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  • 1.
    Asadpoordarvish, Amir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC, Umeå, Sweden.
    Sandström, Andreas
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bollström, Roger
    Toivakka, Martti
    Österbacka, Ronald
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Light-Emitting Paper2015In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 25, no 21, p. 3238-3245Article in journal (Refereed)
    Abstract [en]

    A solution-based fabrication of flexible and light-weight light-emitting devices on paper substrates is reported. Two different types of paper substrates are coated with a surface-emitting light-emitting electrochemical cell (LEC) device: a multilayer-coated specialty paper with an intermediate surface roughness of 0.4 μm and a low-end and low-cost copy paper with a large surface roughness of 5 μm. The entire device fabrication is executed using a handheld airbrush, and it is notable that all of the constituent layers are deposited from solution under ambient air. The top-emitting paper-LECs are highly flexible, and display a uniform light emission with a luminance of 200 cd m−2 at a current conversion efficacy of 1.4 cd A−1.

  • 2.
    Ekspong, Joakim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rajukumar, Lakshmy Pulickal
    Terrones, Mauricio
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst2018In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 46, article id 1802744Article in journal (Refereed)
    Abstract [en]

    The metastable 1T′ polymorph of molybdenum disulfide (MoS2) has shown excellent catalytic activity toward the hydrogen evolution reaction (HER) in water‐splitting applications. Its basal plane exhibits high catalytic activity comparable to the edges in 2H MoS2 and noble metal platinum. However, the production and application of this polymorph are limited by its lower energetic stability compared to the semiconducting 2H MoS2 phase. Here, the production of stable intercalated 1T′ MoS2 nanosheets attached on graphitic nanoribbons is reported. The intercalated 1T′ MoS2 exhibits a stoichiometric S:Mo ratio of 2.3 (±0.1):1 with an expanded interlayer distance of 10 Å caused by a sulfur‐rich intercalation agent and is stable at room temperature for several months even after drying. The composition, structure, and catalytic activity toward HER are investigated both experimentally and theoretically. It is concluded that the 1T′ MoS2 phase is stabilized by the intercalated agents, which further improves the basal planes′ catalytic activity toward HER.

  • 3.
    Ekspong, Joakim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Klechikov, Alexey
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stabilizing Active Edge Sites in Semicrystalline Molybdenum Sulfide by Anchorage on Nitrogen-Doped Carbon Nanotubes for Hydrogen Evolution Reaction2016In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 26, no 37, p. 6766-6776Article in journal (Refereed)
    Abstract [en]

    Finding an abundant and cost-effective electrocatalyst for the hydrogen evolu-tion reaction (HER) is crucial for a global production of hydrogen from water electrolysis. This work reports an exceptionally large surface area hybrid catalyst electrode comprising semicrystalline molybdenum sulfi de (MoS 2+ x) catalystattached on a substrate based on nitrogen-doped carbon nanotubes (N-CNTs), which are directly grown on carbon fiber paper (CP). It is shown here that nitrogen-doping of the carbon nanotubes improves the anchoring of MoS 2+ xcatalyst compared to undoped carbon nanotubes and concurrently stabilizes a semicrystalline structure of MoS 2+ x with a high exposure of active sites for HER. The well-connected constituents of the hybrid catalyst are shown to facilitate electron transport and as a result of the good attributes, the MoS 2+ x/N-CNT/CPelectrode exhibits an onset potential of −135 mV for HER in 0.5 M H2SO4, a Tafel slope of 36 mV dec −1, and high stability at a current density of −10 mA cm −2.

  • 4.
    Fang, Junfeng
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    The design and realization of flexible light-emitting electrochemical cells with record-long lifetime2009In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 19, no 16, p. 2671-2676Article in journal (Refereed)
    Abstract [en]

    Polymer light-emitting electrochemical cells (LECs) offer an attractive opportunity for low-cost production of functional devices in flexible and large-area configurations, but the critical drawback in comparison to competing light-emission technologies is a limited operational lifetime. Here, it is demonstrated that it is possible to improve the lifetime by straightforward and motivated means from a typical value of a few hours to more than one month of uninterrupted operation at significant brightness (>100 cd m−2) and relatively high power conversion efficiency (2 lm W−1 for orange-red emission). Specifically, by optimizing the composition of the active material and by employing an appropriate operational protocol, a desired doping structure is designed and detrimental chemical and electrochemical side reactions are identified and minimized. Moreover, the first functional flexible LEC with a similar promising device performance is demonstrated.

  • 5.
    Mindemark, Jonas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Chemistry, Ångström Laboratory, Uppsala University.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Li, Hu
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ion Transport beyond the Polyether Paradigm: Introducing Oligocarbonate Ion Transporters for Efficient Light-Emitting Electrochemical Cells2018In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 32, article id 1801295Article in journal (Refereed)
    Abstract [en]

    The light-emitting electrochemical cell (LEC) is fundamentally dependent on mobile ions for its operation. In polymer LECs, the mobile ions are commonly provided by dissolving a salt in an ion transporter, with the latter almost invariably being an ether-based compound. Here, the synthesis, characterization, and application of a new class of carbonate-based ion transporters are reported. A polymer LEC, comprising a star-branched oligocarbonate endowed with aliphatic side groups as the ion transporter, features a current efficacy of 13.8 cd A(-1) at a luminance of 1060 cd m(-2), which is a record-high efficiency/luminance combination for a singlet-emitting LEC. It is further established that the design principles of a high-performance carbonate ion transporter constitute the selection of an oligomeric structure over a corresponding polymeric structure and the endowment of the oligomer with functional side chains to render it compatible with the polymeric emitter.

  • 6.
    Munar, Antoni
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Andreas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shedding light on the operation of polymer light-emitting electrochemical cells using impedance spectroscopy2012In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 22, no 7, p. 1511-1517Article in journal (Refereed)
    Abstract [en]

    A combination of impedance spectroscopy, device characterization, and modeling is used to pinpoint key processes in the operation of polymer light-emitting electrochemical cells (LECs). At low applied voltage, electric double layers with a thickness of similar to 23 nm are shown to exist at the electrode interfaces. At voltages exceeding the bandgap potential of the conjugated polymer (V = 2.5 V for superyellow), a light-emitting pn junction forms in situ, with a steady-state structure that is found to depend strongly on the applied voltage. This is exemplified by that the effective pn junction thickness (dpn) for a device with an interelectrode gap of 90 nm decreases from similar to 23 nm at 2.5 V to similar to 6 nm at 3.9 V. The current increases with decreasing dpn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when dpn similar to 10 nm. The existence of an optimum dpn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing dpn, due to an increasing current, and a simultaneous decrease in the exciton radiative decay rate, when an increasing fraction of excitons diffuses away from the pn junction into the surrounding non-radiative doping regions.

  • 7.
    Skrypnychuk, Vasyl
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yu, Victor
    Hilke, Michael
    Mannsfeld, Stefan C. B.
    Toney, Michael F.
    Barbero, David R.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Enhanced Vertical Charge Transport in a Semiconducting P3HT Thin Film on Single Layer Graphene2015In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 25, no 5, p. 664-670Article in journal (Refereed)
    Abstract [en]

    The crystallization and electrical characterization of the semiconducting polymer poly(3-hexylthiophene) (P3HT) on a single layer graphene sheet is reported. Grazing incidence X-ray diffraction revealed that P3HT crystallizes with a mixture of face-on and edge-on lamellar orientations on graphene compared to mainly edge-on on a silicon substrate. Moreover, whereas ultrathin (10 nm) P3HT films form well oriented face-on and edge-on lamellae, thicker (50 nm) films form a mosaic of lamellae oriented at different angles from the graphene substrate. This mosaic of crystallites with - stacking oriented homogeneously at various angles inside the film favors the creation of a continuous pathway of interconnected crystallites, and results in a strong enhancement in vertical charge transport and charge carrier mobility in the thicker P3HT film. These results provide a better understanding of polythiophene crystallization on graphene, and should help the design of more efficient graphene based organic devices by control of the crystallinity of the semiconducting film.

  • 8.
    van Reenen, Stephan
    et al.
    Eindhoven University of Technology.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dzwilewski, Andrzej
    Eindhoven University of Technology.
    Janssen, Rene A
    Eindhoven University of Technology.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kemerink, Martijn
    Eindhoven University of Technology.
    Salt concentration effects in planar light-emitting electrochemical cells2011In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 21, no 10, p. 1795-1802Article in journal (Refereed)
    Abstract [en]

    Incorporation of ions in the active layer of organic semiconductor devices may lead to attractive device properties like enhanced injection and improved carrier transport. In this paper, we investigate the effect of the salt concentration on the operation of light-emitting electrochemical cells, using experiments and numerical calculations. The current density and light emission are shown to increase linearly with increasing ion concentration over a wide range of concentrations. The increasing current is accompanied by an ion redistribution, leading to a narrowing of the recombination zone. Hence, in absence of detrimental side reactions and doping-related luminescence quenching, the ion concentration should be as high as possible.

  • 9.
    Wang, Jia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Enevold, Jenny
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Photochemical Transformation of Fullerenes2013In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 23, no 25, p. 3220-3225Article in journal (Refereed)
    Abstract [en]

    Experimental findings and associated theoretical insights regarding the photochemical transformation of fullerenes are reported, which challenge the conventional wisdom in the field and point out a viable path towards improved fullerene-based electronic devices. It is shown that the efficiency of the photochemical monomer-to-dimer transformation of the fullerene [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) is strongly dependent on the light intensity, and this is utilized to demonstrate that direct patterning of an electroactive PCBM film can be effectuated by sub-second UV-light exposure followed by development in a tuned developer solution. By straightforward analytical reasoning, it is demonstrated that the observed intensity-dependent monomer-to-dimer transformation dictates that a significant back-reaction to the ground state must be in effect, which presumably originates from the excited-triplet state. By a combination of numerical modeling and analytical argumentation, it is further shown that the final dimer formation must constitute a bi-excited reaction between two neighboring monomers photo-excited to the triplet state.

  • 10.
    Wang, Jia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Direct UV patterning of electronically active fullerene films2011In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 21, no 19, p. 3723-3728Article in journal (Refereed)
    Abstract [en]

    We utilize UV light for the attainment of high-resolution, electronically active patterns in [6,6]-phenyl C61-butyric acid methyl ester (PCBM) films. The patterns are created by directly exposing selected parts of a solution-cast PCBM film to UV light, and thereafter developing the film by immersing it in a tuned developer solution. We demonstrate that it is possible to attain complex, large-area PCBM structures with a smallest demonstrated-feature size of 1 μm by this method, and that the patterned PCBM material exhibits a high average electron mobility (1.2 × 10−2 cm2 V−1 s−1) in transistor experiments. The employment of UV light for direct patterning of PCBM for electronic applications is attractive, because PCBM exhibits high absorption in the UV range, and no sacrificial photoresist is needed. The patterning is achieved through the transformation by UV light of the soluble PCBM monomers into insoluble dimers with retained attractive electronic properties.

  • 11. Wu, Zhongbin
    et al.
    Luo, Jiajia
    Sun, Ning
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhu, Liping
    Sun, Hengda
    Yu, Ling
    Yang, Dezhi
    Qiao, Xianfeng
    Chen, Jiangshan
    Yang, Chuluo
    Ma, Dongge
    High-Performance Hybrid White Organic Light-Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll-Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter2016In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 26, no 19, p. 3306-3313Article in journal (Refereed)
    Abstract [en]

    Thermally activated delayed fluorescence (TADF)-based white organic light-emitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior efficiency/color rendering index/color stability and low efficiency roll-off simultaneously. In this paper, high-performance hybrid WOLEDs are demonstrated by employing an efficient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum efficiency, current efficiency, and power efficiency of 23.0%, 51.0 cd A(-1), and 51.7 lm W-1, respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m(-2). The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue-green-red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high-performance hybrid WOLEDs.

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