Fully Non‐Fused Ring Electron Acceptors Enable Effective Additive‐Free Organic Solar Cells with Enhanced Exciton Diffusion Length

Low-cost photovoltaic materials and additive-free, non-halogenated solvent processing of photoactive layers are crucial for the large-scale commercial application of organic solar cells (OSCs). However, high-efficiency OSCs that possess all these advantages remain scarce due to the lack of insight into the structure-property relationship. In this work, three fully non-fused ring electron acceptors (NFREAs), DTB21, DTB22, and DTB23, are reported by utilizing a simplified 1,4-di(thiophen-2-yl)benzene (DTB) core with varied alkoxy chain lengths on the thiophene bridge. The material-only costs of these acceptors are only 11-13$ per gram. Importantly, DTB22 has an exciton diffusion length (L_D) of up to 25.5 nm. The DTB21 and DTB23 exhibit decreased L_Ds of 20.1 and 23.1 nm, respectively. After blending with the polymer donor PBQx-TF, the PBQx-TF:DTB22 film exhibits the fastest hole transfer and the longest carrier recombination lifetime among these OSCs. Consequently, the optimal PBQx-TF:DTB22-based OSC achieves an excellent PCE of 17.00%, which is among the highest values for fully NFREAs. In contrast, the PBQx-TF:DTB21- and PBQx-TF:DTB23-based OSCs show relatively lower PCEs of 15.13% and 15.63%, respectively. Notably, all these OSCs are fabricated using toluene as the solvent, without any additives. Additionally, the DTB22-based OSC also demonstrates good stability, retaining 95% of its initial efficiency after 500 h without encapsulation in a glovebox.