Veit Wagner

Unconventional Face‐On Texture and Exceptional In‐Plane Order of a High Mobility n‐Type Polymer

Substantial in-plane crystallinity and dominant face-on stacking are observed in thin films of a high-mobility n-type rylene-thiophene copolymer. Spun films of the polymer, previously thought to have little or no order are found to exhibit an ordered microstructure at both interfaces, and in the bulk. The implications of this type of packing and crystalline morphology are discussed as they relate to thin-film transistors.

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.

Zinc oxide nanowire arrays for silicon core/shell solar cells.

The optics of core / shell nanowire solar cells was investigated. The optical wave propagation was studied by finite difference time domain simulations using realistic interface morphologies. The interface morphologies were determined by a 3D surface coverage algorithm, which provides a realistic film formation of amorphous silicon films on zinc oxide nanowire arrays. The influence of the nanowire dimensions on the interface morphology and light trapping was investigated and optimal dimensions of the zinc oxide nanowire were derived.

Stability and spacial trap state distribution of solution processed ZnO-thin film transistors

Solution processed zinc oxide thin film transistors (TFTs) were investigated for spacial identification of instability inducing electronic trap states by utilizing surface-to-active-channel distance dependent analysis. It is shown that the performance and stability of zinc oxide TFTs deposited by spray pyrolysis strongly depend on the surface-to-channel distance and herewith on the film thickness in the investigated regime from 1 nm to 30 nm. In thin layers, the charge transport process is dominated by the number of percolation paths and near channel trapping processes due to coulomb interactions with surface charges. This leads to a high thickness of 3 nm for the percolation threshold. As soon as a closed layer is formed and the charge separation of 7 nm between surface and active channel is exceeded, bulk properties become more dominant. A maximum linear mobility of 11cm2 V−1 s−1 and an on-set voltage of 2 V were obtained for a film thickness of 30 nm. An increase of the film thickness from 10 nm to 30 ...

Tuning the contact resistance in nanoscale oligothiophene field effect transistors

Nanoscale organic transistors for high frequency applications are often limited by contact resistances. We report on tuning of those resistances by shifting the transport level for dihexyl-n-thiophene (DHnT) semiconductors by variation of the number of thiophenes n from 4 to 7. The intrinsic mobility as well as contact resistance were determined from individual transfer curves of bottom-contact transistors with channel lengths down to 50nm. Best values were found for DH7T with μ=0.12cm2∕Vs and Rc=1kΩcm. While the contact resistance remains fairly constant for a given n as expected, the intrinsic mobility still decreases with decreasing channel length.Nanoscale organic transistors for high frequency applications are often limited by contact resistances. We report on tuning of those resistances by shifting the transport level for dihexyl-n-thiophene (DHnT) semiconductors by variation of the number of thiophenes n from 4 to 7. The intrinsic mobility as well as contact resistance were determined from individual transfer curves of bottom-contact transistors with channel lengths down to 50nm. Best values were found for DH7T with μ=0.12cm2∕Vs and Rc=1kΩcm. While the contact resistance remains fairly constant for a given n as expected, the intrinsic mobility still decreases with decreasing channel length.

Chemical composition and temperature dependent performance of ZnO-thin film transistors deposited by pulsed and continuous spray pyrolysis

Zinc oxide thin film transistors (TFTs) deposited by continuous and pulsed spray pyrolysis were investigated to analyze process kinetics which make reduction of process temperature possible. Thus, fluid mechanics, chemical composition, electrical performance, and deposition and annealing temperature were systematically analyzed. It was found that ZnO layers continuously deposited at 360 °C contained zinc oxynitrides, CO3, and hydro carbonate groups from pyrolysis of basic zinc acetate. Statistically, every second wurtzite ZnO unit cell contained an impurity atom. The purity and performance of the ZnO-TFTs increased systematically with increasing deposition temperature due to an improved oxidation processes. At 500 °C the zinc to oxygen ratio exceeded a high value of 0.96. Additionally, the ZnO film was not found to be in a stabilized state after deposition even at high temperatures. Introducing additional subsequent annealing steps stabilizes the film and allows the reduction of the overall thermal stress...

Long-term stabilization of sprayed zinc oxide thin film transistors by hexafluoropropylene oxide self assembled monolayers

Surface functionalization of solution processed zinc oxide layers was studied in transistors with bottom-gate bottom-contact configuration aiming at suppression of trapping processes to increase device stability. Saturation of electrically active surface sites and formation of a moisture barrier to decrease the impact of humid atmosphere was successfully shown by binding hexafluoropropylene oxide (HFPO) on the metal oxide semiconductor. Deep trap level related electrical parameters, i.e., stability, hysteresis, and on-set voltage, improved rapidly within 60 s of exposure which was attributed to occupation of sites characterized by low adsorption energies, e.g., at edges. In contrast, shallow trap level related parameters, i.e., mobility, showed a much slower process of improvement. Identical behavior was determined for the contact angle. A physical model is presented by applying first order reaction kinetics equation to Youngs law and multiple trapping and release model which relates the dependence of th...

Fabrication and Charge Transport Modeling of Thin-Film Transistor Based on Carbon Nanotubes Network

In this paper, the fabrication and modeling of thin-film transistors (TFTs) based on random network of single-walled carbon nanotubes (SWCNTs) are presented. The thin film is obtained by dispersing 99% semiconductive SWCNTs with an effective deposition technique at room temperature that combines vacuum filtration and silanization of substrate. The case of TFT structures with channel length varying from 2 to 50 μm is experimentally studied. In the device with channel length equal to 8 μm, an apparent mobility of 40.75 cm2/V·s, a current density of 0.06 μm, and ION/IOFF ratio of 1.8 × 104 have been measured. In order to obtain a numerical model as close as possible to the real structure, a 3-D model for the thin-film layer is developed, rather than the 2-D type commonly used in the literature, to reproduce the electric transport properties of the CNTs network in the channel of TFTs devices. The spatial arrangement of the random network CNTs in the channel of the thin-film structure is also analyzed by considering the percolation theory. According to this theory, an exponent of the power law equal to α = 1.7 is experimentally detected, indicating that the devices operate close to the percolation region. A good agreement is found between the transport characteristics of the simulated and fabricated devices. The adopted model opens new routes to understand the transport properties of the film. The proposed fabrication approach can be easily transformed to large areas leading to a suitable use in industrial application.

Feasible industrial fabrication of thin film transistor based on randomized network of single walled carbon nanotubes

In this paper, the fabrication of thin film transistor based on randomized network of single walled carbon nano tubes (SWCNTs-TFT) is presented. The randomized network is obtained by deposition of dispersed SWCNTs on the substrate with a novel technique combining vacuum filtration and silanization of substrate. This approach, which is compatible with all kind of substrates, allows a fabrication process at room temperature that is capable to overcome the high temperature procedure for CNTs deposition. The drain and source electrodes of the TFT are based on an interdigitated electrode (IDE) with 8 μm channel length and 3mm channel width. The obtained device shows output performance with an apparent mobility of 40.7 cm2/Vs, current density 0.05 μA/μm and ION /IOFF ratio 2×103. A comparison of the model describing the SWCNTs-TFT with that of (metal-oxide-semiconductor) MOS-like device confirms a p-type behavior. The proposed approach can be easily transformed to large areas leading to a suitable use in low cost industrial application.

Controlled growth of ZnO nanorods via self-assembled monolayer

AbstractA simple method to achieve tailored growth of ZnO nanorods by employing self-assembled monolayer of alkanethiol molecules on a conductive substrate is introduced. Defects or pinholes in self-assembled monolayer are used to define the nucleation sites of ZnO during electrochemical deposition. The density of ZnO nanorods is tuned by the quality of self-assembled monolayer. A corresponding growth model for the growth of ZnO on self-assembled monolayer-modified substrate is proposed. The dimensions and the nucleation density of ZnO nanorods are tailored by systematically varying the quality of self-assembled monolayer and the parameters of electrochemical deposition. Furthermore, it is shown that this method also allows for laterally patterned growth of ZnO nanorods via microcontact printing of self-assembled monolayer.Graphical AbstractElectrochemical deposition of ZnO on Au surface results in dense coverage of ZnO nanorods. Self-assembled monolayers are applied on Au to tune the density of ZnO nanorods. The pinholes in self-assembled monolayer are used to define the nucleation site of ZnO.