Andreas Elschner

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.

PEDT/PSS for efficient hole-injection in hybrid organic light-emitting diodes

Light-emitting diodes have been prepared by depositing three organic layers successively by spin-coat and evaporation techniques. The first layer of PEDT/PSS smoothens the ITO surface, reduces the probability of electrical shorts and is beneficial for a high overall yield of the operating devices. Above all, this layer promotes hole-injection as an important parameter for higher efficiencies and prolonged operation life. The second layer of spin-coated dendritic phenylamines (TDAPB) with high glass transition temperature modulates the injection of holes into the emitting layer, formed by evaporated Alq. By comparing characteristics and operation lifetime data of devices with and without PEDT/PSS it is shown that the combination of polymeric and monomeric organic layers leads to highly efficient devices, opening new ways to modify device architectures.

Composition and growth mechanisms of alumina scales on FeCrAl-based alloys determined by SNMS

The composition and growth phenomena of alumina scales on a conventional wrought alloy Fe-20Cr-5Al and on an yttria containing ODS alloy of similar base composition were investigated. The oxide scales were formed during two-stage oxidation at temperatures between 900 and 1100°C in air and in air containing 18O tracer. The in-depth concentration profiles of the various elements as well as the oxygen isotope distributions in the oxide scales were determined by secondary neutrals mass spectrometry (SNMS). It was found that detailed knowledge about the time and temperature dependence of scale composition is necessary for deriving unequivocal information on the scale growth process from the oxygen isotope distributions. It appeared that the growth of the oxide scales mainly occurs on the scale/alloy and the scale/gas interface. The existence of growth within the scale could not clearly be confirmed although some results indicated that this effect might have occured. The scales on the ODS alloy and the conventional alloy differed mainly in the relative contributions of cation diffusion to the overall growth process.

Organic molecular films on gold versus conducting polymer: Influence of injection barrier height and morphology on current–voltage characteristics

The current–voltage characteristics I(V) of model organic devices are studied under ultra-high-vacuum conditions. Active materials are N,N′-bis-(1-naphthyl)-N,N′-diphen-yl1-1,1-biphenyl1-4,4′-diamine (α-NPD) and pentacene, electrode materials are polycrystalline Au and the conductive polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Despite a similar work function of electrode material surfaces (∼5 eV), hole injection from PEDOT/PSS is significantly more efficient than from Au, due to a smaller hole injection barrier. Hole injection characteristics from Au electrodes for devices made from α-NPD are independent of deposition sequence and substrate used. Pentacene devices exhibit serious asymmetries in that respect. These are caused by a strong dependence of morphology and preferred molecular orientation on the substrate for the crystalline material.

Influence of water on the work function of conducting poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)

The influence of water exposure on the work function (ϕ) and surface composition of conducting poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDT:PSS) was investigated with ultraviolet and x-ray photoelectron spectroscopies. It was found that annealing PEDT:PSS in vacuum to 220°C yields a high ϕ of 5.65eV. Subsequent exposure to water vapor or air reduces ϕ to ∼5.15eV, and the film surface becomes enriched with PEDT. These observations were fully reversible for repeated annealing–water exposure cycles. The reduction in ϕ is attributed to (i) the inclusion of water leading to a larger dielectric constant and (ii) polymer swelling-induced rearrangements of surface dipoles.

Impedance spectroscopy as a probe for the degradation of organic light-emitting diodes

Impedance spectroscopy is a powerful method for characterizing the electrical properties of materials and their interfaces. In this study we use capacitance measurements to investigate the degradation of electrically aged bottom-emitting organic light-emitting diodes with different polymeric hole injection layers. The devices comprise a heterojunction between a hole transporting triphenyl-diamine and an electron transporting and green emitting aluminum chelate complex [Alq3, tris-(8-hydroxyquinoline) aluminum]. A detailed analysis of the capacitance as function of frequency and dc bias yields information about trapped and interfacial charges as well as the dynamics of injected charges. We find that the loss of luminance and the increase in drive voltage of stressed devices is accompanied by a deterioration of hole injection and the formation of positively charged quenching centers at or close to the organic heterojunction. Using a new polymeric hole injection layer leads to improved device stability.

Inhomogeneous luminance in organic light emitting diodes related to electrode resistivity

In organic light emitting diodes (OLEDs) with transparent electrodes, the luminance usually becomes inhomogeneous if the size of the pixel increases above 10mm. A theoretical model for inhomogeneous voltage and luminance in OLEDs is provided together with an approximate analytical solution for the problem in case of cylindrical symmetry. Experimental observations of inhomogeneous luminance are compared with numerical simulations based on the theoretical model, proving the applicability of the approximations made in the theoretical model.

The oxidation behaviour of niobium containing γ-TiAl based intermetallics in air and argon/oxygen

The oxidation behaviour of γ-TiAl based alloys with different Nb contents (2–10 At.%) was investigated in air and in argon-20% oxygen at 900 °C using thermogravimetric analysis. The oxide scales were characterized by a combination of optical microscopy, SEM/EDX and X-ray diffraction analyses. Although in all studied cases the presence of niobium improves the oxidation resistance of γ-TiAl, the oxidation kinetics, scale morphology and composition in air differed strongly from that in argon-oxygen. In air the oxidation resistance increases with increasing niobium-content. In Ar/O2 the niobium dependence is far more complex because internal oxidation occurs which is favoured by the presence of niobium. SNMS analysis revealed that the differences in behaviour in the two atmospheres are related to the formation of Ti-rich nitride at the scale/alloy interface during air oxidation. The positive effect of niobium on the oxidation resistance of γ-TiAl is mainly caused by a decrease of the transport processes in the heterogeneous TiO2/Al2O3-surface scale. Nitride formation and/or niobium enrichment at the scale/alloy interface also affect the oxidation behaviour, however these factors are believed to be the result of the decreased transport processes rather than the main reason for the niobium effect.

Gallium Complexes in Three‐Layer Organic Electroluminescent Devices

Organic light-emitting diodes fabricated by subsequently spin-coating two layers-a hole-transporting followed by a metal chelate emissive layer - onto poly(3,4-ethylene-dioxythiophene)/poly(styrenesulfonate) are presentedfor the first time. The electron-hole recombination occurs in a layer consisting of Ga complexes (see Figure), which exhibit high fluorescence quantum yields, and their emission spectra are blue-shifted relative to that of tris(8-hydroxyquinoline) aluminum. By doping this spin-coated emission layer with fluorescent emitters the emission band can be shifted within the visible spectral range.

Lifetime of organic light emitting diodes on polymer anodes

We report on the use of a thin layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as anode for bottom emission organic light emitting diodes (OLEDs). The combination of polymer anodes with OLEDs having either electrically doped or undoped hole transport layers in direct contact with the polymer is shown. We discuss the impact of the annealing conditions of the polymer on the OLED lifetime in comparison to indium tin oxide anodes. Supported by a differential thermal analysis of PEDOT:PSS, a strong influence of residual water in the polymer on the device lifetime is found. Additional heating of the polymer anode in a dry ambient prior to OLED deposition is necessary to achieve high device lifetimes. At a luminance of 260 cd/m2, pin-OLEDs on a PEDOT:PSS anode show no measurable device degradation during 5200 h of operation.