Studies On Sub-monolayer Emission And Top-emitting Organic Light-emitting Devices

Today it is knowledge economy times focusing on information industry.Flat Panel Displays (FPDs) takes an important role as the interface formankind obtaining information,it's effect has become more singnifi-cant .Organic Light-Emitting Devices (OLEDs) is an important member inFPDs family due to its many merits, such as light weight, low cost, wide visualangle, high response speed, spontaneous light-emitting, high brightness andefficiency. OLEDs attracts to world wide attention in the fields of science andindustry. Through the continuing efforts of world known companies andresearch institutes, the performances of OLEDs have been made significantimprovement and have reached to the level of commercialization. Althoughthe development of OLEDs is relatively perfect, how to further improve theperformances of the devices is also the research focus in the world.In the filed of FPDs, Top-emitting OLEDs has more advantage than thetraditional down-emitting OLEDs. Top-emitting OLEDs can hide the drivingcircuit under the organic emitting layer to improve the resolution of displays.On the other hand, it can be fabricated on an opaque silicon substrate ,onwhich can be a variety of functions integrated. Because of these advantagesTop-emitting OLEDs are more suitablet for active matrix displays.In this dissertation, we have done some meaningful works: aQuinacridone(QAD)sub-monolayer was inserted in the emitting layer ofOLED, which used as an exciton probe in device . Different devices werefabricated by changing the deposited position of the QAD sub-monolayer. Theformation and diffusion of excitons were detected from the different theelectroluminescence (EL) spectrum and luminescence intensity. The QADmolecule can't trap carriers directly, but emit through energy transfer from thehost material in doping OLEDs. In respect that the interface betweenNPB/Alq3 has a higher electron transport barrier, we induced a BCP holes andexcitons blocking layer in the electron transport layer to limited the excitonand holes in the narrow zone between the interface of NPB/ Alq3 and BCPlayer,and then inserted the QAD sub-monolayer in this narrow zone,we foundthat the energy transfer from Alq3 molecule to QAD molecule improved andalmost QAD electroluminescence without Alq3 spectrum was obtained. Wealsso optimized the thickness of this narrow zone, and the highest EL currentefficiency of 7.2cd/A was attained by inserting a QAD sub-monolayer in themiddle of this 10nm-thick Alq3 narrow zone. Compared to the doping method,it is an easier way to design high-efficiency OLEDs.We investigated two different structures for the top emitting OLEDs. Wefabricated the top emitting OLEDs with single crystal silicon as substrate, andthermal oxidation method grown SiO2 as insulated layer,metal Au as anode, asemi-transparent LiF/Al/Ag as cathode. The injection of carriers at metalcontact to organic semiconduntors is the key problem in this devices. Atmetal/organic semiconductor interface, the hole and electron barriers(φBhandφBe) depend on the position of highest occupied molecular orbital (HOMO)and lowest unoccupied molecular orbital (LOMO) with respect to the metalFermi level (EF).Now the view of dipole barriers existing at interface isaccepted and approved widely. Our experiment also indicated that dipolebarriers are existed at all meta/organic semiconductor interface and play animportant role in the electric properties. In order to decrease the dipole barrierbetween Au anode and NPB layer, we used CuPc layer as buffer layer tomodify the anode, how the buffer layer thickness affect the properties ofdevices was discussed. The optimized device show an maximum luminance of15440 cd/m2 at 19V and the maximum EL efficiency of 3.29 cd/A at 13V.CuPc is a very well known organic semiconductor, it has very good chemicaland thermal stability, and it can form compact and smooth thin films easily.Inducing CuPc layer can not only improve the hole injection efficiency of topemitting OLEDs, but also balance the carriers injection, which can enhance theemitting efficiency of devices. Our result indicated that CuPc can reduce thehole injection barrier of Au anode/NPB interface in the top emitting OLEDs,so it is an ideal anode modified material. Au has higher metal work function,good conductivity and is difficult to react with oxygen and water. It is an goodanode material fcandidate in top emitting OLEDs.Another top-emitting OLEDs was fabricated with an evaporated metalAl on the glass substrate as anode, spin coating PEDOT:PSS as anodemodified layer, semi-transparent LiF/Al as cathode. An maximum luminanceof 21450 cd/m2 at 17V and maximum current efficiency is 6.4cd/A was got. Inorder to decrease the turn-on voltage,the lower work function metal Al anodewas replaced by higher work function metal Ni anode. And the turn-on voltageof device lowered from 6V to about 3V,an maximum luminance of 25400cd/m2 at 12.6V and maximum current efficiency 4.3cd/A was apdated. MetalNi has such higher work function as to enhance the hole injection from anodeand decrease the driving voltage. As the same time, the enhancement of holeinjection lead imbalance of carriers injection, it is the reason why the currentefficiency of device with Ni anode reduced. Comparing the two differentanode top emitting OLEDs, we find that the device with Al/Ni as anode canobtain higher power efficiency, the maximum power efficiency is 2.5 lm/W at4.5V. Considering the system integration, we moved the same configurationdevice from glass substrate to silicon substrate and also get an excellent topemitting OLEDs. Compared the two different top emitting OLEDs, withPEDOT:PSS and without PEDOT:PSS , it is clearly shown that PEDOT:PSS isa nice anode modified material and could reduced the hole injection barrierbetween metal and organic material remarkably. PEDOT:PSS modified anodeis another effective way to improve the properties of top emitting OLEDs.