Spatially Orthogonal 2D Sidechains Optimize Morphology in All‐Small‐Molecule Organic Solar Cells

Abstract Organic semiconductors consist of a conjugated backbone and flexible sidechains. Compared to the meticulous design of backbones, less attention has been paid to the investigation of sidechains, in particular their spatial orientation. Herein, three non‐fullerene acceptors, anti‐PDFC, syn‐PDFC, and PDFC‐Ph, are applied in all‐small‐molecule organic solar cells (ASM‐OSCs) to reveal the varied effects of sidechains on morphology and device performance. With spatially orthogonal alkyl chains, anti‐PDFC and syn‐PDFC show unique bimodal lamellar packing and moderate crystallinity. When blending with an efficient binary BTR‐Cl/Y6 system, anti‐PDFC as well as syn‐PDFC not only form their own crystal phase but also improve the packing order of BTR‐Cl, consequently enhancing the power conversion efficiency (PCE) of ternary ASM‐OSC to be 14.56%. However, although PDFC‐Ph has an identical backbone with anti‐PDFC, the alternated sidechains make it relatively amorphous, which is prone to damage the original packing of the host donor/acceptor, and thus deteriorating the device performance. When PC 71 BM is added to optimize the morphology further, the triple‐acceptor device involving anti‐PDFC realizes a PCE of 15.67%, which is among the best efficiencies in ASM‐OSCs. This study demonstrates that a multi‐dimensional sidechain can optimize the morphology of a bulk heterojunction as effective as a conjugated backbone.