Alfred Neuhold

Efficient Blue-Light-Emitting Polymer Heterostructure Devices: The Fabrication of Multilayer Structures from Orthogonal Solvents

[*] Prof. E. J. W. List, Dr. S. Sax, A. Neuhold NanoTecCenter Weiz Forschungsgesellschaft mbH Franz-Pichler-Straße 32, A-8160 Weiz (Austria) E-mail: list@ntc-weiz.at Prof. K. Müllen, N. Rugen-Penkalla Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz (Germany) E-mail: muellen@mpip-mainz.mpg.de Prof. S. Schuh, Prof. E. Zojer, E. J. W. List Institute of Solid State Physics Graz University of Technology Petersgasse 16, A-8010 Graz (Austria)

Solution-Processable Septithiophene Monolayer Transistor

Septithiophene with endgroups designed to form liquid crystalline phases and allows controlled deposition of an electrically connected monolayer. Field effect mobilies mobilities of charge carriers and spectroscopic properties of the monolayer provide evidence of sustainable transport and delocalization of the excitation through intermolecular interactions within the layer.

Charge trapping by self-assembled monolayers as the origin of the threshold voltage shift in organic field-effect transistors

The threshold voltage is an important property of organic field-effect transistors. By applying a self-assembled monolayer (SAM) on the gate dielectric, the value can be tuned. After electrical characterization, the semiconductor is delaminated. The surface potentials of the revealed SAM perfectly agree with the threshold voltages, which demonstrate that the shift is not due to the dipolar contribution, but due to charge trapping by the SAM.

Color tuning of nanofibers by periodic organic-organic hetero-epitaxy.

We report on the epitaxial growth of periodic para-hexaphenyl (p-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of p-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the p-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and p-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of p-6P spacer thickness (Δdp–6P) we have determined a critical value of Δdp–6P ≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and p-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from p-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/p-6P multilayer structures depends on the p-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue via white to the yellow-green spectral range.

Diffusion of Ag into Organic Semiconducting Materials: A Combined Analytical Study Using Transmission Electron Microscopy and X-ray Reflectivity

This study shows that the morphology of organic/metal interfaces strongly depends on process parameters and the involved materials. The interface between organic n-type blocking layer materials and the top Ag cathode within an organic photodiode was investigated. Ag was deposited on either amorphous tris-8-hydroxyquinolinato-aluminum (Alq(3)) or crystalline 4,7-diphenyl-1,10-phenanthroline (Bphen) using different deposition techniques such as electron beam deposition, ion beam sputtering, and vacuum thermal evaporation at various deposition rates. The interfaces were studied by transmission electron microscopy and X-ray reflectivity. It was found that Bphen does not show any Ag diffusion no matter which deposition technique was used, whereas the Ag diffusion into Alq(3) depends on the deposition technique and the deposition rate. The highest amount of Ag diffusion into Alq(3) occurred by using thermal vacuum deposition at low deposition rates.

Grazing‐incidence in‐plane X‐ray diffraction on ultra‐thin organic films using standard laboratory equipment

A novel grazing-incidence in-plane X-ray diffraction setup based on a commercial four-circle diffractometer with a sealed-ceramic copper X-ray tube, upgraded with parabolic graded multilayer X-ray optics and a one-dimensional position-sensitive detector, is presented. The high potential of this setup is demonstrated by a phase analysis study of pentacene thin films and the determination of in-plane lattice constants of pentacene mono- and multilayers. The quality of the results compare well to studies performed at synchrotron radiation facilities.

Structure and morphology of an organic/inorganic multilayer stack: An x-ray reflectivity study

The internal morphology and crystallographic properties of a complete organic thin film multilayer stack are characterized via x-ray scattering techniques, atomic force microscopy (AFM), and scanning electron microscopy. The stack consists of the three organic layers – copper(II)phthalocyanine (CuPc), perylene tetracarboxylic bisbenzimidazole (PTCBI), and aluminum-tris(8-hydroxychinolin) (Alq3) – sandwiched between an optically semitransparent gold layer and a top silver electrode. The interface roughness progress is determined by the x-ray reflectivity, which is confirmed by the surface roughness determination via AFM. The crystallographic properties are characterized via x-ray diffraction. The CuPc layer is highly crystalline with preferentially oriented crystallites but forms a rough interface (σRMSu2009=u20095.5u2009nm) toward the PTCBI layer. The PTCBI layer grows with randomly distributed crystallites in a worm-like morphology with an interface roughness of σRMSu2009=u20096.4u2009nm toward the Alq3 layer. The amorphous Alq...

N-type self-assembled monolayer field-effect transistors

Within this work we present the synthesis and applications of a novel material designed for n-type self-assembled monolayer field-effect transistors (SAMFETs). Our novel perylene bisimide based molecule was obtained in six steps and is functionalized with a phosphonic acid linker which enables a covalent fixation on aluminum oxide dielectrics. The organic field-effect transistors (OFETs) were fabricated by submerging predefined transistor substrates in a dilute solution of the molecule under ambient conditions. Investigations showed a thickness of about 3 nm for the organic layer which is coincides to the molecular length. The transistors showed bulk-like electron mobilities up to 10-3 cm2/Vs. Due to the absence of bulk current high on/off-ratios were achieved. An increase of the electron mobility with the channel length and XPS investigations point to a complete coverage of the dielectric with a dense monolayer. In addition, a p-type SAMFET based on a thiophene derivative and our new n-type SAMFET were combined to the first CMOS bias inverter based solely on SAMFETs.