Victor Christou

High resolution x-ray photoemission study of plasma oxidation of indium–tin–oxide thin film surfaces

The influence of plasma oxidation and other surface pretreatments on the electronic structure of indium–tin–oxide (ITO) thin films has been studied by high resolution x-ray photoemission spectroscopy. Plasma oxidation compensates n-type doping in the near surface region and leads to a reduction in the energy of plasmon satellite structure observed in In 3d core level spectra. In parallel, the Fermi level moves down within the conduction band, leading to a shift to low binding energy for both core and valence band photoemission features; and the work function increases by a value that corresponds roughly to the core and valence band binding energy shifts. These observations suggest that the conduction band of ITO is fixed relative to the vacuum level and that changes of work function are dominated by shifts of the Fermi level within the conduction band.

New molecular lanthanide materials for organic electroluminescent devices

Organic electroluminescent (EL) devices based upon the new lanthanide EL material Tb[Ph2P(O)NP(O)Ph2]3 (Tbpip3) are described. Several device structures are reported and the effect of charge transporting material and layer thickness on device performance critically assessed. Device performance is optimised in a three-layer structure containing TPD and Alq as the charge transport layers. This device has an efficiency of 0.7 cd A−1 at 20 cd m−2 at 25 V and 1 mA cm−2.

A low reflectivity multilayer cathode for organic light-emitting diodes

In this paper, a vacuum-deposited multilayer cathode for organic light-emitting diodes (OLEDs) with reduced reflectivity is described. The reduced reflectivity (58% at 550 nm) is due to the addition of a smooth and compact carbon film between a thin semi-transparent magnesium layer and the top aluminum contact. For the following light-emitting organic structure deposited on indium-tin-oxide substrates: N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/terbium tris(1-phenyl-3-methyl-4-(tertiarybutyryl)pyrazol-5-one) triphenylphosphine oxide [(tb-PMP)3Tb(Ph3PO)]/3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), the injection properties of such a multilayer cathode are presented and compared to those of Al/Mg and C/Al cathodes. Such a cathode shows promise for contrast improvements in OLEDs.

Circularly polarized luminescence from an organoterbium emitter embedded in a chiral polymer

The periodic helical structure of an aligned cholesteric liquid crystal gives rise to circular Bragg reflection such that circularly polarised light of the same handedness as the helix is reflected while counter circularly polarised light is transmitted. The resulting circularly polarised 1D photonic band gap can be used to suppress or enhance circularly polarised photoluminescence from a fluorescent guest material embedded in a chiral host whose resonance region coincides with the emission of the fluorophor. The periodic structure of the host suppresses emission of one circular handedness of certain frequencies and enhances the suppressed emission at the edges of the reflection band. To avoid the effects of redistributing the photon density of states across the spontaneous emission spectrum and to observe inhibition of spontaneous emission of one handedness a very narrow fluorophor is needed. In this paper, an organoterbium complex is embedded in a cholesteric reactive mesogen (RM) host, which is subsequently polymerized by UV-exposure to generate a chiral polymer. The reflection band of the chiral host is tuned to completely overlap the different emission lines of the organolanthanide as well as to lie in between them. We investigate how the suppression of one emission channel causes a redistribution of probabilities for the remaining pathways resulting in a spectral and spatial redistribution of emission.

Improving Colour Purity and Device Efficiency in Molecular Lanthanide TFEL Devices

Organic EL devices with excellent red CIE colour coordinates, based upon the new lanthanide EL material Eu[Ph2P(O)NP(O)Ph2]3(Eupip3) are described. Several device structures are reported and the effects of charge transport layers on device structure critically assessed. Improved device performance from an unencapsulated device using an aluminum cathode is achieved in a three layer structure with TPD and TAZ as the charge transport layers. This device has an efficiency of 0.02 % at 20 cd/m2 at 25 V and 1 mA/cm2.