Infrarotspektroskopische Untersuchung von Elektrodenmodifikationen und deren Auswirkung auf den angrenzenden organischen Halbleiter

In dieser Arbeit werden die chemische Zusammensetzung, die Orientierung und der Ladungstransfer an Grenzflächen mit Hilfe von Infrarotspektroskopie im mittelinfraroten und ferninfraroten Bereich untersucht. An der Anodengrenzfläche wurde der Einfluss der Modifikation von Indiumzinnoxid durch flüssigprozessiertes Nickeloxid (sNiO) und thermisch verdampftes Molybdänoxid (MoO3) auf das Donormaterial, in diesem Fall fluoriertes Zinkphtalocyanin (F4ZnPc), analysiert. Die Messungen wurden in situ während des Aufdampfvorgangs des F4ZnPc durchgeführt. So konnte eine chemische Veränderung des Moleküls an der Grenzfläche zu ITO und sNiO festgestellt werden und ein Ladungstransfer, aber keine chemische Veränderung auf MoO3. Der Ladungstransfer führte zur Bildung des F4ZnPc-Kations, wobei sich eine Raumladungszone mit einer Ausdehnung von 8nm formte. Die Orientierung der Moleküle in der F4ZnPc-Schicht wurde durch die Modifikation für Schichtdicken über 20nm nicht signifikant beeinflusst. Auf der Kathodenseite wurden selbstorganisierende Monolagen (SAMs) aus Dimethylamin- Biphenyl-Phosphonaten dazu verwendet die Austrittsarbeit zu verkleinern, aber nicht den Kontaktwinkel von ITO zu verändern. Die Moleküle der SAM wurden zunächst auf ITO charakterisiert und dabei Neigungswinkel, Austrittsarbeitsänderung und Kontaktwinkel bestimmt. Danach konnte der organische n-Typ Halbleiter N,N’-bis(2- phenylethyl)Perylen-3,4,9,10-bis-(dicarboximid) (BPE-PTCDI) aufgedampft werden und im IR vermessen werden. Eine Korrelation zwischen Orientierung des BPE-PCTDI und der Kontaktwinkeländerung des Substrats konnte gefunden werden. Außerdem wurde die elektronische Wechselwirkung zwischen ITO/ SAM und BPE-PTCDI gemessen, die mit der Austrittsarbeitsänderung durch die SAM einhergeht, und substratabhängig ist

Highly efficient polaritonic light-emitting diodes with angle-independent narrowband emission

Authors acknowledge funding by the Volkswagen Foundation (no. 93404; M.C.G.), the Leverhulme Trust (RPG-2017-213; M.C.G), the European Research Council under the European Union Horizon 2020 Framework Programme (FP/2014-2020)/ERC grant agreement no. 640012 (ABLASE; M.C.G) and the Alexander von Humboldt Foundation (Humboldt Professorship; M.C.G.). A.M. acknowledges funding through an individual fellowship of the Deutsche Forschungsgemeinschaft (404587082; A.M.) and from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement no. 101023743 (PolDev; A.M.).Angle-independent narrowband emission is required for many optoelectronic devices, ranging from high-definition displays to sensors. However, emerging materials for electroluminescent devices, such as organics and perovskites, show spectrally broad emission due to intrinsic disorder. Coupling this emission to an optical resonance reduces the linewidth, but at the cost of inheriting the severe angular dispersion of the resonator. Strongly coupling a dispersionless exciton state to a narrowband optical microcavity could overcome this issue; however, electrically pumped emission from the resulting polaritons is typically hampered by poor efficiencies. Here we present a universal concept for polariton-based emission from organic light-emitting diodes by introducing an assistant strong coupling layer, thereby avoiding quenching-induced efficiency losses. We realize red- and green-emitting, narrowband (full-width at half-maximum of less than 20 nm) and spectrally tunable polaritonic organic light-emitting diodes with up to 10% external quantum efficiency and high luminance (>20,000 cd m−2 at 5 V). By optimizing cavity detuning and coupling strength, we achieve emission with ultralow dispersion (<10 nm spectral shift at 60° tilt). These results may have wide-reaching implications for on-demand polariton emission and demonstrate the practical relevance of strong light–matter coupling for next-generation optoelectronics, particularly display technology.Publisher PDFPeer reviewe

Improving the thermal stability of top-emitting organic light-emitting diodes by modification of the anode interface

This research was financially supported by the EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207), the DARPA-NESD program (N66001-17-C-4012) and the Leverhulme Trust (RPG-2017-231). Y.D. acknowledges a stipend from the Chinese Scholarship Council (CSC). C.K. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1A6A3A03012331). M.C.G. acknowledges support from the Alexander von Humboldt Stiftung through the Humboldt-Professorship.Top‐emitting organic light‐emitting diodes (OLEDs) are of interest for numerous applications, in particular for displays with high fill factors. To maximize efficiency and luminance, molecular p‐doping of the hole transport layer (p‐HTL) and a highly reflective anode contact, for example, made from silver, are used. Atomic layer deposition (ALD) is attractive for thin film encapsulation of OLEDs but generally requires a minimum process temperature of 80 °C. Here it is reported that the interface between the p‐HTL and the silver anode of top‐emitting OLEDs degrades during an 80 °C ALD encapsulation process, causing an over fourfold reduction in OLED current and luminance. To understand the underlying mechanism of device degradation, single charge carrier devices are investigated before and after annealing. A spectroscopic study of p‐HTLs indicates that degradation is due to the interaction between diffusing silver ions and the p‐type molecular dopant. To improve the stability of the interface, either an ultrathin MoO3 buffer layer or a bilayer HTL is inserted at the anode/organic interface. Both approaches effectively suppress degradation. This work shows a route to successful encapsulation of top‐emitting OLEDs using ALD without sacrificing device performance.Publisher PDFPeer reviewe

High-brightness blue polariton organic light-emitting diodes

H2020 Marie Sklodowska-Curie Actions - 101023743; University of St Andrews; Alexander von Humboldt-Stiftung; European Research Council - 101097878; Deutsche Forschungsgemeinschaft - RTG2591Polariton organic light-emitting diodes (POLEDs) use strong light-matter coupling as an additional degree of freedom to tailor device characteristics, thus making them ideal candidates for many applications, such as room temperature laser diodes and high-color purity displays. However, achieving efficient formation of and emission from exciton-polaritons in an electrically driven device remains challenging due to the need for strong absorption, which often induces significant nonradiative recombination. Here, we investigate a novel POLED architecture to achieve polariton formation and high-brightness light emission. We utilize the blue-fluorescent emitter material 4,4′-Bis(4-(9H-carbazol-9-yl)styryl)biphenyl (BSBCz), which exhibits strong absorption and a highly horizontal transition-dipole orientation as well as a high photoluminescence quantum efficiency, even at high doping concentrations. We achieve a peak luminance of over 20,000 cd/m2 and external quantum efficiencies of more than 2%. To the best of our knowledge, these values represent the highest reported so far for electrically driven polariton emission from an organic semiconductor emitting in the blue region of the spectrum. Our work therefore paves the way for a new generation of efficient and powerful optoelectronic devices based on POLEDs.Peer reviewe

Accurate efficiency measurements of organic light-emitting diodes via angle-resolved spectroscopy

Funding: EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207), the Leverhulme Trust (RPG-2017-231), the Volkswagen Foundation (No. 93404), and the Alexander von Humboldt Stiftung (Humboldt-Professorship to M.C.G.). C.K. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1A6A3A03012331). Diese Arbeit wurde mitfinanziert durch Steuermittel auf der Grundlage des vom Sächsischen Landtag beschlossenen Haushaltes.The accurate characterization of thin-film LEDs – including organic light emitting diodes (OLEDs), perovskites and quantum dot LEDs – is crucial to our understanding of the factors that influence their efficiency and thus to the fabrication of LEDs with improved performance and stability. In addition, detailed information about the angular characteristics of LED emission is useful to assess the suitability of individual architectures, e.g. for display applications. Here, the implementation of a goniometer-based measurement system and corresponding protocol are described that allow to accurately determine the current-voltage-luminance characteristics, external quantum efficiency and luminous efficacy of OLEDs and other emerging thin-film LEDs. The system allows recording of angle-resolved electroluminescence spectra and accurate efficiency measurements for devices with both Lambertian and non-Lambertian emission characteristics. A detailed description of the setup and a protocol for assembling and aligning the required hardware are provided. Drawings of all custom parts and the open-source Python software required to perform the measurement and to analyze the data are included.Publisher PDFPeer reviewe

High-density integration of ultrabright OLEDs on a miniaturized needle-shaped CMOS backplane

This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) under Contract N6600117C4012, by the National Institutes of Health under Grant U01NS090596, and by the Leverhulme Trust (RPG-2017-231). C.K.M. acknowledges funding from the European Commission through a Marie Skłodowska Curie individual fellowship (101029807). M.C.G. acknowledges funding from the Alexander von Humboldt Stiftung (Humboldt-Professorship). We thank Aaron Naden for the FIB/STEM measurements (Engineering and Physical Sciences Research Council under grant numbers EP/L017008/1, EP/R023751/1 and EP/T019298/1).Direct deposition of organic light-emitting diodes (OLEDs) on silicon-based complementary metal–oxide–semiconductor (CMOS) chips has enabled self-emissive microdisplays with high resolution and fill-factor. Emerging applications of OLEDs in augmented and virtual reality (AR/VR) displays and in biomedical applications, e.g., as brain implants for cell-specific light delivery in optogenetics, require light intensities orders of magnitude above those found in traditional displays. Further requirements often include a microscopic device footprint, a specific shape and ultrastable passivation, e.g., to ensure biocompatibility and minimal invasiveness of OLED-based implants. In this work, up to 1024 ultrabright, microscopic OLEDs are deposited directly on needle-shaped CMOS chips. Transmission electron microscopy and energy-dispersive X-ray spectroscopy are performed on the foundry-provided aluminum contact pads of the CMOS chips to guide a systematic optimization of the contacts. Plasma treatment and implementation of silver interlayers lead to ohmic contact conditions and thus facilitate direct vacuum deposition of orange- and blue-emitting OLED stacks leading to micrometer-sized pixels on the chips. The electronics in each needle allow each pixel to switch individually. The OLED pixels generate a mean optical power density of 0.25 mW mm−2, corresponding to >40 000 cd m−2, well above the requirement for daylight AR applications and optogenetic single-unit activation in the brain.Publisher PDFPeer reviewe

High brightness, highly directional organic light-emitting diodes as light sources for future light-amplifying prosthetics in the optogenetic management of vision loss

Funding: Engineering and Physical Sciences Research Council (Grant Number(s): EP/R010595/1). National Science Foundation (Grant Number(s): 1706207). Defense Sciences Office, DARPA (Grant Number(s): N66001-17-C-4012). Leverhulme Trust (Grant Number(s): RPG-2017-231). Alexander von Humboldt-Stiftung (Grant Number(s): Humboldt Professur). National Research Foundation of Korea (GrantNumber(s): 2017R1A6A3A03012331). China Sponsorship Council.Optogenetic control of retinal cells transduced with light-sensitive channelrhodopsins can enable restoration of visual perception in patients with vision loss. However, a light intensity orders of magnitude higher than ambient light conditions is required to achieve robust cell activation. Relatively bulky wearable light amplifiers are currently used to deliver sufficient photon flux (>1016 photons/cm2/s in a ±10° emission cone) at a suitable wavelength (e.g., 600 nm for channelrhodopsin ChrimsonR). Here, ultrahigh brightness organic light-emitting diodes (OLEDs) with highly directional emission are developed, with the ultimate aim of providing high-resolution optogenetic control of thousands of retinal cells in parallel from a compact device. The orange-emitting phosphorescent OLEDs use doped charge transport layers, generate narrowband emission peaking at 600 nm, and achieve a luminance of 684 000 cd m–2 at 15 V forward bias. In addition, tandem-stack OLEDs with a luminance of 1 152 000 cd m–2 and doubled quantum efficiency are demonstrated, which greatly reduces electrical and thermal stress in these devices. At the photon flux required to trigger robust neuron firing in genetically modified retinal cells and when using heat sinking and realistic duty cycles (20% at 12.5 Hz), the tandem-stack OLEDs therefore show a greatly improved half-brightness lifetime of 800 h.Publisher PDFPeer reviewe

Wireless magnetoelectrically powered organic light-emitting diodes

This work was supported by a scholarship to J.F.B. donated by Beverly and Frank MacInnis to the University of St Andrews, the European Union Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement no. 101023743 (PolDev to A.M.), The Leverhulme Trust (RPG-2017-231), the Alexander von Humboldt Foundation (Humboldt Professorship to M.C.G.), the DFG-funded Research Training Group “Template-Designed Organic Electronics (TIDE)” (RTG2591), and the EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207).Compact wireless light sources are a fundamental building block for applications ranging from wireless displays to optical implants. However, their realization remains challenging because of constraints in miniaturization and the integration of power harvesting and light-emission technologies. Here, we introduce a strategy for a compact wirelessly powered light-source that consists of a magnetoelectric transducer serving as power source and substrate and an antiparallel pair of custom-designed organic light-emitting diodes. The devices operate at low-frequency ac magnetic fields (~100 kHz), which has the added benefit of allowing operation multiple centimeters deep inside watery environments. By tuning the device resonance frequency, it is possible to separately address multiple devices, e.g., to produce light of distinct colors, to address individual display pixels, or for clustered operation. By simultaneously offering small size, individual addressing, and compatibility with challenging environments, our devices pave the way for a multitude of applications in wireless displays, deep tissue treatment, sensing, and imaging.Peer reviewe

Orientation distributions of vacuum-deposited organic emitters revealed by single-molecule microscopy

This work was supported by the Volkswagen Foundation (No. 93404) and the DFG-funded Research Training Group “Template-Designed Organic Electronics (TIDE)”, RTG2591. M.C.G. acknowledges support from the Alexander von Humboldt Stiftung through the Humboldt-Professorship. A.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement No. 101023743 (PolDev).The orientation of luminescent molecules in organic light-emitting diodes strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual emitter molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10−6 wt%) of C545T and minimising background levels to reliably measure its photoluminescence. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed devices. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for organic light-emitting devices.Publisher PDFPeer reviewe