Organic polymeric and small molecular electron acceptors for organic solar cells

In organic solar cells (OSCs), the electron donor (D) and electron acceptor (A) blended active layer is the most crucial component for governing the power conversion efficiency (PCE). Various efficient donor materials with wide structural variations have been developed to couple with high-electron mobility fullerene-based acceptors, giving PCEs beyond 12%. However, fullerene-embedded OSCs encounter great challenges of low flexibility for structural modifications, poor absorption and blend morphological stability. The demand for alternative acceptors drives current OSC research towards non-fullerene acceptors (NFAs). Tailor-made NFAs of polymer or small molecule (SM) can typically exhibit tunable optical and electrochemical properties, high solubility, air stability, and favorable intermolecular interactions leading to compact packing and good nano-phase segregation in the active blend. In this review, we systematically depict the effects of molecular structures on the physical properties and device performances. The promising/most popular cores and general molecular design strategies of NFAs are outlined. The polymeric and SM NFAs were classified into several sub-groups based on their structural features, and in every sub-group, the structural evolution, current status, the champion case as well as the future challenges were highlighted and discussed. For polymeric NFAs, naphtalene diimide (NDI) and perylene diimide (PDI) are most promising and widely explored due to their easy synthesis, high electron affinity and mobility, leading to promising PCE when NDI and PDI units are conjugated with various electron-rich/deficient aromatics. Various electron-deficient core-based polymeric NFAs were also employed. Aromatic diimides (NDI and PDI) were also widely employed as the central core or terminal unit for SM NFAs. In particular, PDI was interested in electron deficient core, and their monomers, dimers, and trimers gave various degrees of success. PDI trimeric NFA showed superior PCE (∼9.3%) because of its twisted 3D or fused geometry capable of interlocking the polymer donor allows optimum molecular packing, morphology and, therefore, efficient charge separation and transport. The excellent photochemical stability, strong absorption and synthetic flexibility of diketopyrrolopyrrole (DPP) produced promising SM NFAs. The rigid and co-planar indacenodithiophene (IDT) cores bearing various electron-deficient terminal groups were extensively explored, and the structural engineering on both the core and side chain groups together with post-treatments produced the highest PCE (∼13.2%). These results conclude that NFAs possess the better possibility for tuning absorption profile, matched energy levels and optimal D/A nano-morphology for delivering promising PCEs. We highlighted the structure-property-performance relationships and future challenges, and hope this article can trigger new ideas for designing more promising NFAs.