Extension of the conjugated framework of non-fullerene electron acceptors toward highly efficient organic photovoltaics†

Extending the π-conjugated framework of non-fullerene electron acceptors (NFAs) has been considered as an effective method to improve their optoelectronic properties; however, the ways in which the conjugation extension affects the molecular packing and aggregation behavior of NFAs and further determines their photovoltaic performance in binary and ternary solar cells remain unclear. In this work, we find that extending the end-group of a cutting-edge NFA, C5-16, with a biphenyl moiety can enlarge its torsional angles and hamper intermolecular interactions, whilst the extension of the quinoline moiety on the backbone core can encourage core-to-core interactions, allowing a prolonged crystallization period during the film-formation process and leading to reduced morphological and excitonic static disorder for the acceptor. When the end-group and core extended NFA C5Qx-B6F is further combined with C5-16, the C5Qx-B6F:C5-16 blend film not only retains improved structural order with reduced excitonic static disorder for acceptors like C5Qx-B6F, but also exhibits transformation of the molecular packing from A-to-A and D-to-D to A-to-D, leading to efficient charge collection ability with suppressed bimolecular recombination. As a result, a maximum PCE of 20.3%, FF of 81.8%, J_SC of 27.6 mA cm⁻² and V_OC of 0.899 V are obtained, with an elevated operational T₈₀ lifetime due to improved morphological stability, which benefits from the reduced free volume in the photoactive film.