H. von Seggern

Luminescence properties of nanocrystalline Y2O3:Eu3+ in different host materials

In this study, the optical properties of nanocrystalline europium doped yttria, Y2O3:Eu3+ were investigated in dependence on different caging hosts such as porous MCM-41, porous silica, and porous alumina with pore sizes ranging between 2.7 to 80 nm. These results were compared to nanopowders measured in air and aqueous solution whose particle sizes were 5 nm and 8 nm, respectively. All these results were compared to a commercial lamp phosphor powder with a grain size of about 5 μm. The structural properties of the samples were determined by x-ray diffraction and transmission electron microscopy. Investigated optical properties are the photoluminescence emission spectra, the excitation spectra, the lifetimes, and the quantum efficiencies. A heavy dependence of the charge transfer process on the surrounding will be reported and discussed.

Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode

The mechanisms of enhanced electron injection into the electron transport layer of Alq3 [tris(8-hydroxyquinoline)-aluminum] via LiF interlayers are studied by means of I–V characteristics, secondary ion mass spectroscopy (SIMS), and Kelvin probe measurements. Devices for single carrier injection were prepared using aluminum electrodes, Alq3 electron transport layers, and thin intermediate layers of LiF. It was found that devices deposited in the order Alq3-LiF-aluminum have a much higher electron injection capability than structures deposited in the order aluminum-LiF-Alq3. SIMS depth profile analysis reveals that the evaporation of Al on LiF leads to a spatial separation of Li and F probably induced by a chemical reaction of Al with LiF. Simple thermodynamic calculations support the energetic feasibility of such a reaction. Titanium cathodes in the same layer sequence also exhibit electron injection enhancement, probably due to their similar chemical reactivity. However, electron injection from Ag electr...

Physical model of photostimulated luminescence of x-ray irradiated BaFBr:Eu2+

A dynamical model of the temporal dependence of photostimulated luminescence of the storage phosphor BaFBr:Eu2+ as a result of preceeding x‐ray irradiation is presented. The model is based on a monomolecular recombination mechanism. The commonly used bimolecular mechanism leads to contradictions with experimental observations. The monomolecular recombination is explained by the existence of a photostimulable luminescence complex with a recombination center and an electron trap in close proximity. Charge transfer after optical excitation occurs through tunneling. The simulation of the transient charge carrier dynamics is performed through rate equations. Good agreement wih experimentally determined temporal responses for different x‐ray doses applied and stimulating light intensities is obtained.

Breakdown-induced polarization buildup in porous fluoropolymer sandwiches : a thermally stable piezoelectret

The buildup of air-breakdown-induced polarization in a one-side-metallized three-layer sandwich structure consisting of fluorinated ethylene propylene copolymer (FEP) / expanded polytetrafluoroethylene (ePTFE) / FEP has been studied utilizing a corona triode for voltage application. The FEP layers form structurally and electrically dense layers, whereas the ePTFE layer consists of 91% air and 9% fibrous PTFE. Upon negative corona charging, breakdown sets in within the pores of the ePTFE, as soon as the electric field strength exceeds the Paschen breakdown value of air. The resulting ion-plasma then separates in the strong electric field of the corona-deposited surface charges whereby ions of the two polarities drift towards opposite FEP layers, where they are trapped, and macroscopic dipoles are formed. These dipoles are responsible for a strong piezoelectricity. It will be demonstrated by thermally stimulated discharge currents that when poling is performed at elevated temperatures, for example, 150°C, t...

Trap engineering in organic hole transport materials

Chemical impurities with known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital energies were incorporated in organic hole transport materials. The effect of these dopants on quantity and depth of trap levels, transport properties, and luminescence of organic light emitting devices was examined. This was achieved by investigating current–voltage characteristics, luminance–voltage characteristics, and utilizing the method of thermally stimulated current for trap level detection. It was found that 4,4′,4″-tris-[N-(1-naphthyl)-N-(phenylamino)]triphenylamine (1-NaphDATA) doped into N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) generates a trap level whose activation energy corresponds to the HOMO energy difference between dopant and matrix molecules. Therefore, the detected electronic states can be assigned to hole traps. The influence of those traps on the charge transport will be reported. For doping α-NPD into 1-NaphDATA no new trap levels could be detected.

Energetic trap distributions in organic semiconductors

Abstract The electronic trap distributions of the vapor deposited electron transport material Alq 3 [tris(8-(hydroxyquinoline) aluminum] and hole transport material 1-NaphDATA [4,4′,4″-tris( N -(1-naphthyl)- N -phenylamino)-triphenylamine] have been analyzed by the fractional TSC method (thermally stimulated current) and the thermally stimulated luminescence (TL) technique. The obtained trap distributions can be described by a Gaussian distribution in case of Alq 3 and two discrete trap levels in case of 1-NaphDATA. In the case of Alq 3 , a distinction between the trapped charge carrier polarity was possible. A correlation between the I – V curves of electron- and hole-only devices and the respective trap distributions suggests that the trap distribution is responsible for the shape of the observed current–voltage characteristics. It will be shown that the observed trap states cannot be explained by intrinsic tail states of regular HOMO and LUMO levels.

The influence of mechanical rubbing on the field-effect mobility in polyhexylthiophene

This paper reports on improvements of the field-effect mobility in regioregular head-to-tail coupled poly(3-hexylthiophene) based transistors by mechanically induced alignment of polymer chains in the active layer. It is demonstrated that mechanical rubbing perpendicular to the source drain contacts can increase the field-effect mobility up to 800% whereas rubbing parallel to the source drain contacts results in a reduced mobility. The polymer alignment is thereby deduced from optically polarized transmission spectroscopy on polymer-coated quartz glass substrates and is shown to directly correlate with the electrical behavior of a bottom-gate field-effect transistor. The influence of layer thickness on rubbing is investigated and it is shown that annealing after mechanical rubbing at high temperature can further increase the alignment. Differences between thick drop-cast and thin spin-coated films are explained in terms of different solvent evaporation rates, allowing the material to order to a different degree. This interpretation is deduced from characteristic optical and electrical features of the differently prepared poly(3-hexylthiophene) films.

Space-charge limited current in regioregular poly-3-hexyl-thiophene

Temperature dependent current–voltage characteristics of an organic diode based on a thin film of regioregular poly-3-hexyl-thiophene (P3HT) are compared with results of a theoretical model assuming space-charge limited currents with a density of states (DOS), nonmonotonous in energy. This DOS was deduced from experiments utilizing the method of thermally stimulated currents. Both, experiments and theory result in an almost power-law dependence of j∝Vm, where the exponent m increases with decreasing temperature assuming a value of m=2 at room temperature. This effect can be accounted for by filling of deep traps at lower temperatures. Transport of charge carriers in P3HT seems to be limited by hopping in disordered regions rather than by the transport via extended states within crystalline grains.

Non-equilibrium transport of charge carriers in disordered organic materials

An analytic theory of non-equilibrium hopping charge transport in disordered organic materials is developed. It rests on the concept of effective transport energy and includes quasi-equilibrium (normal) and extremely non-equilibrium (dispersive) regimes of hopping transport as limiting cases at long and short times, respectively. Special attention is paid to the regime of moderately weak non-equilibrium transport. In this regime the quasi-equilibrium value of the mobility is nearly established, whereas the coefficient of field-assisted diffusion continues to increase at long times. Analytic expressions for relaxation times in the context of field-assisted diffusion and carrier drift have been obtained. The results of the theory are in agreement both with the data of time-of-flight experiments for molecularly doped polymers and the results of numerical simulations of the Gaussian disorder model. The impact of non-equilibrium effects on the transit time of charge carriers in thin organic films with a thickness of the order of 100 nm, which is typical for organic light-emitting diodes, is outlined.

Enhancement of organic magnetoresistance by electrical conditioning

We demonstrate that electrical conditioning can be used as an efficient method to enhance the organic magnetoresistance effect in organic light emitting diodes. Depending on duration and intensity of the conditioning process the absolute value of the magnetoresistance effect can be increased from ∼1% to values exceeding 15% at 40mT in devices based on poly(paraphenylene vinylene). Qualitatively, the increase in magnetoresistance can be correlated with a decrease in luminance during the conditioning process. From this we conclude that device degradation mechanisms are responsible for the enhancement of organic magnetoresistance.