| Organic light-emitting diodes(OLEDs)have been widely used in lighting and display fields due to their advantages of low power consumption,wide viewing angle,flexibility and fast response speed.Among many luminescent materials,phosphorescent materials are widely concerned because they can make full use of singlet and triplet excitons to achieve 100%theoretical quantum efficiency.However,the high operation voltage affects the practical application of OLEDs.Therefore,in this paper,based on three primary colors phosphorescent materials,the device structure is designed and optimized to reduce the operation voltage.The details are as follows:Based on the blue phosphorescent material Bis(2,4-difluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(?)(FIr6),9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole(CzSi)was firstly determined as the host material,and inserted a 4,4',4"-Tris(carbazol-9-yl)triphenylamine(TcTa)film as a ladder layer between the hole transport layer and the light-emitting layer to prepare devices with single light-emitting layer.Molybdenum trioxide(MoO3)and 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile(HAT-CN)were selected as the hole injection layer to compare the performance of the devices,and it was found that the latter had a better effect in reducing the operation voltage.The optimal thickness of HAT-CN was 6 nm.On the basis of the above work,compared with 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane(TAPC)and HAT-CN:TAPC as the hole transport layer,the study found that utilizing HAT-CN:TAPC as the hole transport layer can not only improve the performance of the devices but also reduce the operation voltage.The optimal doping concentration of HAT-CN was 0.2 wt%.The improvement of hole injection and transport reduced the operation voltage.The turn-on voltage of the device was 2.8 V,and the maximum current efficiency,external quantum efficiency and power efficiency were 39.14 cd/A,24.6%and 38.68 lm/W,respectively.Even at the brightness of 1000 cd/m2,the current efficiency and external quantum efficiency as high as 36.73 cd/A and 23.1%,the operation voltage was 3.7 V.Based on the red phosphorescent material iridium(?)bis(2-phenylquinoly-N,C2')-dipivaloylmethane(PQ2Ir(dpm)),the hole transport material TAPC and the electon transport material 1,3,5-tris(6-(3-(pyridine-3-yl)phenyl)pyridine-2-yl)(Tm3PyP26PyB)were selected as the host materials to prepare devices with single light-emitting layer.Little difference in efficiency and operation voltage between these devices.Then,based on the two host materials,devices with double light-emitting layers were prepared,and the thickness of each functional layer was optimized.It's found that the overall film thickness of the devices with double light-emitting layers was increased compared to that with single light-emitting layer,the simplified structure led to lower operation voltage.The widened exciton recombination zone and balanced distribution of electrons and holes on the emitters improved the efficiency and brightness of devices.And in all optimization processes,whether it was to increase the concentration,increase the thickness or increase the operation voltage,the electroluminescent spectra of the device remained stable.Finally,the turn-on voltage was 2.4 V,the maximum brightness was 34280 cd/m2,and the maximum current efficiency,maximum external quantum efficiency and maximum power efficiency were 40.59 cd/A,14.7%and 47.20 lm/W,respectively.Even at the brightness of 1000 cd/m2,the current efficiency and external quantum efficiency were maintained at 36.34 cd/A and 13.1%,respectively,and the operation voltage was only 3.5 V.Based on the green phosphorescent material iridium(?)bis(2',6'-bis(trifluoromethyl)-2,3'-bipyridine)tetraphenylimidodiphosphinate(Ir1),TcTa and 9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H-carbazole(mCPPO1)were selected as the host materials to prepare the devices with single light-emitting layer.It's found that the devices exhibited inferior performance and high operation voltage.Due to the excellent electron transport ability,Irl was doped into the hole transport layer TAPC to prepare devices with simplified structure,which improved the performance of the device while reducing the operation voltage.Finally,the turn-on voltage of the device was only 2.5 V,and the maximum current efficiency,external quantum efficiency and power efficiency were 71.6 cd/A,22.9%and 75.0 lm/W,respectively.At the brightness of 1000 cd/m2,the current efficiency and external quantum efficiency were maintained at 67.2 cd/A and 21.5%,respectively,and the operation voltage was only 3.4 V.Based on the bluish green material bis(2-phenylpyridinato-N,C2')iridium(2-(2',4'-difluorophenyl)-4-methylpyridine)((ppy)2Ir(dfpmpy))as the emitter,firstly,devices with single light-emitting layer were prepared by utilizing hole-type material TcTa and the TADF material 2,4-diphenyl-6-bis(12-phenylindolo)[2,3-a]carbazole-11-yl)-1,3,5-triazine(DIC-TRZ)as the host materials.By comparison,the devices with TcTa as the host material exhibited low efficiency and brightness,and high operation voltage.The devices with DIC-TRZ as the host material exhibited obvious improvement in brightness and efficiency,and the operation voltage was also reduced.By doping(ppy)2Ir(dfpmpy)into TcTa and DIC-TRZ respectively,we fabricated a series of devices with double light-emitting layers.It's found that the performance of the devices was significantly improved,and operation voltage was reduced.The performance of the optimal device was as follows:turn-on voltage was 2.3 V,the maximum current efficiency,external quantum efficiency and power efficiency were 89.19 cd/A,27.9%and 106.78 lm/W,respectively.At the brightness of 1000 cd/m2,the current efficiency and external quantum efficiency as high as 82.97 cd/A and 25.4%.In terms of operation voltage,at the brightness of 1000 cd/m2,5000 cd/m2 and 10000 cd/m2,the operation voltage of the device was 3.1 V,3.6 V and 4.2 V,respectively. |
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