Synthesis And Electro-optical Property Investigation Of Thermally Activated Delayed Fluorescence Materials Used For Solution-processed Oleds

Solution-processed organic light-emitting diodes(OLEDs)have attracted great attention as a device for application in large-size OLEDs because of simple fabrication process,low-cost manufacturing technology and easy scalability of the solution process.As critical component of OLEDs,pure organic fluorescence materials with thermally activated delayed fluorescence(TADF)that can realize 100%internal quantum efficiency are regarded as promising materials for the third generation of OLEDs.Although TADF OLEDs can approach 100%internal quantum efficiency,the excited state quenching induced by the triplet-triplet annihilation should be minimized by utilizing the host-guest systems to achieve highly efficient electroluminance.However,a host-guest system is normally composed of two/multi-component mixtures,which makes solution-based process challenging derives from inevitable phase separation.In this work,to address this issue,a series of novel TADF materials,which forms with polymers and dendrimers,was developed to be an alternative strategy to restrain the concentration quenching,improve the luminous efficiency,balance the carrier transporting and reduce the operating voltage.The main content of this dissertation is as follows:1.We designed and synthesized two thermally cross-linkable materials,DV-CDBP and DV-MOC-DPS,which act as a host material and a TADF emitter for solution-processed OLEDs,respectively.Utilizing the method of in-situ crosslink,the cross-linked films with the different mass ratios of DV-CDBP and DV-MOC-DPS were investigated.The cross-linked films possess excellent thermal,morphological stabilities and corrosion-resistance.Employing the cross-linked film as emitting layers,an efficient blue solution-processed OLED with a maximum external quantum efficiency of 2.0%was achieved.The molecular design reported here provides a novel route toward highly efficient solution-processed OLEDs.2.We synthesized two high efficient TADF dendrimers based on the orientation of molecular design and the utilization of peripheral dendrons.Introduced the phenyl bridge to increase the distance of the emission core TA between donor(D)and acceptor(A)is a promising route to simultaneously achieve small singlet-triplet energy splitting(ΔEST)and enhanced PL quantumyields(PLQYs).Furthermore,non-conjugated aliphatic chains carbazole/tricarbazole dendrons were conveniently introduced to the TADF core,which can effective encapsulate the emission core to restrain the concentration quenching effect and make the fluorescence of the core independent.By utilizing the emitter as the non-doped solution-processed emissive layers,the resulting yellow OLED achieved low driving voltage of 2.4 V and superior external quantum efficiency of 11.8%,which is comparable to the most-efficient nondoped solution-processed fluorescent OLED.3.Three near-infrared(NIR)dendrimers with TADF characteristics are reported to develop the non-doped solution-processed OLED for the first time.We select the well-known red TADF molecule as principal ingredients because of their excellent performances for OLED application.Introducing the methoxy substituents for the enhanced electron-donating ability and the longer conjugation length owing to the existence of lone-pair electrons.Thus,the modified core with the characteristic of TADF and the emission of NIR can be anticipated.The rigid ring end-capped aliphatic chain dendrons are introduced to improve the dissolvability and film-forming ability.Simultaneously,the functional dendrons can encapsulate the core to prevent concentration quenching and render balanced charge transportation.Employing MPPA-MCBP as the emitter,the non-doped solution-processed device exhibits a maximum external quantum efficiency(EQE)of 0.62%with a peak wavelength at 698 nm,which is comparable to the most-efficient solution-processed NIR FOLEDs with similar electroluminescent spectra,which indicates that the molecular modification is very cost-effective and competitive for improving the device performance.4.As we know,the utilization of exciplex-forming has been considerd as an idea strategy for realizing the power-efficient OLEDs.With this idea in mind,two bifunctional TADF dendrimers have been designed and synthesized.Non-conjugated connection can make the photophysical properties of TADF core and the exciplex-forming function of dendrons independent.With the utilization of the hole-transporting dendrons and the adjacent electron-transporting layer recombination for the formation of the interfacial exciplex,ensures efficient charge injection and transporting and efficient singlet and triplet energy transfers from the interfacial exciplex to the TADF core.Consequently,the nondoped device of G-mCP achieves extremely low driving voltage of 2.7 V and a high power efficiency of 46.6 lm W-1,which is among the highest power efficiency of the reported TADF s-OLED.