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Keisuke Tajima

Keisuke Tajima
RIKEN | RIKEN AICS · Center for Emergent Matter Science (CEMS)

Ph.D

About

188
Publications
29,860
Reads
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8,695
Citations
Citations since 2016
76 Research Items
4150 Citations
20162017201820192020202120220100200300400500600
20162017201820192020202120220100200300400500600
20162017201820192020202120220100200300400500600
20162017201820192020202120220100200300400500600
Introduction
We work on the development of new organic semiconducting polymer materials and their application to organic electronic devices. Specifically, we seek the methodology and the molecular design to control the precise structures in molecular- and nano-scale at our will, and try to find breakthroughs to drastically enhance the performance. Targets of our research are organic solar cells, field-effect transistors, and the organic electronic devices with new functions based on the structure controls.
Additional affiliations
April 2013 - present
RIKEN
Position
  • Group Leader
October 2011 - present
Japan Science and Technology Agency (JST)
Position
  • PRESTO Researcher
July 2011 - November 2012
The University of Tokyo
Position
  • Professor (Associate)

Publications

Publications (188)
Article
To improve the performance of organic field‐effect transistors (OFETs) employing π‐conjugated polymers, a basic understanding of the relationships between the material properties and device characteristics is crucial. Although the density of states (DOS) distribution is one of the essential material properties of semiconducting polymers, insights i...
Article
Full-text available
Carrier extraction in mixed tin-lead perovskite solar cells is improved by modifying the top and bottom perovskite surfaces with ethylenediammonium diiodide and glycine hydrochloride, respectively. Trap densities in perovskite layers are reduced as a result of surface passivation effects and an increase in film crystallinity. In addition, the orien...
Article
Full-text available
A diradicaloid molecule with high semiconducting performance is synthesized based on the quinoidal benzo[1,2‐b:4,5‐b′]dithiophene structure. The diradical character is investigated by quantum chemical calculations and variable temperature electron spin resonance. The diode devices based on this molecule show a large change in electric current in ma...
Article
Full-text available
Ultrathin (thickness less than 10 µm) organic photovoltaics (OPVs) can be applied to power soft robotics and wearable electronics. In addition to high power conversion efficiency, stability under various environmental stresses is crucial for the application of ultrathin OPVs. In this study, the authors realize highly air‐stable and ultrathin (≈3 µm...
Article
Bulk heterojunction (BHJ) structures prepared by coating mixed solutions of a donor and an acceptor have been used for efficient organic photovoltaics (OPVs), but this approach allows little control of the molecular orientations in the films. Herein we show that a sequential deposition strategy, in which the molecular acceptor is deposited sequenti...
Article
A series of π-conjugated copolymers containing V-shaped electron-deficient units of triphenyleno[1,2-c:7,8-c′]bis([1,2,5]thiadiazole) (TPTz) or its non-fused precursor were synthesized. Both the ring fusion in TPTz and the proper spacer units between...
Article
Full-text available
Photo-stimuli response in materials is a fascinating feature with many potential applications. A photoresponsive gel of poly(heptazine imide), PHI, termed PHIG, exhibits photochromism, photoconductivity, and photo-induced charge accumulation, and is generated using ionic liquids and PHI. Although there are several examples of ionic liquid gels that...
Article
Full-text available
Recent progress in organic photovoltaics (OPVs) has led to an increased importance of laboratory-scale fabrication in ambient air using solution processes. However, the effect of the existence of both ambient air and light during the formation of a photoactive layer on the performance of fabricated devices has not been elucidated thus far in detail...
Preprint
Full-text available
Carrier extraction is a key issue which limits the efficiency of perovskite solar cells. In this work, carrier extraction is improved by modifying the perovskite layers with a combination of ethylenediammonium diiodide post-treatment and glycine hydrochloride additive. Ethylenediammonium dications primarily affect the top surface of the perovskite...
Article
Full-text available
Despite the critical importance of the density of states (DOS) to the electrical properties of molecular semiconductors, there are few reliable measurement methods for the DOS, especially for the edge regions with low DOS that determine the charge conduction. Here, we evaluate the DOS for occupied states with high sensitivity (down to 10–5 relative...
Preprint
Full-text available
Photo-stimuli response in materials is a fascinating feature with many potential applications. A photoresponsive gel of poly(heptazine imide), PHI, termed PHIG, exhibits photochromism, photoconductivity, and photo-induced charge accumulation, and is generated using ionic liquids and PHI. Although there are several examples of ionic liquid gels that...
Article
A chiral polythiophene surfactant based on poly(3-(S)-2-methylbutylthiophene) ((S)-P3MBT) with a semifluoroalkyl group at one end of the main chain was synthesized and used to form surface-segregated monolayers (SSMs). Films of pure (S)-P3MBT mainly adopted the edge-on orientation, whereas (S)-P3MBT films with a SSM of the polymer surfactant (S)-P3...
Article
For electronic devices with molecular semiconductors, surface of the molecular semiconductor thin-films plays a key role in the device performance. Despite its importance, the method to control the surface structure without changing bulk properties is limited. Here we introduce a concept of a surface-segregated monolayer that enables us to control...
Article
Surface modification of SiO 2 using a catalyst‐free quantitative reaction between an amine and an ethynyl‐π‐extended naphthalenediimide was investigated. A post‐reaction method, in which the catalyst‐free reaction was performed at the surface after the formation of amino‐terminated self‐assembled monolayers (SAMs), resulted in dense, uniform modifi...
Article
Full-text available
The chirality of organic semiconductors is important for various applications in optoelectronics and spintronics. Here, we propose a new strategy to induce structural chirality in achiral organic semiconductors in thin films. Enantiomeric fullerene derivatives (S)-pSi and (R)-pSi, which have oligo(dimethylsiloxane) as a low-surface-energy moiety, w...
Article
Full-text available
Although the physical and biological functions of the skin layer of spider dragline have been studied and partially clarified, the morphology and elemental contents of the skin layer of silk fibers have not been investigated in detail to date. Here, the surface of Nephila clavata spider dragline was evaluated by field emission scanning electron mic...
Article
Full-text available
Significance We have developed ultraflexible 3-µm-thick organic photovoltaics (OPVs) with good environmental stability and a high efficiency of over 13% by a simple postannealing process. By combining the stable polymer donor/nonfullerene small molecule acceptor blends and thermally stabilized interfaces due to doping, our OPVs achieve extremely lo...
Article
In article number 1900762 by Kenjiro Fukuda, Shinjiro Umezu, Someya Takao, and co‐workers, the fabrication of ultraflexible organic photovoltaics on nanostructured substrates is reported. The organic photovoltaics have a high power conversion efficiency and a low dependence on incident light angle. By combining nanograting structures in other layer...
Article
A new π-conjugated polymer based on naphthalenediimide with a branched semifluoroalkyl side chain (FNDIT2) was synthesized and used to change the polymer orientations in thin films. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy revealed that FNDIT2 spontaneously forms surface-segregated monolayers in bl...
Article
The nanopatterning of the surfaces of polymer substrates enhances the performances of photovoltaics. Ultraflexible organic photovoltaics (OPVs) are one of the promising energy harvesters for wearable electronics. A reduction in incident light angle dependence while maintaining the power conversion efficiency (PCE) is desirable for wearable electron...
Article
Full-text available
Accurate determination of charge carrier density in organic solar cells under light irradiation is essential because charge carrier density is directly related to the bimolecular recombination rate and open-circuit voltage of the cells. We investigate the robustness of transient photovoltage/current (TPV/C) and impedance spectroscopy (IS) to interf...
Article
This focus review provides an overview of the formation and functions of surface-segregated monolayers (SSMs) on organic semiconductor films. Careful design of SSM molecules allows the formation of highly ordered surface monolayers with high coverage densities. SSMs can be used to induce changes in the molecular structure and the orientation in the...
Article
Full-text available
Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In this study, we systematically investigate 16 combinations of four donor polymers and four...
Article
Diketopyrrolopyrrole (DPP)-based semiconducting polymers were synthesized with the alkyl side-chains of 3,7,11,15-tetramethylhexadec-2-en-1-yl (TMHDe) and 3,7,11,15-tetramethylhexadecyl (TMHD) that are derived from phytol, a naturally-occurring diterpene alcohol. The properties of these polymers were compared to those of DPP-2-octyldodecyl (DPP-OD)...
Article
Two thiophene-fused perylene-3,4,9,10-tetracarboxy diimides, peryleno[2,1-b]thiophene diimide (PTIa) and peryleno[1,2-b]thiophene diimide (PTIb), were synthesized as electron-deficient building units for electronic and optoelectronic materials. Both PTIs have an absorption band in the 480–600 nm range, fluorescence emission with high quantum yield...
Article
Development of light and flexible power harvester has been attracting great attention as a next breakthrough power solution especially in wearable device field. Organic photovoltaics (OPVs) based on polymer active layers is one of the promising energy harvesters in such a field. The improvement of power conversion efficiency (PCE) is one of the mos...
Article
Integrating an additional component featuring complementary light absorption into binary polymer solar cells is a superior tactic to ameliorate solar cell efficiency and stability. An appropriate additive not only extends absorption range, but may also facilitate charge separation and transport processes. In this work, we elucidate the effects of i...
Article
The growing number of new π-conjugated polymers and small-molecules has promoted the rapid development of organic solar cells (OSCs) over the past two decades. Among the most widely used building blocks, dithieno[3,2-b:2ʹ,3ʹ-d]pyrrole (DTP) and its derivatives have occupied an important position in both electron-rich and electron-deficient semicond...
Article
Designing donor/acceptor (D/A) interfaces that can efficiently generate free carriers is an attractive research target for organic photovoltaics (OPVs). While many reports suggest that the molecular orientation of the donor at the D/A interface influences the free-charge generation and recombination, the effects of the acceptor orientation on these...
Article
Molecular alignment at the interface between the electron donor and acceptor (D/A) materials in thin films may have a large effect on the charge generation process in organic solar cells (OSCs). In this study, the effects of the polymer chain orientations at the D/A interface were investigated in OSCs with planar heterojunction (PHJ) structures. Po...
Article
Full-text available
Next-generation biomedical devices1-9 will need to be self-powered and conformable to human skin or other tissue. Such devices would enable the accurate and continuous detection of physiological signals without the need for an external power supply or bulky connecting wires. Self-powering functionality could be provided by flexible photovoltaics th...
Article
A poly(3-butylthiophene) derivative with an end group of 1,1,1,3,3,5,5-heptamethyl trisiloxane group (P3BT-Si) was synthesized for the application to surface segregated monolayer (SSM) in polymer films. The surface segregation behaviors, thermal properties and chain orientation of P3BT-Si were investigated. The formation of the surface layer of P3B...
Article
Precise control of the molecular arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging, despite its vital importance for achieving high efficiency in organic photovoltaics (OPVs). Here, we report that substitution patterns of linear and branched alkyl side chains on electron‐d...
Article
Precise control of the molecular arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging, despite its vital importance for achieving high efficiency in organic photovoltaics (OPVs). Here, we report that substitution patterns of linear and branched alkyl side chains on electron‐d...
Presentation
Full-text available
We reported some new NFAs based on indacenodithiophene (IDT) and naphtho[2,3-b]thiophene diimide (NTI), which output two strong absorption bands at 600−780 nm and 400−500 nm. The NTI unit allowing easy functionalization on its imide part, we created and reported two differently substituted IDT-NTIs: 2-ethylhexyl (2EH) and 1-hexylheptyl (1HH). After...
Article
Full-text available
Linear polydimethylsiloxane (PDMS) was investigated as a solubilizing group for π-conjugated polymers with the aim of combining high solubility in organic solvents with the molecular packing in solid films that is advantageous for charge transport. Diketopyrrolopyrrole-based copolymers with different contents and substitution patterns of the PDMS s...
Article
Full-text available
Understanding molecular diffusion across the interfaces in planar heterojunctions is fundamentally important to improving the performance and stability of organic electronic devices. In this study, we quantitatively evaluated the diffusion of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) across the interface of planar heterojunctions into the p...
Article
Surface-segregated monolayers (SSMs) based on two poly(3-alkylthiophene)s with semifluoroalkyl groups at either the side chains (P3DDFT) or one end of the main chain (P3BT-F17) were used as self-organized buffer layers at the electrode interfaces in bulk heterojunction (BHJ) organic photovoltaic devices (OPVs). Both of the SSMs greatly shifted the...
Article
Full-text available
To maximize the short-circuit current density (JSC) and the open circuit voltage (VOC) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole-based p-type polymer (J61) and three benzotriazole-based nonfullerene small molecule acceptors (BTA1-3) are chosen to investigate the energeti...
Article
To investigate the effect of symmetry of copolymerized unit on the properties of naphthalene diimide (NDI)-based photovoltaic polymers, three n-type semiconductor polymers, PNDI-BDT, PNDI-T-BTh, and PNDI-T-P-T are synthesized by copolymerizing NDI with benzodithiophene (BDT), thienylene-benzothiophene (T-BTh), and thienylene-phenylene-thienylene (T...
Article
Full-text available
Preparation of highly crystalline organic semiconductor films is vital to achieving high performance in electronic devices. Here we report that surface segregated monolayers (SSMs) on top of phenyl-C61-butyric acid methyl ester (PCBM) thin films induce crystal growth in the bulk, resulting in a dramatic change in the structure to form a new crystal...
Article
Based on naphthalene diimide (NDI) unit connected through a thiophene (T) as π-bridge with thiophenes (T), thienothiephene (TT) and dithienothiophene (DTT) units, three new copolymers of PDTNDI-T, PDTNDI-TT and PDTNDI-DTT, respectively, are synthesized and applied to all polymer solar cells (all-PSCs). The relationship among the strucutures, optoel...
Article
In this research, a haptacyclic carbazole-based dithienocyclopentacarbazole (DTCC) ladder-type structure was formylated to couple with two 1,1-dicyanomethylene-3-indanone (IC) moieties, forming a new non-fullerene acceptor DTCCIC-C17 using a bulky branched 1-octylnonayl side chain at the nitrogen of the embedded carbazole and four 4-octylphenyl gro...
Article
Novel naphtho[2,3-b]thiophene diimide (NTI)-based molecules, IDT-NTIs, in which two NTI units are linked to indaceno[1,2-b:5,6-b´]dithiophene (IDT), were developed as acceptors for organic photovoltaics (OPVs). The characteristic features of IDT-NTIs were their absorption spectra with two bands at 400-500 and 600-780 nm covering a wide range of the...
Article
Full-text available
Active layers for organic photovoltaic devices (OPVs) were prepared by hand drawing with paint marker pens containing solutions of the materials. Although the pen-coated organic films were visually non-uniform with quite high surface roughness, OPV devices using these films exhibited similar or slightly better performances than those using spin-coa...
Article
We demonstrate systematic work function tuning of thiol-based SAM-modified gold electrodes with high controllability and sensitivity as high as 0.05 eV using vacuum ultraviolet technique (VUV). Under different irradiation times, both work function and wettability of the metal surface is modified. Fine tuning of the electrode work function is demons...
Article
The Photopolymer Science and Technology Award No.172200, the Best Paper Award 2017, was presented to Kyohei Nakano, Yujiao Chen, Kaori Suzuki and Keisuke Tajima (The Center for Emergent Matter Science (CEMS), RIKEN) for their outstanding contribution published in Journal of Photopolymer Science and Technology, 29, (2016) 533-536, entitled "Modifica...
Article
Full-text available
Single-walled carbon nanotubes (SWCNTs) have received much attention because of their potential in optoelectronic applications. Pristine SWCNTs exhibit substantial van der Waals interactions and hydrophobic characteristics, so precipitation occurs immediately in most organic solvents and water. Highly toxic and hazardous chemicals are often used to...