Intrinsically Stretchable and Efficient Fully Π‐Conjugated Polymer via Internal Plasticization for Flexible Deep‐Blue Polymer Light‐Emitting Diodes with CIE_y = 0.08

Abstract Intrinsically stretchable polymeric semiconductors are essential to flexible polymer light‐emitting diodes (PLEDs) owing to their excellent strain tolerance capacity under long‐time deformation operation. Obtaining intrinsic stretchability, robust emission properties, and excellent charge‐transport behavior simultaneously from fully π‐conjugated polymers (FCPs) is difficult, particularly for applications in deep‐blue PLEDs. Herein, an internal plasticization strategy is proposed to introduce a phenyl‐ester plasticizer into polyfluorenes (PF‐MC4, PF‐MC6, and PF‐MC8) for narrowband deep‐blue flexible PLEDs. Compared with controlled poly[4‐(octyloxy)‐9,9‐diphenylfluoren‐2,7‐diyl]‐co‐[5‐(octyloxy)‐9,9‐diphenylfluoren‐2,7‐diyl] (PODPFs) (2.5%), the freestanding PF‐MC8 thin film shows a fracture strain of >25%. The three stretchable films exhibit stable and efficient deep‐blue emission (PLQY > 50%) because of the encapsulation of π‐conjugated backbone via pendant phenyl‐ester plasticizers. The PF‐MC8‐based PLEDs show deep‐blue emission, which corresponds to CIE and EQE values of (0.16, 0.10) and 1.06%, respectively. Finally, the narrowband deep‐blue electroluminescence (FWHM of ≈25 nm; CIE coordinates: (0.15, 0.08)) and performance of the transferred PLEDs based on the PF‐MC8 stretchable film are independent of the tensile ratio (up to 45%); however, they show a maximum brightness of 1976 cd m⁻² at a ratio of 35%. Therefore, internal plasticization is a promising approach for designing intrinsically stretchable FCPs for flexible electronics.