Roland Schmechel

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.

A pentacene ambipolar transistor: Experiment and theory

An ambipolar pentacene transistor with top-gold and top-calcium contacts has been realized by utilizing a parallactic shadow mask effect during vapor deposition. The pentacene deposited on top of a silicon dioxide gate insulator is doped by Ca at the pentacene/SiO2 interface in order to compensate electron traps. An equivalent circuit model based on a resistor-capacitor network has been developed to describe the basic electrical properties of the transistor. Shockley-like analytical expressions for the output and transfer characteristic, as well as an analytical expression for the potential and charge-carrier distribution in the channel, are derived under the assumption of a high electron-hole recombination probability. The model has been fitted to our experimental results and yields comparable mobilities for both holes and electrons in the order of 0.1cm2∕Vs. The increasing threshold voltages, with an increase in gate voltage, are discussed as an indication for trapped charge carriers within the insulato...

Light emission from a polymer transistor

We report on light emission from a polymeric transistor that utilizes interdigitated source and drain electrodes with channel length of 5 μm in a bottom gate configuration based on a Si/SiO2 substrate. The polymer investigated is poly[9,9-di(ethylhexyl)fluorene] deposited by spin coating from chloroform solution to achieve an active layer thickness of 40 nm. Light emission occurs above drain source voltages of −60 V and the light intensity can be controlled by the gate voltage. Emission occurs close to the drain electrode as determined by optical microscopy. The transistor operates in hole accumulation mode without saturation of the output characteristics.

n-type organic field-effect transistor based on interface-doped pentacene

The realization of an n-type pentacene field-effect transistor based on interface-doped pentacene is demonstrated, laying a headstone for an organic complementary-metal–oxide–semiconductor technology. The doping is performed by depositing traces of calcium onto the gate insulator before applying the organic semiconductor. Electron field-effect mobilities of 0.19cm2V−1s−1 are achieved. The field effect, i.e., the electron accumulation behavior, is studied by impedance spectroscopy and charge measurements on a metal–insulator–semiconductor (MIS) diode. A good correlation between the physical properties of the transistor and the MIS diode can be reported. A temporal dynamics and a hysteresislike accumulation behavior are observed, both explainable by the influence of deep electron traps.

Correlation between structural defects and electronic properties of icosahedral boron-rich solids

From fine-structure investigations it is well known that in many icosahedral boron-rich solids the occupation densities of specific atomic sites are considerably reduced. Investigations of the electronic properties have proved that the electronic properties of these semiconductors are strongly influenced by high densities of intrinsic states in the band gaps. For -rhombohedral boron and boron carbide, the best investigated icosahedral boron-rich solids, it is shown that the concentrations of structural defects and electronic gap states are quantitatively correlated, and that this way the electron deficiencies theoretically calculated for the valence bands of corresponding idealized structures are compensated. Obviously, the structural defects in these crystals are the necessary consequence of the valence electron deficiency. It is suggested that this correlation holds for the icosahedral boron-rich solids in general.

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.

Hopping transport in doped organic semiconductors: A theoretical approach and its application to p-doped zinc-phthalocyanine

A detailed approach to the complex hopping transport in organic semiconductors is presented and used to describe experimental data from Maennig et al. [Phys. Rev. B 64, 195208 (2001)] on the effect of doping on conductivity, mobility and thermopower. In this approach, the energetic distribution of the charge carriers in a Gaussian shaped density of states (DOS) is calculated under thermal equilibrium conditions and compared to the energetic distribution of the current. The description is based on the Miller–Abraham model for hopping in a disordered material and utilizes the so-called transport energy concept. To include also the case of higher electron concentrations in the tail states of the DOS the Fermi distribution was taken into account. Furthermore, additional trap states in the gap are considered to describe the experimental data at low doping concentration more correctly. In the framework of the model there is no indication of a thermally activated ionization of the dopants. In contrast to other descriptions, the position of the Fermi energy and transport energy are calculated from the model. It is demonstrated that the principal behavior of the transport parameter can be well explained in terms of classical semiconductor physics.

On the electronic properties of β-rhombohedral boron interstitially doped with 3d transition metal atoms

Abstract The controlled modification of the p-type character of pure boron-rich solids to n-type by suitable doping is an essential prerequisite for technical applications. Interstitial doping of β-rhombohedral boron with V, Cr, Fe and Ni fulfils this requirement, while Cu and Co do not. Comparative investigations of the Seebeck coefficient, the DC electrical conductivity, the optical absorption and the dynamical conductivity of BV, BCo and BFe lead to the conclusion that donor levels positioned between the conduction band and the uppermost intrinsic electron trapping level and of sufficient density to overcompensate unoccupied valence and gap states are necessary for n-type conductivity. In contrast to Fe, the interstitial accommodation of V atoms seems to reduce the concentration of possible gap states.