Polymerselektierte Kohlenstoffnanoröhren für lichtemittierende Feldeffekttransistoren

Carbon nanotubes hold remarkable optoelectronic properties. Beside their high conductivity and field-effect mobility, which predetermines their use in future electronic applications, they feature a direct bandgap that enables luminescence in the near-infrared. Yet, carbon nanotubes still suffer from heterogeneous source material that commonly consists of various nanotube species with different metallicity, diameter and emission wavelength. This dissertation investigates the selective sorting of carbon nanotubes by polyfluorene-polymers as well as the application of these polymer/nanotube dispersions in light-emitting field-effect transistors. Some polyfluorene-polymers show a striking selectivity for the dispersion of certain nanotube species that is not yet completely understood. In this work, the influence of the solvent type and the polymer molecular weight on the selective dispersion behavior is analyzed, to produce nanotube/polymer dispersions with tailored composition and yield. It was found that meta- or non-stable nanotube species are stabilized by a low diffusion constant. Therefore a low dispersion viscosity leads to high selectiveness, however at at the expense of overall yield. Carbon nanotube dispersions can be used with high excess of polymer as semiconducting polymer layers with improved injection behavior. The application of these films in lightemitting field-effect transistors reveals an improved charge injection from the metal source and drain contacts. As shown in electrostatic simulations, the one-dimensionally confined structure of carbon nanotubes leads to an enhancement of the applied electric field at the tube-tips. The nanotube doping reduces threshold voltages for both electron and holes and increases ambipolar currents and light emission. Hence, it represents an easy-to-achieve performance improvement for polymer transistors where charge injection might be an issue. A further purification of the dispersion and removal of the excess polymer leads to solution processable nanotube inks with near-monochiral distribution. Their performance in nanotube-network field-effect transistors with polymer dielectrics and electrolyte-gated transistors has been evaluated. While monochiral and narrowband excitonic light emission was obtained from transistors with polymer dielectric, the high charge carrier densities in ion-gel gated devices revealed electrically stimulated trion emission and low voltage operation with the nanotube network films. The dependence of exciton and trion emission on charge carrier density has been shown, effectively creating a photoluminescence emitter with voltage-controllable emission wavelength.. Defined emission in the near-infrared and good electrical performance promotes the use of polymer-selected carbon nanotubes as a bridge between electric and optical signal transmission, especially for upcoming generations of printable and flexible electronics.Kohlenstoffnanoröhren besitzen bemerkenswerte optoelektronische Eigenschaften. Neben hoher Leitfähigkeit und Feldeffekt-Mobilität ermöglicht eine direkte Bandlücke Lumineszenz im nahen infraroten Spektrum. Das heterogene Ausgangsmaterial der Kohlenstoffnanoröhrenherstellung erweist sich allerdings als Nachteil für optoelektronische Anwendungen, da es in der Regel verschiedene Spezies mit leitenden oder halbleitenden Eigenschaften sowie unterschiedlichen Durchmessern und Emissionwellenlängen enthält. Diese Dissertation behandelt das selektive Dispergieren von Nanoröhren mit Polyfluoren-Polymeren und die Anwendung dieser Dispersionen in lichtemittierenden Feldeffekttransistoren. Bestimmte Polyfluorene zeigen eine noch nicht eindeutig verstandene Selektivität für spezifische Nanoröhren-Arten. Diese Arbeit analysiert hierbei den Einfluss des Lösungsmittels und des Polymer-Molekulargewichts auf das selektive Dispergierverhalten, mit dem Ziel, maßgeschneiderte Nanoröhren-Zusammensetzungen herzustellen. Die Ergebnisse zeigen eine Stabilisierung von halb- und instabilen Röhren durch niedrige Diffusionskonstanten. Niedrigviskose Dispersionen zeigen deshalb eine höhere Selektivität, allerdings verbunden mit einer niedrigeren Gesamtausbeute. Erzeugte Nanoröhren-Dispersionen können mit hohem Polymerüberschuss als halbleitende Polymerfilme benutzt werden. Die Anwendung dieser Schichten zeigt ein verbessertes Injektionsverhalten der Ladungsträger. Elektrostatische Simulationen deuten auf eine Erhöhung der angelegten elektrischen Felder um die Röhrenspitzen, verursacht durch die eindimensionale Struktur der Kohlenstoffnanoröhren, hin. Dies erleichtert das Tunneln von Ladungsträgern durch die Schottkybarrieren an Source- und Drain-Kontakten, was zu einer Verringerung der Schwellspannung und erhöhten ambipolaren Strömen führt. Mit einem weiteren Zentrifugierschritt kann die selektive Dispersion von überschüssigem Polymer gereinigt werden. Dies erzeugt lösungsprozessierbare Nanoröhren-Formulierungen mit fast monochiralen Zusammensetzungen. Diese wurden in Nanoröhrennetzwerken als Halbleiter für Feldeffekttransistoren getestet. Während mit Polymerdielektrikas eine exzitonische und schmalbandige Emission erreicht werden kann, zeigen Elektrolytkontaktierte Transitoren weitere Emmisionsbänder bei niedrigerer Energie, die der Emission von Trionen zugeordent werden kann. Es wird die Abhängigkeit bei Exzitonen- und Trionenbildung von der Ladungsträgerdichte aufgezeigt, was effektiv zu einem spannungsgesteuerten Photolumineszenzemitter mit wechselbarer Wellenlänge führt. Die schmalbandige Infrarotlumineszenz und die guten elektrischen Eigenschaften ermöglichen den Einsatz der Polymer-selektierten Nanoröhren als Brücke zwischen elektrischer und optischer Signalübermittlung, insbesondere in neuen Anwendungsfeldern wie druckbarer und flexibler Elektronik

In Situ Raman Mapping of Charge Carrier Distribution in Electrolyte-Gated Carbon Nanotube Network Field-Effect Transistors

Solution-processed networks of purely semiconducting single-walled carbon nanotubes (s-SWNTs) can be used to create high-mobility field-effect transistors (FETs) for flexible electronics. In order to optimize network alignment and density, an understanding of carrier distribution within the FET channel is necessary. Here, we used confocal Raman microscopy to investigate charge accumulation and doping in electrolyte-gated FETs with asymmetric layers of only (7,5) nanotubes, that were selected by dispersion in poly­(9,9-dioctylfluorene). The nanotube FETs exhibited hole mobilities of up to 7.5 cm² V^–1 s^–1 and on/off ratios of 10⁵. All Raman modes decreased in intensity with hole accumulation. The G′-peak and D-peak shifted linearly with negative gate voltages to higher wavenumbers. Using the G′-peak shift, the charge carrier distribution in an operating FET was mapped at different gate and source-drain voltages with high spatial resolution (∼300 nm) and over large areas. With this simple technique, we were able to visualize directly how the assignment of source and drain electrodes determined channel pinch-off and the onset of the saturation regime in FETs with nonuniform carbon nanotube distributions along the channel. In situ Raman mapping could also be applied to other semiconductors that show significant changes in their Raman spectra with doping

Determining the coating speed limitations for organic photovoltaic inks

To determine the output capability of present organic photovoltaic (OPV) materials, it is important to know the theoretical maximum coating speeds of the used semiconductor formulations. Here, we present a comprehensive investigation of the coating stability window of several prototype organic semiconductor inks relevant for organic solar cells. The coating stability window was first determined experimentally by a sheet to sheet coater at velocities of up to 10 m/min. A numerical simulation model based on the Coating Window Suite 2010 software was established to give insight into the coating stability limitations at higher coating velocities. An analysis of PEDOT:PSS [poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate)] in a water/isopropyl alcohol mixture as well as P3HT:PCBM [poly(3-hexylthiophene-2,5-diyl):Phenyl-C61-butyric acid methyl ester] in chlorobenzene, o-xylene and tetrahydronaphthalene showed the possibility of coating speeds up to 60 m/min. The simulation further revealed the maximum coating head distances for a given wet film thickness. Finally, we show a solar-cell with slot-die coated PEDOT:PSS and P3HT:PCBM-layer based on the parameters obtained by the simulated data, which exhibits reasonable performance. © 2012 Elsevier B.V. All rights reserved

Trion Electroluminescence from Semiconducting Carbon Nanotubes

Near-infrared emission from semiconducting single-walled carbon nanotubes (SWNTs) usually results from radiative relaxation of excitons. By binding an additional electron or hole through chemical or electrochemical doping, charged three-body excitons, so-called trions, are created that emit light at lower energies. The energy difference is large enough to observe weak trion photoluminescence from doped SWNTs even at room temperature. Here, we demonstrate strong trion electroluminescence from electrolyte-gated, light-emitting SWNT transistors with three different polymer-sorted carbon nanotube species, namely, (6,5), (7,5) and (10,5). The red-shifted trion emission is equal to or even stronger than the exciton emission, which is attributed to the high charge carrier density in the transistor channel. The possibility of trions as a radiative relaxation pathway for triplets and dark excitons that are formed in large numbers by electron–hole recombination is discussed. The ratio of trion to exciton emission can be tuned by the applied voltages, enabling voltage-controlled near-infrared light sources with narrow line widths that are solution-processable and operate at low voltages (<3 V)

Enhanced Ambipolar Charge Injection with Semiconducting Polymer/Carbon Nanotube Thin Films for Light-Emitting Transistors

We investigate the influence of small amounts of semiconducting single-walled carbon nanotubes (SWNTs) dispersed in polyfluorenes such as poly(9,9-di-<i>n</i>-octylfluorene-<i>alt</i>-benzothiadiazole (F8BT) and poly(9,9-dioctylfluorene) (F8) on device characteristics of bottom contact/top gate ambipolar light-emitting field-effect transistors (LEFETs) based on these conjugated polymers. We find that the presence of SWNTs within the semiconducting layer at concentrations below the percolation limit significantly increases both hole and electron injection, even for a large band gap semiconductor like F8, without leading to significant luminescence quenching of the conjugated polymer. As a result of the reduced contact resistance and lower threshold voltages, larger ambipolar currents and thus brighter light emission are observed. We examine possible mechanisms of this effect such as energy level alignment, reduced bulk resistance above the contacts, and field-enhanced injection at the nanotube tips. The observed ambipolar injection improvement is applicable to most conjugated polymers in staggered transistor configurations or similar organic electronic devices where injection barriers are an issue

Mapping Charge Transport by Electroluminescence in Chirality-Selected Carbon Nanotube Networks

We demonstrate random network single-walled carbon nanotube (SWNT) field-effect transistors (FETs) in bottom contact/top gate geometry with only five different semiconducting nanotube species that were selected by dispersion with poly(9,9-dioctyl­fluorene) in toluene. These FETs are highly ambipolar with balanced hole and electron mobilities and emit near-infrared light with narrow peak widths (<40 meV) and good efficiency. We spatially resolve the electroluminescence from the channel region during a gate voltage sweep and can thus trace charge transport paths through the SWNT thin film. A shift of emission intensity to large diameter nanotubes and gate-voltage-dependent photoluminescence quenching of the different nanotube species indicates excitation transfer within the network and preferential charge accumulation on small band gap nanotubes. Apart from applications as near-infrared emitters with selectable emission wavelengths and narrow line widths, these devices will help to understand and model charge transport in realistic carbon nanotube networks

Effect of Polymer Molecular Weight and Solution Parameters on Selective Dispersion of Single-Walled Carbon Nanotubes

The selective dispersion of single-walled carbon nanotube species (n,m) with conjugated polymers such as poly­(9,9-dioctylfluorene) (PFO) and poly­(9,9-dioctylfluorene-<i>co</i>-benzothiadiazole) (F8BT) in organic solvents depends not only on the type of solvent but also on the molecular weight of the polymer. We find an increasing amount of nanotubes and altered selectivities for dispersions with higher molecular weight polymers. Including the effects of different aromatic solvents, we propose that solution viscosity is one of the factors influencing the apparent selectivity by changing the reaggregation rate of the single-walled carbon nanotubes (SWNT). The type of solvent, polymer molecular weight, concentration, and viscosity should thus be taken into account when screening for new polymers for selective SWNT dispersion