Microscopic charge transport mechanisms and exciton annihilation in organic thin films and single crystals

Um die Natur der Transportdynamik von Ladungsträgern auch auf mikroskopischen Längenskalen nicht-invasiv untersuchen zu können, wurde im ersten Schwerpunkt dieser Arbeit das PL- (Photolumineszenz-) Quenching (engl.: to quench: löschen; hier: strahlungslose Rekombination von Exzitonen) in einer organischen Dünnschicht durch die injizierten und akkumulierten Löcher in einer Transistorgeometrie analysiert. Diese Zusammenführung zweier Methoden - der elektrischen Charakterisierung von Dünnschichttransistoren und der Photolumineszenzspektroskopie - erfasst die Änderung des strahlenden Zerfalls von Exzitonen infolge der Wechselwirkung mit Ladungsträgern. Dadurch werden räumlich aufgelöste Informationen über die Ladungsverteilung und deren Spannungsabhängigkeit im Transistorkanal zugänglich. Durch den Vergleich mit den makroskopischen elektrischen Kenngrößen wie der Schwell- oder der Turn-On-Spannung kann die Funktionsweise der Transistoren damit detaillierter beschrieben werden, als es die Kenngrößen alleine ermöglichen. Außerdem wird die Quantifizierung dieser mikroskopischen Interaktionen möglich, welche beispielsweise als Verlustkanal in organischen Photovoltaikzellen und organicshen Leuchtdioden auftreten können. Die Abgrenzung zu anderen dissipativen Prozessen, wie beispielsweise der Exziton-Exziton Annihilation, Ladungsträgerrekombination, Triplett-Übergänge oder Rekombination an Störstellen oder metallischen Grenzflächen, erlaubt die detaillierte Analyse der Wechselwirkung von optisch angeregten Zuständen mit Elektronen und Löchern. Im zweiten Schwerpunkt dieser Arbeit werden die Transporteigenschaften des Naphthalindiimids Cl2-NDI betrachtet, bei dem der molekulare Überlapp sowie die Reorganisationsenergie in derselben Größenordnung von etwa 0,1 eV liegen. Um experimentell auf den mikroskopischen Transport zu schließen, werden nach der Optimierung des Kristallwachstums Einkristalltransistoren hergestellt, mit Hilfe derer die Beweglichkeit entlang verschiedener kristallographischer Richtungen als Funktion der Temperatur gemessen werden kann. Die einkristalline Natur der Proben und die spezielle Transistorgeometrie ermöglichen die Analyse der räumlichen Anisotropie des Stromflusses. Der gemessene Beweglichkeitstensor wird daraufhin mit simulierten Tensoren auf der Basis von Levich-Jortner Raten verglichen, um auf den zentralen Ladungstransfermechanismus zu schließen.In order to study charge transport in organic thin-film transistors on a microscopic length scale noninvasively, photoluminescence quenching by injected holes in transistor geometry was analyzed. The combination of these two techniques – the electrical characterization of transistors and the photoluminescence spectroscopy – captures the variation of radiative recombination of excitons, which results from the interaction with the accumulated charge carriers. Thereby, spatially resolved information about the charge distribution and its voltage dependence in the transistor channel become accessible. By comparison with the macroscopic electrical parameters, such as the threshold voltage or the turn-on voltage, the mode of operation of the transistors can thus be described in more detail than the characteristic values alone permit. In addition, the quantification of these microscopic interactions becomes possible, which can occur, for example, as a loss channel in organic photovoltaic cells and organic light-emitting diodes. The delimitation to other dissipative processes, such as exciton-exciton annihilation, charge carrier recombination, triplet transitions or recombination at impurities or metallic interfaces, allows the detailed analysis of the interaction of optically excited states with electrons and holes. The second focus of this work is on the transport properties of the naphthalene diimide Cl2-NDI in which the molecular overlap as well as the reorganization energy are of the same order of magnitude of approximately 0.1 eV. In order to close experimentally on the microscopic transport, after the optimization of crystal growth, single crystal transistors are produced by means of which the mobility along different crystallographic directions can be measured as a function of the temperature. The single crystal nature of the samples and the special transistor geometry allow the analysis of the spatial anisotropy of the current flow. The measured mobility tensor is then compared with simulated tensors based on Levich-Jortner rates to infer the central charge transfer mechanism

15th INTERNATIONAL SEMINAR ON POWER SEMICONDUCTORS

Several approaches for close integration of power switches with CMOS logic are subject of technical evaluations and academic discussions. This paper identifies the commercially relevant processing steps of different integration methods (HV and SOI CMOS, monolithic integration in SOI and in GaN, Direct Wafer Bonding, micro-Transfer-Printing of GaN on CMOS and CMOS on GaN) and compares them on simple cost per wafer and cost per chip models. Four examples of real ICs verify the simple costs per chip model. Commercially attractive high voltage to logic partitionings are identified for the different integration approaches

Single-crystal field-effect transistors of new Cl2_{2}-NDI polymorph processed by sublimation in air

Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode ($\beta$-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution ($\alpha$-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm$^{2}$V$^{-1}$s$^{-1}$ ($\alpha$-phase) and up to 3.5 cm$^{2}$V$^{-1}$s$^{-1}$ ($\beta$-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on $\beta$-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm$^{2}$V$^{-1}$s$^{-1}$. The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps

High-mobility organic thin-film transistors based on a small-molecule semiconductor deposited in vacuum and by solution shearing

The small-molecule organic semiconductor 2,9-di-decyl-dinaphtho-[2,3-b: 2',3'-f]-thieno[3,2-b]-thiophene (C-10-DNTT) was used to fabricate bottom-gate, top-contact thin-film transistors (TFTs) in which the semiconductor layer was prepared either by vacuum deposition or by solution shearing. The maximum effective charge-carrier mobility of TFTs with vacuum-deposited C-10-DNTT is 8.5 cm(2)/V s for a nominal semiconductor thickness of 10 nm and a substrate temperature during the semiconductor deposition of 80 degrees C. Scanning electron microscopy analysis reveals the growth of small, isolated islands that begin to coalesce into a flat conducting layer when the nominal thickness exceeds 4 nm. The morphology of the vacuum-deposited semiconductor layers is dominated by tall lamellae that are formed during the deposition, except at very high substrate temperatures. Atomic force microscopy and X-ray diffraction measurements indicate that the C-10-DNTT molecules stand approximately upright with respect to the substrate surface, both in the flat conducting layer near the surface and within the lamellae. Using the transmission line method on TFTs with channel lengths ranging from 10 to 100 mu m, a relatively small contact resistance of 0.33 k Omega cm was determined. TFTs with the C-10-DNTT layer prepared by solution shearing exhibit a pronounced anisotropy of the electrical performance: TFTs with the channel oriented parallel to the shearing direction have an average carrier mobility of (2.8 +/- 0.3) cm(2)/V s, while TFTs with the channel oriented perpendicular to the shearing direction have a somewhat smaller average mobility of (1.3 +/- 0.1) cm(2)/V s. (C) 2013 Elsevier B.V. All rights reserved

Triphenylene Silanes for Direct Surface Anchoring in Binary Mixed Self-Assembled Monolayers

New triphenylene-based silanes 2-(ω-(chlorodimethylsilyl)-<i>n</i>-alkyl)-3,6,7,10,11-penta-<i>m</i>-alkoxytriphenylene <b>4 (T<i>m</i>-C<i>n</i>)</b> with <i>n</i> = 8 or 9 and <i>m</i> = 7, 8, 9, 10, or 11 were synthesized, and their self-assembly behavior in the liquid state and at glass and silicon oxide surfaces was investigated. The mesomorphic properties of triphenylene silanes <b>4 (T<i>m</i>-C<i>n</i>)</b> and their precursors <b>3 (T<i>m</i>-C<i>n</i>)</b> were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. From the small-angle X-ray scattering (SAXS) regime, a preferential discotic lamellar mesophase can be deduced, and wide-angle X-ray scattering (WAXS) highlights the liquid-like characteristics of the alkyl side chains. To transfer these bulk structural properties to thin films, self-assembled monolayers (SAMs) were obtained by adsorption from solution and characterized by water contact angle measurements, null ellipsometry, and atomic force microscopy (AFM). Employing the concentration as an additional degree of freedom, binary SAMs of 2-(ω-(chlorodimethylsilyl)-undecyl)-3,6,7,10,11-penta-decyloxytriphenylene <b>4 (T10-C11)</b> were coassembled with chlorodecyldimethylsilane or chlorodimethyloctadecylsilane, and their capability as model systems for organic templating was evaluated. The structure of the resulting binary mixed SAMs was analyzed by water contact angle measurements, null ellipsometry, and X-ray reflectivity (XRR) in combination with theoretical modeling by a multidimensional Parratt algorithm and AFM. The composition dependence of film thickness and roughness can be explained by a microscopic model including the steric hindrance of the respective molecular constituents