Structural Design And Performance Study Of Hirh-performance Phosphorescent Organic Light Emitting Devices

As a new type of lighting and display device,organic light-emitting diode?OLED?has become a hot point in flat panel display and solid-state lighting fields,because of its flexibility,fast response time,excellent color gamut,and low power consumption.Threr are two types of OLEDs,fluorescent OLEDs and phosphorescent OLEDs.Phosphorescent OLEDs have received considerable attention compared with fluorescent OLEDs,owing to their almost 100%internal quantum efficiency by efficiently utilizing both singlet and triplet excitons.However,there is a serious problem for phosphorescent OLEDs that its efficiency decreases sharply with increasing the current density.In the practical application,the roll-off of efficiency greatly influences the performance of phosphorescent OLEDs in high brightness.In this thesis,we aim for solving this problem.A series studies have been carried out around how to reduce the roll-off of efficiency in phosphorescent OLEDs.The research includes the following aspects:1.A high efficiency and low efficiency roll-off phosphorescent OLED is demonstrated to reduce the triplet exciton concentration based on a homojunction stacked emissive layer by alternating[CBP:4%Ir?ppy?₃?5 nm?]and[CBP:8%Ir?ppy?₃?5 nm?]ultrathin films.The results show that the maximum current efficiency?CE?of 48.8 cd/A and the maximum external quantum efficiency?EQE?of 19.6%are obtained and kept until 1×10³ cd/m².As the luminance further increases from 1×10³ to 1×10⁵ cd/m²,CE drops less than25.1%and EQE drops less than 25.5%.We design experiments using phosphorescence probe to investigate the diffusion process of triplet excitons.It is confirmed that the improved properties for the proposed phosphorescent OLED are attributed to the following reasons:the abovementioned stacked emissive structure can facilitate triplet exciton diffusion from the high doping regions to the neighboring low doping regions.As a result,the triplet exciton quenching is greatly weakened.2.To construct carrier balanced transport system,modulate the triplet exciton distribution in the emitting layer and weaken the triplet exciton quenching,several thin Bphen layers are inserted into the emitting layer to form the multiple quantum well?MQW?structure.The emitting layer is consequently thus divided into several sub layers.Such structure can not only improve device brightness and efficiency,but also suppress the efficiency roll-off.A maximum CE of 49.2 cd/A,power efficiency?PE?of 31.3 lm/W and EQE of 19.8%are achieved in the optimized device with six quantum wells.As the current density increases from 100 to 200 mA/cm²,there only exists a less than 5.53%drop in CE,12.12%drop in PE and 5.58%drop in EQE.The exciton generation region is detected by experimental method using fluorescent probe.The results show that excitons in the reference device highly concentrate in a narrow area near the EML/Bphen interface.In contrast,the exciton formation region is averagely distributed in the whole light emitting layer in the optimized MQW device.As a result,the triplet exciton quenching is weakened.3.In order to reduce the efficiency roll-off and enhance the light extraction,we fabricate phosphorescent OLEDs with spontaneously distributed embossed structure by a simple and low-cost spin-coating method.Two kinds of Al₂O₃nano-particles with mean diameters of 300 and 500 nm,mixed with weight ratio of 1:1,are implanted into phosphorescent device as a single layer and the subsequent functional layers are deposited on the surface of Al₂O₃ NPs.When the concentration of Al₂O₃ ethanol solution is chosen as 0.08 mg/ml,the maximum CE of 52.1 cd/A,PE of 32.7 lm/W and EQE of 20.9%are obtained,which are 1.52,1.95 and 1.53 times those of the reference device.When the current density increases from 100 to 200 mA/cm²,a less than 3.37%drop in CE,7.05%drop in PE and 3.23%drop in EQE are found in the embossed device,with the decrease factor of 83.44%,74.15%and 84.35%,compared with the reference device.The improved performances are attributed to the greatly enlarged charge-carrier-recombination region and enhanced light extraction.The enlarged charge carrier recombination region can reduce triplet exciton density and suppress the exciton annihilation,which lead to the increase of internal quantum efficiency.