The Fabrication And Research Of Top-emitting Organic Electroluminescent Devices

In active displays,organic light-emitting diodes were controlled by thin-film transistors.If the device is emitted in the form of bottom emission,light passing through the substrate would be blocked by the TFT and metal lines on the substrate,thus affecting the actual light-emitting area.If the light is emitted from the top of the devices,the lines on the substrate do no affect the light-emitting area of the devices.Therefore,the operation voltage of devices is lower at the same brightness,and longer lifetime can be obtained.Consequently,the research on top-emitting organic electroluminescent devices is of great importance.The specific research works are as follows:1.Bottom-emitting devices were prepared using iridium complex Ir（mppy）₃ as the phosphorescent emitter,and the doping concentration of Ir（mppy）₃ was optimized.Then the top-emitting organic electroluminescent devices were prepared using Ag as the bottom-reflecting electrode and top-transmitting electrode.The effects of electrode thickness,cathode modification layer thickness,cavity length（including hole transport layer and electron transport layer）on the efficiency,emission spectrum and external quantum efficiency of the top emitter devices were investigated.Compared with the bottom emitting device,the top-emitting device achieves a significant increase in performance and brightness with low operating voltage.By increasing the thickness of the top Ag layer,full width at half maximum of less than 20 nm emission spectrum（peak at 552 nm）is obtained.Finally,the device achieves an ultra-high current efficiency of up to 161.17 cd/A with EQE of 38%and ultra-low efficiency roll-off.2.Phosphorescent iridium complex Ir（ppy）₂（dfpmpy）was introduced as the emitter,and TCTA and DIC-TRZ matching its energy level were selected as the host materials,and the doping concentration of Ir（ppy）₂（dfpmpy）was optimized,and then the top-emitting organic electroluminescent devices were prepared.The effects of the electrode thickness,thickness of each functional layer（including hole transport layer and electron transport layer）,and thickness of the capping layer on the efficiency,emission spectrum,angular distribution characteristics,and external quantum efficiency of the top-emitting devices were studied.Compared to the bottom emitter device（507 nm）,the top emitter device（502 nm）achieves a huge improvement of 47%in current efficiency.By controlling the thickness of the silver layer,a narrow emission spectrum with full width at half maximum of only 18 nm is obtained,which greatly improves the color purity of the device.3.The top-emitting devices were fabricated using gold as the top translucent electrode material with phosphorescent materials Ir（ppy）₂（dfpmpy）and Ir（mppy）₃ as the emitters,respectively.The effects of electrode thickness and cavity length on efficiency,emission spectra and the lifetime of top-emitting devices were investigated.It was demonstrated that top-emitting devices prepared by using Au as top translucent electrode material were unable to form strong microcavity effects,resulting in poor resonance selectivity for emission wavelengths.The EL spectra were generally characterized by two or three emission peaks,which unfavorably affected the color purity of devices.The transmittance test results of Au electrodes with different thicknesses exhibited that Au thin-film electrodes featured high transmittance in the wavelength range from 500 nm to 600 nm,which basically covered the wavelength range of emission peaks appearing in EL spectra and well explained the poor resonance selectivity of Au electrodes in this wavelength range.4.The TADF material DIC-TRZ and the phosphorescent material FIrpic were used as emitters to prepare top-emitting devices of various structures,respectively.The effects of the electrode,cavity length and ladder layer on the comprehensive performance and spectra of top-emitting devices were investigated.We found that the electroluminescence spectrum of TADF top-emitting organic light-emitting diodes is substantially narrower than that of conventional bottom-emitting devices under the microcavity effect,which is only about half of the full width at half maximum of bottom-emitting devices,and significantly improves the color purity of devices.By combining faster radiation recombination rates,higher external coupling efficiency,and higher radiation efficiency,the current efficiency and external quantum efficiency of the TADF top-emitting devices increased more than twice as much as the bottom-emitting TADF devices.In addition,top-emitting devices prepared using phosphorescent dopant FIrpic also achieved significant spectral narrowing through optimization of the transport layer.