Research Of High Performance Solution-Processed Red Phosphorescent Organic Light-Emitting Diodes

Organic light-emitting diode（OLED）is one of the most promising technologies for the future of lighting and display.At present,most OLEDs are still fabricated by vacuum evaporation technology,but the equipment operation cost is high,and the device structure and operation flow are complex.In contrast,the solution method is easy to operate and shows great potential in low-cost and large-scale preparation of OLEDs.In this paper,high performance red phosphorescent organic light emitting diodes（PhOLEDs）were prepared by solution method.We mainly optimized the light-emitting layer and the electron transport layer of the devices.We also investigated the effects of doping concentration of guests,types and proportions of mixed hosts,types and doping concentration of assistant dopants,types and thickness of the electron transport materials on device performance.The fabrication process and method of the devices were improved.Based on the device structure and preparation method,the performance of red phosphorescent OLEDs is greatly improved compared with the conventional solution method.The main contents are as follows:The high efficient red phosphorescent OLEDs were fabricated by selecting TAPC and 26DCzPPy as co-hosts,PQ₂Ir（dpm）as red emitter,and rare earth europium complex Eu（DBM）₃Phen as sensitizer,which has appropriate energy level and higher triplet energy.With the increase of doping concentration,the exciton was utilized more and more efficiently,and the brightness and efficiency of the device were also gradually increased.With the increase of doping concentration,excitons were more and more fully utilized,the brightness and efficiency of devices were also gradually increased.However,high doping concentration could lead to severe concentration quenching.The device with 4 wt%PQ₂Ir（dpm）realized the best performance.The exciplex was formed when the ratio of TAPC and 26DCzPPy was 1:1.The proportion of the co-hosts has significant effect on the performance of OLED by solution processing method.The introduction of Eu（DBM）3Phen can capture the excess electrons to widen recombination zone and balance the carriers’distribution,thus improving the device performance.Finally,the best performance parameters were obtained when the ratio of TAPC and 26DCzPPy was 3:1,the doping concentration of Eu（DBM）₃Phen was 0.4 wt%and the thickness of TmPyPB was 70 nm,the turn-on voltage,maximum current efficiency and external quantum efficiency of the device were 2.9 V,47.12 cd/A and 27.3%,respectively.High performance red phosphorescent OLED was prepared by using Ir（MDQ）2（acac）as guest material.The effects of the composition of co-hosts,the types and concentration of auxiliary dopants and the types of electron transport material on the device performance were studied.Experiments showed that the mixture of different functional hosts can promote carriers’ balance,therefore,the comprehensive performance of co-hosts devices is better than single-host device.Then the doping concentration of red emitter and the proportion of co-hosts were optimized,three assistant dopants were selected to co-doped into the light-emitting layer to further improve the performance of the device,respectively.When the doping concentration of assistant dopant PQ₂Ir（dpm）reached the optimum value,the maximum brightness,efficiency and efficiency roll-off of the device were significantly improved.Compared with undoped device,the brightness was increased by 2.4 times,and the efficiency roll-off was decreased from 17.9%to 7.5%.This is because PQ₂Ir（dpm）can be used as a platform for exciton storage and transfer,at the same time,the red emitter Ir（MDQ）₂（acac）adds an exciton source,which makes energy transfer processes between materials more effective.Then we selected different kinds of electron transport materials for experiments and found that the electron mobility,energy level and the thickness of electron transport layer would significantly affect the performance of devices.Finally,red phosphorescent OLED with brightness of 14628 cd/m²,maximum current efficiency arriving at 39.09 cd/A and external quantum efficiency arriving at 22.7%was obtained.