Study On The Performance Improvement Of Simplified Organic Light-emitting Diodes Based On Co-host Structure

Organic light-emitting diodes(OLEDs)have great potential application in full-color flat-panel displays and backlights for liquid-crystal display,due to their high-efficiency,flexible property,active light emitting,wide angle of view,high-brightness and high contrast ratio,and so on.However,there are a lot of technical problems in the commercialization.It restrict the development of OLED that the complicated structures,the unmanageable manufacture process and high production costs and so on.In this paper,we investigated device performance by simplifying the device structure and employing co-host structure,which balance charge carrier transport.Firstly,we demonstrated an simplified orange OLED based on4,4'-Bis(carbazol-9-yl)biphenyl(CBP)with ambipolar transporting property,which is used as comparison.The simplified orange OLED based on CBP has not any charge carrier transport layers.The device has the maximum current efficiency and power efficiency values of 20.7 cd/A and 6.9 lm/W due to CBP possessing weaker electron-transporting property than hole-transporting property.In order to optimize the device performance,we demonstrated simplified OLEDs comprising a single orange emission layer based on CBP:TPBi.We utilized the co-host of CBP:TPBi in the emission layer.We obtain balanced charge carrier transport and recombination and high device performances in the simplified OLEDs by controlling the doping ratio.The resulting device consisting of only single orange emission layer has the maximum current efficiency and power efficiency values of 37.1 cd/A and 33.3 lm/W.It is of interest to note that Iridium(?)bis(4-phenylthieno[3,2-c]pyridinato-N,C2')acetylacetonate(PO-01)has the positive effect on the charge transport and device performance by analysing single charge carrier devices.Then,we fabricated color-stable and highly efficient simplified white phosphorescent organic lightemitting diodes aboutBis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(?)(FIr Pic)and PO-01 based on above co-host system.Thereinto,we introduce co-host system of4,4',4''-tris(Ncarbazolyl)triphenylamine(TCTA):TPBi into blue EML and employ TPBi with good electron-transporting property in orange EML.We obtain color-stable and highly efficient simplified white phosphorescent organic lightemitting diodes about FIr Pic and PO-01 by controlling the doping ratio.The resulting device has the maximum power efficiency and current efficiency of 45.5lm/W and 43.5 cd/A.Thereinto,the device with TCTA:TPBi(1:5)exhibits excellent spectra stability with negligible shift in CIE coordinates of(-0.016,-0.006)when the luminance increases from 114 cd/m~2 to 10 550 cd/m~2.Encouraged by the excellent color-stability based on two complementary colors,we introduce red phosphorescent dye Ir(MDQ)2(acac)into the orange EML.We obtain color-stable simplified white phosphorescent organic lightemitting diodes based on three complementary colors.The resulting device has the maximum power efficiency and current efficiency of32.8 lm/W and 31.3 cd/A.