Research On Solution-processed High-efficiency Small-molecule Organic Light Emitting-diodes

Organic light-emitting diodes?OLEDs?have been widely concerned and favored due to their advantages of ultra-thin film,light weight,flexibility,low power consumption and high color purity.In the future,OLEDs with large screen,low cost and high resolution will be the main trend of the development of novel display technology.Solution process can effectively achieve the above development demand.On the other hand,organic materials are the key technology of OLEDs,small molecules have superiority in easy synthesis and purification,small batch differences and wide selection.Therefore,it is of vital significance to develop solution-processed small-molecule OLEDs for China to seize the future market of new display equipments.In this thesis,some effective schemes are proposed to improve the performance of solution-processed small-molecule OLEDs and a series of innovative results are obtained.1.Three interfacial exciplex hosts with high triplet energy level and thermally activated delayed fluorescence?TADF?properties were innovatively introduced into the solution process to prepare TADF-OLEDs based on the blue small emitter 4CzFCN.All of the blue OLEDs achieved more than 20%EQE,low driving voltages and moderate efficiency decrease.Among them,the device based on the CDBP/PO-T2T interfacial exciplex host realized a high maximum EQE of 21.0%,a low turn-on voltage of 3.6 V.Besides,the driving voltages of the device are only 4.4 V and 5.8 V at the working brightness of 100 and 1000 cd/m²,and EQEs are still remain 20.2%and 13.6%.The performance is one of the best reported blue TADF-OLEDs based on solution process.2.Based on the above-mentioned high-efficiency blue OLED of CDBP/PO-T2T system,the red small-molecule phosphorescent Ir?MDQ?₂acac was introduced into the emitting layer to prepare high-performance two-color white-OLEDs?WOLEDs?.The optimal solution-processed WOLED shows an excellent maximum EQE of 20.8%and can still maintain high level EQEs of 20.2%and 11.8%at working brightness of 100and 1000 cd/m².In addition,EL spectrum of the WOLED is stable that in the actual working brightness range of 10²000 cd/m²,Commission Internationale de I'Eclairage?CIE?coordinates are stable near?0.33,0.39?,and Color Rendering Index?CRI?values are all above 75.6.Our WOLED realizes one of the highest EQE in the reported solution-processed WOLED based on TADF emitters.3.The environmental friendly and low-cost 1,8-Octanediol?8-OD?was introduced into the emitting layer to improve the OLEDs performance.The results show that the solution-processed film quality can be improved by adding 8-OD with a mass ratio of 1³%.When 1%and 2%8-OD were added into the emitting layer,the turn-on voltages of blue and green small-molecules TADF-OLEDs are 0.2⁰.3 V lower than that of standard devices,and EQEs were increased with 20%.In the absence of PVK as the hole transport layer,the solution-processed blue and green TADF-OLEDs realized very high maximum EQEs of 19.3%and 22.3%respectively,which are one of the highest reported EQE.The research results reported in this thesis provide some effective schemes for the preparation of high performance solution-processed small-molecule OLEDs,which is beneficial to simplify and optimize the device structure and improve the performance of the device.