Research On Thermally Activated Delayed Fluorescence-based White Organic Light-emitting Devices

Organic light-emitting Device?OLED?has been used in a certain proportion of mid-and high-end mobile phones and the proportion is increasing year by year.Large-size OLED displays have also gradually joined in people's lives,becoming a choice for some high-income families to improve their life quality.White OLED desk lamps also appear on the market.However,the high cost of OLED still hinders its further increase in market share.Therefore,reducing the cost is the key to popularize OLED production.Commercial OLEDs are mainly fabricated by vacuum thermal evaporation progress,which is the most mature technology of OLED at present.This research also adopts this technology.For now,most of the high efficiency luminescent materials are phosphorescence containing heavy rare metal,which not only increases the production cost of devices,but also pollutes the environment.Therefore,Thermally Activated Delayed Fluorescence?TADF?,a pop luminescent material in recent years,has attracted wide attention.TADF material with small?E_ST value accelerates exciton transferring from triplet?T₁?to singlet?S₁?by absorbing heat from the environment,which is called Reverse Intersystem Crossing?RISC?process.Finally,the exciton achieves 100%utilization.Currently,the lifetime of TADF-based OLED is not ideal.We designed TADF material with stable performance as the host of phosphorescent material to alleviate the efficiency roll-off of phosphorescent OLED and keep a high efficiency at the same time,so as to design a device with better stability for further research.We select a weak donor,carboline derivative?nitrogen heteroatom at?-position?,design a TADF material,DCb-BPP.DCb-BPP as the host of phosphorescent material,achieves high efficiency?External Quantum Efficiency,EQE=21.5%?and low efficiency roll-off(the brightness of which the efficiency drops to half of the maximum values of EQE,B_1/2=52000 cd/m²).At the same time,the stability of the device is very excellent.The LT50?time to 50%of initial luminance of 1000 cd/m²?of the unpacked device reaches more than 400 hours,which is 20 times longer than the classical host CBP-based device.Due to the unbalanced carrier transport of DCb-BPP,we choose to introduce a hole transfer material to balance the carrier transport of the host.The optimized device with balanced carrier transfer achieves a better efficiency?EQE=24.5%?and efficiency roll-off(B_1/2=72000 cd/m²).And then,we design a tri-layer phosphorescent white OLED with DCb-BPP as the host of yellow and red phosphorescent materials,and combined with the classical blue phosphorescent material FIrpic.The resulting white light OLED achieves high color rendering index?CRI>88?,low color temperature?CCT<3000 K?,and small D_UV<0.0054 that meets lighting requirements.For now,many reported WOLEDs based on TADF materials have the disadvantages of serious efficiency roll-off,low brightness and a low color quality.Therefore,we designed a bluish-green TADF material,and cooperated with the yellow light and deep red light phosphor materials to realize the high-efficiency and low-efficiency roll-off of the fluorescent/phosphorescent hybrid WOLED,and at the same time meeting the standard of high-quality white light with high CRI,low CCT and blue light hazard free.Next,we choose acridine group as strong donor with pyrimidine design in TADF,DMAC-BPP.DMAC-BPP-based bluish-green fluorescent device achieves EQE of 13%,which can be called as an efficient emitter.The yellow phosphorescent device with the host of DMAC-BPP has an outstanding efficiency roll-off(B_1/2=77000 cd/m²).Based on these researches,we design a fluorescent and phosphorescent hybrid white OLED with DMAC-BPP as host and emitter.The white OLED achieves high color rendering index?CRI>80?,low color temperature?CCT<2500 K?,small D_UV<0.0054,and blue light hazard free.TADF material assists traditional fluorescent materials emitting is an effective method to get a fluorescent OLED with high efficiency.The reported fluorescent OLED cannot ignore the emission from TADF,which affect the CIE of the whole device.For this reason,we designed a green fluorescent device with high efficiency and high color purity,and further designed a fluorescent white light device based on this theory.We choose TADF,DMAC-BP,promoting the classical fluorescent material C545T emitting.The fluorescent device achieves a high efficiency,EQE=12%.At the same time,the CIE of the device meets the requirements of sRGB,CIE?0.285,0.627?.The test proves that the accelerated exciton radiation is benefit to improve the stability of the device.Then,we selected the blue TADF material and the red fluorescent guest to obtain the double-layer full-fluorescent WOLED,corresponding to EQE of 8.5%.At the brightness of 5500 cd/m²,the device's CRI is65,and the CCT is 5300 K.