Stefan Höfle

Enhanced Electron Injection into Inverted Polymer Light‐Emitting Diodes by Combined Solution‐Processed Zinc Oxide/Polyethylenimine Interlayers

Inverted device architectures for organic light-emitting diodes (OLEDs) require suitable interfaces or buffer layers to enhance electron injection from highwork-function transparent electrodes. A solution-processable combination of ZnO and PEI is reported, that facilitates electron injection and enables efficient and air-stable inverted devices. Replacing the metal anode by highly conductive polymers enables transparent OLEDs.

Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture.

Fully solution processed monochromatic and white-light emitting tandem or multi-photon polymer OLEDs with an inverted device architecture have been realized by employing WO3 /PEDOT:PSS/ZnO/PEI charge carrier generation layers. The luminance of the sub-OLEDs adds up in the stacked device indicating multi-photon emission. The white OLEDs exhibit a CRI of 75.

Tungsten Oxide Buffer Layers Fabricated in an Inert Sol‐Gel Process at Room‐Temperature for Blue Organic Light‐Emitting Diodes

WO3 deposition from tungsten ethoxide precursor solutions at room temperature is demonstrated. The W(OEt)6 precursor can be converted under inert conditions and hence avoids sample contamination with oxygen, opening a pathway to more stable devices. The stoichiometry of all WO3 layers and the optoelectronic performance of the respective SMOLEDs well match thermally evaporated WO3 and its corresponding SMOLEDs. The solution processed WO3 hole injection layers enable the fabrication of blue phosphorescent OLEDs with low onset voltage and current efficiencies of up to 14 cd A(-1) .

N-Fused quinoxalines and benzoquinoxalines as attractive emitters for organic light emitting diodes

We describe the modular synthesis of quinoxaline fluorophores exhibiting strong fluorescence over the whole visible spectrum. Based on their fluorescence quantum yield, frontier orbital energies and solid-state aggregation behavior, several compounds work well in fluorescent organic light emitting diodes (OLEDs) with luminances over 1000 cd m−2 at 8 V driving voltage.

Charge generation layers for solution processed tandem organic light emitting diodes with regular device architecture.

Tandem organic light emitting diodes (OLEDs) utilizing fluorescent polymers in both sub-OLEDs and a regular device architecture were fabricated from solution, and their structure and performance characterized. The charge carrier generation layer comprised a zinc oxide layer, modified by a polyethylenimine interface dipole, for electron injection and either MoO3, WO3, or VOx for hole injection into the adjacent sub-OLEDs. ToF-SIMS investigations and STEM-EDX mapping verified the distinct functional layers throughout the layer stack. At a given device current density, the current efficiencies of both sub-OLEDs add up to a maximum of 25 cd/A, indicating a properly working tandem OLED.

TIPS‐Tetracene‐ and TIPS‐Pentacene‐Annulated Poly(norbornadiene)s: Synthesis and Properties

The synthesis of tetracene- and pentacene-annulated norbornadienes, formed through the Diels-Alder reaction of a dehydroacene with cyclopentadiene is reported. Ring-opening metathesis polymerization (ROMP) leads to polymers that are investigated with respect to their physical, optical, and electronic properties by gel permeation chromatography (GPC), UV-vis spectroscopy, and cyclic voltammetry. The pentacene-containing polymer P1 is successfully integrated into an organic field-effect transistor (OFET); the tetracene-containing polymer P2 is integrated into an organic light-emitting diode (OLED).

OLED Luminaires: Device Arrays with 99.6% Geometric Fill Factor Structured by Femtosecond Laser Ablation

The plethora of design opportunities renders organic light emitting diodes (OLEDs) ideal luminaires for general lighting applications. Progressing from lab-scale device concepts to large-area applications calls for smart device designs that are scalable and, at the same time, unsusceptible to resistive losses within the electrodes. By employing direct pulsed femtosecond laser structuring, we fabricate OLED luminaires comprising monolithically interconnected OLED arrays. We determine the laser ablation thresholds and the optimized process parameters for all functional layers. The clean laser cuts with precise ablation depths show no detectable damage to adjacent layers or any ridges, hence avoiding unwanted short-cuts or device isolation. All processes are scalable. The 3-fold structuring of the OLED luminaire is confined within 45 μm and hence below the resolution limit of the human eye, yielding a geometric fill factor beyond 99% and therefore a very homogeneous device perception.

Charge generation layers for all-solution processed organic tandem light emitting diodes with regular device architecture

We present multi-photon OLEDs where enhanced light emission was achieved by stacking two OLEDs utilizing a regular device architecture (top cathode) and an intermediate charge carrier generation layer (CGL) for monolithic device interconnection. With respect to future printing processes for organic optoelectronic devices, all functional layers were deposited from solution. The CGL comprises a low-work function zinc oxide layer that was applied from solution under ambient conditions and at moderate processing temperatures and a high-work function interlayer that was realized from various solution processable precursor-based metal oxides, like molybdenum-, vanadium- and tungsten-oxide. Since every injected electron-hole pair generates two photons, the luminance and the current efficiency of the tandem OLED at a given device current are doubled while the power efficiency remains constant. At a given luminance, the lower operating current in the tandem device reduces electrical stress and improves the device life-time. ToF-SIMS, TEM/FIB and EDX analyses provided evidence of a distinct layer sequence without intermixing upon solution deposition.