Research On Efficient Trilayer Phosphorescent OLED Without Electrode Modification Layer And Its Working Mechanism

As is known to all that organic light-emitting devices?OLEDs?have attracted considerable attention for solid state lighting,full color displays and so on.A good deal of functional layers,such as the anode modification layer?AML?,cathode modification layer?CML?,hole blocking layer?HBL?and electron blocking layer?EBL?,have been introduced in the OLEDs to achieve high efficiency and low turn-on voltage.The AML and CML are used to enhance the hole or electron injection,respectively.While the HBL and EBL can efficiently block the diffusion of the exciton from the luminescent layer into the transport layer.Obviously,the multilayer structure becomes a frequently-used way to improve device performance.However,since one more layer means an extra preparation process,excess function layers would also cause the long period and high cost that limit the development of their industrialization.With the improvement of the organic material system,some materials could play multiple roles in OLEDs due to their prominent properties.For example,deoxyribonucleic acid-cetyltrimetylammonium[DNA-CTMPYPBA]complex can act as hole transporting layers?HTL?because of high hole mobility,meanwhile the low lowest unoccupied molecular orbital?LUMO?energy level makes it suit for the EBL.4,4',4"-tris?carbazol-9-yl?-triphenylamine[TCTA]is usually used to be HTL,besides,it can also serve as the host in emitting layer?EML?because of its high triplet energy.Hence,it is possible to simplify the structure without sacrificing the device performance by choosing appropriate material.However,few studies have been carried out on phosphorescent white OLEDs?PHWOLEDs?with simple structure.In this paper,we fabricate efficient trilayer PHOLED with a simple structure-HTL/EML/electron transporting layer?ETL?.Firstly,from the previousstudy in the laboratory we find that larger value of current density exists in the device,wherethe TAPC serving as hole transport layer is disposited directly on the anode ITO,instead of the conventional device insertingaAML.In order to investigate the effects on the performance of devices with or w/o AML,we prepare multilayer devices with green phosphorescence dye Ir?ppy?₃as a contrast experiment,the former device is prepared merely with the HTL-TAPC,while MoO3,PEDOT:PSSare introduced in the latter two devices actingas AML.Capacitance-voltage characteristic and transient voltage discharge curve of the device were measured respectively by impedance spectroscopy and transient measurement.Combining with the electroluminescent?EL?characteristics of the device in steady state,i.e.the current density-voltage-luminance characteristic curve,we find that TAPC serving as HTL in device own the best hole-injection abilitywithout anode modification layer.So we deposit TAPC directly on the anode to simplify the device in the first step.Subsequently,the effects of CML on the performance of devices are studied.TmPyPB and Bphen act as ETL without the CML,which are compared with the conventional TmPyPB/LiF structure.The result shows that the electron injection of device with Bphen is compared favourably with this of formal device,so as to further simplificationof the device are developed.Secondly,compared with the conventional multilayer device,the simplified trilayer green device with the structure of ITO/TAPC?50 nm?/CBP:Ir?ppy?₃?30 nm?/Bphen?50 nm?/Mg:Ag?15:1 120 nm?,possesses a better efficiency roll-off.It can be explained via the model between EQE and J simulatingthe internal exciton quenching mechanism.TherecombinationmechanismsinphosphorescentOLEDs?PHOLEDs?are also studied.We prepare devices with CBP doped Ir?ppy?₃,Ir?MDQ?₂?acac?and FIrpic as the EMLin green,red and blue OLEDs,respectively.With the energy level diagram and capacitance-voltage curves,we believe that two recombinationmechanismsexistinthegreenandred PHOLEDs-Langevinrecombination and trap-assited recombination.At the same time,theoretical simulation analysis is also carried out.By further changing the host of the EMLs,we choose the most suitable hostCBP to optimize the trilayer OLED structure.Finally,we prepared efficient single-layer white OLED?WOLED?with structure:ITO/TAPC?50 nm?/CBP:25wt%FIrpic:0.2wt%Ir?MDQ?₂?acac??30nm?/Bphen?50 nm?/Mg:Ag?15:1,120 nm?.The maximum current efficiency of the trilayer WOLED is 21 cd/A.