Abstract:

The design of high-performance small-molecule acceptors (SMAs) for organic solar cells (OSCs) remains a central challenge, particularly under the growing demand for environmentally friendly processing conditions. While halogenation has been widely employed to optimize electronic structures and molecular packing, its reliance on toxic halogenated solvents and the limited tunability of intermolecular interactions highlight the need for alternative strategies. In this context, core functionalization with cyano (CN) groups provides a unique opportunity, as the CN unit combines strong electron-withdrawing ability, high polarity, and linear geometry, potentially offering synergistic regulation of both optoelectronic properties and supramolecular assembly. However, systematic studies on core cyanation remain scarce, and its precise role in balancing charge transfer, molecular ordering, and energy loss in OSCs has not been thoroughly clarified.Here, we report a cyano-functionalized benzo[a]phenazine (BP)-core SMA, denoted as NA8, to explore how core cyanation influences device performance. The introduction of the CN group reduces the intramolecular charge transfer, resulting in a blue-shifted absorption and a slightly enlarged optical bandgap compared with the non-cyanated analogue NA1. Despite this apparent drawback, NA8 demonstrates superior molecular packing, as evidenced by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showing a crystalline coherence length more than twice that of NA1 (101.3 Å vs. 44.6 Å). This improvement originates from the significantly enhanced dipole moment of NA8 (4.26 D vs. 2.21 D for NA1), which facilitates stronger electrostatic and noncovalent interactions (e.g., S···N and H···N contacts), thereby stabilizing more ordered packing motifs.At the blend-film level, atomic force microscopy (AFM) reveals that PM6:NA8 exhibits a rougher yet more clearly phase-separated morphology compared with PM6:NA1, providing continuous transport pathways. Photo-CELIV measurements confirm higher carrier mobility (2.36 × 10⁻⁴ cm² V⁻¹ s⁻¹ vs. 1.29 × 10⁻⁴ cm² V⁻¹ s⁻¹), while transient absorption spectroscopy shows faster exciton dissociation and reduced bimolecular recombination. Together, these synergistic effects explain why the PM6:NA8 device achieves an outstanding power conversion efficiency of 19.04% using non-halogenated o-xylene, compared with 15.14% for PM6:NA1. The improvement primarily arises from the significantly enhanced short-circuit current density (27.35 mA cm⁻²) and fill factor (78.3%), while the open-circuit voltage is only moderately reduced (0.889 V vs. 0.914 V) due to increased reorganization energy associated with C–C bond vibrations in the CN-substituted BP core. Our study identifies core cyanation as a powerful molecular engineering strategy to concurrently tune energy levels, strengthen molecular packing, and optimize nanoscale morphology, providing valuable design guidance for next-generation organic photovoltaics.

Key words: Small molecular acceptor, Organic solar cells, Power conversion efficiency, Cyano substitution, Molecular interaction