Study On Organic Light-Emitting Diodes For Active Display

Organic light-emitting diodes(OLEDs)which have the advantages of simple synthesis of materials,easy preparation process,light weight,self-luminous,wide color gamut and integration of opaque or flexible substrate,etc.,are considered as the new generation of display technology following cathode ray tube display(CRT),liquid crystal(LCD).Although OLED-related display products have been quietly into our daily life from laboratory with three decades of development,researchers have never stopped improving device performance.There still exist many problems in OLED industry,such as working life,cost,integration,etc.,and the comprehensive performance of OLED needs to be further enhanced.This thesis is for practical active display application,and focuses on simple structure tandem OLED(TOLED)and top-emitting OLED(TEOLED)compatible with low-voltage driving circuit.The main contents of this paper are as follows:(1)We firstly proposed the design idea of "simple structure of TOLED",and so-called simple not only included simple configuration of light-emitting units,but also a simple structure of charge generation unit(CGL).We connected two simple yellow light emitting units(CBP/CBP:PO-01/TPBi)through the simplest CGL(Mo Ox)and electron extraction layer(Bphen:Li F)to obtain a simple structure yellow phosphorescent TOLED.Furthermore,we decreased the injection potential barrier between CGL and bottom EL unit,and improved carrier balance of the light-emitting layer.Eventually,we fabricated high-performance simple structure TOLED with doubling current efficiency and almost same power efficiency compared to traditional single EL unit device.(2)We broke through the limitations in material selection and structural design of CGL for the first time,using a buffer-modified p/n-type planar organic heterojunction(OHJ)as CGL consisting of common organic materials,and the configuration of this p/n-type CGL was “Li F/NPB/Bphen/Mo Ox”.We systematically studied the influence of organic layers thickness on TOLED performance,and the thickness of optimized green phosphorescent TOLED,whose current efficiency was greatly enhanced from38.5 cd/A for the single-unit device to 77.6 cd/A,was about 150 nm.Impressively,p-type NPB and n-type BPhen were not only fist introduced in CGL,but also used to be hole transporting layer(HTL)and electron transporting layer(ETL),respectively.With this new and doping-free p/n-type CGL,we could simplify tandem device structure and obtain a low-cost fabrication process,suggesting its great commercial prospect as compared to the conventional CGLs.We believe that the design concept of p/n-type CGLs provides guidance towards the tandem organic light-emitting devices.(3)We also developed effective buffer layer modified p/n-type and n/p-type CGLs by using Ir(ppz)3 and Bphen.The influence of both Mo Ox and Li F as carrier extraction layers on the charge generation process of CGLs was conscientiously investigated.The working mechanisms of two CGLs were discussed on the basis of thermal emission model and tunneling effect,respectively.In order to further improve device characteristics,2 nm Ir(ppz)3:Mo Ox was introduced in n/p-type CGL between p-type organic semiconductor and hole extraction layer to decrease charge injection barrier,twofold enhancement in current efficiency of this n/p-type CGL based TOLED was obtained compared to reference device.In addition,we compared the interface charge transfer properties of Bphen/NPB and Bphen/Ir(ppz)3,and we thought Bphen/NPB was not suitable for non-doped n/p-type CGL.(4)Due to Mo Ox doping layer having the advantages of good hole injection characteristics,high conductivity,good thermal stability,etc.,we prepared TEOLED and inverted TEOLED based on Mo Ox doping technology for p-type and n-type TFT pixel circuit,separately.In TEOLED,bipolar emitting layer Tm PPPy Tz:Ir(ppy)3 led to direct carrier recombination and excitons formation on the phosphor dopants.With the 50 nm Tc Ta capping layer,light out-coupling efficiency of TEOLED was further improved.Our optimized TEOLED achieved a luminance of 4755 cd/m2 at 4 V.Inaddition,we also demonstrated blue,green and red inverted TEOLEDs.Just as other TEOLEDs,EQE of both blue and green inverted devices were 15 % larger than those of conventional bottom-emitting OLEDs,while it was surprising that the red inverted TEOLED exhibited higher efficiency nearly twice as that of the bottom-emitting ounterpart.We explained that the role of the microcavity effect improved the spontaneous emission of emitters in all inverted TEOLEDs,and only the intrinsic quantum yield of the red emitters was improved due to its narrower energy band.Finally,we integrated our high performance TEOLED on p-type circuit substrate to realize super video graphics array(SVGA)OLED microdisplay.