research

Intrinsic Charge Carrier Mobility Probed by Microwaves

Charge carrier mobility is an essential parameter providing control over the performance of semiconductor devices fabricated using a variety of organic molecular materials. Recent design strategies toward molecular materials have been directed at the substitution of amorphous silicon-based semiconductors; accordingly, numerous measurement techniques have been designed and developed to probe the electronic conducting nature of organic materials bearing extremely wide structural variations in comparison with inorganic and/or metal-oxide semiconductor materials. In the present project, we have developped non-contact measurement technique with microwave probes (TIme-Resolved Microwave Conductivity Measurement, TRMC) as a versatile tool for charge carrier mobility in organic molecules, crystals, and supramolecular assemblies for semiconductor applications. Beyond the simple substitution of amorphous silicon, we have attempted to address the systematic use of measurement techniques for future development of organic molecular semiconductors.

研究イメージ
Keywords
Semiconductors, Microwave, Mobility, Effective Mass, Charge Carrier Transport
References
S. Seki, A. Saeki, T. Sakurai and D. Sakamaki, "Charge carrier mobility in organic molecular materials probed by electromagnetic waves" Phys. Chem. Chem. Phys., 2014, 16, 11093-11113.
A. Saeki, Y. Koizumi, T. Aida, and S. Seki, "Comprehensive Approach to Intrinsic Charge Carrier Mobility in Conjugated Organic Molecules, Macromolecules, and Supramolecular Architectures " Acc. Chem. Res., 2012, 45, 1193-1202.

Development of Non-Contact Non-Destructive Technique to Evaluate Charge Carrier Transport at Semiconductor-Insulator Interfaces

In most organic electronic devices, charge carriers are transported during device operation at the material interfaces such as electrode/semiconductor, insulator/semiconductor, and semiconductor/semiconductor. However, analytical tools to address interfacial carrier transporting phenomena have been less developed. Here we successfully developed a novel technique named field-induced time-resolved microwave conductivity (FI-TRMC) for the evaluation of charge carrier mobility at the insulator/semiconductor interfaces without grain boundary effects. In this method, we introduce a simple metal-insulator-semiconductor (MIS) device into the microwave cavity, where the charge carrier generation takes place at the insulator-semiconductor interface by applying a gate bias and charge carrier motions are probed by reflected microwave changes.

研究イメージ
Keywords
Organic Semiconductor, Insulator, Interface, Charge Carrier Transport
References
W. Choi, T. Miyakai, T. Sakurai, A. Saeki, M. Yokoyama, and S. Seki, "Non-contact, non-destructive, quantitative probing of interfacial trap sites for charge carrier transport at semiconductor-insulator boundary" Appl. Phys. Lett., 2014, 105, 033302.
Y. Honsho, T. Miyakai, T. Sakurai, A. Saeki, and S. Seki, “Evaluation of Intrinsic Charge Carrier Transport at Insulator-Semiconductor Interfaces Probed by a Non-Contact Microwave-Based Technique” Sci. Rep., 2013, 3, 3182.

π-Molecular Figuration under Ultra-High Pressure with Unique Electronic Properties Probed by Combined Electromagnatic Wave Spectroscopy

Continuous tuning of conjugated molecular design toward the optimized inter-molecular stacking and assemblies is an essential prerequisite to unveil the inherent electrical and optical features of organic electronics. This is also the case for the modulation of backbone conformation and interchain distance of conjugated polymers, aiming their application to "plastic electronics." To this end, applying pressure in a hydrostatic medium or diamond anvil cell is a facile approach without the need for synthetic molecular engineering of conjugated molecules / polymers. Here we have developped high-pressure, time-resolved microwave conductivity (HP-TRMC) for evaluation of transient photoconductivity in conjugated polymeric materials. X-ray diffraction experiments under high pressure were performed to detail the pressure dependence of backbone configuration of conjugated polymer materials as well as phase and molecular stacking structures of molecular assemblies. The HP-TRMC results are further correlated with high-pressure Raman spectroscopy and density functional theory calculation. A mechanistic insight into the interplay of intra- and intermolecular mobilities is a key to tailoring the dynamic π-figuration associated with electrical properties, which may lead to the use of HP-TRMC in exploring divergent π-conjugated materials at the desired molecular arrangement and conformation.

研究イメージ
Keywords
Ultra-high Pressure, Microwave, Modulation, Microwave, Raman Spectroscopy
References
Y. Noguchi, A. Saeki, T. Fujiwara, S. Yamanaka, M. Kumano, T. Sakurai, N. Matsuyama, M. Nakano, N. Hirao, Y. Ohishi, and S. Seki, "Evaluation of Pressure Modulation of Backbone Conformation and Intermolecular Distance of Conjugated Polymers Toward Understanding the Dynamism of π-Figuration of their Conjugated System" J. Phys. Chem. B, 2015, 105, on web.

Design and Development of π-Conjugated Molecules with Unique Structures and Electronic States

Structures and electronic properties of molecules are closely related to each other, and therefore, we think that designing molecular structures are synonymous with designing molecular properties and functions. From this viewpoint, we are developing novel p-conjugated molecules with unique 3D structures and unusual electronic states based on the principles of quantum chemistry, as potential materials for future organic electronics.

研究イメージ
Keywords
Molecular Magnetism, Radicals, Helicenes, Biradicals, Heteroatoms
References
D. Sakamaki, D. Kumano, E. Yashima, and S. Seki, "A Facile and Versatile Approach to Double N-Heterohelicenes: Tandem Oxidative C[BOND]N Couplings of N-Heteroacenes via Cruciform Dimers" Angew. Chem. Int. Ed., 2015, 127, on web.

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Design of Novel Organic Semiconductors Utilizing Unique Molecular Shapes

Because of the varieties of molecular designs for organic molecules, many kinds of organic semiconductors based on π-conjugated systems have been reported in recent years. In this study, we focus on the design of unique organic semiconducting molecules, macromolecules, and their blends that can realize novel functions or propose new concepts. For example, by using specific macromolecules called "Shish-kebab" polymers featuring one-dimensionally connected phthalocyanine arrays together with electron-accepting molecules, we have developed electron donor-acceptor segregated nanostructures showing high photoconductivity.

研究イメージ
Keywords
Organic Semiconductor, π-Conjugated Molecules, Self-Assembly, Photoconductivity, Microwave
References
S. Yoneda, T. Sakurai, T. Nakayama, K. Kato, M. Takata, and S. Seki, ”Systematic Studies on Side-Chain Structures of Phthalocyaninato-Polysiloxanes: Polymerization and Self-Assembling Behaviors”, J. Porphyrins Phthalocyanines, 2015, 19, 160–170.

Side Chains-Directed Assembly of Organic Semiconducting Materials Featuring Unique Molecular Shape

Functionalization of large π-conjugated molecules with flexible chains at the peripheral positions enables development of organic semiconducting molecules that can show liquid crystalline nature, self-healing property, solution-processable capability, and so on. We focus on the design of peripheral positions of the central π-systems using immiscible side chain pairs such as hydrophobic/hydrophilic, hydrophobic/superhydrophobic, and hydrophilic/superhydrophobic combinations. Based on this particular side-chain engineering, we have developed variety of nanostructures in the materials, leading to the improved electronic/optoelectronic performance as soft organic semiconductors.

研究イメージ
Keywords
Organic Semiconductor, π-Conjugated Molecules, Peripheral Side Chain, Liquid Crystal, Self-Assembly
References
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Microwave-Based Evaluation of Intrinsic Charge Carrier Mobility for Charge Transporting Materials

Our group has joint research projects with other groups that focus on the development of new materials. By means of our FP- or FI-TRMC techniques as well as transient absorption spectroscopy, we evaluated charge carrier transporting property of such newly-developed materials. As recent representative works, for example, we evaluated, through FP-TRMC/TAS combined method, the charge carrier mobility of polyrotaxane-based materials composed of semiconducting phenylene-ethynylene wires isolated by insulating cyclodextrin covers. Self-assembled polymers and supramolecular materials are of interest for our study of clarifying the relationship between structure and semiconducting property. On the other hand, by using FI-TRMC technique, we studied the local-scale charge carrier mobility of π-extended acene- and heteroacene-based thin films at the semiconductor–insulator interfaces. Furthermore, recently we tried to evaluate the charge carrier transporting property of MOFs, COFs, and PCPs that have not been used for semiconductors. In particular, crystalline thin films developed by crystal growth on substrates are the potential targets for the precise evaluation.

研究イメージ
Keywords
Organic Semiconductor, π-Conjugated Molecules, Microwave, Charge Carrier Mobility, One-Dimensional, Two-Dimensional
References
J. Terao, A. Wadahama, A. Matono, T. Tada, S. Watanabe, S. Seki, T. Fujihara, and Y. Tsuji "Design principle for increasing charge mobility of π-conjugated polymers using regularly localized molecular orbitals" Nature Commun., 2013, 4, 1691.
T. Mondal, T. Sakurai, S. Yoneda, S. Seki, and S. Ghosh "Semiconducting Nanotubes by Intrachain Folding Following Macroscopic Assembly of a Naphthalene-Diimide (NDI) Appended Polyurethane" Macromolecules, 2015, 48, 879-888.
Y. Tsutsui, T. Sakurai, S. Minami, K. Hirano, T. Satoh, W. Matsuda, K. Kato, M. Takata, M. Miura and S. Seki "Evaluation of Intrinsic Charge Carrier Transporting Properties of Linear- and Bent-Shaped π-Extended Benzo-Fused Thieno[3,2-b]thiophenes" Phys. Chem. Chem. Phys., 2015, 17, 9624-9628.
J. Guo, Y. Xu, S. Jin, L. Chen, T. Kaji, Y. Honsho, M. A. Addicoat, J. Kim, A. Saeki, H. Ihee, S. Seki, S. Irle, M. Hiramoto, J. Gao, and D. Jiang "Conjugated organic framework with three-dimensionally ordered stable structure and delocalized π clouds" Nature Commun., 2013, 4, 2736.
J. Liu, W. Zhou, J. Liu, I. Howard, G. Kilibarda, S. Schlabach, D. Coupry, M. Addicoat, S. Yoneda, Y. Tsutsui, T. Sakurai, S. Seki, Z. Wang, P. Lindemann, E. Redel, T. Heine, and C. Wöll "Photoinduced Charge Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Band Gap?" Angew. Chem. Int. Ed., 2015, in press.
L. Sun, T. Miyakai, S. Seki, and M. Dinca "Mn2(2,5-disulfhydrylbenzene-1,4-dicarboxylate): A Microporous Metal−Organic Framework with Infinite (−Mn−S−)∞ Chains and High Intrinsic Charge Mobility" J. Am. Chem. Soc., 2013, 135, 8185-8188.

Comming Soon 4

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Keywords
References
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Fabrication of functional nanowires by single particle nano-fabrication technique

Single particle nano-fabrication technique (SPNT) as a fabrication method of nano-structures was established in our laboratory. High-energy charged particles (ion beam) induced a non-homogeneous cross-linking reaction in the nanometer-scale cylindrical area of the polymer films along their trajectories. Development of irradiated samples using good solvents to remove a non-cross-linked polymer afforded nanowires. The length, thickness and number density of the fabricated nanowire can be controlled by changing several parameters for the incident ion beam. Application of this technique for functional polymers such as conductive polymers and biopolymers is expected to be very effective for fabrication of various functional nanowires.

研究イメージ
Keywords
High Energy Particle, Nanowire, Cross-linking, Ion beam
References
H. L. Cheng, M. T. Tang, W. Tuchinda, K. Enomoto, A. Chiba, Y. Saito, T. Kamiya, M. Sugimoto, A. Saeki, T. Sakurai, M. Omichi, D. Sakamaki, and S. Seki, "Reversible Control of Radius and Morphology of Fluorene-Azobenzene Copolymer Nanowires by Light Exposure", Adv. Mater. Interfaces, 2015, 2, 1400450.
.DOI: 10.1002/admi.201400450
M. Omichi, A. Asano, S. Tsukuda, K. Takano, M. Sugimoto, A. Saeki, D. Sakamaki, A. Onoda, T. Hayashi, and S. Seki, "Fabrication of enzyme-degradable and size-controlled protein nanowires using single particle nano-fabrication technique", Nature. Commun., 2014, 5, 3718.
.DOI: 10.1038/ncomms4718
Y. Maeyoshi, A. Saeki, S. Suwa, M. Omichi, H. Marui, A. Asano, S. Tsukuda, M. Sugimoto, A. Kishimura, K. Kataoka, and S. Seki, "Fullerene nanowires as a versatile platform for organic electronics", Sci. Rep., 2012, 2, 600/1-6.

Conducting Conjugated Polymers for Optoelectronic Applications and Explosive Sensors

Conjugated polymers are of special interest in photovoltaic applications because of several reasons such as low-cost, light weight, solution processability, and flexibility. It is possible to tune the optoelectronic properties and hence the photovoltaic efficiencies of these polymers by modifying the structure of the polymers. However, we were more interested in modulating the optoelectronic and photovoltaic properties of conjugated polymers by improving their structural ordering using small amounts of external additives. Interestingly, the presence of a very small amount of additive induces polarity (p/n-type) switching in a bithiazole-benzothiadiazole based polymer in photovoltaic cells. Organic conjugated polymers also find immense application in explosive detection due to cost efficiency, sensitivity, portability and faster signal analysis. Using a fluorescent conjugated polymer based on bithiazole, we were able to show that detection and distinction of explosives such as DNT and TNT can be achieved by employing time resolved microwave conductivity technique.

研究イメージ
Keywords
Conducting polymers, Time-resolved microwave conductivity, Explosive detection.
References
B. Balan, C. Vijayakumar, A. Saeki, Y. Koizumi, S. Seki, "p/n Switching of ambipolar bithiazole-benzothiadiazole based polymers in photovoltaic cells" Macromolecules, 2012, 45, 2709−2719.
B. Balan, C. Vijayakumar, A. Saeki, S. Seki, "Detection and Distinction of DNT and TNT with a Fluorescent Conjugated Polymer Using Microwave Conductivity Technique" J. Phys. Chem. B, 2012, 116, 10371-10378.

Dicyanofluorene based n-Type Polymers

Donor-acceptor type conjugated copolymers consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Detailed study of the optoelectronic properties by UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry, space-charge-limited current (SCLC), flash-photolysis time-resolved microwave conductivity (FP-TRMC), and density functional theory (DFT) revealed the electron accepting and transporting (n-type) nature of the polymers. All-polymer solar cells were succesfully fabricated using these polymers in combination with P3HT.

研究イメージ
Keywords
Dicyanofluorene, Conducting materials, TRMC
References
C. Vijayakumar, A. Saeki, S. Seki, "Optoelectronic Properties of Dicyanofluorene based n-Type Polymers" Chem. -Asian J., 2012, 7, 1845-1852.

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Conjugated Polymer – Gold Nanoparticle Hybrid Assemblies

A composite of bithiazole-benzothiadiazole based semiconducting conjugated copolymer and gold nanoparticles (AuNPs) was prepared in situ and studied the optoelectronic properties. The polymer interacts with the nanoparticle surface through the non-bonding electrons of the nitrogen and sulfur atoms, which provides stability to the nanoparticles as well as planarity and rigidity to the polymer backbone. As a result, the effective conjugation length and delocalization of π-electrons of the polymer improved leading into the enhancement of short range as well as long range mobilities of the polymer in the presence of AuNPs. The electron transfer properties of the hybrid material was also studied in the presence of PCBM and PBI.

研究イメージ
Keywords
Self-assembly, Conjugated polymer, Gold nanoparticle, Charge carrier mobility
References
C. Vijayakumar, B. Balan, A. Saeki, T. Tsuda, S. Kuwabata, S. Seki, "Gold Nanoparticle Assisted Self-Assembly and Enhancement of Charge Carrier Mobilities of a Conjugated Polymer", J. Phys. Chem. C, 2012, 116, 17343-17350.
.DOI: 10.1021/jp3039253

Comming Soon 2

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References
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Comming Soon 3

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References
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