Andreas Sandström

Ambient fabrication of flexible and large-area organic light-emitting devices using slot-die coating

The grand vision of manufacturing large-area emissive devices with low-cost roll-to-roll coating methods, akin to how newspapers are produced, appeared with the emergence of the organic light-emitting diode about 20 years ago. Today, small organic light-emitting diode displays are commercially available in smartphones, but the promise of a continuous ambient fabrication has unfortunately not materialized yet, as organic light-emitting diodes invariably depend on the use of one or more time- and energy-consuming process steps under vacuum. Here we report an all-solution-based fabrication of an alternative emissive device, a light-emitting electrochemical cell, using a slot-die roll-coating apparatus. The fabricated flexible sheets exhibit bidirectional and uniform light emission, and feature a fault-tolerant >1-μm-thick active material that is doped in situ during operation. It is notable that the initial preparation of inks, the subsequent coating of the constituent layers and the final device operation all could be executed under ambient air.

Separating Ion and Electron Transport: The Bilayer Light-Emitting Electrochemical Cell

The current generation of polymer light-emitting electrochemical cells (LECs) suffers from insufficient stability during operation. One identified culprit is the active material, which comprises an intimate blend between an ion-conducting electrolyte and an electron-transporting conjugated polymer, as it tends to undergo phase separation during long-term operation and the intimate contact between the ion- and electron-transporting components provokes side reactions. To address these stability issues, we present here a bilayer LEC structure in which the electrolyte is spatially separated from the conjugated polymer. We demonstrate that employing this novel device structure, with its clearly separated ion- and electron-transport paths, leads to distinctly improved LEC performance in the form of decreased turn-on time and improved light emission. We also point out that it will allow for the utilization of combinations of active materials having mutually incompatible solubilities.

Encapsulating light-emitting electrochemical cells for improved performance

We present a functional and scalable encapsulation of light-emitting electrochemical cells (LECs), which results in a measured ambient operation of >400 h at a brightness of >300 cd/m2 with a maximum efficacy of 6 lm/W, and a linearly extrapolated ambient operation of ∼5600 h at >100 cd/m2. Our findings suggest that previous studies have underestimated the practical stability of appropriately encapsulated LECs. We also report that the dominant ambient degradation for non-encapsulated LECs is water-induced delamination of the cathode from the active layer, while encapsulated LECs in contrast are found to decay from spatial variations in the active layer composition.

Luminescent line art by direct-write patterning

We present a direct-write patterning method for the realization of electroluminescent (EL) line art using a surface-emissive light-emitting electrochemical cell with its electrolyte and EL material separated into a bilayer structure. The line-art emission is achieved through subtractive patterning of the electrolyte layer with a stylus, and the single-step patterning can be either manual for personalization and uniqueness or automated for high throughput and repeatability. We demonstrate that the light emission is effectuated by cation-assisted electron injection in the patterned regions and that the resulting emissive lines can be as narrow as a few micrometers. The versatility of the method is demonstrated through the attainment of a wide range of light-emission patterns and colors using a variety of different materials. We propose that this low-voltage-driven and easy-to-modify luminescent line-art technology could be of interest for emerging applications, such as active packaging and personalized gadgets.

A light–emission textile device: conformal spray-sintering of a woven fabric electrode

We report on the realization of an ultra-flexible, light-weight and large-area emissive textile device. The anode and active material of a light-emitting electrochemical cell (LEC) were deposited b ...

Design rules for light-emitting electrochemical cells delivering bright luminance at 27.5 percent external quantum efficiency

The light-emitting electrochemical cell promises cost-efficient, large-area emissive applications, as its characteristic in-situ doping enables use of air-stabile electrodes and a solution-processed single-layer active material. However, mutual exclusion of high efficiency and high brightness has proven a seemingly fundamental problem. Here we present a generic approach that overcomes this critical issue, and report on devices equipped with air-stabile electrodes and outcoupling structure that deliver a record-high efficiency of 99.2 cd A−1 at a bright luminance of 1910 cd m−2. This device significantly outperforms the corresponding optimized organic light-emitting diode despite the latter employing calcium as the cathode. The key to this achievement is the design of the host–guest active material, in which tailored traps suppress exciton diffusion and quenching in the central recombination zone, allowing efficient triplet emission. Simultaneously, the traps do not significantly hamper electron and hole transport, as essentially all traps in the transport regions are filled by doping.Cost-efficient light-emitting electrochemical cells can deliver unique functions, but suffer because strong brightness has only been obtained at modest efficiency. Tang et al. report on rationally designed devices that are both bright and efficient, thus representing a major step towards commercialization.

Challenging Conventional Wisdom: Finding High-Performance Electrodes for Light-Emitting Electrochemical Cells

The light-emitting electrochemical cell (LEC) exhibits capacity for efficient charge injection from two air-stable electrodes into a single-layer active material, which is commonly interpreted as implying that the LEC operation is independent of the electrode selection. Here, we demonstrate that this is far from the truth and that the electrode selection instead has a strong influence on the LEC performance. We systematically investigate 13 different materials for the positive anode and negative cathode in a common LEC configuration with the conjugated polymer Super Yellow as the electroactive emitter and find that Ca, Mn, Ag, Al, Cu, indium tin oxide (ITO), and Au function as the LEC cathode, whereas ITO and Ni can operate as the LEC anode. Importantly, we demonstrate that the electrochemical stability of the electrode is paramount and that particularly electrochemical oxidation of the anode can prohibit the functional LEC operation. We finally report that it appears preferable to design the device so that the heights of the injection barriers at the two electrode/active material interfaces are balanced in order to mitigate electrode-induced quenching of the light emission. As such, this study has expanded the set of air-stable electrode materials available for functional LEC operation and also established a procedure for the evaluation and design of future efficient electrode materials.