The Application Of High-efficiency Materials In Organic Light-emitting Diodes

With the development of science and technology,organic light-emitting diodes(OLEDs)have become more important in flat panel displays and solid state lighting sources due to their unique advantages.Thus,OLEDs have been widely interested by academic and industrial communities.For monochromatic devices,general methods to improve the device efficiency include the design of efficient emitters,suitable host materials and efficient device structures.However,reports on the study of device lifetime are still rare.For white-emitting OLEDs(WOLEDs),simple device structure,good color stability and high color rendering index(CRI)are important for their practical applications.In this thesis,WOLEDs with simple device structures based on Pt(?)and Ir(?)complexes and the effect of n-doped electron-transporting layer(ETL)upon the efficiency and operational lifetime of green-emitting OLEDs with Ir(?)or Pt(?)emitters were studied.The main research contents of the study are as follows:1.A simple device structure adopting double emitting layers(EMLs)for WOLEDs based on blue emission from a Ir(?)complex(Fir Pic)in the blue EML and green/red emission from a single tetradentate Pt(?)complex Pt-X-4 in the green/red EML is described.The energy transfer from the blue emitter to the aggregation state of Pt-X-4in optimized WOLED,leads to an imbalance of three colors in the device.A space layer is therefore inserted between the two emitters to block the energy transfer from blue emitting layer to the green/red emitting layer in the optimized device to stabilize the electroluminescent spectrum.High CRI of 81,maximum external quantum efficiency(EQE)of up to 19.85%,maximum current efficiency(CE)of 37.29 cd/A,maximum power efficiency(PE)of 32.54 lm/W are achieved for optimized WOLED with Commission International de l'Eclairage(CIE)coordinates of(0.38,0.42).In addition to the broad emission spectrum and high efficiency,the short emission lifetime of Pt-X-4 in aggregation form enables a high luminance of nearly 100000 cd/m² and low efficiency roll-offs of 2.1%and 11%at 1000 and 10000 cd/m²,respectively.2.A simple device structure engaging n-doping ETL enables low efficiency roll-off and long operational lifetime in green phosphorescent OLEDs based on tris[2-(p-tolyl)pyridine]iridium(III)(Ir(mppy)₃)as emitter.The n-doped ETL not only decreasing driving voltage and efficiency roll-off,but also improving electroluminescence performances of the device,thus enlengthening device lifetime.The effect of dopant concentration in EML and the thicknesses of ETL upon the device performances were studied.The device with 8 wt%Ir(mppy)₃ in the EML and 70-nm-thick ETL displayed maximum EQE of 16.95%,maximum CE of 55.89 cd/A and maximum PE of 48.38lm/W.To compensate the higher driving voltage caused by the thicker,Liq was doped into ETL as an n-type dopant.By optimizing the doping concentration of Liq and the thickness of ETL,the device with Liq concentration of 10 wt%and 60-nm-thick ETL displayed a lower turn-on voltage of 3.2 V,maximum EQE of 16.84%,maximum CE of 58.02 cd/A,and a maximum PE of 56.72 lm/W.In addition,because of the broadened exciton recombination zone and improved charge balance in the EML,a high brightness of nearly 90000 cd/m² and low efficiency roll-offs of 2.5%and 15.2%at high luminances of 1000 and 10000 cd/m² were achieved in the device,respectively.The estimated operational half-lifetime of the optimized n-doped device was 99.10 h at the luminescence of 100 cd/m²,which was improved by 69.55%when compared with reference device.3.The efficiency roll-off and operational lifetime of OLEDs with a tetradentate Pt(II)emitter(tetra-Pt-S)and stable host material 3,3'-di(9H-carbazol-9-yl)biphenyl(m CBP)is improved by engaging an n-doped ETL and a blocking layer.And the exciton recombination zone of this kind of green devices with different concentration of Liq were studied by inserting ultra-thin emitters at different positions of the EML.Compared to those of the device with a non-doped ETL,the driving voltage is lowered,the carrier density is balanced,and the exciton density in the EML is decreased in the device Liq-doped ETL.High luminance of almost 70000 cd m^-2 and high current efficiency of 40.5 cd A^-1 at high luminance of 10000 cd m^-2 was achieved in the device with doped ETL with 50 wt%Liq.More importantly,the extended operation lifetime of 1945 h was recorded at the initial luminance of 1000 cd m^-2 in the 50 wt%-Liq-doped device,which is longer than that of the device with non-doped ETL by almost 10 times.