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  • 1.
    Annamalai, Alagappan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mühlbacher, Inge
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Influence of Sb5+ as a Double Donor on Hematite (Fe3+) Photoanodes for Surface-Enhanced Photoelectrochemical Water Oxidation2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 19, p. 16467-16473Article in journal (Refereed)
    Abstract [en]

    To exploit the full potential of hematite (α-Fe2O3) as an efficient photoanode for water oxidation, the redox processes occurring at the Fe2O3/electrolyte interface need to be studied in greater detail. Ex situ doping is an excellent technique to introduce dopants onto the photoanode surface and to modify the photoanode/electrolyte interface. In this context, we selected antimony (Sb5+) as the ex situ dopant because it is an effective electron donor and reduces recombination effects and concurrently utilize the possibility to tuning the surface charge and wettability. In the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes, as confirmed by X-ray photoelectron spectroscopy, we observed a 10-fold increase in carrier concentration (1.1 × 1020 vs 1.3 × 1019 cm–3) and decreased photoanode/electrolyte charge transfer resistance (∼990 vs ∼3700 Ω). Furthermore, a broad range of surface characterization techniques such as Fourier-transform infrared spectroscopy, ζ-potential, and contact angle measurements reveal that changes in the surface hydroxyl groups following the ex situ doping also have an effect on the water splitting capability. Theoretical calculations suggest that Sb5+ can activate multiple Fe3+ ions simultaneously, in addition to increasing the surface charge and enhancing the electron/hole transport properties. To a greater extent, the Sb5+- surface-doped determines the interfacial properties of electrochemical charge transfer, leading to an efficient water oxidation mechanism.

  • 2.
    Chateau, Denis
    et al.
    Laboratoire de Chimie, Université de Lyon, Université Claude Bernard Lyon 1, ENS Lyon, CNRS UMR5182, Lyon 69364, France.
    Chaput, Frederic
    Laboratoire de Chimie, Université de Lyon, Université Claude Bernard Lyon 1, ENS Lyon, CNRS UMR5182, Lyon 69364, France.
    Lopes, Cesar
    Information Systems, Swedish Defence Research Agency (FOI), Linköping SE-581 11, Sweden.
    Lindgren, Mikael
    Department of Physics, Norwegian University of Science and Technology, Trondheim NO-7491, Norway.
    Brännlund, Carl
    Information Systems, Swedish Defence Research Agency (FOI), Linköping SE-581 11, Sweden.
    Öhgren, Johan
    Information Systems, Swedish Defence Research Agency (FOI), Linköping SE-581 11, Sweden.
    Djourelov, Nikolay
    Institut de Physique Nucléaire, Université de Lyon, Université Claude Bernard Lyon 1, CNRS, Villeurbanne, France.
    Nedelec, Patrick
    Institut de Physique Nucléaire, Université de Lyon, Université Claude Bernard Lyon 1, CNRS, Villeurbanne, France.
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Kindahl, Tomas
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lerouge, Frederic
    Laboratoire de Chimie, Université de Lyon, Université Claude Bernard Lyon 1, ENS Lyon, CNRS UMR5182, Lyon 69364, France.
    Andraud, Chantal
    Laboratoire de Chimie, Université de Lyon, Université Claude Bernard Lyon 1, ENS Lyon, CNRS UMR5182, Lyon 69364, France.
    Parola, Stephane
    Laboratoire de Chimie, Université de Lyon, Université Claude Bernard Lyon 1, ENS Lyon, CNRS UMR5182, Lyon 69364, France.
    Silica hybrid sol-gel materials with unusually high concentration of Pt-organic molecular guests: Studies of luminescence and nonlinear absorption of light2012In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 4, no 5, p. 2369-2377Article in journal (Refereed)
    Abstract [en]

    The development of new photonic materials is a key step towards improvement of existing optical devices and for the preparation of a new generation of systems. Therefore synthesis of photonic hybrid materials with a thorough understanding and control of the microstructure-to-properties relationships is crucial. In this perspective, a new preparation method based on fast gelation reactions using simple dispersion of dyes without strong covalent bonding between dye and matrix has been developed. This new sol-gel method is demonstrated through synthesis of monolithic siloxane-based hybrid materials highly doped by various platinum(II) acetylide derivatives. Concentrations of the chromophores as high as 400 mM were obtained and resulted in unprecedented optical power limiting (OPL) performance at 532 nm of the surface-polished solids. Static and time-resolved photoluminescence of the prepared hybrid materials were consistent with both OPL data and previous studies of similar Pt(II) compounds in solution. The impacts of the microstructure and the chemical composition of the matrix on the spectroscopic properties, are discussed.

  • 3.
    Elhaj, Ahmed
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Irgum, Knut
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Monolithic space-filling porous materials from engineering plastics by thermally induced phase separation2014In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 6, no 18, p. 15653-15666Article in journal (Refereed)
    Abstract [en]

    Six different uncompounded engineering and commodity polymers were evaluated for their ability to produce space-filling monolithic entities by thermally induced phase separation (TIPS) from 22 different solvents. Attempts were first made to dissolve the polymers at elevated temperatures, selected below the boiling point of each solvent. Then the solutions of polymers that were homogeneous dissolved underwent a controlled temperature decrease to induce a phase separation as the upper critical solution temperature was passed. Twelve of the solvents gave monolithic entities by this procedure, materials that were characterized with regard to their specific surface area and pore size distribution. These measured parameters were then correlated with their macroporous morphology, assessed by scanning electron microscopy. Monolithic materials with widely different mesoporous properties were obtained with specific surface areas ranging from 169 m(2)/g to structures with essentially nonporous skeletons and distinct mesopore size distribution modes from 6 to 15 nm. The materials furthermore had a wide variation in their macroporous morphologies-among the same polymer processed in different solvents and between different polymers dissolved in the same solvent. TIPS processing therefore appears to be a viable route to prepare space-filling meso- and macroporous support materials for a wide variety of purposes in separation science and heterogeneous chemistry.

  • 4. Gerz, Isabelle
    et al.
    Lindh, E. Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thordarson, Pall
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kullgren, Jolla
    Mindemark, Jonas
    Oligomer Electrolytes for Light-Emitting Electrochemical Cells: Influence of the End Groups on Ion Coordination, Ion Binding, and Turn-on Kinetics2019In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 11, no 43, p. 40372-40381Article in journal (Refereed)
    Abstract [en]

    The electrolyte is an essential constituent of the light-emitting electrochemical cell (LEC), since its operating mechanism is dependent on the redistribution of mobile ions in the active layer. Recent developments of new ion transporters have yielded high-performance devices, but knowledge about the interactions between the ionic species and the ion transporters and the influence of these interactions on the LEC performance is lacking. We therefore present a combined computational and experimental effort that demonstrates that the selection of the end group in a star-branched oligomeric ion transporter based on trimethylolpropane ethoxylate has a paramount influence on the ionic interactions in the electrolyte and thereby also on the performance of the corresponding LECs. With hydroxyl end groups, the the salt is strongly coordinated to the ion transporter, which leads to suppression of ion pairing, but the penalty is a hindered ion release and a slow turn-on for the LEC devices. With methoxy end groups, an intermediate coordination strength is seen together with the formation of contact ion pairs, but the LEC performance is very good with fast turn-on. Using a series of ion transporters with alkyl carbonate end groups, the ion transporter:cation coordination strength is lowered further, but the turn-on kinetics are slower than what is seen for devices comprising the methoxy end-capped ion transporter.

  • 5.
    Jia, Xueen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tai, Cheuk-Wai
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Synthesis of Palladium/Helical Carbon Nanofiber Hybrid Nanostructures and Their Application for Hydrogen Peroxide and Glucose Detection2013In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 5, no 22, p. 12017-12022Article in journal (Refereed)
    Abstract [en]

    We report on a novel sensing platform for H2O2 and glucose based on immobilization of palladium-helical carbon nanofiber (Pd-HCNF) hybrid nanostnictures and glucose oxidase (GOx) with Nafion on a glassy carbon electrode (GCE). HCNFs were synthesized by a chemical vapor deposition process on a C-60-supported Pd catalyst. Pd-HCNF nanocomposites were prepared by a one-step reduction free method in dimethylformamide (DMF). The prepared materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The Nafion/Pd-HCNF/GCE sensor exhibits excellent electrocatalytic sensitivity toward H2O2 (315 mA M-1 cm(-2)) as probed by cyclic voltammetry (CV) and chronoamperometry. We show that Pd-HCNF-modified electrodes significantly reduce the overpotential and enhance the electron transfer rate. A linear range from 5.0 mu M to 2.1 mM with a detection limit of 3.0 mu M (based on the S/N = 3) and good reproducibility were obtained. Furthermore, a sensing platform for glucose was prepared by immobilizing the Pd-HCNFs and glucose oxidase (GOx) with Nafion on a glassy carbon electrode. The resulting biosensor exhibits a good response to glucose with a wide linear range (0.06-6.0 mM) with a detection limit of 0.03 mM and a sensitivity of 13 mA M-1 cm(-2). We show that small size and homogeneous distribution of the Pd nanoparticles in combination with good conductivity and large surface area of the HCNFs lead to a H2O2 and glucose sensing platform that performs in the top range of the herein reported sensor platforms.

  • 6.
    Jin, Xu
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; School of Mechanical Engineering, Dongguan University of Technology, Dongguan 523808, China.
    Sandström, Andreas
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Linnaeus Vag 24, SE-901 87 Umeå, Sweden.
    Lindh, E. Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yang, Wei
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Linnaeus Vag 24, SE-901 87 Umeå, Sweden.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Linnaeus Vag 24, SE-901 87 Umeå, Sweden.
    Challenging conventional wisdom: finding high-performance electrodes for light-emitting electrochemical cells2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 39, p. 33380-33389Article in journal (Refereed)
    Abstract [en]

    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.

  • 7.
    Kaihovirta, Nikolai
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Improving the Performance of Light-Emitting Electrochemical Cells by Optical Design2014In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 6, no 4, p. 2947-2954Article in journal (Refereed)
    Abstract [en]

    The organic light-emitting electrochemical cell (LEG) has emerged as an enabling technology for a wide range of novel and low-cost emissive applications, but its efficiency is still relatively modest. The focus in the field has so far almost exclusively been directed toward limiting internal loss mechanisms, whereas external losses resulting from poor light-outcoupling have been overlooked. Here, we report a straightforward procedure for improving the efficiency and emission quality of LECs. We find that our high-performance glass-encapsulated LECs exhibit a near-ideal Lambertian emission profile but that total internal reflection at the glass/air interface and a concomitant edge emission and self-absorption represent a significant loss factor. We demonstrate a 60% improvement in the outcoupled luminance in the forward direction by laminating a light-outcoupling film, featuring a hexagonal array of hemispherical microlenses as the surface structure, onto the front side of the device and a large-area metallic reflector onto the back side. With this scalable approach, yellow-emitting LEC devices with a power conversion efficiency of more than 15 lm W-1 at a luminance of 100 cd m(-2) were realized. Importantly, we find that the same procedure also can mitigate problems with spatial variation in the light-emission intensity, which is a common and undesired feature of large-area LECs.

  • 8. Liu, Yu Ping
    et al.
    Guo, Si-Xuan
    Ding, Liang
    Ohlin, C. Andre
    Bond, Alan M.
    Zhang, Jie
    Lindqvist Polyoxoniobate Ion-Assisted Electrodeposition of Cobalt and Nickel Water Oxidation Catalysts2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 30, p. 16632-16644Article in journal (Refereed)
    Abstract [en]

    A method has been developed for the efficient electrodeposition of cobalt and nickel nanostructures with the assistance of the Lindqvist ion [Nb6O19](8). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry, and a range of electrochemical techniques have been used to characterize the morphology, composition, catalytic water oxidation activity and stability of the films in alkaline solution. SEM images show that films consisting of nanoparticles with diameters of ca. 30 to 40 nm are formed after 40-50 potential cycles of deposition. Nb and Co/Ni are detected in the films by EDX. ICP-MS results show an elemental ratio of 1:1 for Co:Nb and 1:3 for Ni:Nb, respectively. Raman spectra reveal the presence of both [Nb6O19](8) and Co(OH)(2)/Ni(OH)(2). The films exhibit excellent stability and efficiency for electrocatalytic water oxidation in alkaline solution. Turnover frequencies of 12.9 and 13.2 s(-1) were determined by rotating ring disk electrode voltammetry at an overpotential of 480 mV for Co and Ni films, respectively. Fourier transformed large amplitude alternating current (FTAC) voltammetry reveals an additional underlying oxidation process for Co under catalytic turnover conditions, which indicates that a Co-IV species is involved in the efficient catalytic water oxidation reactions. FTAC voltammetric data also suggest that the Ni films undergoes a clear phase transformation upon aging in aqueous 1 M NaOH and the electrogenerated higher oxidation state Ni from beta-NiOOH is the more active form of the catalyst.

  • 9.
    Lundberg, Petter
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Lindh, Mattias
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Toward Efficient and Metal-Free Emissive Devices: A Solution Processed Host Guest Light-Emitting Electrochemical Cell Featuring Thermally Activated Delayed Fluorescence2017In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 34, p. 28810-28816Article in journal (Refereed)
    Abstract [en]

    The next generation of emissive devices should preferably be efficient, low-cost, and environmentally sustainable, and as such utilize all electrically generated excitons (both singlets and triplets) for the light emission, while being free from rare metals such as iridium. Here, we report on a step toward this vision through the design, fabrication, and operation of a host guest light-emitting electrochemical cell (LEC) featuring an organic thermally activated delayed fluorescence (TADF) guest that harvests both singlet and triplet excitons for the emission. The rare-metal-free active material also consists of a polymeric electrolyte and a polymeric compatibilizer for the facilitation of a cost-efficient and scalable solution-based fabrication, and for the use of air-stable electrodes. We report that such TADF-LEC devices can deliver uniform green light emission with a maximum luminance of 228 cd m(-2) when driven by a constant-current density of 770 A m(-2), and 760 cd m(-2) during a voltage ramp, which represents a one-order-of-magnitude improvement in comparison to previous TADF-emitting LECs.

  • 10.
    Sharifi, Tiva
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Comprehensive study of an earth-abundant bifunctional 3D electrode for efficient water electrolysis in alkaline medium2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 51, p. 28148-28155Article in journal (Refereed)
    Abstract [en]

    We report efficient electrolysis of both water splitting half reactions in the same medium by a bifunctional 3D electrode comprising Co3O4 nanospheres nucleated on the surface of nitrogen-doped carbon nanotubes (NCNTs) that in turn are grown on conductive carbon paper (CP). The resulting electrode exhibits high stability and large electrochemical activity for both oxygen and hydrogen evolution reactions (OER and HER). We obtain a current density of 10 mA/cm(2) in 0.1 M KOH solution at overpotentials of only 0.47 and 0.38 V for OER and HER, respectively. Additionally, the experimental observations are understood and supported by analyzing the Co3O4:NCNT and NCNT:CP interfaces by ab initio calculations. Both the experimental and the theoretical studies indicate that firm and well-established interfaces along the electrode play a crucial role on the stability and electrochemical activity for both OER and HER.

  • 11. Subramanian, Arunprabaharan
    et al.
    Annamalai, Alagappan
    Division of Biotechnology, Safety, Environment and Life Science Institute, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan, Republic of Korea.
    Lee, Hyun Hwi
    Choi, Sun Hee
    Ryu, Jungho
    Park, Jung Hee
    Jang, Jum Suk
    Trade-off between Zr Passivation and Sn Doping on Hematite Nanorod Photoanodes for Efficient Solar Water Oxidation: Effects of a ZrO2 Underlayer and FTO Deformation2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 30, p. 19428-19437Article in journal (Refereed)
    Abstract [en]

    Herein we report the influence of a ZrO2 underlayer on the PEC (photoelectrochemical) behavior of hematite nanorod photoanodes for efficient solar water splitting. Particular attention was given to the cathodic shift in onset potential and photocurrent enhancement. Akaganite (beta-FeOOH) nanorods were grown on ZrO2-coated FTO (fluorine-doped tin oxide) substrates. Sintering at 800 degrees C transformed akaganite to the hematite (alpha-Fe2O3) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrates and surface migration, shallow doping of Zr atoms from the ZrO2 underlayer. The ZrO2 underlayer-treated photoanode showed better water oxidation performance compared to the pristine (alpha-Fe2O3) photoanode. A cathodic shift in the onset potential and photocurrent enhancement was achieved by surface passivation and shallow doping of Zr from the ZrO2 underlayer, along with Sn doping from the FTO substrate to the crystal lattice of hematite nanorods. The Zr based hematite nanorod photoanode achieved 1 mA/cm(2) at 1.23 V-RHE with a low turn-on voltage of 0.80 V-RHE. Sn doping and Zr passivation, as well as shallow doping, were confirmed by XPS, I-ph, and M-S plot analyses. Electrochemical impedance spectroscopy revealed that the presence of a ZrO2 underlayer decreased the deformation of FTO substrate, improved electron transfer at the hematite/FTO interface and increased charge-transfer resistance at the electrolyte/hematite interface. This is the first systematic investigation of the effects of Zr passivation, shallow doping, and Sn doping on hematite nanorod photoanodes through application of a ZrO2 underlayer on the FTO substrate.

  • 12. Swaminathan, Jayashree
    et al.
    Puthirath, Anand B.
    Sahoo, Mihir Ranjan
    Nayak, Saroj Kumar
    Costin, Gelu
    Vajtai, Robert
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ajayan, Pulickel M.
    Tuning the Electrocatalytic Activity of Co3O4 through Discrete Elemental Doping2019In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 11, no 43, p. 39706-39714Article in journal (Refereed)
    Abstract [en]

    To gain constructive insight into the possible effect of doping on the electrocatalytic activity of materials, a catalytic framework with a discrete distribution of dopants is an appropriate model system. Such a system assures well-defined active centers, maximum atom utilization efficiency, and hence enhanced selectivity, catalytic activity, and stability. Herein, a comprehensive investigation of the electrocatalytic activity of iron-doped cobalt oxide (Fe-Co3O4) nanosheets is presented. In order to understand the contribution of dopants, a series of materials with controlled doping levels are investigated. By controlled iron inclusion into the structure of Co3O4, an apparent improvement in the oxygen evolution reaction activity which is reflected in the decrease of 160 mV in the overpotential to reach the current density of 10 mA/cm(2) is manifested. Additionally, it is shown that there exists an optimum doping content above which the catalytic activity fades. Further investigation of the system with density functional calculations reveals that, along with the optimization of adsorption energy toward the reaction intermediates, substantial downshift of the Fermi level and delocalization of electron density occurs on introducing iron ions into the structure.

  • 13.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Umeå, Sweden.
    Buchholz, Herwig A
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Umeå, Sweden.
    White Light from a Light-Emitting Electrochemical Cell: Controlling the Energy-Transfer in a Conjugated Polymer/Triplet-Emitter Blend2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 46, p. 25955-25960Article in journal (Refereed)
    Abstract [en]

    We report on the attainment of broadband white light emission from a hostguest light-emitting electrochemical cell, comprising a blue-emitting conjugated polymer as the majority host and a red-emitting small-molecule triplet emitter as the minority guest. An analysis of the energy structure reveals that host-to-guest energy transfer can be effectuated by both Forster and Dexter processes, and through a careful optimization of the active material composition partial energy transfer and white emission is accomplished at a low guest concentration of 0.5%. By adding a small amount of a yellow-emitting conjugated polymer to the active material, white light emission with a high color rendering index of 79, and an efficiency of 4.3 cd/A at significant luminance (>200 cd/m2), is realized.

  • 14.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pan, Junyou
    Merck KGaA.
    Buchholz, Herwig
    Merck KGgA.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    White light-emitting electrochemical cell2011In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 3, no 9, p. 3384-3388Article in journal (Refereed)
    Abstract [en]

    We report a light-emitting electrochemical cell (LEC) with air-stable electrodes and a solution-processed single-layer active material that emits warm-white light (CIE coordinates = (0.39, 0.43); color rendering index =83) with significant brightness (B) at a low voltage. The carefully tuned trichromatic device exhibits a short turn-on time (5 s to B > 100 cd/m2), high efficiency (3.1 cd/A at B = 240 cd/m2) and good operational stability (17 h at B > 100 cd/m2). We also report a blue LEC with a highly respectable set of device properties in the form of a turn-on time of 5 s, an efficiency of 3.6 lm/W and 5.6 cd/A, and an uninterrupted operational lifetime of 25 h. Finally, by analyzing data from trichromatic and monochromatic devices as well as from the constituent fluorescent CPs, we are able to point out a viable path toward further improvements in the performance of the white-emitting LEC.

  • 15.
    Wang, Jia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, SE-90719 Umea, Sweden.
    Sandström, Andreas
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, SE-90719 Umea, Sweden.
    Combining an Ionic Transition Metal Complex with a Conjugated Polymer for Wide-Range Voltage-Controlled Light-Emission Color2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 4, p. 2784-2789Article in journal (Refereed)
    Abstract [en]

    We report on voltage-controlled electroluminescence (EL) over a broad range of colors from a two-luminophor (2L) light-emitting electrochemical cell (LEC), comprising a blend of a majority blue-emitting conjugated polymer (blue-CP), a minority red-emitting ionic transition metal complex (red-iTMC), and an ion-transporting compound as the active layer. The EL color is reversibly shifted from red, over orange, pink, and white, to blue by simply changing the applied voltage from 3 to 7 V. An analysis of our results suggests that the low concentration of immobile cations intrinsic to this particular device configuration controls the electron injection and thereby the EL color: at low voltage, electrons are selectively injected into the low-barrier minority red-iTMC, but with increasing voltage the injection into the high-barrier majority blue-CP is gradually improved.

  • 16. Wu, Zhongbin
    et al.
    Sun, Ning
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhu, Liping
    Sun, Hengda
    Wang, Jiaxiu
    Yang, Dezhi
    Qiao, Xianfeng
    Chen, Jiangshan
    Alshehri, Saad M.
    Ahamad, Tansir
    Ma, Dongge
    Achieving Extreme Utilization of Excitons by an Efficient Sandwich-Type Emissive Layer Architecture for Reduced Efficiency Roll-Off and Improved Operational Stability in Organic Light-Emitting Diodes2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 5, p. 3150-3159Article in journal (Refereed)
    Abstract [en]

    It has been demonstrated that the efficiency roll-off is generally caused by the accumulation of excitons or charge carriers, which is intimately related to the emissive layer (EML) architecture in organic light-emitting diodes (OLEDs). In this article, an efficient sandwich-type EML structure with a mixed-host EML sandwiched between two single-host EMLs was designed to eliminate this accumulation, thus simultaneously achieving high efficiency, low efficiency roll-off and good operational stability in the resulting OLEDs. The devices show excellent electroluminescence performances, realizing a maximum external quantum efficiency (EQE) of 24.6% with a maximum power efficiency of 105.6 lm W-1 and a maximum current efficiency of 93.5 cd A(-1). At the high brightness of 5 000 cd m(-2), they still remain as high as 23.3%, 71.1 lm W-1, and 88.3 cd A(-1), respectively. And, the device'lifetime is up to 2000 h at initial luminance of 1000 cd m(-2), which is significantly higher than that of compared devices with conventional EML structures. The improvement mechanism is systematically studied by the dependence of the exciton distribution in EML and the exciton quenching processes. It can be seen that the utilization of the efficient sandwich-type EML broadens the recombination zone width, thus greatly reducing the exciton quenching and increasing the probability of the exciton recombination. It is believed that the design concept, provides a new avenue for us to achieve high-performance OLEDs.

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