Felix Hermerschmidt

We report the synthesis of a fluorescent polycyclic aromatic hydrocarbon dye with a “symmetry‐broken” core, derived from the related hexa‐peri‐benzocoronene (HBC) core with a fluoranthene subunit. The fluorophore is composed of a pure carbon skeleton without heteroatoms and exhibits remarkable photoluminescence properties with a photoluminescence quantum yield (PLQY) of up to 67% in toluene, exceeding that of the parent HBC by a factor of 30. The single crystal X‐ray structure reveals the distorted polycyclic aromatic hydrocarbon structure, which is responsible for the optoelectronic properties, as supported by density functional theory calculations. We show that the new fluorescent dye can be readily used for the fabrication of organic light‐emitting diodes (OLED) without extensive optimization, whereby solubility in a variety of solvents and successful film formation are decisive.

Silver nanoparticles were sintered at atmospheric pressure using nitrogen plasma generated by diffuse coplanar surface barrier discharge (DCSBD). Compared to the standard thermal sintering of 1 h at 140 °C only 8 min of nonthermal plasma treatment at 50 °C were necessary to sufficiently anneal the silver nanoparticle film. Electrical resistivity as low as 9 × 10^–6 Ω cm was achieved, i.e., 17% of the bulk conductivity of silver. X-ray photoelectron spectroscopy was used to study the removal of organic moieties from the surface of the inkjet-printed layers. Scanning electron microscopy revealed in detail the process of formation of interconnection between nanoparticles. Our findings pave the way for implementing low-cost and eco-friendly DCSBD plasma sintering into continuous roll-to-roll processing in the future for the annealing of silver nanoparticles on substrates that cannot tolerate high temperatures.

Using a sequential inkjet printing process, we realize bicoloured PeLEDs by selectively dissolving a bromide-based perovskite film with an iodide-based perovskite ink, resulting in red light-emitting features on a green light-emitting background.

The photooxidative stability of a series of doubly bridged anthracenes was evaluated after their preparation via twofold macrocyclization of a bis(resorcinyl)anthracene. Lightfastness correlates with the energy levels of the highest occupied molecular orbital (HOMO), resulting in superior stability of the tetraesters compared to the tetraethers. The lengths and steric demand of the linker only plays a minor role for the ester‐based compounds, which can be prepared in reasonable yields and thus tested in proof‐of‐concept organic light‐emitting diodes. Double ester‐bridging allows deep blue electro‐luminescence, highlighting the importance of the choice of the functional groups used for macrocyclization.

Quantum dot (QD) materials have found increasing use in display applications because of their high color purity and fluorescence quantum yield, enabling devices with higher brightness and efficiency. However, to access large-area printing and coating methods that are carried out in ambient conditions, it is necessary to, first, move away from toxic cadmium, and second, to target materials that can be air-processed. Herein, we synthesize zinc selenide-based blue QD material and air-fabricate light-emitting diodes (LEDs) and single-carrier devices. The encapsulated devices were also measured under ambient conditions. Multi-shell-structured ZnSeTe/ZnSe/ZnS (core/shell/shell) QDs show pure deep blue/purple fluorescence emission with a high photoluminescence quantum yield of 78%. The blue QD-LED devices are fabricated in a conventional structure with bottom light emission with two electron transport materials (ZnO and ZnMgO). The QD-LED devices with ZnO electron transport layer show a maximum luminance of ∼6200 cd m−2 at 9 V with a turn-on voltage of 3.5 V and current efficacy of 0.38 cd A−1, while with ZnMgO electron transport layer, the devices show a maximum luminance of 3000 cd m−2 at 7 V with a turn-on voltage of 3 V and current efficacy of 0.6 cd A−1. Electron-only and hole-only devices were fabricated to show and confirm the underlying charge transport mechanisms. To our knowledge, these results show for the first-time air-fabricated ZnSe-based QD-LEDs, paving the way for scaling up display applications and moving toward high-performance printed electronics.

Large area metal halide perovskite LEDs were fabricated by inkjet printing. Homogeneous drying and crystallization was facilitated by a guided and optimized gas flow.

Metal halide perovskites have received great attention in recent years, predominantly due to the high performance of perovskite solar cells. The versatility of the material, which allows the tunability of the bandgap, has led to its use in light‐emitting diodes, photo, and X‐ray detectors, among other optoelectronic device applications. Specifically in photodetectors, the tunability of the bandgap allows fabrication of spectrally selective devices. Utilizing a combinatorial inkjet printing approach, multiple perovskite compositions absorbing at specific wavelengths in a single printing step are fabricated. The drop‐on‐demand capabilities of inkjet printing enable the deposition of inks in a precise ratio to produce specific perovskite compositions in the printed thin film. By controlling the halide ratio in the compositions, a mixed halide gradient ranging from pure MAPbI₃ via MAPbBr₃ to MAPbCl₃ is produced. The tunability in the absorption onset from 410 to 790 nm is demonstrated, covering the whole visible spectrum, with a precision of 8 nm steps for MAPb(Br_xCl_1−x)₃ compositions. From this range of mixed halide perovskites, photodetectors which show spectral selectivity corresponding to the measured absorption onset are demonstrated, paving the way for use in a printed visible light spectrometer without the need for a dispersion element.

Artificial neural networks (ANNs) are inspired by the biological nervous system. The high performance of such ANNs is achieved through the dynamic change of the synaptic weights by applying self-optimizing learning algorithms. Despite the simple operations for each single element in an ANN, a network with a huge amount of simulated elements consumes lots of computing capacity using von Neumann computer architectures. To overcome this issue, neuromorphic devices facilitate the design of hardware ANNs that emulate the synaptic functions. Here we demonstrate the viability of such an approach using photonic waveguides in combination with a photochromic diarylethene (DAE) molecule. By positioning and irradiating DAE molecules on single waveguides, we modulate the intensity and thereby emulate the plasticity of the synaptic weights. To run the photonic device as an ANN we firstly characterize the modulation range and encode a learning procedure accordingly. As the proof of concept we operate a y-shaped waveguide performing basic AND/OR logic gate functions, with the capability to switch between these two gate functions by using specific training sets.