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  • 101.
    Tang, Shi
    et al.
    The Organic Photonics and Electronics Group, Umeå University: LunaLEC AB, Umeå University, Umeå, Sweden Umeå, Sweden Umeå, Sweden.
    Murto, Petri
    Wang, Jia
    The Organic Photonics and Electronics Group, Umeå University, Umeå, Sweden.
    Larsen, Christian
    The Organic Photonics and Electronics Group, Umeå University; LunaLEC AB, Umeå University, Umeå, Sweden Umeå, Sweden Umeå, Sweden.
    Andersson, Mats R.
    Wang, Ergang
    Edman, Ludvig
    The Organic Photonics and Electronics Group, Umeå University; LunaLEC AB, Umeå University, Umeå, Sweden Umeå, Sweden.
    On the Design of Host-Guest Light-Emitting Electrochemical Cells: Should the Guest be Physically Blended or Chemically Incorporated into the Host for Efficient Emission?2019In: Advanced Optical Materials, ISSN 2162-7568, E-ISSN 2195-1071, Vol. 7, no 18, article id 1900451Article in journal (Refereed)
    Abstract [en]

    It has recently been demonstrated that light-emitting electrochemical cells (LECs) can be designed to deliver strong emission with high efficiency when the charge transport is effectuated by a majority host and the emission is executed by a minority guest. A relevant question is then: should the guest be physically blended with or chemically incorporated into the host? A systematic study is presented that establishes that for near-infrared-(NIR-) emitting LECs based on poly(indacenodithieno[3,2-b]thiophene) (PIDTT) as the host and 4,7-bis(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b ']dithiophen-2-yl)benzo[c][1,2,5]-thiadiazole (SBS) as the guest the chemical-incorporation approach is preferable. The host-to-guest energy transfer in LEC devices is highly efficient at a low guest concentration of 0.5%, whereas guest aggregation and ion redistribution during device operation severly inhibits this transfer in the physical-blend devices. The chemical-incorporation approach also results in a redshifted emission with a somewhat lowered photoluminescence quantum yield, but the LEC performance is nevertheless very good. Specifically, an NIR-LEC device comprising a guest-dilute (0.5 molar%) PIDTT-SBS copolymer delivers highly stabile operation at a high radiance of 263 mu W cm(-2) (peak wavelength = 725 nm) and with an external quantum efficiency of 0.214%, which is close to the theoretical limit for this particular emitter and device geometry.

  • 102.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Tvistevagen 47, SE-90719 Umea, Sweden.
    Murto, Petri
    Xu, Xiaofeng
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Tvistevagen 47, SE-90719 Umea, Sweden.
    Wang, Ergang
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Tvistevagen 47, SE-90719 Umea, Sweden.
    Intense and Stable Near-Infrared Emission from Light-Emitting Electrochemical Cells Comprising a Metal-Free Indacenodithieno[3,2-b]thiophene-Based Copolymer as the Single Emitter2017In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 29, no 18, p. 7750-7759Article in journal (Refereed)
    Abstract [en]

    We report on the synthesis, characterization, and application of a series of metal-free near-infrared (NIR) emitting alternating donor/acceptor copolymers based on indacenodithieno[3,2-b]thiophene (IDTT) as the donor unit. A light-emitting electrochemical cell (LEC), comprising a blend of the copolymer poly[indacenodithieno[3,2-b]thiophene-2,8-diyl-alt-2,3-diphenyl-5,8-di(thiophen-2-y1)- quinoxaline-5,5'-diy1] and an ionic liquid as the single-layer active material sandwiched between two air-stable electrodes, delivered NIR emission (lambda(peak) = 705 nm) with a high radiance of 129 mu W/cm(2) when driven by a low voltage of 3.4 V. The NIR-LEC also featured good stress stability, as manifested in that the peak NIR output from a nonencapsulated device after 24 h of continuous operation only had dropped by 3% under N-2 atmosphere and by 27% under ambient air. This work accordingly introduces IDTT-based donor/acceptor copolymers as functional metal-free electroluminescent materials in NIR-emitting devices and also provides guidelines for how future NIR emitters should be designed for further improved performance.

  • 103.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pan, Junyou
    Buchholz, Herwig A.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    White Light from a Single-Emitter Light-Emitting Electrochemical Cell2013In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 135, no 9, p. 3647-3652Article in journal (Refereed)
    Abstract [en]

    We report a novel and generic approach for attaining white light from a single-emitter light-emitting electrochemical cell (LEC). With an active-layer comprising a multifluorophoric conjugated copolymer (MCP) and an electrolyte designed to inhibit MCP energy-transfer interactions during LEC operation, we are able to demonstrate LECs that emit broad-band white light with a color rendering index of 82, a correlated-color temperature of 4000 K, and a current conversion efficacy of 3.8 cd/A. It is notable that this single-emitter LEC configuration eliminates color-drift problems stemming from phase separation, which are commonly observed in conventional blended multiemitter devices. Moreover, the key role of the electrolyte in limiting undesired energy-transfer reactions is highlighted by the observation that an electrolyte-free organic light-emitting diode comprising the same MCP emits red light.

  • 104.
    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.

  • 105.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Andreas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fang, Junfeng
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A Solution-Processed Trilayer Electrochemical Device: Localizing the Light Emission for Optimized Performance2012In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 134, no 34, p. 14050-14055Article in journal (Refereed)
    Abstract [en]

    We present a solution-processed trilayer light-emitting device architecture, comprising two hydrophobic and mobile-ion-containing "transport layers" sandwiching a hydrophilic and ion-free "intermediate layer", which allows for lowered self-absorption, minimized electrode quenching, and tunable light emission. Our results reveal that the transport layers can be doped in situ when a voltage is applied, that the intermediate layer as desired can contribute significantly to the light emission, and that the key to a successful operation is the employment of a porous and (similar to 5-10 nm) thin intermediate layer allowing for facile ion transport. We report that such a solution-processed device, comprising a thick trilayer material (similar to 250 nm) and air-stable electrodes, emits blue light (lambda(peak) = 450, 484 nm) with high efficiency (5.3 cd/A) at a low drive voltage of 5 V.

  • 106.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB.
    Sandström, Andreas
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB.
    Lundberg, Petter
    Lanz, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    LunaLEC AB.
    van Reenen, Stephan
    Kemerink, Martijn
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB.
    Design rules for light-emitting electrochemical cells delivering bright luminance at 27.5 percent external quantum efficiency2017In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 8, article id 1190Article in journal (Refereed)
    Abstract [en]

    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.

  • 107.
    Tang, Shi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tan, Wan-Yi
    Zhu, Xu-Hui
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Small-molecule light-emitting electrochemical cells: evidence for in situ electrochemical doping and functional operation2013In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 49, no 43, p. 4926-4928Article in journal (Refereed)
    Abstract [en]

    We demonstrate that non-ionic small molecules (SMs) can function as the doping and emissive compound in light-emitting electrochemical cells (LECs), and that high brightness and decent efficiency can be attained for such devices. It is plausible that the expansion of the LEC library, to include easy-to-purify and tunable non-ionic SM compounds, could represent a viable path towards improved LEC devices.

  • 108.
    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.

  • 109.
    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.
    Jenssen, Rene A.J.
    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 cellsArticle in journal (Refereed)
  • 110.
    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.
    Rene A. J., Jenssen
    Eindhoven University of Technology.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Martijn, Kemerink
    Eindhoven University of Technology.
    A unifying model for the operation of light-emitting electrochemical cells2010In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 132, no 39, p. 13776-13781Article in journal (Refereed)
    Abstract [en]

    The application of doping in semiconductors plays a major role in the high performances achieved to date in inorganic devices. In contrast, doping has yet to make such an impact in organic electronics. One organic device that does make extensive use of doping is the light-emitting electrochemical cell (LEC), where the presence of mobile ions enables dynamic doping, which enhances carrier injection and facilitates relatively large current densities. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two competing models that seem physically distinct: the electrochemical doping model and the electrodynamic model. Both models are supported by experimental data and numerical modeling. Here, we show that these models are essentially limits of one master model, separated by different rates of carrier injection. For ohmic nonlimited injection, a dynamic p−n junction is formed, which is absent in injection-limited devices. This unification is demonstrated by both numerical calculations and measured surface potentials as well as light emission and doping profiles in operational devices. An analytical analysis yields an upper limit for the ratio of drift and diffusion currents, having major consequences on the maximum current density through this type of device.

  • 111. Vehse, M.
    et al.
    Liu, B.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    Bazan, G.C.
    Heeger, Alan J.
    Light Amplification by Optical Excitation of a Chemical Defect in a Conjugated Polymer2004In: Advanced Materials, Vol. 16, p. 1001-4Article in journal (Refereed)
  • 112.
    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.

  • 113.
    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.

  • 114.
    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.

  • 115.
    Wang, Jia
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Resist-free laser patterning of perfluoro-alkyl functionalized fullerene films: attaining pattern and stability by order2010In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 11, no 10, p. 1595-1604Article in journal (Refereed)
    Abstract [en]

    We report that it is possible to establish well-defined semiconducting patterns in a perfluoro-alkyl functionalized fullerene (C60-F) film using a straightforward, benign and scalable method. The patterning technique comprises a direct laser light exposure of pre-select film areas, and a subsequent development in a heptane developer solution that selectively removes the non-exposed areas of the film. It is notable that no sacrificial photo-resist material is utilized, and that the remaining patterned C60-F material exhibits high electron mobility (>4 × 10−2 cm2/Vs, as quantified in transistor experiments) and improved ambient stability, both in comparison to the pristine material and to the more commonly utilized fullerene PCBM. We demonstrate that the patterning process has left the remaining C60-F material chemically unaltered, but that its degree of crystallinity has increased. The latter rationalizes the high electron mobility, the improved air stability, and the decreased solubility in the developer solution.

  • 116.
    Wågberg, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Hania, Ralph
    Robinson, Nate
    Shin, Joon Ho
    Umeå University, Faculty of Science and Technology, Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    On the Limited Operational Lifetime of Light-Emitting Electrochemical Cells2008In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 20, no 9, p. 1744-1749Article in journal (Refereed)
  • 117.
    Wågberg, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Liu, Bin
    Natl Univ Singapore, Dept Chem & Biomol Engn.
    Orädd, Greger
    Umeå University, Faculty of Science and Technology, Chemistry.
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Chemistry.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    Cationic polyfluorene: conformation and aggregation in a "good" solvent2009In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, ISSN 0014-3057, Vol. 45, no 11, p. 3228-3233Article in journal (Refereed)
    Abstract [en]

    The conjugated polyelectrolyte (CPE) poly{9,9'-bis[6 ''-(N,N,N-trimethylammonium)-hexylfluorene-alt-co-phenylene] dibromide} (PFPN+Br-) demonstrates a high solubility in methanol in comparison to other more hydrophilic or hydrophobic solvents. We have employed a combination of pulsed-field-gradient-NMR, photoluminescence (PL), and Raman spectroscopy to establish the conformation and aggregation behavior of PFPN+Br- in methanol, with the aim to attain information on how to design CPEs with a high solubility in a preferred solvent. We find that the diffusion coefficient and PL spectrum of PFPN+Br-, as well as the Raman-active methyl rocking mode of methanol, all exhibit a strong dependence on PFPN+Br- concentration. We rationalize our findings with a model in which PFPN+Br- forms aggregates via ∏-∏ interactions between main-chain segments, while the ionic side chains are surrounded and electrostatically screened by the methanol solvent. Accordingly, the notably high solubility of PFPN+Br- in methanol is rationalized by favorable interactions between the ionic side chains and the methanol molecules. We propose that an appropriate design of a high-solubility CPE should consider a matching of the mixed hydrophobic/hydrophilic character of the ionic side chain with that of the preferred solvent.

  • 118. Xiong, Wenjing
    et al.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Murto, Petri
    Zhu, Weiguo
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Ergang
    Combining Benzotriazole and Benzodithiophene Host Units in Host-Guest Polymers for Efficient and Stable Near-Infrared Emission from Light-Emitting Electrochemical Cells2019In: Advanced Optical Materials, ISSN 2162-7568, E-ISSN 2195-1071, Vol. 7, no 15, article id 1900280Article in journal (Refereed)
    Abstract [en]

    A set of host-guest copolymers with alternating benzodithiophene and benzotriazole (BTz) derivatives as host units and 4,7-bis(5-bromothiophen-2-yl)-benzo[c][1,2,5]thiadiazole as the minority guest are synthesized, characterized, and evaluated for applications. A light-emitting electrochemical cell (LEC) comprising such a host-guest copolymer delivers fast-response near-infrared (NIR) emission peaked at 723 nm with a high radiance of 169 mu W cm(-2) at a low drive voltage of 3.6 V. The NIR-LEC also features good stability, as the peak NIR output only drops by 8% after 350 h of continuous operation. It is, however, found that the LEC performance is highly sensitive to the detailed chemical structure of the host backbone, and that the addition of electron-donating thiophene bridging units onto the BTz unit is highly positive while the inclusion of fluorine atoms results in a drastically lowered performance, presumably because of the emergence of hydrogen bonding within the active material.

123 101 - 118 of 118
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