Mechanisms And Properties Of Doped Phosphorescent Organic Light-emitting Diodes

In recent years,organic light emitting diodes(OLEDs)have attracted a lot of attention due to the applications in flat-panel displays and solid-state lighting,which have extraordinary advantages such as high electroluminescence efficiency,ultra-thin,flexibility,wide viewing angle,fast response,energy saving and easily lagre-area manufacturing.The phosphorescent OLEDs,can use all 5%triplets and 25%singlets to achieve nearly 100%internal quantum efficiency,which has become an important research direction for high-efficiency OLEDs.This paper systematically studies the influence of the relationship between the host and guest energy levels on the EL performance of phosphorescent OLEDs,which provides a basis for the preparation of high-efficiency phosphorescent OLEDs;the use of exciplex host with thermally activated delayed fluorescence characteristics doping phosphorescent guest for the EML,low-voltage,high-efficiency phosphorescent OLEDs were prepared,and the light-emitting mechanism was studied.The main work of this paper is summarized as follows:1.In this work,we used five different fluorescent hosts and six phosphorescent guests to study the effect of the relationship between the energy levels of host and guest on the EL performance of phosphorescent organic light-emitting diodes(PHOLEDs).It can be seen that the triplet energy level of host should be higher than that of guest and the distance between them is as close as possible,ensuring the effective energy transfer from host to guest.Furthermore,it is also necessary to have low injection barriers into EML for electrons and holes,and high electron mobility of ETL,which guarantee electrons and holes injected effectively and evenly.This further explains the fundamental reason why the fabrication of high efficiency PHOLEDs must optimize the host,guest and match transporting layers.Our studies will help to establish better design rules for host-guest systems to achieve high performance PHOLEDs.2.PHOLEDs with exciplex as the host are studied,because exciplex can efficiently utilize triplet excitons,as the host of phosphorescence,it can reduce the working voltage of the device and improve the efficiency of the device.Based on m CBP:B4Py PPM exciplex as the host of green,orange and red phosphorescent materials,the turn-on voltage,maximum external quantum efficiency and power efficiency are 2.2 V,18.9%and 97.8 lm W^-1,2.4 V,22.1%,86.5 lm W^-1,And,2.4V,21.6%and 41.8 lm W^-1.Based on 26DCz PPy:PO-T2T exciplex as the host of green,orange and red phosphorescent materials,the turn-on voltage,maximum external quantum efficiency and power efficiency are respectively 2.8 V,19.2%and 66.0 lm W^-1,2.4V,19.9%and78.2 lm W^-1,2.6 V,23.8%and 36.6 lm W^-1.It is significantly better than the device performance with m CBP and 26DCz PPy as the single host,which fully proves that the selection of exciplex as the host is an effective method for realizing high-performance phosphorescent organic light-emitting diodes.