Blue light-emitting conjugated polymer with low modulus and better EL spectra stability

Intrinsic stretchable light-emitting conjugated polymers (LCPs) with low modulus play increasingly significant roles in flexible or skin-like displays. In this work, we focus on investigating the optoelectronic and mechanical properties of polyfluorene-based blue LCPs, of which the polymer side chains were straight alkyl chains (octyl) and branched alkyl chains (2-hexyldecyl). Comparatively, the solution-processed film of the polymer with branched alkyl chains exhibited substantially improved photoluminescence quantum yield (PLQY) from 18.9% to 46.5%. Benefiting from the internal plasticization of branched alkyl chains, the elastic modulus of polymer film reduced from 350 MPa to 180 MPa, confirming the validity of enhancing the alkyl/π ratio in lowering film stiffness and necessity for characterizing the intrinsic mechanical properties. Finally, polymer light-emitting diodes (PLED) that exhibited stable blue emission were achieved with the polyfluorene-based LCPs functionalized by branched alkyl chains. Overall, these observations manifest the feasibility of side-chain branching in tuning the mechanical properties of LCPs, and provide in-depth insights for developing intrinsic stretchable semiconductors. Investigated and optimized the intrinsic mechanical properties of polyfluorene-based conjugated polymers without sacrificing the optoelectronic properties by side-chain engineering.