Stable Deep-Blue Polymer Light-Emitting Diodes with Well-Resolved Emission from a Planar Conformational Chain of Polydiarylfluorenes via Alternating Copolymerization

Control of the secondary conformational behavior of light-emitting conjugated polymers (LCPs) is essential to obtain deep-blue emission with a narrowband and stable color purity. Naturally pi-pi interactions induced by the planar aromatic segments may be easily observed in liner-type LCPs, which lead to reduced emission efficiency, color purity, and energy bandgap. Herein, an alternating copolymerization strategy is proposed to obtain the singlet exciton behavior of a planar conformational segment from polydiarylfluorenes-copolymers (P7-6DPF, P7-8DPF, and P7-9DPF) toward deep-blue polymer light-emitting diodes (PLEDs). Compared to the highly crystalline capacity of P7DPF, P7-8DPF, and P8DPF, alternative P7-6DPF and P7-9DPF present a feature and well-resolved emission from a planar conformational segment of polyfluorene without obvious polaron features and charge transfer states. More interestingly, P7-6DPF and P7-9DPF show thickness-independent electroluminescence (EL) behavior with a stable deep-blue color purity, which is the precondition to fabricate large-scale and high-quality PLEDs. Finally, compared to control devices, PLEDs based on the P7-6DPF and P7-9DPF planar (beta) conformational films present a well-resolved and narrowband emission, stable EL spectra, high brightness (two- to threefold), and high current efficiency (1.3-fold). Therefore, optimizing hierarchical structure via alternating copolymerization is an effective strategy to improve the performance and stability of deep-blue PLEDs.

Automated summary

This study proposes an alternating copolymerization strategy to control the secondary conformational behavior of light-emitting polymers and achieve deep-blue emission with narrowband and stable color purity. By optimizing the hierarchical structure, deep-blue polymer light-emitting diodes (PLEDs) with stable electroluminescence, high brightness, and high current efficiency are successfully fabricated on a large scale. This strategy improves the performance and stability of deep-blue PLEDs.