Enhancing the Deep-Blue Emission Property of Wide Bandgap Conjugated Polymers through a Self-Cross-Linking Strategy

Interchain aggregation is adverse to the stability and performance of wide bandgap light-emitting conjugated polymers (LCPs) and their optoelectronic devices, associated with the electron coupling of the backbone chain. Chain-segmental motion is a fundamental factor to cause interchain aggregation under film post-treatment, device fabrication, and Joule self-heating in device operation. Herein, the suppression of the chain motion through the cross-linking chemical successfully avoided the aggregation-induced residual physical defect emission in wide band gap polyfluorene (POEYDPF). Compared to pristine ones, high Young's modulus (Er) and hardness (H) of cross-linked film effectively confirmed the rigid physical molecular landscape of the POEYDPF chain with low chain segmental motion and dense random interchain packing. According to the ultrafast spectroscopic spectral analysis, this residual defect emission at long wavelength can be suppressed in this rigid cross-linked network. As a result, the cross-linked film presents a deep-blue lasing behavior with an extremely low threshold of 0.70 μJ/cm2, attributed to the low optical loss in the cross-linked topological network. Finally, efficient and stable deep-blue PLEDs with three well-resolved emission peaks of single-chain are also fabricated with a high current efficiency (C.E.) of 1.57 cd/A (twice that of the pristine ones).