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.