Lif Modified Electrode Study On Electro - Optical Characteristics Of Organic Electroluminescent Devices

Although the breakthrough of the research and development of organic light-emitting diodes(OLED) has been gained,some major basic problems remain ambiguous,which lead to short lifetime and low efficiency of devices.In order to solve a series of problems,it should intensify the understanding of luminescence mechanism and the performance of thin film organic materials,transport characteristics of carriers of thin film materials,interfacial characteristics of devices and interfaces engineering.The experiment and numerical simulation were performed mainly surrounding the metal/organic semiconductor interface, in this paper.By using TPP doped 8-hydroxyquinoline,a red device with structure of ITO/PVK/Alq₃:TPP/Al is fabricated and its electroluminescent spectra is measured.In the device,the electron injection is enhanced by inserted LiF and DCL thin film between Alq₃,TPP and Al,and the hole injection is limited by inserting LiF thin films between ITO and PVK. The influence of LiF and DCL thin film on current-voltage and brightness-voltage characteristics is studied.It is found that high brightness and efficiency of the devices are largely ascribed to the balance of electron and hole injection and that of suitable electron and hole recombination zone.These studies are a useful exploration for the preparation of highly efficient devices.The insulating buffer layer modification of the metal/organic interface has been one of the effective ways to improve the carrier injection of OLED.The delay time is an important index of performance evaluation in OLED.Based on the mechanism of injection,transport and recombination of the charge carriers,a model has been developed to calculate the delay time of electroluminescence(EL) from bilayer organic light emitting diodes.The relations of the recombination time versus the applied voltage and injection barrier or internal interfacial barrier or the thickness of organic layer was revealed.The optimal thickness of inserted LiF in bilayer organic light-emitting devices has been calculated.The results show that:(1) the EL delay time is dominated by recombination process at lower applied voltage and transport process at higher applied voltage;(2) whenδ_e/δ_h＜2,metal/organic(M/O) interface is ohmic contact,whenδ_e/δ_h＞2,M/O becomes contact limited;(3) when the internal interface barrier exceeds 0.3eV,the effect of H_h on t_rec becomes very weak and t_rec is fundamentally governed by the applied voltage or other factors;(4) when increasintg L_h/L,t_rec has a rising trend at low voltage,when the applied voltage exceeds a threshold value,t_rec is not to change with L_h/L;(5)Considering OLED with LiF/Ag cathode on different applied voltage,when L_d≈3.1nm,both of the recombination time and the electroluminescent delay time:are shortest.It has a certain referenced meaning to respond rapidly to the functional materials and structure options of EL devices.