An Ionic Liquid That Dissolves Semiconducting Polymers: A Promising Electrolyte for Bright, Efficient, and Stable Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are composed of blends of semiconducting polymers and electrolytes, in which a unique cooperative action of ions and electrons induces a dynamic p–n junction for efficient emission. One of the crucial issues remaining in LECs is uniformity in blends of polymer and electrolyte; phase separation in between the two components results in poor performance or failure of operation. Here, we overcome this issue by developing an ionic liquid-based electrolyte of alkylphosphonium-phosphate that shows notable compatibility high enough to dissolve even light-emitting polymers. This exceptional compatibility enabled us to prepare uniform film blends with various blue to red emitting polymers, and offered bright and efficient LECs. Especially, a blue-emitting LEC showed excellent performance: the luminance reached ∼20 000 cd m^–2 with a high luminance efficiency of ∼5 cd A^–1, of which performances significantly exceed a light-emitting diode using the same polymer. The ionic liquid was further applied to the LECs with state-of-the-art light-emitting dendrimers showing thermally activated delayed fluorescence under electrical excitation, giving a high efficiency of 11 cd A^–1. These demonstrations remind us of the great importance of the polymer–electrolyte compatibility and the usefulness of ILs for electrolyte of LECs

An Ionic Liquid That Dissolves Semiconducting Polymers: A Promising Electrolyte for Bright, Efficient, and Stable Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are composed of blends of semiconducting polymers and electrolytes, in which a unique cooperative action of ions and electrons induces a dynamic p–n junction for efficient emission. One of the crucial issues remaining in LECs is uniformity in blends of polymer and electrolyte; phase separation in between the two components results in poor performance or failure of operation. Here, we overcome this issue by developing an ionic liquid-based electrolyte of alkylphosphonium-phosphate that shows notable compatibility high enough to dissolve even light-emitting polymers. This exceptional compatibility enabled us to prepare uniform film blends with various blue to red emitting polymers, and offered bright and efficient LECs. Especially, a blue-emitting LEC showed excellent performance: the luminance reached ∼20 000 cd m^–2 with a high luminance efficiency of ∼5 cd A^–1, of which performances significantly exceed a light-emitting diode using the same polymer. The ionic liquid was further applied to the LECs with state-of-the-art light-emitting dendrimers showing thermally activated delayed fluorescence under electrical excitation, giving a high efficiency of 11 cd A^–1. These demonstrations remind us of the great importance of the polymer–electrolyte compatibility and the usefulness of ILs for electrolyte of LECs

Highly Flexible MoS₂ Thin-Film Transistors with Ion Gel Dielectrics

Molybdenum disulfide (MoS₂) thin-film transistors were fabricated with ion gel gate dielectrics. These thin-film transistors exhibited excellent band transport with a low threshold voltage (<1 V), high mobility (12.5 cm²/(V·s)) and a high on/off current ratio (10⁵). Furthermore, the MoS₂ transistors exhibited remarkably high mechanical flexibility, and no degradation in the electrical characteristics was observed when they were significantly bent to a curvature radius of 0.75 mm. The superior electrical performance and excellent pliability of MoS₂ films make them suitable for use in large-area flexible electronics

Fluorescent Ferroelectrics of Hydrogen-Bonded Pyrene Derivatives

Organic materials with diverse molecular designs show multifunctional properties such as coupled ferroelectric, optical, ferromagnetic, and transport properties. We report the design of an alkylamide-substituted pyrene derivative displaying fluorescent ferroelectric properties coupled with electron transport properties. In solution phase, this compound displayed concentration-dependent fluorescence, whereas in xerogels, a fluorescent green organogel (>0.1 mM) and entangled nanofibers were observed. A discotic hexagonal columnar liquid crystalline phase was observed above 295 K due to intermolecular hydrogen bonding and π-stacking interactions. The direction of the hydrogen-bonded chains could be inverted by the application of an external electric field along the π-stacked column, resulting in ferroelectric polarization-electric field (<i>P</i>–<i>E</i>) hysteresis. The local electric field arising from the ferroelectric macrodipole moment arrangement along the π-stacking direction affected the electron transport properties on the π-stack of pyrenes, thus confirming the current-switching phenomena according to <i>P</i>–<i>E</i> hysteresis. We report that multifunctional properties such as ferroelectricity, fluorescence, and electron transport switching were successfully achieved in hydrogen-bonded dynamic π-molecular assemblies

Tuning of the Thermoelectric Properties of One-Dimensional Material Networks by Electric Double Layer Techniques Using Ionic Liquids

We report across-bandgap p-type and n-type control over the Seebeck coefficients of semiconducting single-wall carbon nanotube networks through an electric double layer transistor setup using an ionic liquid as the electrolyte. All-around gating characteristics by electric double layer formation upon the surface of the nanotubes enabled the tuning of the Seebeck coefficient of the nanotube networks by the shift in gate voltage, which opened the path to Fermi-level-controlled three-dimensional thermoelectric devices composed of one-dimensional nanomaterials

Highly Fluorescent [7]Carbohelicene Fused by Asymmetric 1,2-Dialkyl-Substituted Quinoxaline for Circularly Polarized Luminescence and Electroluminescence

A new 1,2-dialkylquinoxaline-fused [7]­carbohelicene ([7]­Hl-NAIQx) was designed and synthesized by asymmetrically introducing two alkyl chains onto the quinoxaline unit. Direct alkylation of the quinoxaline ring of quinoxaline-fused helicene leads to discontinuity in the conjugated structure. In the single-crystal analysis, the parent quinoxaline-fused [7]­carbohelicene ([7]­Hl-Qx) was found to have a helical structure formed by two phenanthrene units and a nonplanar twisted angle between the phenanthrene and quinoxaline units. In contrast, [7]­Hl-NAIQx possesses an almost planar aromatic structure between the alkyl-quinoxaline and phenanthrene units (torsion angle: 179°), in addition to the similar helical structure between the two phenanthrene units. The steady-state absorption, fluorescence, and circular dichroism (CD) spectra of [7]­Hl-NAIQx were significantly red-shifted compared to those of [7]­Hl-Qx and [7]­carbohelicene ([7]­Hl). These spectral changes were mainly explained by electrochemical measurements and density functional theory calculations. Moreover, the absolute fluorescence quantum yield (Φ_FL) of [7]­Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]­Hl (Φ_FL = 0.02). Such a large enhancement of the fluorescence of [7]­Hl-NAIQx has provided excellent circularly polarized luminescence (CPL). The value of the anisotropy factor <i>g</i>_lum (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 4.0 × 10^–3. The electroluminescence of an organic light-emitting diode utilizing [7]­Hl-NAIQx was successfully observed

Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units

A series of [5]­carbohelicene derivatives substituted by electron-withdrawing maleimide and electron-donating methoxy, such as maleimide-substituted [5]­carbohelicene (HeliIm) and methoxy-substituted HeliIm (MeO-HeliIm), were newly designed and synthesized to examine the electrochemical properties, excited-state dynamic and circularly polarized luminescence (CPL). First, electrochemical measurements and DFT calculations of [5]­carbohelicene derivatives were performed by comparing with the structural isomers: picene derivatives. Introduction of an electron-withdrawing maleimide group onto a [5]­carbohelicene core contributes to the stabilized LUMO state in HeliIm as compared to that of [5]­carbohelicene (Heli), whereas the energy level of HOMO state in MeO-HeliIm increases by introducing electron-donating methoxy (MeO) groups onto a HeliIm skeleton. The HOMO–LUMO gap of MeO-HeliIm is smaller than those of HeliIm and Heli, which is similar to the steady-state spectroscopic measurements. The absolute fluorescence quantum yield (Φ_FL) of HeliIm (0.37) largely increased as compared to [5]­carbohelicene, Heli (0.04), whereas Φ_FL of MeO-HeliIm (0.22) was slightly smaller than that of HeliIm. Theses photophysical processes including intersystem crossing are successfully explained by the kinetic discussions. Since [5]­carbohelicene derivatives show the chirality, measurements of circular dichroism (CD) and circularly polarized luminescence (CPL) were successfully performed. In particular, HeliIm and MeO-HeliIm have provide excellent circularly polarized luminescence (CPL) and the values of the anisotropy factor <i>g</i>_lum were estimated to be ∼2.4 × 10^–3 and ∼2.3 × 10^–3, relatively. This is the first observation of CPL in [5]­carbohelicene derivatives

Controlled Excited-State Dynamics and Enhanced Fluorescence Property of Tetrasulfone[9]helicene by a Simple Synthetic Process

Tetrasulfone­[9]­helicene (PTSH) was newly synthesized to improve and evaluate its fluorescence and excited-state dynamics through a single-step oxidation reaction of tetrathia[9]­helicene (PTTH). In electrochemical measurements, the reduction potential of PTSH was shifted in a positive direction by approximately 1.0 V when compared to that of PTTH because of its electron-accepting sulfone units. The results of the electrochemical measurements agree with the energy levels calculated by density functional theory (DFT) methods and steady-state spectroscopic measurements. Furthermore, a significant enhancement of the absolute fluorescence quantum yield (Φ_FL) was achieved. The absolute fluorescence quantum yield of PTSH attained 0.27, which is approximately 10 times larger than that of PTTH (Φ_FL = 0.03). Such an enhancement of Φ_FL can be successfully explained by the corresponding kinetic comparison. The reason is mainly the increased energy gap Δ<i><i>E</i></i>_ST between the lowest singlet (S₁) and triplet (T₁) excited states. Finally, excellent circularly polarized luminescence of PTSH was also observed. The value of the anisotropy factor <i>g</i>_CPL was estimated to be 8.3 × 10^–4 in PTSH

Highly Efficient and Stable Perovskite Solar Cells by Interfacial Engineering Using Solution-Processed Polymer Layer

Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly­(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices

Large-Area Synthesis of Highly Crystalline WSe₂ Monolayers and Device Applications

The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics, and optoelectronic devices. Recent reports have demonstrated the growth of large-size two-dimensional MoS₂ layers by the sulfurization of molybdenum oxides. However, the growth of a transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO₃, where large-size WSe₂ monolayer flakes or thin films can be successfully grown. The top-gated field-effect transistors based on WSe₂ monolayers using ionic gels as the dielectrics exhibit ambipolar characteristics, where the hole and electron mobility values are up to 90 and 7 cm²/Vs, respectively. These films can be transferred onto arbitrary substrates, which may inspire research efforts to explore their properties and applications. The resistor-loaded inverter based on a WSe₂ film, with a gain of ∼13, further demonstrates its applicability for logic-circuit integrations