Emitter And Device Designs Of Blue-Hazard-Free White Organic Light-Emitting Diodes Based On Blue MultiResonance Delayed Fluorescent Emitters

This thesis mainly focuses on the multiple resonance(MR)-type thermally activated delayed fluorescence(TADF)emitters based blue-hazard-free white organic light emitting diodes(WOLED).A series of studies have been conducted around the design of blue MR-TADF emitters for WOLED,as well as the optimization of the WOLED device performance.The main achievements are summarized as follows:(1)Conventional donor-acceptor-type TADF emitters generally suffer large structural relaxation and thus exhibit broad emission.WOLED containing such emitters as blue components thus generally generate high energy blue photons with wavelength shorter than 440 nm.To address this,here we employ a reported pure blue MR-TADF emitter t-DABNA as the blue component in WOLED.Thanks to its MR-TADF feature,t-DABNA exhibits a blue narrowband emission peaked at ca.464 nm with a full width at half maximum(FWHM)of ca.31 nm,which allows t-DABNA to precisely avoid contribution of the high-energy photons before 440 nm and thus be a promising blue component to construct human-eye-friendly WOLED.Particularly,in double-emittinglayer WOLED using t-DABNA and a high-performance TADF emitter BPPZ-DPXZ respectively as blue and orange components,blue-hazard-free white emissions were thus achieved with a maximum external quantum efficiency(EQE)of 26.5%.Moreover,the EL spectra were rather stable with a variation of Commission Internationale de l’Eclairage coordinates of only(0.00,0.01)in a wide luminance range from 100 to 10000 cd m-2 due to the stable exciton allocation and harvest.It is expected that our strategy can pave a new way for developing highly efficient all-TADF WOLED without blue-hazard emission.(2)Blue emitters are indispensable in fabricating WOLED.Their properties such as efficiency and color purity are closely related to the performance of WOLED.MR-TADF emitters can achieve efficient TADF while restricting molecular relaxations relying on the MR effect.Therefore,MR-TADF emitters are regarded as the promising candidates to fabricate high-performance WOLED.However,MR-TADF emitters are generally with planar molecular structures and thus easily lead to strong intermolecular π-π stacking.And it will not only give rise to unavoidable concentration quenching and energy loss,but also hinder the spectral modulation of WOLED.To address this problem,herein,a new blue MR-TADF emitter DPMX-CzDABNA was synthesized by introducing a diphenyl fluorene group as peripheral steric hindrance onto a previous reported framework CzDABNA.DPMX-CzDABNA displays a blue emission band with an emission onset beyond 450 nm.Meanwhile,thanks to the introduction of steric hinderance groups which can largely suppress the intermolecular π-π stacking,concentration quenching of DPMX-CzDABNA can be well relieved.Thus all the fabricated blue OLEDs based on DPMX-CzDABNA(with doping concentrations<20 wt%)can realize maximum EQEs around 25%with onsets beyond 440 nm.By further employing DPMX-CzDABNA as the blue component in two-complementarycomponent-based WOLED,the fabricated devices all exhibit warm white emissions with stable EL spectrum and blue-hazard-free emissions,as well as a maximum EQE of 26.5%.The efficiency roll-off and color render index are also relatively improved compared to those in the former work.This work not only indicates that introducing steric hindrance can effectively suppress the concentration sensitivity of blue MR-TADF emitters,but more importantly proves the great potential of MR-TADF emitters in developing high performance blue-hazard-free WOLED.