Study On High-Efficiency Red And White Organic Light-Emitting Diodes Based On Exciplex System

Organic light-emitting diodes are widely used in the display and lighting fields due to their advantages of thinness,flexibility,self-illumination,wide viewing Angle,wide color gamut,low power consumption,high contrast,and no blue light hazard.In recent years,with the proposed theory of thermally activated delayed fluorescence,which can convert triplet excitons into singlet excitons through reverse intersystem crossing,exciplex with the phenomenon of thermally activated delayed fluorescence have aroused a great deal of interest.However,white OLEDs based on exciplex usually need to prepare blue exciplex,which inhibits its further development.Meanwhile,the efficiency and color rendering index of white OLEDs need to be further improved.In this paper,the white light emission is achieved by yellow exciplex co-host doped with red dye and ultra-thin layer,which provides a feasible solution for yellow exciplex to realize high-efficiency white OLEDs.Detail contents in this thesis are as follows:(1)Firstly,it was verified by photoluminescence spectra and transient delay curve that the mixed film of donor material N-([1,1'-biphenyl]-2-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9'-spirobi[fluoren]-4-amine(FSF4A)and acceptor material 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine(PO-T2T)can form exciplex.Prepared pure exciplex emission devices with different molar ratios and studied their electroluminescence properties.We obtained a yellow OLED,achieving a maximum external quantum efficiency of 6.48%,a maximum current efficiency of 16.4 cd/A and a maximum power efficiency of 17.2 lm/W,while achieving a low turn-on voltage of2.4 V and a maximum brightness of 18513 cd/m~2.Such superior performance can be attributed to the highly efficient RISC process of the exciplex.Subsequently,exciplex is used as co-host to dope different concentrations of red phosphorescent material to prepare high-performance red OLEDs.The optimal devices achieve a maximum external quantum efficiency of 17.3%,a maximum current efficiency of 30.6 cd/A and a maximum power efficiency of 38.5 lm/W,the red OLEDs also achieve an ultra-low turn-on voltage of only 1.95 V and a maximum brightness of 35490 cd/m~2.(2)White OLEDs are prepared by employing yellow exciplex as co-host doped with red phosphorescent material and blue ultra-thin layer.To avoid energy transfer,different interlayer materials are inserted between the blue ultra-thin layer and the red light layer.The white OLED of the interlayer material MCP achieves a maximum current efficiency of 29.6 cd/A and a maximum power efficiency of 18.6 lm/W.We believe that this is due to the formation of interface exciplex between MCP and PO-T2T,which improves the utilization rate of blue materials for excitons.The white OLEDs are optimized by adjusting the thickness of the interlayer.The optimized white OLED achieves standard white light with CIE coordinates(0.33,0.33)and a low turn-on voltage of only 2.2 V,while obtaining a maximum current efficiency of 32.6 cd/A,a maximum power efficiency of 34.1 lm/W,and a maximum external quantum efficiency of 12.4%,but the color rendering index of the device is only 52.In order to further improve the efficiency and color rendering index of white OLEDs,we insert high-efficiency green phosphorescent ultra-thin layer,and adjust the thickness of the interlayer between the two ultra-thin layers to optimize the white OLEDs.The optimized white OLED achieves a maximum current efficiency of 39.8 cd/A,a maximum power efficiency of 41.7 lm/W,and a maximum external quantum efficiency of 17.3%,and the color rendering index is improved to 80.Blue materials generally have the problem of insufficient efficiency.Therefore,inserting an ultra-thin green light layer can effectively reduce the emission of blue light materials,and further enhance the green light emission to improve the performance of white OLEDs.