M.J.A. de Voigt

Stability of the interface between indium-tin-oxide and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) in polymer light-emitting diodes

A cause for degradation of polymer light-emitting diodes is the oxidation of the polymer by oxygen diffusing out of the indium-tin-oxide (ITO) anode. This problem can be solved by the introduction of an organic hole-injecting film, poly-(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS), between the ITO and the emissive polymer. Indeed, a dramatic improvement of the lifetime and also the luminous efficiency has been observed. However, our Rutherford backscattering (RBS) studies show that the ITO/PEDOT:PSS interface is not stable. In as prepared glass/ITO/PEDOT:PSS samples 0.02 at. % indium was found in the PEDOT:PSS film. Annealing in a nitrogen atmosphere at 100 °C during 2500 h increased the indium concentration to 0.2 at. %. Upon exposure to air much faster degradation of the ITO/PEDOT:PSS interface was observed; after several days in air the amount of indium reached a saturation concentration of 1.2 at. %. The degradation of the interface can be explained by etching of the IT...

Indium diffusion in model polymer light-emitting diodes

The diffusion of indium into poly-(phenylenevinylene) (PPV) in model polymer light-emitting diodes (p-LEDs) was studied with Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and particle induced X-ray emission (PIXE). The model p-LEDs consisted of a glass substrate, an indium–tin-oxide (ITO) electrode, a PPV layer obtained by thermal conversion of sulfonium precursor PPV, and a patterned aluminium electrode. From RBS measurements it was concluded that about 0.01 at.% indium was present in the PPV, homogeneously distributed in depth. Annealing at 230°C for 19 h caused the amount of In in the PPV layer to increase by roughly an order of magnitude. Under the patterned aluminium electrode, the annealing treatment resulted in accumulation of In at the PPV/Al interface, whereas the depth distribution of In remained homogeneous in the uncovered region of the model LEDs. XPS spectra on annealed model LEDs show that In was present in the near surface region of the PPV films, although LEIS analysis showed that In was not situated in the outermost atomic layer. LEIS measurements on as-prepared model LEDs showed that the patterned Al electrode had caused surface contamination of the uncovered PPV film with Al, which can have impact on the diffusion of In to the outermost surface during annealing treatments.

A model for ion-irradiation induced hydrogen loss from organic materials

In the study of interfacial diffusion processes in polymer light-emitting diodes, the use of high-energy ion-scattering techniques can be of great value due to the possibility of quantitative elemental depth profiling. However, ion irradiation of polymers is known to cause various degradation effects, including the loss of hydrogen. Since the hydrogen loss determines the accuracy of depth profiling, it is an interesting subject for study in order to define experimental conditions in which the degradation is suppressed. The loss of hydrogen from porphyrins (organic solar cells) has been measured by means of elastic recoil detection analysis with 2, 4, and 7.6 MeV He+ beams. A theoretical model is proposed in which the hydrogen loss is described through the formation and recombination of free hydrogen radicals. A distinct difference is introduced between direct recombination processes and the diffusion of radicals out of the ion track.

SEARCH FOR HIGH-SPIN ISOMERIC STATES IN THE RARE-EARTH REGION

Several new high-spin (I>10) isomeric states in the mass region A=144–164 are located in a systematic search with 12C induced reactions, employing a sixteen NaI (Tl)Ge(Li) detector multiplicity filter. The proximity to the N=82 neutron shell closure strongly suggests that shell effects contribute significantly to this isomerism.

Collective and quasiparticle excitations in 180Pt

Abstract The 144Nd(40Ar, 4n)180Pt reaction has been used to study collective and quasiparticle excitations by means of several γ and electron spectroscopic measurements. Four bands are observed in addition to the ground-state, β-, γ- and S-bands up to a maximum spin of 26+. A sharp bandcrossing is observed at k = 0.33 MeV of the g.s.b. in 180Pt with the S-band. The crossing is discussed in terms of possible v i 13 2 and π h 9 2 alignments. The observed 02+ state at Ex = 478keV can be interpreted as the head of a β-band in the rotation-vibration model, or as a somewhat oblately deformed band head.

In-beam γ-ray studies of complex band structures and of isomeric states in 152, 153Dy nuclei

Abstract The high-spin level structures of 152 Dy and 153 Dy were studied experimentally with 154, 155 Gd(α xnγ ) in-beam reactions, and for 152 Dy also with 144, 146 Nd ( 12 C, x n γ) reactions. The experiments included measurements of singles γ-ray and conversion-electron spectra, γ-ray angular distributions and E γ - t and E γ - E γ - t coincidences. A multiplicity filter set-up was used to study the feeding and decay of isomeric states in 152 Dy. In 152 Dy about twenty so far unknown levels were found, including two high-spin isomeric states with T 1 2 ≈ 60 and ≈ 13 ns at excitation energies E x ≈ 5.04 and 6.08 MeV, respectively. These states are compared with recent calculations on yrast traps. The level scheme of 153 Dy contains 28 levels up to E x = 4.1 MeV and J π = ( 37 2 + ) . Band structures in both nuclei are discussed in comparison with other N = 86 and N = 87 isotones.

Experimental investigation of the quasi-continuum γ-ray cascades following 160Gd(α, xn)164 − xDy reactions

Abstract Multiplicity shape parameters have been determined for the 160 Gd( α , x n) 164 − x Dy reactions with x = 4–8 for E α = 40–110 MeV. Gamma-ray entry lines for the (α, 4n) and (α, 6n) reactions have been deduced from the side-feeding multiplicities as a function of γ-ray energy for various bombarding energies. The γ-ray multiplicities and the excitation energies of the entry lines increase steeply with the bombarding energy in the region of compound-nucleus formation and retain rather constant values when pre-equilibrium decay is important. These saturation effects locate the neutron “drip line” in the energy versus spin plane. The role of angular momentum and of excitation energy on the γ-ray decay process is investigated. The multiplicity data are compared with a combined statistical-master-equation-IBA calculation, which yields insight in the physical parameters that govern the decay process.

Degradation effects in poly para-phenylene vinylene derivatives due to controlled oxygen exposure

Abstract The influence of oxygen exposure on three chemically different poly para -phenylene vinylene (PPV) derivatives used in polymeric light emitting devices (PLEDs) has been investigated. During device preparation, PPV layers have been exposed to oxygen either in the dark or during the irradiation with visible light, before the cathode was applied. Device efficiency was studied by electrical and optical characterisation and correlated to oxygen depth profiles measured with Elastic Recoil Detection Analysis (ERDA). Treatment with oxygen during light exposure leads to a decrease in current, light output and efficiency. It was found that two different PPV derivatives show the same current and light output reduction with different oxygen uptake. This behaviour is explained in terms of a different chemical structure and the number of structural defects (tolane-bisbenzyl moieties (TBBs)) incorporated. For two PPV derivatives treated with oxygen in the dark a reduction in current and light output was found, while the efficiency was unchanged. Another PPV derivative, however, shows a shift in the on-set voltage of the light output accompanied by an increased oxygen level at the PPV/Ca interface. It is concluded that after exposure in the dark, oxygen is incorporated in the PPV by Van der Waals interaction. During evaporation of the cathodes, oxygen will diffuse to the cathode and will be gettered by the calcium, which results in the formation of an electron injection barrier.

Monomer diffusion assisted preparation of polymer gratings: A nuclear microprobe study

Abstract Polymers with an ordered molecular structure can be applied in optical systems for e.g. data transport, data storage and displays. Patterned UV photo-polymerization is used to prepare polymer gratings from a mixture of two acrylate monomers. A 3 MeV proton microprobe is used to study these gratings, prepared from two different monomers, each containing a different easily detectable label element, e.g. Cl, Si or F. During the preparation process, the difference in reactivity and mobility of these two monomers in combination with polymer–monomer interaction results in diffusion of monomers. Since this diffusion process takes place on length scales of micrometers, a scanning ion microprobe is a powerful tool for the quantitative analysis of the polymer films, obtained after complete polymerization. The microprobe is equipped with PIXE, PIGE and RBS, to quantify both the label elements and C and O. This makes it possible to determine the concentration of monomer units as a function of position and thus to study the diffusion process. Two combinations of different monomers are studied. In the case of a 0.5:1 mixture of a monofunctional and a difunctional monomer, both monomers migrate to the illuminated areas and large thickness variations are observed. When a 1:1 mixture of two difunctional monomers is used, opposite migration of the two monomers is observed, while the film shows no variation in thickness.

Influence of a partially oxidized calcium cathode on the performance of polymeric light emitting diodes

We investigated the influence of the presence of oxygen during the deposition of the calcium cathode on the structure and on the performance of polymeric light emitting diodes (pLEDs). The oxygen background pressure during deposition of the calcium cathode of polymeric LEDs was varied. Subsequently, the oxygen depth distribution was measured and correlated with the performance of the pLEDs. The devices have been fabricated in a recently built ultraclean setup. The polymer layers of the pLEDs have been spincoated in a dry nitrogen atmosphere and transported directly into an ultrahigh vacuum chamber where the metal electrodes have been deposited by evaporation. We used indium–tin–oxide as anode, OC1C10 PPV as electroluminescent polymer, calcium as cathode, and aluminum as protecting layer. We achieved reproducibility of about 15% in current and brightness for devices fabricated in an oxygen atmosphere of ≪10−9 mbar. For further investigations the calcium deposition was carried out in an oxygen atmosphere fr...