Synthesis And Characterization Of Low-Triplet-Energy Host Materials Encapsulated With Carbazole Dendrons

Compared with fluorescent counterparts,phosphorescent emitters can harvest both singlet and triplet excitons to obtain a theoretical 100% internal quantum efficiency.Owing to the strong concentration quenching,they are usually doped into an appropriate host matrix for the fabrication of phosphorescent organic light-emitting diodes(PhOLEDs).In such host-guest systems,the triplet energy of the host is at least 0.1 eV higher than that of the phosphorescent guest to avoid the triplet energy back transfer from host to guest.This means that a wide bandgap host is required in blue PhOLEDs,leading to the increased driving voltage and poor device performance as a result of the enhancement of the charge injection barrier.To solve the afore-mentioned problem,in this dissertation,we propose a promising strategy to realize high-performance blue PhOLEDs based on a low-triplet-energy host.By encapsulating the low-triplet-energy host with large-size dendrons at its periphery,the direct contact between the low-triplet-energy host and blue phosphor could be prevented so as to avoid the unwanted triplet energy back transfer.Therefore,high efficiency is within our expectation for blue PhOLEDs when the triplet energy of the host is lower than or close to that of the blue phosphor.With this idea in mind,a series of dendritic hosts(F-G1,F-G2,F-G3 and F-G4)have been designed and synthesized using a planar fluorene unit as the core and oligocarbazole as the dendron.It is found that the triplet energies of F-G1~F-G4(2.60-2.68 eV)are mainly decided by the central fluorene core and nearly independent on the dendron generation.However,as for their corresponding blue electrophosphorescent devices containing FIrpic as the blue dopant,the peak luminous efficiency is significant increased from 4.0 cd/A(1.9 lm/W,1.8%)of F-G1 to 26.5 cd/A(16.4 lm/W,11.5%)of F-G4.The obtained 5.6-fold improvement can be ascribed to the elimination of the triplet energy back transfer because of the effective encapsulation to the lower-triplet-energy host core with the increasing dendron generation.We believe that this work will shed light on the development of low-triplet-energy hosts used for efficient blue PhOLEDs in the future.