Arko Graf

Arko Graf
Universität Heidelberg · Department of Chemistry

Master of Science

About

44
Publications
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926
Citations

Publications

Publications (44)
Article
Exciton-polaritons are hybrid light–matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due...
Article
Full-text available
Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped...
Article
Full-text available
Selective dispersion of semiconducting single-walled carbon nanotube (SWCNTs) with conjugated polymers typically involves harsh sonication methods that damage and shorten the nanotubes. Here, we use simple high speed shear force mixing (SFM) to disperse nearly monochiral (6,5) SWCNTs with poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6′-{2,2′-bipy...
Article
Full-text available
The orientation of the emissive dipole moment of seven iridium-based phosphorescent emitter molecules commonly used in organic light-emitting diodes (OLEDs) is investigated. The orientation of Ir(ppy)3, Ir(ppy)2(acac), Ir(chpy)3, Ir(dhfpy)2(acac), Ir(BT)2(acac), Ir(MDQ)2(acac), and Ir(piq)3 is determined by measuring the angle dependent spectral ra...
Article
Thus far, research activities of 2D materials in optics, photonics and optoelectronics predominantly focus on micromechanically cleaved or grown nanosheets. Here, we show that high quality liquid-exfoliated nanosheets offer an alternative approach. Starting from well-defined, monolayer rich WS2 dispersions obtained after liquid exfoliation and size...
Conference Paper
Orientation of transition dipoles can strongly improve the outcoupling efficiency of or-ganic LEDs (OLEDs). We present two new classes of OLED emitters that may achieve horizontal alignment: columnar liquid crystals and carbon nanotubes.
Article
Full-text available
The application of light emitting field effect transistors (LEFET) is an elegant way of combining electrical switching and light emission in a single device architecture instead of two. This allows for a higher degree of miniaturisation and integration in future opto-electronic applications. Here, we report on a LEFET based on lead sulphide quantum...
Article
Current state‐of‐the‐art organic solar cells (OSCs) still suffer from high losses of open‐circuit voltage (VOC). Conventional polymer:fullerene solar cells usually exhibit bandgap to VOC losses greater than 0.8 V. Here a detailed investigation of VOC is presented for solution‐processed OSCs based on (6,5) single‐walled carbon nanotube (SWCNT): [6,6...
Article
Full-text available
Strong coupling between plasmons and excitons leads to the formation of plexcitons: quasiparticles that combine nanoscale energy confinement and pronounced optical nonlinearities. In addition to these localized modes, the enhanced control over the dispersion relation of propagating plexcitons may enable coherent and collective coupling of distant e...
Article
Full-text available
We demonstrate the formation and tuning of charged trion-polaritons in polymer-sorted (6,5) single-walled carbon nanotubes in a planar metal-clad microcavity at room temperature. The positively charged trion-polaritons were induced by electrochemical doping and characterized by angle-resolved reflectance and photoluminescence spectroscopy. The dopi...
Article
Full-text available
Dense layers of semiconducting single-walled carbon nanotubes (SWNTs) serve as electrochromic materials in the near-infrared with high optical density and high conductivity. Electrochromic cells with tunable notch filter properties instead of broadband absorption are created via highly selective dispersion of specific semiconducting SWNTs through p...
Article
While organic light-emitting diodes (OLEDs) covering all colors of the visible spectrum are widespread, suitable organic emitter materials in the near-infrared (nIR) beyond 800 nm are still lacking. Here, the first OLED based on single-walled carbon nanotubes (SWCNTs) as the emitter is demonstrated. By using a multilayer stacked architecture with m...
Article
Full-text available
Exciton-polaritons are quasiparticles with hybrid light–matter properties that may be used in new optoelectronic devices. Here, electrically pumped ultrastrongly coupled exciton-polaritons in a high-mobility donor–acceptor copolymer are demonstrated by integrating a light-emitting field-effect transistor into a metal-clad microcavity. Near-infrared...
Article
Monochiral (7,5) single walled carbon nanotubes (SWCNTs) are integrated into a field effect transistor device in which the built-in electric field at the nanotube/metal contact allows for exciton separation under illumination. Variable wavelength spectroscopy and 2D surface mapping of devices consisting of 10 – 20 nanotubes are performed in the vis...
Article
Full-text available
The integration of periodic nanodisk arrays into the channel of a light-emitting field-effect transistor leads to enhanced and directional electroluminescence from thin films of purified semiconducting single-walled carbon nanotubes. The maximum enhancement wavelength is tunable across the near-infrared and is directly linked to the periodicity of...
Article
We review recent progress in controlling the alignment of the transition dipole moment of electroluminescent molecules to maximize the light extraction efficiency in OLEDs. Particular focus devoted to ways of measuring molecular orientation and on linking molecular structure to alignment.
Article
Full-text available
In this work, a comprehensive methodology for the fitting of single-walled carbon nanotube absorption spectra is presented. Different approaches to background subtraction, choice of line profile, and calculation of full width at half maximum are discussed both in the context of previous literature and the contemporary understanding of carbon nanotu...
Article
Composites of single-walled carbon nanotubes (SWNT) and conjugated polymers are highly interesting as active semiconducting layers in solution-processable and flexible electronics. Understanding the percolation behavior and percolation threshold for electrical conductivity in these composites is a prerequisite for future applications. Here, we inve...
Article
Full-text available
Hybrid photonic-plasmonic modes in periodic arrays of metallic nanostructures offer a promising trade-off between high-quality cavities and subdiffraction mode confine- ment. However, their application in electrically driven light-emitting devices is hindered by their sensitivity to the surrounding environment and to charge injecting metallic elect...
Data
Supplementary Figures 1-5 and Supplementary Table 1
Article
Full-text available
Their high oscillator strength and large exciton binding energies make single-walled carbon nanotubes (SWCNTs) highly promising materials for the investigation of strong-light matter interactions in the near-infrared and at room temperature. To explore their full potential, high-quality cavities - possibly with nanoscale field localization - are re...
Article
In this work, for the first time, the diameter limit of surfactant wrapped single walled carbon nanotubes (SWCNTs) in SWCNT:C60 solar cells is determined through preparation of monochiral small and large diameter nanotube devices as well as those from polychiral mixtures. Through assignment of the different nanotube chiralities by photoluminescence...
Article
Full-text available
We demonstrate broadband tunability of light emission from dense (6,5) single-walled carbon nanotube thin films via efficient coupling to periodic arrays of gold nanodisks that support surface lattice resonances (SLRs). We thus eliminate the need to select single-walled carbon nanotubes (SWNTs) with different chiralities to obtain narrow linewidth...