Dietmar Knipp

Pentacene thin film transistors on inorganic dielectrics: Morphology, structural properties, and electronic transport

The structural and transport properties of evaporated pentacene organic thin film transistors (TFTs) are reported, and they show the influence of the deposition conditions with different inorganic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. X-ray diffraction and atomic force microscopy show a clear correlation between the morphology and the structure of the highly polycrystalline films for all dielectrics investigated. The roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on smooth thermal oxide are in general highly ordered and independent of the deposition conditions. The ordered films exhibit a “thin film” and a bulk phase, and the bulk phase volume fraction increases with the deposition temperature and the film thickness. Careful control of the deposition conditions gives virtually identical films on thermal oxide and silico...

Morphology and electronic transport of polycrystalline pentacene thin-film transistors

Temperature-dependent measurements of thin-film transistors were performed to gain insight in the electronic transport of polycrystalline pentacene. Devices were fabricated with plasma-enhanced chemical vapor deposited silicon nitride gate dielectrics. The influence of the dielectric roughness and the deposition temperature of the thermally evaporated pentacene films were studied. Although films on rougher gate dielectrics and films prepared at low deposition temperatures exhibit similar grain size, the electronic properties are different. Increasing the dielectric roughness reduces the free carrier mobility, while low substrate temperature leads to more and deeper hole traps.

Visible light communication using OFDM

In this paper wireless communication using white, high brightness LEDs (light emitting diodes) is considered. In particular, the use of OFDM (orthogonal frequency division multiplexing) for intensity modulation is investigated. The high peak-to-average ratio (PAR) in OFDM is usually considered a disadvantage in radio frequency transmission systems due to non-linearities of the power amplifier. It is demonstrated theoretically and by means of an experimental system that the high PAR in OFDM can be exploited constructively in visible light communication to intensity modulate LEDs. It is shown that the theoretical and the experimental results match very closely, and that it is possible to cover a distance of up to one meter using a single LED

Light trapping in thin-film silicon solar cells with submicron surface texture

The influence of nano textured front contacts on the optical wave propagation within microcrystalline thin-film silicon solar cell was investigated. Periodic triangular gratings were integrated in solar cells and the influence of the profile dimensions on the quantum efficiency and the short circuit current was studied. A Finite Difference Time Domain approach was used to rigorously solve the Maxwells equations in two dimensions. By studying the influence of the period and height of the triangular profile, the design of the structures were optimized to achieve higher short circuit currents and quantum efficiencies. Enhancement of the short circuit current in the blue part of the spectrum is achieved for small triangular periods (P<200 nm), whereas the short circuit current in the red and infrared part of the spectrum is increased for triangular periods (P = 900nm) comparable to the optical wavelength. The influence of the surface texture on the solar cell performance will be discussed.

Light trapping in thin-film silicon solar cells with integrated diffraction grating

The optics of microcrystalline silicon thin-film solar cells with integrated light trapping structures was investigated. Periodic grating couplers were integrated in microcrystalline silicon thin-film solar cells and the influence of the grating dimensions on the short circuit current and the quantum efficiency was investigated by the numerical simulation of Maxwell’s equations utilizing the finite difference time domain algorithm. The grating structure leads to scattering and higher order diffraction resulting in an increased absorption of the incident light in the silicon thin-film solar cell. The influence of the grating period and the grating height on the short circuit current and the quantum efficiency was investigated. Enhanced quantum efficiencies are observed for the red and infrared parts of the optical spectrum. Optimal dimensions of the grating coupler were obtained.

Hole transport in polycrystalline pentacene transistors

Measurements of pentacene thin-film transistors are used to explore hole transport mechanisms. The grain-boundary barrier model does not account for the data, since no field dependence of the mobility is observed over a wide range of gate and drain voltages. Instead, trapping provides a more satisfactory qualitative and quantitative interpretation. The subthreshold characteristics are attributed to deep acceptors in the pentacene film, and the conclusions are supported by numerical modeling.

Studying nanostructured nipple arrays of moth eye facets helps to design better thin film solar cells.

Nipples on the surface of moth eye facets exhibit almost perfect broadband anti-reflection properties. We have studied the facet surface micro-protuberances, known as corneal nipples, of the chestnut leafminer moth Cameraria ohridella by atomic force microscopy, and simulated the optics of the nipple arrays by three-dimensional electromagnetic simulation. The influence of the dimensions and shapes of the nipples on the optics was studied. In particular, the shape of the nipples has a major influence on the anti-reflection properties. Furthermore, we transferred the structure of the almost perfect broadband anti-reflection coatings to amorphous silicon thin film solar cells. The coating that imitates the moth-eye array allows for an increase of the short circuit current and conversion efficiency of more than 40%.

On the origin of contact resistances of organic thin film transistors.

A model is presented that describes the gate-voltage-dependent contact resistance and channel-length-dependent charge carrier mobility of small-molecule-based organic thin-film transistors in top and bottom drain/source contact configuration.

Optical enhancement and losses of pyramid textured thin-film silicon solar cells

The optical enhancement and losses of microcrystalline thin-film silicon solar cells with periodic pyramid textures were investigated. Using a finite difference time domain algorithm, the optical wave propagation in the solar cell structure was calculated by rigorously solving the Maxwell’s equations. The influence of the profile dimensions (the period and height of the pyramid) and solar cell thickness on the quantum efficiency and short circuit current were analyzed. Furthermore, the influence of the solar cell thickness on the upper limit of the short circuit current was investigated. The numerically simulated short circuit currents were compared to fundamental light trapping limits based on geometric optics. Finally, optical losses in the solar cell were analyzed. After identifying these key losses, strategies for minimizing the losses can be discussed.

Influence of impurities and structural properties on the device stability of pentacene thin film transistors

The influence of environmental conditions on the electronic transport and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ and ex situ measurements of pentacene thin film transistors were carried out to study the influence of dry oxygen and moisture on the device operation. The staggered thin film transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Exposing the pentacene films to oxygen leads to the creation of acceptorlike states in the band gap. The acceptorlike states cause a shift of the onset of the drain current towards positive gate voltages. A simple analytical model will be presented which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film. Exposing the pentacene film to moisture causes a drop of the charge carrier mobility, a reduction of the threshold voltage, and a shift of the onset voltage. Besides the creation of acceptorlike states in the...