Modulating Molecular Orientation Enables Efficient Nonfullerene Small-Molecule Organic Solar Cells

In bulk-heterojunction organic solar cells (BHJ-OSCs), exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in blend films. Efficient photovoltaic small molecules (SMs) typically possess an acceptor–donor–acceptor structure that causes intrinsic anisotropy, limiting the control over molecular packing because of the lack of an effective method for modulating molecular orientation. In this report, we design a group of model compounds, named DRTB-T-CX (X = 2, 4, 6, and 8), to demonstrate that adjusting the length of the end alkyl chain can be used to modify the molecular orientation. A top-performance power conversion efficiency (PCE) of up to 11.24% is achieved with a DRTB-T-C4/IT-4F-based device, which is the best performance reported for a state-of-the-art nonfullerene SM organic solar cell (NFSM-OSC).