Study On Large Area Tandem Organic Light-emitting Devices

Organic light-emitting devices (OLEDs) are attracting singnificant interest inpresent display and lighting products due to the superior advantages in high efficiency,flat emission, healthy light, and flexible feasibility. The use of OLEDs in commercialproducts is limited by the fact that the fabrication of large area OLEDs generallyinvolves a lot of issues as enlargement in size. This opens an opportunity to get insightinto the methodology and understanding of the large area OLED fabrication. In thispaper, we primarily study on improving the emission uniformity of large area OLEDsfor practical applications, and predominantly focus on the fundamental investigation ofconventional OLEDs including the electron injection mechanism, n-type electricaldopants, tandem OLEDs and exciplex emission materials. These work, which showsgreat significance in realizing large area OLEDs fabrication, including the followingparts:In Chapter1, we introduce the background for making OLEDs, describe theprinciple of carrier injection, transportation and recombination process, and demonstratethe current progresses and challenges in large area OLEDs.In Chapter2, LiNH2has been utilized as the alternative n-type dopant to lithiummetal, as its much more ambient stability. Tandem OLEDs that are fabricated with twokinds of intermediate connectors having Cs2CO3and LiNH2, respectively, show acomparable EL efficiency, but the former device has better EL stability. The interfacialchemical reaction between n-type dopant and HAT-CN, leading to the increase inelectron injection barrier from the intermediate connector into the bottom EL unit, is thedominant mechanism behind the degradation of tandem devices. The results prove thatthe low reducibility and the small ionic radius of metallic Li versus Cs can suppress theinterfacial chemical reaction and therefore improve the stability of tandem OLEDs.Using the intermediate connector of LiNH2-doped BPhen/HAT-CN/NPB, a highly efficient tandem white OLED is demonstrated with CIE coordinate (0.40,0.47) andmaximum current efficiency of103cd/A satisfactory for promising applications.In Chapter3, operational mechanism of intermediate connectors are addressed inemphasizing on roles of HILs in the performances of tandem OLEDs. HAT-CN andMoO3-doped NPB that have high electrical conductivity are used as HILs forintermediate connectors with negligible voltage drop, resulting in tandem OLEDs withhigh power efficiency. On the other hand, intermediate connectors using MoO3andMoO3-doped NPB as HILs have high charge generation capability, which help inrealizing high current efficiency tandem OLEDs. The correlation between theeffectiveness of intermediate connectors and the properties of tandem OLEDs is wellestablished, which can shed light on choosing suitable component materials to optimizeintermediate connectors.In Chapter4, we investigate the effect of hole injection barriers from intermediateconnectors on the performance of tandem OLEDs. The hole injection barriers arecaused by the offset of the highest occupied molecular orbital (HOMO) energy levelsbetween hole transporting materials (HTLs) contained in the intermediate connector andthe top EL electroluminescence unit, respectively, We also find that although chargegeneration can occur at the interfaces between the TMO and a wide variety of HTLs ofdifferent HOMO values, an increase in the hole injection barrier however limits theelectroluminescence efficiency of the top EL units. In the case of large hole injectionbarriers, significant charge accumulation in the HTLs makes the intermediate connectorlose its functionality gradually over operating time, and limits device stability.In Chapter5, we use photoluminescence (both steady state and time resolved) anddelayed electroluminescence measurements to study phosphorescent and fluorescentOLEDs using an exciplex-forming host. We find that in an exciplex-forming host:guestsystem, excitons are predominantly formed on the host and not on the guest. Theefficiency enhancement when used in phosphorescent OLEDs is caused by efficientenergy transfer from the exciplex to the dopant and lower triplet-polaron quenchingeffects. This energy transfer is found to occur mainly by Dexter mechanism. The resultsexplain the high efficiency of phosphorescent OLEDs based on exciplex-forming hostsand shed light on the reasons behind the significant difference in the efficiency ofOLEDs utilizing exciplex-forming hosts when used with fluorescent versusphosphorescent emitters. In Chapter6, we demonstrate low driving voltage tandem OLEDs via utilizingexciplex-forming hosts in the EL units instead of conventional host materials. The useof exciplex-forming host reduces the charge injection barriers and the trapping ofcharges on guest molecules, resulting in the lower driving voltage. The use of exciplex-forming hosts also allows using fewer layers, hence simpler EL configuration which isbeneficial for reducing the fabrication complexity of tandem OLEDs.In Chapter7, we investigate the influences of LiF thickness on the performance ofOLEDs with various electron transporting layers (ETLs). The interfacial chemicalreaction and tunneling injection are thought to be the predominant mechanisms indifferent cases of OLEDs with various ETLs. In the cases of using reducible ETLs, theinterfacial chemical reaction is responsible for improving electron injection of LiF/Al,and the electron injection barrier becomes independent on the LiF thicknesses whenETLs have enough strong reducibility. In the cases of using non-reducible ETLs, thetunneling injection is the predominant mechanism in the electron injection of LiF/Albilayer cathode, happening the improved electron injection when LiF is3~4nm.In Chapter8, we improve the line source to obtain the uniform films with thedeviation <3%in the range of30cm×30cm, which is essential for uniform emissionof large area OLEDs. The use of a buffer layer and tandem structure has beendemonstrated to reduce the short circuit in the large area OLEDs. By designing properdriving circuit, we can achieve various sizes OLED panel without using the metal grid.This work demonstates the fundamental study on improving the fabrication processof large area OLEDs, electron injection machenism, operational mechanism of tandemOLEDs, and exciplex emission materials, which shed light on the significance inrealization of large area OLEDs for practical applications.