Design And Peak Regulation Of Organic Electroluminescent Device Based On Inverted Structure

As a new-generation cold light source,organic light-emitting diodes(OLED)have many advantages such as low cost,high efficiency,wide viewing angle,large contrast,fast response,and flexible manufacturing.They have very important applications in lighting and display technology.Inverted organic light-emitting diodes are easier to match the drain of the n-type oxide in thin-film transistor(TFT)of AMOLED compared to traditional upright OLED devices,which is of great significance to the stability of light-emitting devices,the simplification of the manufacturing process and the improvement of resolution.This paper is devoted to designing inverted OLED devices with short wavelength,high quantum efficiency and high radiance.The device design and peak wavelength regulation based on inverted structure were studied.The following are the main contents:(1)ZnO was used as the electron transport layer of the inverted OLED,and the PEI self-assembled layer was used to modify the surface of ZnO.The PEI layer can reduce the surface roughness of the ZnO layer and increase its LUMO level,which is beneficial to improve the injection of electrons and reduce the injection barrier between the electrode and the organic layer.Based on the inverted structure,an ultra-short peak wavelength of 369 nm was obtained,which is the shortest emission peak of the currently reported organic substance as the emission layer.By comparing four different inverted OLED devices,it can be found that the peak of the emission spectrum shows a blue shift as the thickness decreases.By measuring the fluorescence lifetime of the TAZ layer in different TAZ-ZnO-ITO glass samples,it can be found that as the thickness of the TAZ decreases,its fluorescence lifetime also decreases.By analyzing the F?rster resonance energy transfer(FRET),we can get the exciton diffusion length,F?rster transfer rate and exciton diffusion coefficient of TAZ.(2)An exciton relaxation model of inverted OLED with ZnO as the electron transport layer is established.If the lifetime of random walker is shortened due to non-radiation channels(such as interface effects),its relaxation will stop at a higher state of DOS(Density of State).Accordingly,electroluminescence emission must be shifted hypsochromically due to the energy increase of the most populated states.After using the Mo O3 / NPB / Mo O3 gradient structure hole injection transport system instead of Mo O3,the change in the thickness of the luminescent layer will bring greater peak shift.(3)The changes in performance and peak of inverted OLED devices brought by different functional layer structures and parameters were studied.After inserting a BPhen buffer layer between the electron transport layer ZnO and TAZ layer,the exciton lifetime was extended,and the device peak appeared red-shifted,which verified the exciton relaxation model.When the BPhen thickness is 10 nm,the maximum radiance of the inverted device is 10.42 m W/cm2(@13 V),which is a 25.1% improvement over the 5 nm BPhen device.And the maximum external quantum efficiency is 1.21%(@3 m A/cm2),which is a 33% improvement over the 5 nm BPhen device.(4)The device changes caused by using multiple-hole transport layers were studied.After using the TCTA layer with a higher hole transport rate,emission peak of the device can reach 384 nm.Its external quantum efficiency is 3.65%,and its maximum radiance is 17.2 m W/cm2,which is 21% higher than the highest radiance reported previously.The variation of the functional layer and parameters can make the emission peak of the device change between 369 nm and 384 nm.After using the NPB layer with a lower hole transmission rate,the device exhibits a typical blue light with the NPB layer as the emission layer and the emission peak is 430 nm.