| Organic light-emitting devices(OLEDs)have considerable advantages and development potential in both lighting and display fields due to their advantages of self-luminescence,lower operating voltage and wider operating temperature.Recently,exciplex systems have become a hot research direction in the field of OLEDs due to their thermally activated delayed fluorescence(TADF)effect.Compared with the traditional host materials,the exciplex system has bipolarity,wider half-peak width and faster energy transfer rate,in addition,the exciplex system can also effectively reduce the turn-on voltage of the device,so the exciplex systems are widely used as host materials for phosphorescent dyes in phosphorescent OLEDs.In this paper,a novel exciplex system formed by doping hole transport material TAPC and TADF material DMIC-TRZ was first explored.The photoluminescence peak is located at 539 nm and the fluorescence quantum yield(PLQY)is 85.8%.Then,the effect of the change of the ratio of donor and acceptor on the electrical properties of exciplex devices was explored.For most exciplex systems,current efficiency(CE)and power efficiency(PE)are optimal when the ratio of donor and acceptor is 1: 1.However,in this paper,when the doping ratio of TAPC:DMIC-TRZ is 1:6,the device obtains the best performance,where the turn-on voltage is 2.49 V and the maximum CE is 25.12 cd/ A,the maximum PE is 30.59 lm/W.Through the preparation of single-loaded devices,it was found that the reason for the best performance in the new exciplex system when the ratio of donor to acceptor was 1:6 was that the transport of holes and electrons was the most balanced,the exciton quenching is reduced.Then,the optimized TAPC:DMIC-TRZ exciplex was doped with an orange phosphorescent material PO-01 to prepare a phosphorescent OLED.When the doping concentration was 8%,the device achieved a maximum current efficiency of 61.15 cd/A and a low turn-on voltage of 2.02 V,while the efficiency roll-off is only 0.13%,the highefficiency ultra-low-efficiency roll-off is attributed to the extremely balanced carrier transport capability and efficient energy transfer mechanism.Secondly,because the TAPC:DMIC-TRZ exciplex cannot be used as the host of the blue phosphorescent dye,blue phosphorescent OLED devices with different hosts were prepared.The experiment found that when MCP was used as the host of the blue phosphorescent dye,the device had the highest luminance efficiency.Therefore,on the basis of the optimized orange phosphorescent OLED,MCP is inserted as the host and FIr Pic is used as the blue light-emitting layer of the dye to prepare a complementary color white phosphorescent OLED.The maximum CE and PE of the optimized white phosphorescent OLED are 46.7 cd/A and 31.3lm/W,respectively,the turn-on voltage is 2.7 V,the color rendering index is 61,and the color coordinates at 5 V are(0.36,0.42).However,as the voltage increases,the exciton recombination region moves,resulting in a decrease in the spectral stability of the white light device,a large color coordinate shift,and a low luminance efficiency of the blue layer,which reduces the overall luminance efficiency of the white light device.Therefore,in order to further improve the efficiency,color rendering index and spectral stability of the white light device,a green light layer is inserted between the orange light layer and the blue light layer on the basis of the two-color white phosphorescent OLED.The green light layer can improve the device efficiency and expand the spectrum coverage,while acting as a spacer layer to improve spectral stability.The optimized three-color white light device,the spectral stability is improved,the color coordinates are shifted from(0.303,0.430)at 6 V to(0.319,0.437)at 9 V,only shifted(0.016,0.007),and the maximum CE is 51.1 cd/A,the maximum PE is45.4 lm/W,the turn-on voltage is 2.38 V,and the color rendering index is 78. |
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