new opportunities for overcoming the trade-offs that limit OPV performance. Finally, we discuss ongoing challenges and outline future perspectives to guide continued development and innovation in asymmetric SMA design.