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Research Activities
The research interest of our group is in electronic semiconductors and thier devices. We perform development, characterization, and controls of performance of electronic semiconductor devices using functional materials such as conducting π-conjugated polymers and oligomers.
We investigate the topics by fabricating devices such as transistors, solar cells, and light-emitting devices, and by using characterization methods such as electron spin resonance (ESR) spectroscopy and transport.
"ESR studies of solar cells and thier materials"
Ternary polymer solar cells:
We have developed and fabricated ternary polymer solar cells using non-fullerene acceptors with measurable ESR, and directly observed charge formation during device fabrication and charge accumulation during device operation by ESR. The charge accumulation correlates with the device performance, which was clarified by simultaneous measurement of ESR and device performance, and the device operation and degradation mechanisms were elucidated from a molecular-level microscopic viewpoint.
npj Flexible Electronics 6 (2022) 22.
Press Release, University of Tsukuba, April 20, 2022.
Perovskite solar cells:
Conventional pervskite solar cells were investigated by ESR, which directly showed the accumulation of photogenerated hole carriers in the solar cells, and have elucidated the operation and deterioration mechanism of the device performance under device operation from a microscopic viewpoint at a molecular level.
Press Release, Unversity of Tsukuba, December 4, 2020.
Perovskite solar-cell materials:
We have investigate a typical hole-transport material spiro-OMeTAD in perovskite solar cells and the layered samples with perovskite materals using ESR spectroscopy under simulated solar irradiation, which shows a remakable enhancement of hole formation in spiro-OMeTAD and a greatly improved charge transfer at the interface between spiro-OMeTAD and perovskite by Li-TFSI doping. The effect of the insertion of a self-assembled monolayer at the interface between the hole transport layer NiOx and the perovskite on device performance was also clarified from a microscopic viewpoint.
Sol. Energy Mater. Sol. Cells 258 (2023) 112428.
Appl. Phys. Lett. 110 (2017) 123904. (APL Editor's Pick)
University of Tsukuba, Press release, March 22, 2017.
Kyushu Institure of Technology, Press release, March 22, 2017.
AIP Publishing in the News, March 21, 2017.
Polymer solar cells:
Bulk-heterojunction polymer solar cells using conducting polymers and fullerene derivative were investigated by ESR, which directly showed the accumulation of photogenerated hole carriers in the solar cells, and have elucidated the deterioration mechanism and the light-soaking phenomenon of the device performance under device operation from a microscopic viewpoint. The effects of device structure on device performance and device degradation are also clarified from a microscopic viewpoint.
ACS Applied Energy Materials 6 (11) (2023) 6299.
JST Press release, February 28, 2013.
Newspapers, February 28, 2013.
Appl. Phys. Lett. 109 (2016) 133301.
Thermal-annealing effects for the electron-collectioning electrodes in polyer solar cells were investigated by ESR, which directly showed the dramatic enhancement of fullerene anion formation in polymer solar cells by thermal annealing, and their correlation with the degradation of the device perfomance was found.
Appl. Phys. Lett. 104 (2014) 243903.
Oligomer solar cells:
Organic thin-film solar cells with pentacene/C60 heterojunction structures were investigated by ESR, which directly showed the charge formation in pentacene layers during the device fabrication under dark conditions and have elucidated the deterioration mechanism of the initial performance of from a microscopic viewpoint.
Adv. Energy Mater. 2 (2012) 591.
JST Press release, March 1, 2012.
Newspapers, March 2, 2012.
"ESR studies of light-emitting devices and their materials"
Direct observation of charge and spin States in thermally activated delayed fluorescent material organic light-emitting diodes:
We fabricated ESR-measurable organic light-emitting diodes using thermally activated delayed fluorescent materials and simultaneously measured ESR and device characteristics. The spin state in the device was then directly observed by ESR method to attribute the molecular species that originated from it, and the change of charge state during device operation was elucidated from a microscopic viewpoint at the molecular level.
Direct observation of the charge doping state of light-emitting electrochemical cells and elucidation of the operating mechanism:
An ESR-measurable light-emitting electrochemical cell was fabricated using a typical fluorescent polymer material, Supper Yellow, and the charge doping state during device operation was directly observed to elucidate the device operation mechanism from a molecular-level microscopic viewpoint.
Press Release, University of Tsukuba, June 2, 2023.
Direct observation of radical states and the correlation with performance deterioreation:
Radical states in a typical Alq3-based light-emitting diodes were directly investigated by ESR spectroscopy under device operation, which clarifies the origin of the radical states at a molecular level, and demonstrates the correlation between the change of the radical states and the peformance deterioration, revealing the degradation mechanism of the diodes from a microscopic viewpoint.
Physica Status Solidi A 215 (2018) 1700731. (Article-selected for cover art)
Gap states at the electron-transport material BCP/metal interface:
The gap states formed at the interfaces between electron-transport material BCP and metal were investigated by ESR using BCP/metal layered thin films, which observed the spin formation due to the gap states and clarified the spin dynamics, the spin concentration, and the interactions among spins.
Chem. Phys. Lett. 607 (2014) 29. (Article-selected for cover art)
Electron-transport and electroluminescent material Alq3:
The charge-carrier states in a typical electron-transport and highly efficient electroluminescent oligomer Alq3 layer were investigated by ESR using Mg-doped Alq3 thin films.
Hole-transport material NPB:
The charge-carrier states in a typical hole-transport NPB layer used for organic light-emitting diodes (OLEDs) were investigated by ESR using iodine-doped NPB thin films.
Chem. Lett. 41 (2012) 191.
"ESR studies of transistors"
Ion gel-gated MoS2 transistors:
The spin states of charge carriers, interface states and charge transport of ion gel-gated transistors were investigated by ESR under high charge density. For MoS2 thin-film transistors, the spin states due to conduction electrons and S or MoS6 atomic vacancies and thier electric-field control were clarifed.
Press Release, University of Tsukuba, March 5, 2021.
Ion gel-gated carbon-material transistors:
The spin states of charge carriers, interface states and charge transport of ion gel-gated transistors were investigated by ESR under high charge density. For single-walled carbon natnotube transistors, the spin formation due to atomic vacanies and thier electrically induced ambipolar spin vanishments were found, contributing to the emergency of high mobility, and the formation of Tomonaga-Luttinger-liquid was demonstrated from a microscopic viewpoint. For monolayer graphene transistors, electrically induced ambipolar spin vanishments similarly observed for carbon nanotube study and electrically induced Pauli paramagnetism in two-dimensional electron system were demonstrated from a microscopic viewpoint.
Ion gel-gated organic transistors:
The spin states of charge carriers, interface states and charge transport of ion gel-gated transistors were investigated by ESR under high charge density. For regioregular poly(3-hexylthiophene) (RR-P3HT) thin-film transistors, two dimensional magnetic interactions and nonmagnetic charge transport was discovered. For rubrene single-crystal transistors, the molecular orientation and charge trapping at organic interfaces were investigated, which demonstrated that the formation of clean organic interfaces. Also, electric-field control of charge states of holes and electrons in RR-P3HT and pentacene was studied.
ACS Appl. Electron. Mater. 1 (2019) 2522.
Appl. Phys. Lett. 102 (2013) 133301.
Appl. Phys. Express 6 (2013) 041603.
Rubrene single-crystal FETs:
Microscopic mechanisms behind very high mobility in rubrene single-crystal FETs (RSC-FETs) were clarified by directly evaluating trapping time of charge carriers using field-induced ESR. Moreover, their clean FET interfaces without deep trapping levels due to a lamination fabrication method were found from a microscopic viewpoint.
Phys. Rev. B 83 (2011) 075302.
Appl. Phys. Express 4 (2011) 085702.
Polymer:fullerene MIS diodes:
Metal-insulator-semiconductor (MIS) diode structures with conducting polymer regioregular poly(3-alkylthiophene) (RR-P3AT) and fullerenes (C60) was invesigated by ESR, which directly observed gate-induced ambipolar charge carries in the device by ESR.
Jpn. J. Appl. Phys. 46 (2007) L1191.
Pentacene FETs:
Gate-induced hole carriers in pentacene FETs were observed by ESR for the first time. This study demonstrates the spatial extent of charge carriers being approximately ten molecules, supporting a band-transport feature in the pentacene FETs.
Phys. Rev. Lett. 97 (2006) 256603.
Newspapers, December 28, 2006.