Highly ductile, stable, and efficient organic photovoltaic blends enabled by polymerized ladder-type heteroheptacene-based small-molecule acceptors

Although many fully-conjugated polymerized small-molecule acceptors (PSMAs) with high photovoltaic per-formance have been reported recently, most of them exhibit high stiffness and are very brittle, which limits their applications in wearable and stretchable electronics. In this work, we show that PSMAs using ladder-type het-eroheptacene-cored SMAs (M-series acceptors) as the main building blocks can achieve high mechanical ductility while maintaining high power conversion efficiencies (PCEs). To finely tune molecular stacking and miscibility of the resulting PSMAs (PQ-HD and PQ-OD), two pendant groups with different alkyl lengths (namely, 2-hexyl-decyl and 2-octyldodecyl) are anchored on their backbones. With the shorter side-chains, a more intermixed bulk-heterojunction blend morphology as well as highly well-ordered molecular packing are achieved, leading to simultaneous enhancement of both photovoltaic and mechanical properties. The best-performing PSC based on PQ-HD with 2-hexyldecyl side-chains shows a high PCE of 13.36% and excellent mechanical stretchability with a crack-onset strain (COS) of 17.5%, both of which are superior to that based on PQ-OD with 2-octyldodecyl side -chains (PCE = 10.87% and COS = 12.8%). Moreover, the PQ-HD-based PSC also exhibits excellent thermal stability. These outstanding properties in combination with its low synthetic complexity shall make PQ-HD a promising candidate for future applications in flexible and wearable devices.