Phosphorescent Luminous Efficiency Of Organic Electroluminescent Devices

In recent years,organic electroluminescent devices(OLEDs)have attracted increasing attention because of their potential advantages in low-power,emissive,flexible,cost-competitive,flat panel displays. Molecularly doped phosphorescenct blends have recently been exploited to facilitate a greater understanding of the physics of organic materials and improve the efficiency of OLEDs.These attributers are a result of the ability of the phosphorescent molecular to harness both triplet and singlet states,leading to OLEDs with internal quantum efficiencies approaching 100%.Based on the research results of other groups,we studied the electroluminescence efficiency in organic electrophosphorescence EPH),the main results are listed as following:1.Based on the energy transfer process from host to dopant in an organic electrophosphorescent(EP)device,the expression of energy transfer probability(η)between the host 4,4'-bis[N-(p-tolyl)-N-phenyl -amino]biphenyl(TPD)and guest tris(2-phenylpyridine)iridium (Ir(ppy)₃)EP systems was proposed.The results show that:(1)The rate of the triplet energy transfer(K_HGand K_GH)increases exponentially with increasing donor-acceptor molecular distance(R),whereas decreases as the intermolecular distance(R_HH)increases from 0.8 to 2.4 nm. Furthermore,K_GHchanges more quickly than K_HG.(2)The energy transfer probability(η)increases as R reduces,and the R_HHchanges can be safely neglected for R＜0.9nm.The situation changes for 0.9 nm＜R＜1.1 nm,R_HH(＜1nm)plays an essential role whenηchanges and increases with the latter.However,if R＞1.1 nm,the transfer probability will be below zero.Here,the energy transfer principle may be less important,and the high electroluminescence(EL)quantum efficiency of phosphorescent system will be attributed to the direct electron-hole recombination in phosphorescent molecules.(3)Theηwill increase when Forster radius (R₀)increases or Gibb's energy decreases.2.Based on the charge injection and recombination processes,and the triplet(T)-triplet(T)annihilation process,a model to calculate the electroluminescent efficiency is presented.The influences of the applied electric field on the injection efficiency and the recombination efficiency and the electroluminescence efficiency are discussed.It is found that:(1) the injection efficiency is increased while the recombination efficiency is decreased with increasing applied electric field;(2)the EL efficiency at low electric field is enhanced slowly but at high electric field is decreased with increasing applied voltage.(3)the EL efficiency is decreased with increasing the distance(R)between host and guest molecular.So we can conclude that the EL efficiency in single-layer organic electrophosphorescent devices is dominated by injection efficiency at lower electric field and recombination efficiency at higher electric field.3.Based on the experience formula and the triplet(T)-triplet(T) annihilation processes,an analytical model to calculate the recombination width and the external quantum efficiency in doped organic electrophosphorescence(EPH)devices was presented.The influences of applied bias,current density and the thickness of the device on the width of recombination zone and the external quantum efficiency were studied thoroughly.It is found that:(1)as applied voltage increases,the recombination width of the device reduces and the external quantum efficiency increases;(2)with increasing the thickness of the device,the recombination width also increases correspondingly,and the external quantum efficiency have different change tendency at a given applied voltage;(3)the external quantum efficiency decreases obviously with increasing recombination current density.