Highly Efficient Orange-Red Thermally Activated Delayed Fluorescence Emitters And Their Applications In Organic Light-Emitting Diodes

Thermally activated delayed fluorescence(TADF)materials have gradually developed into a new generation of organic light-emitters and are widely used in organic light-emitting diode(OLED).While blue and green TADF materials are booming,the development of red TADF materials lag behind due to the limitation of the energy-gap law.In order to solve the problem of scarcity of high-efficiency red TADF materials and devices,we used different molecular design strategies to develop a series of highly efficient orange-red emission TADF materials,and applied them to TADF-OLEDs.Firstly,we successfully designed and synthesized two new orange-red TADF materials CN-BP-TPA and CF3-BP-TPA by secondary acceptor modification.The experimental results demonstrated that the introduction of secondary acceptors can promote the red shift of the wavelength,and effectively improve the photoluminescence quantum yield(ΦPL)and device efficiencies.Linking different secondary acceptors resulted in a large difference in the device performances of CN-BP-TPA and CF3-BPTPA.In the doped devices,the CN-BP-TPA-based device showed efficiencies of 26.0%,76.8 lm W-1,and 61.1 cd A-1,the wavelength was 580 nm.However,the CF3-BP-TPAbased device exhibited efficiencies of only 16.6%,59.2 lm W-1,and 50.9 cd A-1 with wavelength of 555 nm.In the non-doped devices,the device efficiencies were significantly reduced by the severe concentration quenching effect,while the wavelength were red-shifted by more than 20 nm.The introduction of the secondary acceptor promoted the wavelength red-shifted,and effectively improved the ΦPL and device efficiencies of the new materials,which provides a new idea for designing efficient red TADF materials.Secondly,we obtained two new materials DPPZ-DMAC and DPPZ-2DMAC by adopting stronger donors and acceptors and improving molecular rigidity.Stronger donors and acceptors promoted the wavelength red-shifted,better rigid structures were beneficial to suppress nonradiative decay processes and improve material’s efficiency.In the doped device,DPPZ-DMAC-based devices exhibited the best efficiencies when the doping concentration was 6 wt%,the device efficiencies were 27.8%,64.2 lm W-1 and 59.7 cd A-1,respectively,the emission wavelength and CIE coordinates were 598 nm and(0.55,0.44),respectively.This is one of the better results of the reported orange-red TADF-OLEDs.Due to the influence of strong non-radiative decay process,the device of DPPZ-2DMAC showed low efficiencies.When the doping ratio was 6 wt%,the device efficiencies were only 10.3%,10.7 Im W-1,and 10.6 cd A-1,and the wavelength shifted to 634 nm,CIE coordinates was(0.62,0.36).This work indicated that enhancing the electron-donating/withdrawing capabilities of the donors/acceptors and improving the molecular rigidity is one of the good methods to obtain efficient orange-red TADF emitters.Finally,we adopted a multi-donor strategy to developed two orange-red TADF materials 2,7,12-triTPA-BPQ and 2,7,12-triDMAC-BPQ.Benefiting from a high ΦPL(84%),2,7,12-triTPA-BPQ achieved excellent device performances with EQEmax,wavelength,and CIE coordinates of 25.1%,590 nm,and(0.53,0.46),respectively.The wavelength of the doped OLED device based on 7,12-triDMAC-BPQ was red-shifted to 620 nm,the color coordinates was(0.59,0.39),while the EQEmax was only 7.5%.This work demonstrated that increasing the number of donors and ensuring efficient reverse intersystem crossing(RISC)process is an effective way to obtain high-efficiency orangered TADF emitters.