Stable Deep‐Blue Electroluminescence from Hierarchical Crystalline Structure Based on Planar Conformational Terfluorenes: Benefits of Preventing Defect Structures Formation

Abstract Liner‐type organic conjugated molecules with 1D π‐electron delocalization present conformation‐dependent intramolecular optoelectronic behavior. Significantly different from non‐planar ones, the coplanar conformational backbone of π‐conjugated molecules can enhance intramolecular π‐electron delocalization and dominate intermolecular π‐electron coupling, which are the fundamental parameters for improving their optoelectronic properties. Herein, the effect of the formation of coplanar conformation on intermolecular packing and optoelectrical properties of liner‐type steric terfluorene (DSFX‐SFSO) toward solution‐processed deep‐blue organic light‐emitting diodes (OLEDs) is systematically demonstrated. Firstly, uniform and continuous crystalline films based on the planar conformational DSFX‐SFSO molecules are achieved via thermal annealing at 160 °C, which can avoid microscale crystallization processing. Contrary to amorphous ones, these hierarchical crystalline films present robust deep‐blue emission with an efficiency of > 60%, due to the suppression of defect structures. More interestingly, this hierarchical crystalline film has a stable and efficient deep‐blue electroluminescence (CIE: 0.15, 0.09) without the obvious defective green‐band emission, even at 500 mA cm⁻², also further confirming the excellent and perfect molecular landscape to obtain single‐chromophore excitonic behavior from single planar‐terfluorene segments. Therefore, obtaining a uniform hierarchical structure based on planar conformational molecules is an effective strategy to optimize the optoelectrical properties for optoelectronic applications.