White Organic Light-emitting Diodes Based On Thermally Activated Delayed Fluorescence

White organic light-emitting diodes(WOLED)have been commercialized after decades of rapid development with its unique advantages of low power consumption,earthquake resistance,flexibility,high efficiency and high brightness.It has unlimited potential in the future development and application fields.However,it is undeniable that the performance of WOLED is not perfect and its preparation cost is not cheap enough.Further research is needed to truly replace the traditional lighting source into thousands of households.In order to reduce device cost and simplify device structure,based on thermally activated delayed fluorescence(TADF)mechanism,a series of single-emitting layer full-TADF WOLEDs,multiple-emitting-layer full-TADF WOLEDs and singleemitting layer TADF/phosphorescence hybrid WOLEDs were prepared and studied.Firstly,two novel orange-red TADF materials(DPPZ-DMAC and DPPZ-2DMAC)combined with blue TADF material 2tCz2CzBn were used to prepare a series of singleemitting layer full-TADF WOLEDs.The WOLED based on DPPZ-DMAC with a nearly zero singlet-triplet energy level difference(0.01 eV)and a high absolute quantum yield(ΦPL,91.6%)was finally achieved high efficiency of 22.7%/66.9 lm W-1/51.9 cd A-1.However,in order to further reduce triplet-triplet annihilation and eliminate the accumulation of triplet excitons on 2tCz2CzBn,we prepared WOLED using a double host(7:3)composed of traditional fluorescent material(mCBP)and TADF sensitizer(2tCz2CzBn)instead of a single TADF host.By using an effective host strategy,several important conditions were achieved at the same time,including inhibition of sensitizersensitizer interaction,balanced carrier transport and reasonable modulated energy transport channel.Finally,a warm white WOLED with 30.1%excellent EQEmax,more than 80 lm W-1 PEmax,low Von of 2.5 V and about 3600 K CCT was obtained.Furthermore,the CIE coordinates were close to standard blackbody radiation and can be precisely within the specific quadrilateral given by the American National Standards Institute(ANSI),which means that this design has broad prospects for indoor health lighting.Then,we used an orange-red TADF material(CN-BP-TPA)and blue TADF material 2tCz2CzBn to prepare a series of multiple-emitting layers full-TADF WOLEDs.We first prepared a single-layer WOLED based on CN-BP-TPA,however,its wavelength is not long enough,we finally chose a more suitable multiple-emitting-layer structure to fabricate WOLED.By increasing the doping ratio of orange-red material from 10 wt%to 20 wt%,the WOLED spectrum was successfully changed from cold white to daylight white to warm white,and the EQEmax/PEmax/CEmax of WOLED increased from 15.0%/38.1 lm W-1/3 1.5 cd A-1 to 18.3%/42.7 lm W-1/35.3 cd A-1.The design of multipleemitting-layer provided a new idea for us to prepare WOLEDs based on emitters with different wavelengths.Finally,we used a yellow phosphorescent material(PO-01)combined with blue TADF material 2tCz2CzBn to fabricate a single-emitting layer hybrid WOLED.Through a series of optimizations,the WOLED based on emitting layer of 2tCz2CzBn:1 wt%PO01 finally achieved the EQEmax of 29.3%,PEmax of 84.2 lm W-1 and CEmax of 77.5 cd A1.Notably,the device exhibited warm white emission,CCT of 3410 K and CIE coordinates of(0.43,0.46).Similarly,this kind of WOLED with less blue light demonstrated the good eye protection function and the broad application in indoor health lighting.Most importantly,the hybrid WOLED adopted blue material 2tCz2CzBn as the host material of PO-01,which not only reduced the cost of device preparation,but also simplified the device structure.