High-Performance CH-Series Non-Fullerene Acceptors for Organic Photovoltaics

ConspectusThe innovative exploration of non-fullerene acceptors (NFAs) such as ITIC, Y6, and others, has boosted the power conversion efficiencies (PCEs) of organic solar cells (OSCs) surpassing 21%. However, organic photovoltaics still suffer from significant efficiency gaps compared to inorganic photovoltaics, particularly in open-circuit voltage under similar bandgaps. This notable disparity is largely driven by the stark difference in nonradiative recombination energy losses: OSCs typically incur losses exceeding 0.2 eV, whereas their inorganic counterparts suffer only minimal losses, ranging from a mere 0.03 to 0.04 eV. This insurmountable nonradiative recombination is closely associated with some intrinsic features of organic photovoltaic light-harvesting materials: relatively flexible molecular frameworks, loose and disordered molecular aggregates, large exciton binding energies, etc. Therefore, a multiscale regulation spanning single-molecular properties and aggregation behaviors in further molecular design is required, if a remarkable PCE improvement is expected.In this Account, we first present a brief review of the development of electron acceptor materials, with a focus on analyzing the prominent merits of current high-efficiency acceptor molecular skeletons (especially Y6 analogs) in terms of intermolecular packing modes, photodynamic, etc. Meanwhile, great challenges for further material design also arises from the quite limited structural optimization room for Y-series backbones. In order to break through the dilemma of molecular design, we developed CH-series NFAs with multi-functionalized central units and an "acceptor-donor-acceptor" architecture. Subsequently, a systematic discussion about CH-series NFAs will be made to reveal their advantages in (1) inducing a directional transformation of molecular packing mode toward a more favorable one, through multiple intermolecular weak interactions such as fluorine-hydrogen/sulfur/π bonds, thus rendering multidimensional long-range ordered molecular stacking to minimize energy loss pathways in OSCs; (2) breaking through the limitations of traditional dimeric/trimeric/polymeric acceptor design by pioneering the construction of relatively rigid central-units-linked dimeric/trimeric NFAs with multiple free terminals to enhance intermolecular packings; (3) proposing a novel "functional reconfiguration" strategy for the central units, aiming to explore new photoelectric conversion mechanisms in organic photovoltaic materials.Thus far, CH-series NFAs based binary OSCs have achieved the highest PCE of approaching 21%, ranking among the best NFAs. If further considering their great structural modification possibilities, CH-series NFAs hold exceptional promise as a versatile platform for developing OSCs with record-breaking PCEs. Therefore, we further propose some perspectives for CH-series NFAs, for example, more precise structural and packing optimization to reduce exciton binding energies and improve molecular packing ordering; further in-depth exploration of a "functional reconfiguration" strategy to apply new photoelectric conversion mechanisms, such as triplet excitons, singlet fission, etc.; extending the absorption edge of NFAs to near-infrared II region to harvest more low-energy photons, especially for tandem OSCs. These strategies may have the potential to overcome the critical challenge existing in OSCs and shrink the PCE gap comparing to inorganic platforms.