Highly Efficient Solution-processed Organic Light-emitting Diodes Based On Thermally Activated Delayedfluorescence Materials

In recent years,small molecular organic light-emitting diodes(OLEDs)based on the solution processes have received increasing attention.Compared with vacuum deposition-based device processing technology,solution-processable small molecule materials provide the possibility of low-cost,large-area devices.Furthermore,small molecule compounds present the advantages of an unequivocal monodisperse chemical structure and thus of a potentially better reproducibility of purification rather than polymer counterparts.Compared with traditional fluorescent and phosphorescent emitters,thermally activated delayed fluorescence(TADF)materials can not only realize 100% internal quantum efficiency,but also have the advantages of low-cost and environmental friendliness,considered as the next generation organic flat panel display and lighting materials.However,high performance TADF materials suitable for solution processes are rare,and the efficiency and stability cannot compare with that of the vacuum deposition-based device.Herein,the purpose of this study is to realize high-efficiency red,yellow,green,blue and white solution-processed OLEDs by selecting soluble TADF materials and optimizing device structure.The electroluminescence properties of the solution-processable TADF molecules,such as ACRDSO2,PXZDSO2 and 3,6-2TPA-TXO,had been studied at large.These small molecular TADF materials combine the advantages of simple synthesis,easy purification,high performance and suitable for solution processing.The superior performance of solutionprocessed green OLED based on ACRDSO2 has been obtained by optimizing carefully the device structure.The maximum external quantum efficiency(EQE)and maximum current efficiency(CE)achieved 17.5% and 53.3 cd A-1,respectively.The EQE of device was maintained 14.0% with the operating voltage of 5.6 V at 1000 cd m-2.By using TmPyPB,with both electron transport and hole blocking properties,as the electron transport layer,the carrier recombination region was limited to the light emitting layer to improve the luminous efficiency of the device.The maximum EQEs of the OLEDs using yellow material PXZDSO2 and orangered material 3,6-2TPA-TXO as emitters achieved 15.2% and 10.2%,respectively.A novel solution-processed all-fluorescent white OLED with high device performance had been developed.The non-doped blue OLED based on DMAC-DPS was fabricated with a maximum EQE of 4.9%.Then,the high properties of all-fluorescent white OLED were obtained by binary blending with 3,6-2TPA-TXO and DMAC-DPS as emission layer.The maximum CE,power efficiency(PE)and EQE were 22.5 cd A-1,14.9 lm W-1and 8.0%,respectively.The dilemma between device efficiency and cost was relaxed by using TADF materials as emitters to harvest triplet exciton.Expanding the TADF-based up-conversion systems for conventional fluorescent emitters,the red solution-processed fluorescent OLEDs with high performance were fabricated,composed of a double-dopant system,which have a host with wide energy gap,a TADF assistant host(DC-TC or DC-ACR)and a conventional red fluorescent emitter(DBP).The maximum CE and EQE of the OLED using DC-TC as assistant host achieved as high as 11.9 cd A-1 and 8.0% with the CIE coordinate of(0.61,0.38),respectively.Due to the reverse intersystem crossing of TADF assistant host and the F?rster energy transfer between TADF assistant host and fluorescent emitter,the triplet excitons of assistant host could be translated effectively to the singlet level of DBP.Therefore,the theoretical limitation(~5%)of traditional fluorescent OLED was broken.Exciton dynamic study of the ternary system revealed that the rapid F?rster energy transfer channel enabled the significant reduction of singlet and triplet exciton density,which contributed to the suppressed roll-off of OLEDs operating at high current density.Regarding TADF assistant host molecular design,maintaining a moderate singlet radiative rate constant(kr S)for large F?rster energy transfer radius is as important as keeping a relatively small singlet-triplet splitting energy(?EST)for efficient triplet exciton utilization in this ternary prototype.