Functional polymer surface passivated zinc oxide hybrid nanomaterial as an efficient cathode interfacial layer for solution processable OLEDs

Abstract

In this dissertation, functional polymer passivated colloidal zinc oxide hybrid nanomaterials have been investigated with conjugated polyelectrolyte and polyethyleneimine polymers, respectively, for the purpose of cathode interfacial layer for solution processable OLEDs. For the case of conjugated polyelectrolyte passivated zinc oxide hybrid nanomaterial, alkoxy side-chain tethered polyfluorene polymer has been synthesized and adopted to surfactant of zinc oxide. Conjugat-ed polyelectrolyte hybridized zinc oxide nanomaterials show good miscibility and higher colloidal stability. Form x-ray photoelectron spectroscopy analysis indicates that oxygen atom and bromide anion on conjugat-ed polyelectrolyte passivated the surface oxygen vacancy of zinc oxide nanoparticle, leading to formulate conjugated polyelectrolyte covering structure on zinc oxide nanoparticle. Solution processable OLED with SY-PPV emitting polymer and conjugated polyelectrolyte hybridized zinc oxide nanomaterials demonstrat-ed 11.7 cd $A^{-1}$ and 8.6 lm $W^{-1}$ performances which are 1.6 (1.6)-fold and 2.5 (3.9)-fold improved values compared with conjugated polyelectrolyte only (zinc oxide only) devices. These results are mainly due to reduced electron injection barrier of zinc oxide after conjugated polyelectrolyte implanted and reduced pho-toluminescence quenching property of emissive layer which directly contracted with cathode interfacial lay-ers. Secondly, for polyethyleneimine hybridized zinc oxide hybrid nanomaterials, zinc oxide colloidal nanoparticles are passivated by coordination bonding with nitrogen atoms from polyethyleneimine and oxy-gen vacancy of zinc oxide nanoparticle confirmed by x-ray photoelectron spectroscopy and photophysical properties of colloidal state hybrid nanomaterials. Applying as cathode interfacial layer, polyethyleneimine hybridized zinc oxide nanomaterials show 13.3 cd $A^{-1}$ and 11.7 lm $W^{-1}$ which is improved compared with conjugated polyelectrolyte hybridized zinc oxide nanomaterial device. Furthermore, it is comparable to vac-uum processed LiF device (14.1 cd $A^{-1}$, 12.1 lm $W^{-1}$) and shows reliably in performances. Considering poly-ethyleneimine hybridized zinc oxide nanomaterial devices (electron only and electron blocking layer em-ployed devices) show enhanced properties, polyethyleneimine zinc oxide hybrid nanomaterials have effi-cient electron injection capability than LiF.