Study On PLEDs Performance By Employing Interfacial Modification And Nanostructure Composite Electrode

Polymer light-emitting diodes (PLEDs) as the solid lighting and display panel,compared with inorganic LED, liquid crystal display (LCD), have many advantages, e.g.self-emission, fast response, full solid device, super-thin thickness, low cost, especiallyflexible and solution processed. PLEDs are considered as the most ideal and potentiallighting technology in21stcentury. However, now there are also some problems toresolve from large scale commercial application such as low luminous efficiency andhigh fabrication cost due to the imbalance of charge carrier injection and transport andthe cost of commercial transport conductive anode indium-tin oxide (ITO) wasincreasing. Aiming at those problems, some new and systematic works have been doneto focus on the fabrication process to obtain high performance device usingmodification and novel nanostructure electrode to replace the ITO anode with highluminous efficiency for phosphorescent PLEDs.1. Solution processed blue electrophosphorescent PLEDs have been fabricatedcontaining an oligo(ethylene oxide)(PEO-DME) additive to enhance theelectroluminescence efficiency. The device uses poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) coated on indium tin oxide as the hole injectionelectrode, cesium fluoride/aluminum as the electron injection electrode. The singleemissive layer is a blend of poly(9-vinylcarbazole)(PVK) as the host,bis[(4,6-difluorophenyl)-pyridinato-N,C2](picolinato)Ir(III)(FIrpic) as thephosphorescent dopant, and1,3-bis[(4-tert-butylphenyl)-1,3,4-oxidiazolyl]phenylene(OXD-7) to enhance electron transporting. The addition of an oligo(ethylene oxide) at5-10wt%effectively lowers both the electron and hole injection barriers. The maximumcurrent efficiency obtained was26.5cd/A at an emission brightness of2500cd/m~2. Thehigh performance is due to (i) improved charge carrier injection at the interfaceresulting from specific interfacial interactions between PEO-DME and aluminum,(ii)improved charge carrier transporting ability and high electric field resulting fromPEO-DME doping.2. White polymer phosphorescent light-emitting diodes (WPLEDs) have been fabricated employing poly(ethylene glycol) dimethyl ether (PEG-DME) blended in thesingle active layer to enhance the emission efficiency. The devices have a simplesandwich architecture of ITO/PEDOT:PSS/emissive layer/CsF/Al. The emissive layeruses a blend of PVK, OXD-7, two or three phosphorescent dopants with complementarycolors. The addition of PEG-DME enhances electron injection, transport, and thebalance of densities of electrons and holes. The measured current efficiency in the frontviewing direction is17.5cd/A at1800cd/cm~2for the two complementary WPLEDs,and35.7cd/A at3000cd/m~2for the three complementary color WPLEDs. The currentefficiencies remain high even at brightness levels up to30,000cd/m~2. The high currentefficiency is ascribed to the improved electron injection ability from the metal cathode,the enhanced charge carrier transport ability and the enhanced red emitting intensity byblending with PEG-DME. Also the low roll-off of the current efficiency was due to thelower triplet-polaron annihilation and the electric field-induced triplet excitonquenching by increased charge carrier transport in unipolar device and movedrecombination zone. The improved charge carrier injection at the interface and theenhanced charge carrier transport were resulted from specific interfacial interactionsbetween PEG-DME and aluminum and higher electric field by blending withPEG-DME.3. We report the demonstration of blue, green, and red electrophosphorescentPLEDs fabricated on silver nanowire (AgNW) polymer composite electrode. Thecurrent efficiency of the ITO-free blue PLEDs falls in the narrow range of20.0-21.5cd/A in the brightness range of450-5500cd/m~2, and decreases slightly to18.5cd/A at10,000cd/m~2. The devices are20%-50%more efficient than control devices onITO/glass. The current efficiency of the green and red phosphorescent PLEDs was alsoenhanced by20-30%. The improved performance can be attributed to the highertransparency of the AgNW electrode than ITO/glass in the blue region, as well asphoton scattering on the surface of AgNWs in the composite electrode that enhanceslight out-coupling efficiency. The blue PLEDs can be repeatedly bent to1.5mm radiusconcave or convex with calculated strain in the emissive layer to be5%(tensile orcompressive).4. Phosphorescent WPLEDs have been fabricated employing AgNW-polymercomposite substrate to enhance the light out-coupling and emission efficiency. The current efficiency in the front viewing direction is20.3cd/A for the two complementarycolors devices and42.3cd/A at4000cd/cm~2for the three primary colors devices, theresults are35%and41%higher than that of a conventional WPLEDs used as acomparison, respectively. The high current efficiency is ascribed to the improved lightout-coupling and the waveguided light in ITO and glass substrate could extract in to airfrom light scattering of the AgNW network on the surface and embedded in the polymercomposite. The WPLEDs on the composite substrate are highly flexible and be bent to amaximum5%compressive strain with little decrease on device efficiency.