S. Schrader

Angle resolved ultraviolet photoelectron spectroscopy of oriented sexiphenyl layers

Ultra-thin films of oriented p-sexiphenyl (6P) on a GaAs(001) wafer were studied by angle resolved ultraviolet photoelectron spectroscopy (ARUPS) using UV photons in the energy range between 20 and 60 eV. The samples were prepared under ultra-high vacuum by controlled evaporation onto cleaned GaAs substrates, which were thermostated at enhanced temperature during the deposition process. The thickness of the deposited films played a very important role for the orientation of 6P molecules. For layers having a thickness of about 35 A, the molecules were oriented perpendicular to the substrate surface and for films of about 300-A thickness the molecules lost their upright orientation. The orientation of 6P molecules was monitored both by the anisotropy as visible in the measured ARUPS spectra and independently by atomic force microscopy (AFM) investigations carried out after UPS measurements. From the experimental photoelectron spectra, the full one-dimensional valence band structure of oriented p-sexiphenyl layers has been determined. While valence states between 2 and 5 eV show only weak dispersion, the bands between 5 and 11 eV show a clear momentum dispersion, which indicates the high degree of order in the investigated 6P layers.

Ultraviolet photoelectron spectroscopy on new heterocyclic materials for multilayer organic light emitting diodes

Electronic band structure of three heterocyclic model compounds: 5,12-dihydro-quinoxalino-2,3-b-phenazine; 7,18-dihydro-5,7,11,16,18,20,22-octa-aza-nonacene; and 1,3,5-Tris[(3-phenyl-6-tri-fluoro-methyl)-quinoxaline-2-yl]benzene was studied by ultraviolet photoelectron spectroscopy. We also determined their ionisation potential, between 5.3 and 6.0 eV, suggesting that these materials can efficiently act as electron transporting layer in organic light emitting devices (OLEDs).

Ultraviolet photoelectron spectroscopic study of heterocyclic model compounds for electroluminescent devices

One approach to increase the overall performance of organic light emitting devices is to separate the light-emitting volume from the ones which are assigned to charge injection or transport. The electronic properties of model heterocyclic compounds, namely low molecular phenylquinoxalines and related derivatives, for electron transport layers have been investigated by Ultraviolet Photoelectron Spectroscopy. The ionization potentials of these materials have been determined.

Valence electronic structure of oxadiazoles and quinoxalines model compounds

Thin films of oxadiazoles and quinoxalines model compounds have been investigated by means of Ultraviolet Photoelectron Spectroscopy (UPS) in order to get a detailed description of the valence electronic band structure. Furthermore, we used semi-empirical quantum-chemical calculations to evaluate the valence electronic structure of the isolated molecules. UPS spectra have been calculated by use of molecular eigenvalues and we assigned the different peaks to the different molecular eigenstates.

Organic light-emitting devices based on new heterocyclic compounds

One approach to increase the overall performance of organic light emitting devices is to separate the light-emitting volume from the ones which are assigned to charge injection or transport. We realized such polymer hetero-layer structures by combination of hole transporting materials like polyparaphenylenevinylene (PPV) with new electron transporting materials, i.e. heterocyclic polymers and heterocyclic low molecular compounds, especially phenyl quinoxalines (PQs). The electronic properties of these heterocyclic compounds have been investigated by various methods including ultraviolet photoelectron spectroscopy. PQs show electron affinities near 3.5 eV and ionization potentials below 6 eV. Measurements of thermally stimulated depolarization currents (TSDC) were carried out in order to study both dipolar relaxation and charge transport processes in single layer devices. The TSDC spectra revealed the prominence of both dipolar relaxation and of charge transport processes. The dipolar processes show activation energies between 0.4 eV and 1 eV which are typical values for small relaxing entities like polymer side groups. Current-voltage and current-luminance characteristics were used to study the prepared heterolayer devices. Double layers made of PPV and polyphenylquinoxaline (PPQ) are characterized by low onset voltages near 2 V and high luminous efficiency of more than 0.8 cd/A. The experimental findings show that PPQs are promising materials in the field of organic electroluminescence.

Analysis of detrapping processes of aromatic 1,3,4-oxadiazoles with thermally stimulated luminescence

Abstract Thermally stimulated luminescence (TSL), and ultraviolet photoelectron spectroscopy (UPS) measurements have been carried out on Langmuir–Blodgett films of a substituted 1,3,4-oxadiazole. UPS provided the experimental electronic band structure of the investigated material and also parameters like ionisation potential, 7.9xa0eV, and electron affinity, 4.8xa0eV. These high values indicate that it can be used as hole blocking/electron transporting layer in emitting devices. Thermally stimulated processes on oxadiazoles thin films have been studied in order to get information about their electronic properties. The fractional glow technique, applied on the thermal stimulated luminescence curves, highlights different peaks. They originate from carrier relaxation processes. We focused our attention on the complex structure between 120 and 160xa0K. By the analysis of the curves performed both with the initial rise method and with a fitting procedure, we estimated the activation energy, between 3.5 (initial rise method) and 4xa0meV (numerical simulation), the frequency factors, 10 4 xa0s −1 , and a Gaussian trap distribution shape.

Electronic structure of aromatic 1,3,4-oxadiazoles studied by ultraviolet photoelectron spectroscopy

Abstract The family of substituted 1,3,4-oxadiazole can be used in organic light-emitting diodes (OLEDs) acting both as hole blocking and as improved emissive layers. Thin films of aromatic 1,3,4-oxadiazoles, prepared in situ by evaporation from pinhole sources onto fresh deposited gold films on flat silicon wafers, were studied by means of ultraviolet photoelectron spectroscopy (UPS) in order to get detailed information about valence electronic band structure. The UPS measurements were performed at the beam line TGM-2 at BESSY (Berlin) in the energy range between 8 and 80xa0eV. Furthermore, we determined the vacuum level shift at the organic/metal interface. We used semi-empirical quantum-chemical calculations to evaluate the valence electronic structure of the isolated molecules. This was also the starting point used as input to simulate the UPS spectra obtained from the films. The good agreement between experimental and theoretical curves allows us to assign the different peaks to different molecular eigenstates and it also provides a more quantitative description of the experimental findings.

Interface properties of organic materials investigated by using ultraviolet photoelectron spectroscopy

Abstract The properties of the interface between a thin layer of native SiO 2 and a low molecular compound were studied by means of ultraviolet photoelectron spectroscopy (UPS). The investigated material, an amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole (NADPO), was prepared by using the Langmuir–Blodgett (LB) technique. It is of interest since its electronic properties do not depend on the preparation conditions and it is also being candidate for use in optoelectronic devices. The UPS measurements were performed at the beam line Seya F2.2 at HASYLAB (Hamburg, Germany). This beamline is of a 1xa0m Seya–Namioka monochromator type, equipped with two gratings, which cover the photon energy range from 4 to 40xa0eV. The photoelectrons were collected with a Vacuum Generators ADES 400 angle resolving spectrometer system at room temperature. The base pressure during the experiment was 1×10 −10 xa0mbar. We investigated the interface properties depending on the number of monolayers (2, 4, 6 and 8) of well oriented LB films, monitoring the highest occupied molecular orbital (HOMO) energy position.

New polymeric heterolayer systems for efficient light-emitting devices

Efficiency of organic light emitting devices can be increased by separating the zones of exciton recombination from the electrodes by the insertion of additional organic layers which act as protecting - and as hole or electron blocking layers. We realized such polymer hetero-layer structures by combination of hole transporting and emitting materials like polyparaphenylenevinylene (PPV) or its derivatives with new electron transporting materials, i.e. heterocyclic polymers and heterocyclic low molecular compounds, especially phenyl quinoxalines. Current-voltage molecular compounds, especially phenyl quinoxalines. Current-voltage and current-luminance characteristics were used to study the prepared heterolayer devices. Optical spectroscopy as well as UV photoelectron spectroscopy were used to characterize the electronic structure of the individual materials. Quantum chemical calculations completed the spectroscopic studies and supported the interpretation of experimental findings. Double layers made of PPV and polyphenylquinoxaline (PPQ) are characterized by low onset voltages of about 2.2 V, high efficiency, and high brightness reaching values of more than 2000 cd/m2 at a driving voltage of 10 V. The experimental findings show that PPQs are promising materials for organic electroluminescence applications.

Thermally stimulated processes in heterocyclic materials suitable for heterolayer organic light emitting diodes

Abstract We studied ultraviolet photoelectron spectroscopy (UPS) and thermally stimulated processes in two types of phenylquinoxalines, and one type of starburst trisphenylquinoxaline in order to characterize the valence band structure and the localized states. An ionization potential of about 6.0xa0eV was determined for each material using UPS. Thermally stimulated discharge currents (TSDC) gave evidence of dipolar and/or charge transport mechanisms. We assigned each TSDC peak to the different elementary processes using in addition dielectric spectroscopy (DES), and from thermally stimulated luminescence (TSL) curves, performed for the first time on these materials, we estimated the mean trap depths.