Host Materials Design And Device Research For Highly Efficient Top-emitting Organic Light-emitting Diode

Owing to the merits of fast response time,self-luminous,high resolution and brightness,flexible in form factor,low power consumption and high aperture ratio,topemitting organic light-emitting diode(TEOLED)has been widely used in active-matrix display products,such as smart phone,smart watch,television,computer and hand-held game player.However,there still exists problems such as low efficiency,high roll-off and low color purity in TEOLED,which limits its further development.To enhance the performances of TEOLED,the following three aspects will be mainly discussed in this dissertation.Firstly,we design interface exciplex complex,mCP/B4PYMPM,as the host,and use the ITO/Ag/ITO anode along with Yb/Ag cathode for the fabrication of TEOLED.The exciplex host ensures 100%excitons harvesting and low driving voltage,while the highly reflective anode and semi-reflective cathode provokes micro-cavity effect.By simulating and optimizing the micro-cavity effect,we successfully achieve a super-highefficiency top-emitting OLED.The device has a turn-on voltage of only 2.4 V and a maximum current efficiency of 159.1 cd A-1.The current efficiency still maintains 144 and 120 cd A-1 at a luminance of 1000 and 5000 cd m-2,respectively.The device also exhibits a much narrower FWHM of 23 nm,which is over 2-fold narrower than the conventional bottom-emitting device(71 nm).Secondly,we optimize the co-deposition ratio and thickness of the Yb:Ag transparent cathode.When the ratio is 1:9 and the thickness is 20 nm,the cathode has a transmittance of 70.9%and a sheet resistance of only 3.1 Ω□-1,which is better than those of Yb/Ag and Mg:Ag.Based on the Yb:Ag,we fabricate red/green/blue top-emitting devices.Benefiting from the excellent performance of cathode and micro-cavity effect,the current efficiencies of the Yb:Ag based devices are greatly improved compared with that of the bottom-emitting devices,and the spectra are also greatly narrowed.We further combine this transparent cathode with the charge generating layer composed of Bphen:Yb/HAT-CN,and eventually super-efficient top-emitting tandem green device is successfully achieved.The turn-on voltage is merely 4.8 V,and the maximum current efficiency is 356.2 cd A-1,which is 5.04 times of the bottom-emitting monolayer device.Finally,we fabricate blue phosphorescent OLEDs based on two newly designed and synthesized spiro-structural host materials,namely Cz-HOST and Ph-HOST.Thanks to the high thermal decomposition temperature,matched HOMO/LUMO energy level,high triplet energy level and excellent carrier transport ability,the devices exhibit superior performance.The maximum current efficiency,power efficiency and external quantum efficiency of blue devices based on Cz-HOST and Ph-HOST are 44.2 and 32.5 cd A-1,43.4 and 29.4 lm W-1,21.1 and 15.1%,respectively.Due to the higher thermal stability and more balanced hole-electron transfer ability of Cz-HOST,Cz-HOST even surpasses the classical commercial host material,mCP,in the BEOLED device.