In the framework of the one-dimensional mean-field (MF) drift-diffusion approach the well-defined boundary conditions far away from the metal/insulator contacts of a planar metal/insulator/metal system are used to determine the boundary condition at the interface itself. The novel self-consistent boundary condition linking the carrier density and the electric field at the interface enables a straightforward description of the current voltage (IV) characteristics in forward and reverse bias bridging space charge and injection-limited regimes and accounting for barrier lowering from the potential drop in the used contact materials. Yet, because of the low carrier density in the insulator under injection limitation, single-particle phenomena, such as the Schottky effect, must be considered. We reconsider the validity of the MF approach, depending on the external bias and the prevailing injection barriers. For the crucial parameter window where the MF approach fails and single-particle phenomena become important, a modification of the boundary conditions at the insulator/metal interface is proposed to account for the discrete nature of carriers. The difference between the thus modified MF and the unmodified MF approach is illustrated by several examples.
In this contribution we emphasise the ambipolar organic field-effect transistors (OFETs) as the prime element for the
realization of various OFET types. It will be shown that ambipolar OFETs can be used to produce on the one hand
complementary unipolar OFETs and thus CMOS elements and on the other hand light-emitting OFETs. Some ambipolar
light-emitting OFETs will be presented and the impact of the contact formation at the source and drain electrodes on the
device characteristics will be discussed. In general, the investigation of ambipolar OFETs provides a deep understanding
of the OFET operation and guides the way to novel aspects of the OFET applicability.
Magnetic field effects in organic light emitting diodes have emerged as subject of intense research activities. We
investigated the recently discovered organic magnetoresistance effect, i. e. the phenomenon that the presence of
an external magnetic field can influence both the current flow through an organic light emitting diode and the
light emission from the device. Magnetoresistance measurements were performed in different device structures
as a function of magnetic field and driving voltage. We demonstrate that electrical conditioning can be used
as an efficient method to enhance the organic magnetoresistance effect in devices based on polymers and small
molecules. Depending on duration and intensity of the conditioning process the magnetoresistance effect can
be increased from ~1% to values exceeding 15% at 40mT in devices with poly(paraphenylene-vinylene) as
light emitting polymer. Qualitatively the increase in magnetoresistance can be correlated with a decrease in
luminance during the conditioning process. From this we conclude that degradation of the bulk emitter material
is responsible for the enhancement of organic magnetoresistance. In addition, we show a dependence of the
magnetoresistance effect on the charge carrier balance within the device. In bipolar devices the magnetoresistance
effect is significantly larger than in hole-dominated devices which suggests that electron-hole pairs play an
important role in the fundamental mechanism causing the organic magnetoresistance effect.
The charge carrier transport in organic field effect transistors (OFETs) is determined by the
transport properties of the insulator / organic semiconductor interface. We demonstrate that an
adequate treatment of this interface results in a polarity change of the OFET charge carrier
transport properties, without further altering the device structure. Illuminating the utilized
PMMA polymer dielectric, by using UV radiation, leads to the introduction of mainly
electron traps at the dielectric interface. This results in the suppression of the electron
transport for an otherwise n-type pentacene OFET. However, as a consequence of trapped
electrons in the near surface layer of the PMMA dielectric, the hole transport of the device is
enabled though a hole blocking source/drain metallization. This effect, as well as the impact
of the UV irradiation on the PMMA dielectric will be discussed in detail. The UV treatment
yields a PMMA interface rich on polar groups. The influence of these groups on the OFET
characteristics is investigated by studying several polymer dielectrics with varying content of
the emerging groups.
In the present paper a new concept towards O-CMOS technology is presented substantiating the importance of the semiconductor/dielectric interface for charge carrier transport in organic semiconductors. It will be demonstrated that by controlling the interface properties of either SiO2 or PMMA, unipolar p- and n-type OFETs can be realized using a single organic semiconductor and even a single metal for source and drain contacts. Two dielectric/semiconductor interface modifications are considered for the realization of complementary OFETs on the basis of pentacene, otherwise known for its exclusive hole transporting properties. Selective modification of the SiO2 dielectric interface with traces of vacuum deposited Ca, allows for electron transport in pentacene and the realization of complementary pentacene OFETs on a single substrate. By this technique electron traps are removed due to a reaction of atomic Ca with oxygen from available hydroxide groups, resulting in the formation of an oxidized Ca layer. In a second approach, it is demonstrated that by selective UV treatment of a PMMA dielectric surface, unipolar n-type pentacene OFETs can be converted to unipolar p-type by the introduction of electron traps in the form of -OH and -COOH groups at the PMMA interface. Both methods allow for the realization of CMOS organic inverter stages with decent electrical properties.
During the deposition of Pentacene on a Si-SiO2 gate structure with Au bottom contacts for source and drain, the film growth was monitored with simultaneous in situ macro Raman spectroscopy and drain current measurements of the OFET device. The deposition of the active layer was carried out under UHV conditions at a growth rate of 0.65 Å/min. The purpose of the in situ characterization was to determine the minimum nominal thickness of the Pentacene layer required for efficient charge transport through the OFET circuit. At a thickness around 1.5 nm nominal coverage, the first percolation paths through the first organic monolayer develop, resulting in a sharp rise of the drain current. Up to a nominal film thickness of 30 nm, a subsequent slower increase of the drain current can be observed, revealing that the percolation of the first monolayer continues on a slower pace up to rather thick organic layers. These in situ measurements were complemented by ex situ isothermal deep level transient spectroscopy (charge QTS).
In this study we report on new concepts to generate light emission in organic thin film transistors. The initial physical understanding of light emission from tetracene based field-effect transistors was proposed to be originated from a strong underetching of the drain and source electrodes. This underetched electrodes in combination with the evaporated tetracene is thereby believed to generate a virtual OLED at the drain electrode. Accumulated holes have to leave the gate
oxide interface to reach the drain electrode by crossing the bulk of the organic semiconductor. Light then occurs by injection of electrons in a large electric field in the bulk. Today's transistors do not show the underetching anymore but are still emitting light only at the drain electrode, again supporting the initial interpretation of a defect state at the edge of the drain electrode. In this context the question how electrons can overcome a potential barrier of 2.7 eV is still open. Therefore an investigation of the gold tetracene interface by UPS and XPS techniques has been started and preliminary data indicate the unexpected result that the barrier for electrons is comparable to that for holes. In a further step the generation of an ambipolar transistor by interface doping with calcium was tried and an n-type pentacene transistor could be fabricated but the strategy failed for tetracene. Finally an electrochemical interface doping was performed by the application of Lithium triflate in PEO to a thin interface layer between gate oxide and tetracene. This leads to light emission but unfortunately also to the loss of the gate voltage influence. Based on these results a possible strategy will be presented.
We report on light emission on organic thin film transistors of tetracene and polyfluorene (Poly(9,9-di(ethylhexyl)fluorene) (PF2/6)). The utilized transistor structure is a bottom gate configuration with interdigitated source and drain electrodes on a Si/SiO2 substrate with a channel length of 5 μm. Light emission occurs above a source drain voltage of 30V even if the gate voltage is higher than the drain voltage. The light output can be controlled by the gate voltage. The light emission occurs close to the drain electrode as observed by light microscope images of operating transistors. In order to understand the functional principle of a light emitting transistor a resistor capacitor equivalent circuit model has been utilized to describe charge carrier transport, carrier distribution and the electrical potential distribution in such a device. The model extends the common thin film transistor theory for unipolar charge transport to ambipolar charge transport. Analytical expressions for output and transfer characteristics as well as for the potential and charge carrier distributions are obtained. Further, the effect of contact resistors on the output and transfer characteristics are simulated. The model is used to explain the underlying mechanisms of the present devices. Imperfections on the contact electrodes, most probably due to under-etching of the electrodes are seen as the main reason for the electron injection.
The effect of trap states on the transport and luminescence properties of organic light emitting diodes (OLEDs) is studied. For trap level detection energy resolved thermally stimulated current (TSC) measurements known as fractional glow are utilized to determine the density of occupied states (DOOS) in various organic semiconductors such as the small molecule systems Alq3 [aluminum tris(8-hydroxyquinoline)], 1-NaphDATA {4,4',4"-tris-[N-(1-naphtyl)-N-phenylamino]-triphenylamine} and α-NPD [N,N'-di-(1-naphthyl)-N,N'-diphenylbenzidine] and the polymeric semiconductor MDMO-PPV {poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene]}. Characteristic differences in the trap spectra are obtained and interpreted in terms of possible structural and compositional origins of the investigated materials. In order to judge the formation process of traps and their practical consequences on the charge carrier transport I-V and L-V characteristics of 1-NaphDATA doped α-NPD devices and α-NPD doped 1-NaphDATA devices were compared to respective non-doped samples. A clearly reduced current and luminescence was found only in the former case. It was possible to conclude that the detected electronic trap states either act as hole traps or as scattering centers. Furthermore, pulsed transport studies on ITO/α-NPD/Alq3/Al devices show thte critical influence of traps on the dynamical performance of the charge transport. In a two-pulse experiment the carrier injection and trap depletion can be separated.
The methods of thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) were employed to reveal the trap structure of the most prominent organic semiconductors materials such as tris-8-(hydroxyquinoline) (Alq3), N-N'-di(1-naphtyl)-N-N'-diphenylbenzidine ((alpha) -NPD), and 4,4',4'-tris-(N-2-naphtyl)-N-phenylamino- triphenylamine (1-Naph-DATA). The energetic trap depths and a lower limit of the trap densities were derived for all investigated materials by means of the initial-rise method and curve fitting techniques. Typical activation energies range between 0.1 and 0.6 eV and trap concentrations differ between 1014 and 1017 cm-3. Most materials exhibit trap levels with a single activation energy, however, in Alq3 a brought distribution of trap depths will be reported. In addition, the polarity of the dominant trap levels was determined by a comparison of TSC spectra from optically and electrically filled traps. Besides the trap detection and characterization the effect of doping and accelerated aging on the trap structure will be shown. TSC and TSL results on rubrene doped Alq3 reveals a characteristic shift in the trap depth indicating new rubrene related trapping site. The effect of aging on the trap structure of organic semiconductors in 'potentially harmful' atmospheres such as oxygen and humidity and their correlation to I-V characteristics will also be reported.
In image plate technology the understanding of the radiation-induced generation of photostimulable luminescence centers (PSLCs) is still an unsolved problem in commercially utilized x-ray storage phosphors, such as BaFBr: Eu2+ and RbBr:Tl+. Two different mechanisms have been proposed: the first one assumes a vacancy-free crystal prior to irradiation known as Itoh process originally proposed for alkali halides whereas the second one relies on a vacancy- containing crystal where the vacancies are assumed to be F+-centers. In the present study we area addressing this problem through the determination of the relative concentrations of spatially-correlated and non-correlated centers for samples with different activator concentrations. The PSLC-concentration ratio meaning the concentration of the spatially correlated centers to the total center concentration is measured by means of photostimulated luminescence in the temperature range from LHe to room temperature. It is found that the Eu2+-concentration is playing a decisive role in the relative occurrence of the two PSLC-types in the sense that for higher Eu2+- contents the ratio of correlated to non-correlated centers is increasing. It will be shown that this result clearly favors the Itoh formation mechanism for x-irradiation induced PSLC generation.
We report on progress in different domains of n-detection: (1) High gain image intensifiers coupled to n-scintillators enable small size n-monitors with high spatial resolution. (2) Standard CCD cameras combined with special frame grabbers provide low cost n-image processing. (3) Progress in diode technology allows direct light detection from n-scintillators with the aim of low effort and very thin n-detectors. (4) New foils with X-ray photography combined with n-converters can be used for efficient large area position-sensitive detectors. The stored information is read out by laser-stimulated emission of light.
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