Design of a Low‐Cost Electron Acceptor for Highly Efficient Organic Solar Cell through Alkyl Chain Branching Position Manipulation

The state‐of‐the‐art bulk‐heterojunction (BHJ) organic solar cells (OSCs) typically include expensive fused‐ring electron acceptors, hindering industrialization. Designing low‐cost and highly efficient electron acceptors remains challenging. Herein, two low‐cost electron acceptors (DTB2 and DTB3) based on a conjugated 1,4‐di(thiophen‐2‐yl)benzene (DTB) core and two fluorinated 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile end‐groups are reported. Their only difference is the alkyl chain branching position on the benzene ring. Both acceptors exhibit similar low optical gaps of ≈1.35 eV but different molecular orientations. DTB2 shows an edge‐on arrangement, while DTB3, with a shift in branching positions toward the conjugated backbone, produces a face‐on arrangement. Such molecular orientations are maintained in their BHJ layers after blending with a polymer donor PBQx‐TF. The PBQx‐TF:DTB3 film demonstrates superior BHJ phase‐separation and faster charge carrier generation (0.44 ps) than those of the PBQx‐TF:DTB2 film (50 ps). As a result, the DTB2‐based OSC achieves a modest power conversion efficiency (PCE) of 8.5%. While the DTB3‐based OSC produces an outstanding PCE of 15.3%, which is much higher than those of the reported DTB‐based OSCs. Besides, DTB3 has a figure of merit up to 0.46, higher than the state‐of‐the‐art fused‐ring electron acceptors. This work provides new insights into designing low‐cost and highly efficient electron acceptors.