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

Abstract 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 π–π 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 (β) 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.