Improved Performance Of Organic Light-Emitting Device Using Metal Nanoparticles

Organic light-emitting devices （OLEDs） have made significant achievements inthe past few years with variety of applications. OLED possess a lot of advantagessuch as light weight, wide viewing angle, high efficiency, flexible and so on.Currently, the small area display panel based on AMOLED technology has come intomarkets. The metal nanoparticles have been widely studied for its special effect onOLED performance and remarkable achievements have been made.In our experiments, the surface Plasmon coupling effect of metal nanoparticlescould weaken exciton annihilation and reduce exciton lifetime, thus enhancing theprobability of radiation recombination, device efficiency, and reducing the efficiencyroll-off of devices. Through a large number of literatures, we conclude that twoconditions should be satisfied to realize the coupling effects: overlap between theemission wavelength and surface Plasmon resonant wavelength; an optimizeddistance between metal nanoparticles and the excitons.Firstly, we studied influence of the silver nanoparticles on performance of greenfluorescent OLED and the device structure is: ITO/NPB （50nm）/DOPPP （30nm）/BCP（20nm）/LiF （0.5nm）/Mg:Ag （150nm）. The silver nanoparticles were introduced intothe NPB and BCP on both sides of emission layer with an average thickness of0.5nmby thermal evaporation and the distance between silver nanoparticles and the emissionlayer was set as0nm,3nm and5nm. Thus, five different devices were fabricated.Simultaneously, another five comparative devices without silver nanoparticles werealso fabricated. The experimental results indicated the device brightness andefficiency were enhanced by17%and14%, respectively, when the distance was3nmand5nm with silver nanoparticles in NPB. However, the device brightness andefficiency decreased with the distance of0nm. This could be attributed to theenhancement of radiation recombination due to the plasmon coupling effect and theexcitons were quenched with the distance of0nm. The device brightness andefficiency poorly decreased when the silver nanoparticles were in the BCP due to the far distance from recombination zone location and the electron traps introduced by Agnanoparticles.Then, we studied the impact of gold nanoparticles on green fluorescent OLEDand the device structure was: ITO/MoO₃（2nm）/NPB （35nm）/CBP:Ir（ppy）₃（10%）（30nm）/BCP （10nm）/Alq₃（20nm）/LiF （1nm）/Al （150nm）. The gold nanoparticleswere introduced into NPB and BCP on both sides of emission layer with an averagethickness of0.5nm by thermal evaporation and the distance between goldnanoparticles and the emission layer was set as0nm,3nm and6nm. Thus, sixdifferent devices were fabricated. Simultaneously, another six comparative deviceswithout gold nanoparticles were also fabricated. The device brightness and efficiencydecreased significantly when the distance was0nm with gold nanoparticles in NPBand BCP due to serious exciton quenching by gold nanoparticles. The devicebrightness and efficiency were enhanced when the distance was3nm and6nm withgold nanoparticles in the BCP due to the coupling effect. The device with the distanceto be3nm showed best performance, of which denoted a brightness enhancement of15%and the efficiency roll-off was17.6%from the brightness at maximum currentefficiency to10000cd/m². When the gold nanoparticles was in NPB with the distanceof3nm and6nm, the device brightness and efficiency simultaneously decreased, butthe device efficiency roll-off was lower. As can be seen, the efficiency roll-off of thedevice with the distance of6nm was12.5%from the brightness at maximum currentefficiency to10000cd/m². We attribute this to the reduced exciton lifetime and thebetter balance of electrons and holes due to holes traps by gold nanoparticles in theNPB.