Research On High-Efficiency Phosphorescent Organic Light-Emitting Diodes Based On Exciplex

The application of Organic light-emitting diodes(OLEDs)in full-color display panels and solid-state lighting has received great attention due to their unique characteristics such as mercury-free,low power consumption and flexibility.In addition,compared with traditional inorganic light-emitting diodes(LEDs),OLED can save 20%home electricity consumption and even up to 50%for office.In recent years,there have been great achievements on the research of exciplex.When the exciplex is used as host,it can greatly improve the device performance due to the properties of thermally activated delayed fluorescence(TADF),low energy barrier and bipolar transport.Based on such advantages of exciplex,in this work,we have successfully designed two new exciplex hosts for highly efficient monochrome and white OLEDs.In order to fabricate high-performance white OLEDs,the research on monochrome devices are indispensable.So in the first work,we designed two exciplex materials for different phosphorescent emitters.Because of the high triplet energy of blue dopants,the host with high triplet energy level is needed to improve the efficiency of energy transfer of the blue device.In addition,for green,yellow,orange and red phosphorescent materials,we chose an exciplex-based TADF host material with low triplet energy level and short exciton lifetime.Finally,upon optimization of the device structures,the blue,green,yellow,orange and red OLED devices exhibit maximum external quantum efficiency(EQE)of 26.3%,25.0%,30.7%,32.5%and 26.2%,respectively.Also,these devices have ultra-high power efficiency(PE)and ultra-low turn-on voltage,which are the requisite for high-efficiency white OLEDs.Based on the previous research on exciplex-based monochrome OLEDs,in the second work,we selected the blue unit and yellow unit in the first work to fabricate high-efficiency two color multiple emitting layer white OLEDs.Based on the advantage of two emitting layers,there is an energy transfer between the two exciplexes which greatly improve the utilization of excitons.By adjusting the thickness of the emitting layers,we have obtained a color stable warm white OLED with high luminous efficiency,low roll-off and color stability.The device has an ultra-high EQE of 36.9%,and its PE has reached 137.4 lm W-1 due to its low turn-on voltage of 2.2 V.Even at the luminance of 1000 cd m-2,the efficiency still remained 34.9%and 105.6 lm W-1.Moreover,the CIE shift of the device from 1000 cd m-2 to 10 000 cd m-2 is only(0.03,0.01).This work gives rise to a new energy transfer mechanism between the two emitting layers,which provides a new idea for designing white OLEDs.Due to the low color rendering index(CRI)of the two color white OLEDs,next,we fabricated a three color multiple emitting layer white OLED to improve the CRI.The deep red and deep blue phosphor dyes are beneficial to improve the CRI of devices,so we used red phosphorecent materials instead of yellow one,and added green dye to further optimize the emission spectra.Finally,by adjusting the thickness and order of the emitting layers,the device not only has a maximum EQE of 26.9%and CRI of 82 at the CIE of(0.418,0.426),but also has a CIE shift of merely(0.001,0.001)from 1000 to 10 000 cd m-2.This work has made a significant contribution to the application of white OLED in solid-state lighting.