Search

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 1 m Seya–Namioka monochromator type, equipped with two gratings, which cover the photon energy range from 4 to 40 eV. 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 mbar. 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.

Electronic structure at the interface between gold and aromatic 1,3,4-oxadiazoles was studied by means of ultraviolet photoelectron spectroscopy (UPS). Thin films of 1,3,4-oxadiazoles were prepared in situ by evaporation from pinhole sources on freshly prepared gold on a Si substrate. We determine the frontier orbitals positions at the interface oxadiazoles/Au finding evidence for the presence of a dipole layer. Furthermore, the complete valence electronic band structure has been determined from the UPS spectra. The spectral features corresponding to the top of the valence band and dominating the electronic properties of this material were assigned to the different molecular eigenstates by comparison with simulated spectra from quantum chemical calculations.

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, 7.9 eV, and electron affinity, 4.8 eV. 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 160 K. 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 4 meV (numerical simulation), the frequency factors, 104 s−1, and a Gaussian trap distribution shape.

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 80 eV. 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.

The electronic structure of thin films of new heterocyclic compounds was studied by means of ultraviolet photoelectron spectroscopy (UPS) and near edge X-ray absorption fine structure (NEXAFS). The complete one-dimensional valence electronic band structure has been determined from the UPS spectra. The spectral features that correspond to the top of the valence band and that dominate the electronic properties of this material were assigned to the different molecular eigenstates by comparison with simulated spectra from quantum chemical calculations. NEXAFS measurements were carried out on the same films.

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).

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 6.0 eV 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.

The properties of the interface Si /SiO 2 /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), is also of interest in photonics, because it can be used as active material in opto-electronic or electro-optic devices. The UPS measurements were performed at the beam line Seya F2.2 in HASYLAB (Hamburg, Germany). The photoelectrons were collected with a Vacuum Generators ADES 400 angle resolving spectrometer system at room temperature. The photoelectron spectra were measured with an angle of incidence of 45 degrees in normal emission for various incident energies and for an incident photon energy, E inc , of 24 eV in off normal emission. We investigated the interface properties depending on the number of monolayers (2, 4, 6, 8) of well oriented Langmuir-Blodgett films, monitoring the frontier orbital position, with particular regard to the highest occupied molecular orbital energy position. The obtained results give evidence that the band structure is depending on the thickness of the organic layer at the interface Si /SiO 2 /NADPO. We also monitored the development of the energetic position of the highest occupied molecular orbital (HOMO) from that of the monolayer towards the "true" HOMO energy position of the thin film, where the word "true" is related to the actual interface properties of the films and not to a single monolayer (ML) or a double layer. As a matter of fact the 2 ML spectra show a different behaviour than those of thicker films since the 2 ML system is strongly influenced by the substrate while the spectra of thicker films are de-coupled from the electronic structure of the substrate.

Quinoxaline model compounds are interesting materials for Organic Light Emitting Devices (OLEDs) because of their thermal stability and their higher ionization potential in comparison to other electron transporting/hole blocking materials. In this work we studied a starburst trisphenylquinoxaline by means of Ultraviolet Photoelectron Spectroscopy (UPS) and Thermally Stimulated Luminescence (TSL). UPS provided not only the characterization of the valence band structure but also parameters like ionization potential, 6 eV, and, combined with absorption spectroscopy, electron affinity, that are of crucial importance in designing optimized OLED configurations. On the other hand, a wide distribution of localized states occurs in organic layers due to several factors and TSL can investigate these states. The combination of the used techniques together with semi-empirical quantum chemical calculation, gave a detailed description not only of the valence and conduction band of the studied materials, i.e. the energy position of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), but also of the trap distributions localized in the band gap: a shallow one at 0.06 eV and a deeper state centered at 0.24 eV. The full spectroscopic and electrical characterization of the material formed the background for understanding its behavior in heterolayer devices.

Electronic structure of ordered films obtained from an amphiphilic substituted 2,5-diphenyl- 1,3,4-oxadiazole (NADPO-11) by Langmuir-Blodgett (LB) technique were studied, for the first time, by means of ultraviolet photoelectron spectroscopy (UPS). The complete one- dimensional valence band structure has been determined from the UPS spectra. The spectral features that correspond to the top of the valence band and that dominate the electronic properties of this material were assigned to the different molecular eigenstates by comparison with simulated spectra from quantum chemical calculations. Good agreement between simulated and experimental curves allowed to assign UPS peaks to certain groups of molecular orbitals (MOs) and to understand the electronic bonding state (σ- or π- type) of the involved MOs. Furthermore, we performed UPS measurements on in-situ evaporated amorphous films of the same substance, comparing their valence band features to the previous ones. The correlation between ionisation potential and electron affinity, determined by combined ultraviolet photoelectron spectroscopy/optical spectroscopy, and carrier-transport and light emitting properties of these material are also discussed.

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.

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.

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.