A guide to qualitative haze measurements demonstrated on inkjet-printed silver electrodes for flexible OLEDs

The search for alternative transparent electrodes to the commonly used indium tin oxide (ITO) in optoelectronic devices has led to solution-based approaches based on inkjet printing. As an additive manufacturing technique that allows drops to be positioned only where necessary, inkjet printing shows reduced waste of starting material compared to other methods such as spin coating. As a result, functional materials can be both coated and structured without the need for masks or lithographic pre-patterning of the substrate. For this contribution, we utilized a particle-free silver ink to produce a transparent electrode by inkjet printing. After printing, the silver ions were reduced to metallic silver by an argon plasma. The process takes place at low temperatures (ca. 40 – 50°C), making it suitable for use with flexible substrates, which are often temperature-sensitive. The printed silver layers show good electrical conductivity and optical transmittance, with a crystalline grain structure being formed and maintained during the metallization process. This structure forms a self-organized nanometer-size grid, whose structure allows light to pass through. Due to its nano-structured property, the haze of the electrode was investigated using a simple experimental setup based on a light source shining through the electrode and analyzing the size of the projected pattern. Such qualitative assessment can be a useful indication of the quality of the electrode and we provide details on how to replicate this setup. The final electrodes were implemented in solution-processed OLEDs, which showed bright luminance and overall low haze compared to ITO-based reference devices.Peer Reviewe

Bicolour, large area, inkjet-printed metal halide perovskite light emitting diodes†

We demonstrate a bicoloured metal halide perovskite (MHP) light emitting diode (LED) fabricated in two sequential inkjet printing steps. By adjusting the printing parameters, we selectively and deliberately redissolve and recrystallize the first printed emissive layer to add a pattern emitting in a different color. The red light emitting features (on a green light emitting background) have a minimum size of 100 μm and originate from iodide-rich domains in a phase-segregated, mixed MHP. This phase forms between the first layer, a bromide-based MHP, which is partially dissolved by printing, and the second layer, an iodide-containing MHP. With an optimised printing process we can retain the active layer integrity and fabricate bicolour, large area MHP-based LEDs with up to 1600 mm2 active area. The two emission peaks at 535 nm and 710 nm are well separated and produce a strong visual contrast.Bundesministerium für Bildung und Forschung 10.13039/501100002347Helmholtz Energy Materials Foundry 10.13039/501100015608Peer Reviewe

Using Combinatorial Inkjet Printing for Synthesis and Deposition of Metal Halide Perovskites in Wavelength‐Selective Photodetectors

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(BrxCl1−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.Peer Reviewe

Blue cadmium-free and air-fabricated quantum dot light-emitting diodes

The article processing charge was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 491192747 and the Open Access Publication Fund of Humboldt-Universität zu Berlin.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.Peer Reviewe

Implementation of Flexible Embedded Nanowire Electrodes in Organic Light-Emitting Diodes

The implementation of silver nanowires (NWs) as flexible transparent electrodes (FTEs) in solution-processed organic light-emitting diodes (OLEDs) still faces two major challenges: the high roughness of NW films and heat sensitivity of the most commonly used transparent substrate poly(ethylene terephthalate) (PET). A solution-based, roll-to-roll, and sheet-to-sheet scalable process to create indium tin oxide (ITO)-free FTEs is reported. This FTE is realized by spraying NWs on PET, without the necessity of postdeposition heat treatment. To overcome the roughness limitation, NW films are reverse transfer embedded on another PET substrate. As a result, the FTE shows a low roughness, as well as high mechanical, thermal, organic-solvent, and plasma stability. This developed FTE shows comparable transmittance with ITO but lower sheet resistance and higher mechanical stability. The FTE is implemented in a solution-processed OLED with PDY-132 (Super Yellow) as the emissive layer. In contrast to many other works in this field, a ZnO-nanoparticle electron-injection layer is used on the NWs instead of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or other organic hole injectors. The use of ZnO nanoparticles instead of organic layers yields many advantages in terms of process and device stability. The resulting devices show greater flexibility, conductivity, and luminance than PET/ITO reference devices, while having the same power efficacy.Peer Reviewe

Highly Efficient Indium Tin Oxide-free Organic Photovoltaics Using Inkjet-printed Silver Nanoparticle Current Collecting Grids

We report an in-depth investigation of an inkjet-printed silver (Ag) nanoparticle grid combined with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) of different conductivities as an alternative to an indium tin oxide (ITO)-based transparent anode for organic solar cell applications. The reported measurements revealed higher transparency of the inkjet-printed Ag nanoparticle-based grid when compared to different thicknesses of ITO on glass substrates. Based on the proposed current collecting grid, a record power conversion efficiency of 2% is achieved for ITO-free organic solar cell

Using Active Surface Plasmons in a Multibit Optical Storage Device to Emulate Long-Term Synaptic Plasticity

Artificial intelligence takes inspiration from the functionalities and structure of the brain to solve complex tasks and allow learning. Yet, hardware realization that simulates the synaptic activities realized with electrical devices still lags behind computer software implementation, which has improved significantly during the past decade. Herein, the capability to emulate synaptic functionalities by exploiting surface plasmon polaritons (SPPs) is shown. By depositing photochromic switching molecules (diarylethene) on a thin film of gold, it is possible to reliably control the electronic configuration of the molecules upon illumination cycles with UV and visible light. These reversible changes modulate the dielectric function of the photochromic film and thus enable the effective control of the SPP dispersion relation at the molecule/gold interface. The plasmonic device displays fundamental functions of a synapse such as potentiation, depression, and long‐term plasticity. The integration of such plasmonic devices in an artificial neural network is deployed in plasmonic neuroinspired circuits for optical computing and data transmission.Peer Reviewe

High performance organic light-emitting diodes employing ITO-free and flexible TiOx/Ag/Al:ZnO electrodes

The broad application of flexible optoelectronic devices is still hampered by the lack of an ITO-free and highly flexible transparent electrode. Dielectric/metal/dielectric (DMD) transparent electrodes are promising candidates to replace ITO, especially in flexible devices due to their mechanical stability to bending, high optical transmittance and low sheet resistance (<6 Ω sq−1). This paper reports on organic light emitting diodes (OLEDs) employing a DMD electrode, specifically TiOx/Ag/Al:ZnO (doped with 2 wt% Al2O3) fabricated by sputter deposition, together with a solution-processed organic polymeric emitting layer. The electrodes were sputtered without substrate heating on rigid glass and flexible polyethylene terephthalate (PET). The results showed that the OLED devices on the DMD electrodes outperform the OLEDs on commercial ITO substrates in terms of maximum luminance as well as current efficacy. Specifically, DMD-based devices achieve up to 30% higher current efficacy on glass and up to 260% higher efficacy on PET, as compared to the ITO-based reference devices. Maximum luminance reaches up to 100 000 cd m−2 for the DMD-based OLEDs on glass and 43 000 cd m−2 for those on PET. This performance is due to the low sheet resistance of the electrodes combined with efficient light outcoupling and shows the potential of DMDs to replace ITO in optoelectronic devices. This outstanding type of optoelectronic device paves the way for the future high throughput production of flexible display and photovoltaic devices.Peer Reviewe

Impact of Different Intermediate Layers on the Morphology and Crystallinity of TiO2 Grown on Carbon Nanotubes by Atomic Layer Deposition

Nanocomposites of TiO2 and carbon nanotubes (CNTs) have been extensively studied in photocatalysis, sensing, and energy conversion and storage over the last decade. The unique properties of these nanocomposites are greatly dependent on the morphology, crystallinity, and homogeneity of the TiO2 coating. However, a fine control of the film microstructure is still challenging due to limited understanding of early stages of the TiO2 growth. The presence of an intermediate buffer layer can induce remarkable changes in the morphological and structural characteristics of the coatings. Here, TiO2 films deposited by atomic layer deposition (ALD) on CNTs without and with different intermediate layers (Al2O3 and ZnO) have been systematically investigated. Compared to bare CNTs, it is suggested that these two intermediate layers with higher surface energy can lead to a delay of the TiO2 crystallization, ultimately resulting in the growth of conformal and crystalline TiO2 films. This study demonstrates a strategy to tailor the microstructure and the properties of thin films via ALD by applying intermediate layers and provides information about the role of surface energy of the substrate in crystallization and growth behavior of ALD thin films.Peer Reviewe