Inkjet‐Printed Self‐Hosted TADF Polymer Light‐Emitting Diodes

Thermally activated delayed fluorescent (TADF) materials are extensively investigated as organic light-emitting diodes (OLEDs) with TADF emitting layers demonstrating high efficiency without the use of heavy metal complexes. Therefore, solution-processable and printable TADF emitters are highly desirable, moving away from expensive vacuum deposition techniques. In addition, using emissive materials not requiring an external host simplifies the fabrication process significantly. Herein, OLEDs using a solution-processable TADF polymer that do not need an external host are introduced. The non-conjugated TADF polymer features a TADF emitter (4-(9H-carbazol-9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)-isoindoline-1,3-dione) as a side chain, as well as a hole-transporting side chain and an electron-transporting side chain on an inactive polymer backbone. All organic layers of the OLEDs are fabricated using solution processing methods. The OLEDs with inkjet-printed emissive layers have comparable maximum current and external quantum efficiency as their spin-coated counterparts, exceeding luminance of 2000 cd m$^{-2}$. The herein-explored strategy is a viable route toward self-hosted printable TADF OLEDs

Structural changes of water molecules during photoelectrochemical water oxidation on TiO2 thin film electrodes

Behaviors of photogenerated charge carriers and structural changes of water molecules on TiO2 photoelectrodes were investigated by using time-resolved visible to mid-IR absorption spectroscopy. From the spectra measured in the visible to NIR region, it was shown that the lifetime of trapped electrons and holes becomes longer upon applying more positive potentials. This result was reasonably explained by the enhancement of the upward band bending at the water/TiO2 interface. On the other hand, from the spectra measured in the mid-IR region, structural changes of the water molecules were observed. When a TiO2 electrode was photoexcited at the potential where the water oxidation starts, a new absorption peak appeared at 3620 cm−1 with a slight decrease in the intensity of hydrogen-bonded water. This new peak was assigned to the isolated O–H band of water molecules. Usually, TiO2 surfaces exhibit super-hydrophilic properties with strong hydrogen-bonding; however, the obtained result was opposite. Therefore, the appearance of this isolated O–H band was ascribed to the cleavage of the hydrogen-bonding networks resulting from the production of reaction intermediates such as OH radicals or H2O2. The intensity of the isolated O–H decreases when applying more positive potentials, where the O2 evolution proceeds more efficiently. This could be ascribed to the rapid consumption of the reaction intermediates. At these potentials, the intensity of hydrogen-bonded water was also further decreased

Fabrication of robust TiO2 thin films by atomized spray pyrolysis deposition for photoelectrochemical water oxidation

Photoelectrodes are highly essential for the photoelectrochemical water splitting process and development of novel fabrication techniques is vital for further enhancement of activity. In this study, we successfully fabricated highly active TiO2 thin films by using novel atomized spray pyrolysis deposition (ASPD) technique. The ASPD technique utilizes a unique atomization process to produce highly fine aerosols which resulted in a highly crystalline TiO2 nanostructure. The deposition process was optimized by controlling deposition temperatures and precursor amounts. XRD and SEM studies confirmed the formation of anatase TiO2 phase and a highly interconnected nano-flakes on FM substrate at 550 degrees C. The photoelectrochemical activity of the optimized thin films showed a photocurrent density of similar to 5 mA cm(-2) at 1.0 V (vs. Ag/AgCl) in 0.1 M Na2SO4 (aq) under 375 nm (150 mW cm(-2)) illumination. This photocurrent was much higher than the two other anatases TiO2 thin films fabricated by conventional spray pyrolysis deposition (SPD) using the same precursor and anatase TiO2 powder (particle size similar to 21 nm). Transient IR absorption study revealed that the SPD powder based thin films have deeply trapped electrons, whereas ASPD thin films consisted with only free and/or shallowly trapped electrons. Higher crystallinity and enhanced electron conductivity of the TiO2 thin films fabricated by ASPD are responsible for this stable and high photoelectrochemical activity. (C) 2017 Elsevier B.V. All rights reserved

Identification of individual electron- and hole-transfer kinetics at CoOx/BiVO4/SnO2 double heterojunctions

The fabrication of heterojunctions with different band gap semiconductors is a promising approach to increase photoelectrochemical (PEC) activity. The PEC activity is determined by the charge separation; hence, the behaviors of charge carriers at the junctions should be elucidated. However, it has been quite challenging since the distinction of carriers located in different layers has been extremely hard. In this work, we succeeded in the identification of the individual electron- and hole-transfer kinetics at CoO/BiVO/SnO double heterojunctions by measuring transient absorption (TA) from the visible to mid-IR region: we found that the absorption peaks of electrons and holes depend on the materials. From the change in spectral shape after the selective photoexcitation of BiVO, it was confirmed that electrons excited in the BiVO rapidly transferred to the SnO layer after ∼3 ps, but the holes remained in the BiVO and further transferred to CoO in a few picoseconds. As a result, recombination of charge carriers was suppressed and 2.4 and 3.6 times a large amount of carriers are surviving at 5 μs on BiVO/SnO and CoO/BiVO/SnO, respectively, compared to bare BiVO. For such picosecond-rapid and effective charge separation, the previously well proposed sole intralayer or interlayer charge separation mechanism is not enough. Hence the synergetic effect of these two mechanisms, the band-bending-assisted charge transfer across the heterojunction, is proposed. The enhanced PEC activity of CoO/BiVO/SnO electrodes was reasonably explained by this synergistic charge separation kinetics. This fundamental knowledge of charge carrier dynamics will be beneficial for the design of superior solar energy conversion systems

Interfacial manipulation by rutile TiO2 nanoparticles to boost CO2 reduction into CO on a metal-complex/semiconductor hybrid photocatalyst

Metal-complex/semiconductor hybrids have attracted attention as photocatalysts for visible-light CO2 reduction, and electron transfer from the metal complex to the semiconductor is critically important to improve the performance. Here rutile TiO2 nanoparticles having 5-10 nm in size were employed as modifiers to improve interfacial charge transfer between semiconducting carbon nitride nanosheets (NS-C3N4) and a supramolecular Ru(II)-Re(I) binuclear complex (RuRe). The RuRe/TiO2/NS-C3N4 hybrid was capable of photocatalyzing CO2 reduction into CO with high selectivity under visible light (lambda > 400 nm), outperforming an analogue without TiO2 by a factor of 4, in terms of both CO formation rate and turnover number (TON). The enhanced photocatalytic activity was attributed primarily to prolonged lifetime of free and/or shallowly trapped electrons generated in TiO2/NS-C3N4 under visible-light irradiation, as revealed by transient absorption spectroscopy. Experimental results also indicated that the TiO2 modifier served as a good adsorption site for RuRe, which resulted in the suppression of undesirable desorption of the complex, thereby contributing to the improved photocatalytic performance. This study presents the first successful example of interfacial manipulation in a metal-complex/semiconductor hybrid photocatalyst for improved visible-light CO2 reduction to produce CO

Clear and transparent nanocrystals for infrared-responsive carrier transfer

赤外光を電気エネルギーや信号に変換する無色透明な材料の開発に成功 --見えない電子デバイスの開発へ道--. 京都大学プレスリリース. 2019-02-13.An Author Correction to this article was published on 17 April 2019. https://doi.org/10.1038/s41467-019-09888-2Infrared-light-induced carrier transfer is a key technology for ‘invisible’ optical devices for information communication systems and energy devices. However, clear and colourless photo-induced carrier transfer has not yet been demonstrated in the field of photochemistry, to the best of our knowledge. Here, we resolve this problem by employing short-wavelength-infrared (1400–4000 nm) localized surface plasmon resonance-induced electron injection from indium tin oxide nanocrystals to transparent metal oxides. The time-resolved infrared measurements visualize the dynamics of the carrier in this invisible system. Selective excitation of localized surface plasmon resonances causes hot electron injection with high efficiency (33%) and long-lived charge separation (~ 2–200 μs). We anticipate our study not only provides a breakthrough for plasmonic carrier transfer systems but may also stimulate the invention of state-of-the-art invisible optical devices

Excited-State Dynamics of Graphitic Carbon Nitride Photocatalyst and Ultrafast Electron Injection to a Ru(II) Mononuclear Complex for Carbon Dioxide Reduction

We have previously developed photocatalytic CO₂ reduction systems using graphitic carbon nitride (g-C₃N₄) and a Ru­(II) mononuclear complex (e.g., <i>trans</i>(Cl)–[Ru^II{4,4′-(H₂PO₃)₂bpy}₂(CO)₂Cl₂] bpy = 2,2′-bipyridine, abbreviated as <b>RuP</b>) hybrids and demonstrated its high activities under visible light (λ > 400 nm). To understand the excited-state dynamics of C₃N₄ and electron-transfer process to <b>RuP</b>, here we examined the photophysical properties of g-C₃N₄ as well as mesoporous g-C₃N₄ (mpg-C₃N₄) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C₃N₄ (as well as mpg-C₃N₄) has at least three emissive excited states with different lifetimes (g-C₃N₄; 1.3 ± 0.4, 3.9 ± 0.9, and 15 ± 4 ns at 269 nm photoexcitation) in aqueous suspension. These excited states were not quenched upon addition of a hole scavenger (e.g., disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (<b>RuP</b>), even though photochemical electron-transfer processes from/to g-C₃N₄ has been experimentally confirmed by photocatalytic reactions. On the other hand, TR-IR spectroscopy clearly indicated that mobile electrons photogenerated in mpg-C₃N₄, which are shallowly trapped and/or free electron in the conduction band, are able to move into <b>RuP</b> with a timescale of a few picoseconds. These results suggest that main emission centers and reaction sites (including charge-transfer interfaces) are separately located in the C₃N₄ materials, and that electron transfer from C₃N₄ to <b>RuP</b> progresses through less- or non-luminescent sites, in which mobile electrons exist with a certain lifetime

Interfacial Manipulation by Rutile TiO₂ Nanoparticles to Boost CO₂ Reduction into CO on a Metal-Complex/Semiconductor Hybrid Photocatalyst

Metal-complex/semiconductor hybrids have attracted attention as photocatalysts for visible-light CO₂ reduction, and electron transfer from the metal complex to the semiconductor is critically important to improve the performance. Here rutile TiO₂ nanoparticles having 5–10 nm in size were employed as modifiers to improve interfacial charge transfer between semiconducting carbon nitride nanosheets (NS-C₃N₄) and a supramolecular Ru­(II)–Re­(I) binuclear complex (<b>RuRe</b>). The <b>RuRe</b>/TiO₂/NS-C₃N₄ hybrid was capable of photocatalyzing CO₂ reduction into CO with high selectivity under visible light (λ > 400 nm), outperforming an analogue without TiO₂ by a factor of 4, in terms of both CO formation rate and turnover number (TON). The enhanced photocatalytic activity was attributed primarily to prolonged lifetime of free and/or shallowly trapped electrons generated in TiO₂/NS-C₃N₄ under visible-light irradiation, as revealed by transient absorption spectroscopy. Experimental results also indicated that the TiO₂ modifier served as a good adsorption site for <b>RuRe</b>, which resulted in the suppression of undesirable desorption of the complex, thereby contributing to the improved photocatalytic performance. This study presents the first successful example of interfacial manipulation in a metal-complex/semiconductor hybrid photocatalyst for improved visible-light CO₂ reduction to produce CO

Visible-light CO2 reduction over a ruthenium(ii)-complex/C3N4 hybrid photocatalyst: the promotional effect of silver species

Hybrid photocatalysts constructed with a mononuclear Ru(II)-complex (RuP), silver nanoparticles, and carbon nitride nanosheets (NS-C3N4) photocatalyze CO2 reduction to selectively form formate under visible light. The structure of the nanoparticulate silver species, which worked as promoters for the reaction, was characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, high-resolution transmission microscopy, and X-ray absorption fine-structure spectroscopy. The silver promoters were loaded on the surface of NS-C3N4 by an impregnation method from an aqueous solution containing AgNO3 or an in situ photodeposition method. Impregnation of NS-C3N4 with 2.0 wt% Ag followed by reduction with H2 at 473 K (further modified with RuP) resulted in the highest photocatalytic activity, giving a turnover number of 5700 (based on RuP), which was the greatest value among the formate-generating hybrid systems with a mononuclear complex. While the optimized photocatalyst contained highly dispersed Ag2O-like nanoclusters as the major silver species, experimental results suggested that highly dispersed Ag0 species are more important for enhancing CO2 reduction activity, that is, the obtained experimental results led us to conclude that there are two major factors affecting activity: one is the feature size of silver species (smaller is better), and the other is the oxidation state of silver (metallic is better)

Inkjet‐Printed Self‐Hosted TADF Polymer Light‐Emitting Diodes

Thermally activated delayed fluorescent (TADF) materials are extensively investigated as organic light-emitting diodes (OLEDs) with TADF emitting layers demonstrating high efficiency without the use of heavy metal complexes. Therefore, solution-processable and printable TADF emitters are highly desirable, moving away from expensive vacuum deposition techniques. In addition, using emissive materials not requiring an external host simplifies the fabrication process significantly. Herein, OLEDs using a solution-processable TADF polymer that do not need an external host are introduced. The non-conjugated TADF polymer features a TADF emitter (4-(9H-carbazol-9-yl)-2-(3′-hydroxy-[1,1′-biphenyl]-3-yl)-isoindoline-1,3-dione) as a side chain, as well as a hole-transporting side chain and an electron-transporting side chain on an inactive polymer backbone. All organic layers of the OLEDs are fabricated using solution processing methods. The OLEDs with inkjet-printed emissive layers have comparable maximum current and external quantum efficiency as their spin-coated counterparts, exceeding luminance of 2000 cd m−2. The herein-explored strategy is a viable route toward self-hosted printable TADF OLEDs.</p