Satoru Ohisa

In this study, a millimeter-thick photoresponsive polymer capable of dynamic viscoelastic modulation under blue light irradiation is reported. This star-shaped polymer has a polydimethylsiloxane (PDMS) backbone networked via photoresponsive hexaarylbiimidazole (HABI) and is referred to as PDMS–HABI. By controlling the molecular weight of the PDMS precursor, the density of HABI was tuned, thereby enabling light penetration and photoresponse in samples with thicknesses up to 2 mm. Compression tests revealed that transmittance played a crucial role than the local elastic modulus in determining the extent of compression force reduction upon light irradiation. The optimized sample exhibited a compression force reduction to 1/28 under 430-nm light irradiation. To explore practical applications, a tactile display that could present hard and soft sensations was fabricated. However, as PDMS–HABI lacked shape recovery when exposed to light, it was integrated into an elastic container to endow it with resilience. The compression properties of the container filled with PDMS–HABI were evaluated under light irradiation, and photoinduced softening reduced the compression force by nearly 50%. These results indicated the feasibility of using PDMS–HABI in tactile displays capable of presenting in-plane distributions of hardness and softness, with potential applications in augmented reality/virtual reality haptic interfaces.

Reducing the driving voltage is a long‐term and important challenge in the development of light‐emitting diodes (LEDs) because the light‐emission threshold voltage cannot be below the theoretical limit of the emitter's bandgap divided by the elementary charge. Here, green perovskite nanocrystal (PeNC) LEDs are inverted that emit light at ultralow voltages (below the theoretical limit) realized by efficient Auger‐assisted energy up‐conversion at the interface of the zinc oxide nanoparticles (ZnO NPs) electron injection layer and PeNC light‐emission layers. The up‐conversion utilizes the recombination energy of the charge‐transfer excitons. After modifying the interface by plasma treatment of the ZnO NPs and by inserting an organic interlayer between the ZnO NPs and PeNCs, the device performance is dramatically improved from that of the unmodified PeNC LED. The threshold voltage at which the emission spectrum can be observed is 1.55 V, substantially lower than 2.34 V (the PeNC emitter's bandgap divided by the elementary charge). Moreover, after mixing an organic aromatic semiconductor compound in the PeNC light‐emission layer, the carrier balance is regulated and the threshold voltage is further reduced to 1.30 V.

Quantum dots (QDs) are essential luminescent materials with applications in wide‐color‐gamut displays requiring exceptional color reproducibility. Multinary semiconductor QDs composed of groups I, III and VI elements are expected to serve as eco‐friendly materials to replace conventional QDs owing to the potential narrow spectral widths and tunable bandgaps of the former. Although optimized Ag–In–Ga–S/Ga–S core/shell QDs (AIGS QDs) have exhibited vibrant green emissions, electroluminescence from QD‐based light‐emitting diodes (QLEDs) incorporating these AIGS QDs is reduced as a consequence of the effects of defect sites. The present work therefore examines the incorporation of electron transport materials (ETMs) into AIGS QD emitting layers. A device incorporating emitting layers composed of AIGS QDs and 2,4,6‐tris(3‐(3‐pyridyl)phenyl)‐1,3,5‐triazine (TmPPyTz), with the latter acting as a highly conductive ETM, exhibits a low driving voltage and high efficiency. Furthermore, the addition of two ETMs — TmPPyTz and tris(2,4,6‐trimethyl‐3‐(pyridin‐3‐yl)phenyl)borane — is found to provide enhanced luminescence properties because these materials are deposited in the emitting layer in different forms and hence has varying effects in terms of improving conductivity and charge balance. The resulting QLEDs have a sharp spectral width of 30 nm, suggesting a level of color purity suitable for wide‐color‐gamut displays.

Organic π-conjugated radicals have recently joined the ranks of high-efficiency light-emitting materials; however, their light-emission mechanism is still a matter of debate. Here, the authors highlight a recently proposed luminescent enhancement mechanism and record-breaking efficiency of a radical organic light-emitting diode. Organic π-conjugated radicals have recently joined the ranks of high-efficiency light-emitting materials, however, their light-emission mechanism is still a matter of debate. Here, the authors highlight a recently proposed luminescent enhancement mechanism and record-breaking efficiency of a radical organic light-emitting diode.

Electrical leakage is one of the fatal faults associated with thin‐film devices (TFDs) including organic light‐emitting devices (OLEDs). To prevent electrical leakage, highly smooth electrodes are required to avoid the concentration of the electric field at local points. However, this requires the use of expensive electrode substrates. Here, OLEDs using inexpensive, aluminum‐foil‐based electrode substrates with an extremely high surface roughness are fabricated. A low leakage current is realized by incorporating extraordinary thick buffer layers of phosphotungstic acid (PWA) exhibiting the negative differential resistance (NDR) property. In its pristine state, PWA exhibits a high electrical resistance. However, upon applying electric current, the resistance is significantly decreased, resulting in increased conductivity and expression of NDR property, owing to the formation of conductive filaments and charge‐storage effects. The OLEDs produced with PWA achieve a low‐driving voltage and high external quantum efficiency simultaneously. Aluminum‐foil‐based OLEDs with a light‐emission area of 64 cm² are fabricated. They emit steady luminescence even in crumpled and torn states. To the best of authors’ knowledge, this is the first report of a large‐area OLED exhibiting steady luminescence in such deformed states.

Metal halides doping of perovskite nanocrystals (NCs) has been shown to precisely control nonradiative pathways and to improve photoluminescence quantum yield (PLQY). Here, we report a trivalent lanthanide halide neodymium (III) chloride (NdCl₃)-doped perovskite NCs prepared with a post-synthetic room temperature treatment for efficient blue light-emitting devices (LEDs). The Nd 3d and Cl 2p core peaks were observed in the NdCl₃-doped NCs, which allowed for simultaneous doping of Nd³⁺ and Cl^– into the pristine CsPbBr₃ NCs. The NdCl₃-doped NCs exhibited blue emission at a peak wavelength of 478 nm with a high PLQY of 97% in solution. We found that the Nd³⁺ cation incorporated into the NCs more effectively suppressed nonradiative recombination compared with common halide anion exchange from temperature dependence of optical properties. Blue LEDs based on NdCl₃-doped NCs had an external quantum efficiency of 2.7%, which represents a considerable performance improvement compared with LEDs based on organic chloride salt-doped NCs.

The surface modification of metal halide perovskite nanocrystals (NCs) significantly impacts their optical properties and colloidal stability. This subsequently affects the performance of light-emitting devices (LEDs). Therefore, numerous surface passivation techniques like ligand exchange and metal halide doping have been explored to passivate the surface defects of perovskite NCs and obtain highly efficient LEDs. In this study, we demonstrated the post-synthetic metal halide doping treatment using lead (II) bromide (PbBr2) to passivate the surface defects of the CsPbBr3 NCs at a moderate reaction temperature of 80 °C. The alkyl quaternary ammonium salt, didodecyldimethylammonium bromide (DC12AB), enabled the complete dissolution of PbBr2 in a non-polar solvent, toluene. Due to surface crystal growth, the particle sizes of the PbBr2-doped CsPbBr3 NCs were higher than that of the as-synthesized CsPbBr3 NCs. The photoluminescence quantum yield (PLQY) of the CsPbBr3 NCs drastically increased from 26% to 80% after the PbBr2 doping treatment. Moreover, the PbBr2-doped CsPbBr3 NCs possessed long-term colloidal stability of more than 2 months that indicated the strong bonding between the NCs and ligands. We observed that the alkyl chain length of the quaternary alkyl ammonium salts affected the luminance and device stability during operations. In this study, a promising strategy was devised to achieve highly luminescent perovskite NCs with excellent colloidal stability that can enhance the performance of LEDs.

Metal halide perovskite nanocrystals (NCs) have been established as promising materials for light‐emitting devices (LEDs) due to their interesting optoelectrical properties, including high photoluminescence quantum yield (PLQY) and high color purity with narrow full width at half maximum (FWHM). However, blue perovskite LEDs, with an emission wavelength below 470 nm, require further investigation to improve their performance. Here, it is demonstrated that mixed‐halide CsPb(Cl/Br)₃ NCs prepared by ligand exchange with the rigid bidentate structure of adamantane‐1,3‐diamine (ADDA) can be used for blue LEDs. The ligand exchange of CsPb(Cl/Br)₃ NCs with ADDA improves PLQY without changing the peak wavelength and FWHM. The LEDs based on ligand exchange NCs with ADDA show an external quantum efficiency (EQE) of 0.49%. Moreover, a quaternary ammonium salt, oleylammonium chloride (OAM‐Cl), is also investigated as an interfacial layer to reduce current density. The LED fabricated with OAM‐Cl reaches an EQE of 1.1% and luminance of 43.2 cd m⁻² at the electroluminescence wavelength of 456 nm, representing significantly higher performance compared to that of LEDs without OAM‐Cl layers. Therefore, the ligand exchange of NCs with ADDA and the use of a passivation layer comprising OAM‐Cl facilitate an improvement in the LED performance.

For efficient electron injection, a method to control the work functions (WFs) of ZnO electrodes in organic light-emitting devices (OLEDs) is reported in this study. First, ZnO was modified by doping of tetraalkylammonium salts (TRAX) into polyethylenimine ethoxylated (PEIE) for the WF control. Tetrabutylammonium salts (TBAX), where X = chloride, bromide, iodide, acetate, thiocyanate, and tetrafluoroborate anions, were doped into PEIE. A WF of nondoped PEIE-modified ZnO was 3.65 eV, whereas TBAX-doped PEIE-modified ZnO exhibited WFs ranging from 3.52 to 3.00 eV depending on the anion. TBAX salts exhibited different electron-donating capabilities depending on the anion, and the doping of TBAX with a large electron-donating capability exhibited a large WF reduction effect. In addition, tetraethyl- and tetrahexylammonium chlorides were doped into PEIE. PEIE doped with TRACl containing long alkyl chains exhibited a large WF reduction effect due to its low electron-accepting capabilities. In addition, the WF reduction mechanism was considered by the depth direction analysis of the PEIE:TBAX films. Finally, the ZnO/PEIE:TRAX bilayers were applied as electron injection layers in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] emissive-layer-based OLEDs with an inverted structure. The ZnO/PEIE:TBAX devices with low WFs exhibited low driving voltages.