A Study Of Hybrid White Organic Light-emitting Diodes Based On Ultrathin Non-doped Emitters

White organic light-emitting diode?WOLED?can be applied to lighting and display fields and has drawn more and more attention because of its large emitting area,thin,flexible substrates,etc.However,the structure of the WOLED is relatively complicated in many studies,and the guest-host doped emitters used greatly increase the difficulty of the device preparation and the waste of organic materials.This thesis studies the ultrathin non-doped emitters in WOLED,using blue fluorophor with thermally activated delayed fluorescence?TADF?and orange-red phosphors as the emitters.Based on this issue,this paper carries out the following work:First,we systematically optimize the OLED device based on ultrathin non-doped light-emitting layer?EML?,including optimizing the thickness of hole/electron injection layer and ultrathin non-doped emitter and selecting suitable materials for hole/electron transporting layers.To compare the performances of the devices with ultrathin non-doped EML and doped EML,the corresponding doped devices are fabricated as well,where the host is either hole-transporting layer?HTL?or electron-transporting layer?ETL?.And the result is the non-doped device performs obviously better than the doped ones,indicating the effectivity of the ultrathin non-doped EML structure.By analyzing the influence of charge-carrier trapping property of emissive material on charge-carrier balance and current efficiency of the device,we find out that the ultrathin non-doped EML structure doesn't alter the electrical characteristics of the devices and won't destroy the charge carrier balance,hence most phosphorescent dyes can achieve high efficiencies with it.To confirm the main exciton recombination region and the spatial distribution of excitons,the orange-red phosphor,iridium?III?bis?2-methyldibenzo-[f,h]quinoxaline??acetylacetonate??Ir?MDQ?₂?acac??of 0.1nm,is used as the sensing layer.It is found that holes are majority carriers and the main exciton recombination zone locates at the vicinity of HTL/ETL interface and excitons are formed mainly in the molecules of electron transport material between which the triplets diffuse efficiently.Based on the discoveries above,a novel structure for hybrid WOLED with ultrathin non-doped EMLs is proposed by inserting the blue thermally activated delayed fluorescence?TADF?material,1,2-bis?carbazol-9-yl?-4,5-dicyanobenzene?2CzPN?,at the interface of HTL and ETL,and two orange-red phosphor layers at both sides of the TADF layer with a distance slightly larger than the F?rster radius to separately harvest the electrically generated singlet and triplet excitons.The maximum EQE of monochrome orange-red and blue OLEDs are 16.9%and 6.4%,respectively,while that of the corresponding WOLED reaches 13.1%,with the Commission Internationale de L'Eclairage?CIE?changing from?0.49,0.42?at 1 mA to?0.46,0.41?at 10 mA.The emission mechanism has been further investigated by calculating the fraction of excitons trapped and transferred to the phosphor layers.And we find out that charge-carrier trapping is the dominant emission mechanism at low current and the emission through energy transfer apparently increases as the current grows.It can be anticipated that this work might be a starting point for further research towards high-performance hybrid WOLEDs with extremely simple design.