Research

Precise Control of Molecular Assembly Structures Creates New Functions

Polymer, Photon, and Electron are the keywords of our laboratory.  We study physical properties and functions related to light or electrons in polymeric systems such as conjugated polymers among polymers.  In other words, there are two major research approaches.  One corresponds to the upward pointing arrows in the figure on the right, which is aiming at the development of optoelectronic functions of polymers.  More specifically, we are dealing with topics such as polymer solar cells, perovskite solar cells, and polymer LEDs.  The other corresponds to the down arrow in the figure on the right, which explores the optoelectronic properties of polymers using light as a tool.  More specifically, we measure the properties and dynamics of excitons and charge carriers by making full use of spectroscopic measurements using ultrashort laser pulses, and discuss radiative and nonradiative transitions, singlet fission, or charge transport and recombination mechanisms.  These two research approaches do not represent opposite vectors of research, but rather they form a circle as shown in the figure.  In order to develop novel functions, it is sometimes necessary to elucidate optoelectronic properties in order to understand elementary processes.  On the other hand, the exploration of optoelectronic properties of polymers often leads to the discovery of new functions.

✓Molecular Time scale

The energy and electron reactions responsible for optical and electronic functions are extremely fast and proceed over a wide range of time scales (10−15 s ~ 10−6 s).  In this laboratory, such ultrafast phenomena of molecules are observed in real time by using ultrashort laser pulses with pulse widths of 100 femtoseconds (10−13 s). This enables us to elucidate photophysical and photochemical phenomena such as energy transfer and electron transfer.

✓molecular spacial scale

The transfer of energy and electrons between molecules proceeds over distances of only a few nanometers (10−9 m).  Therefore, the structure of materials must also be controlled on the nanometer scale.   In this laboratory, we are developing new optical and electronic functional materials by designing the molecular size by controlling the phase-separated structure of polymers on the nanometer scale and by modifying the phase-separation interface with functional molecules.

research theme


１. Photophysics and photochemistry of conjugated polymers

Ultrashort pulsed laser spectroscopy reveals photophysical elementary processes on molecular time scales.

"Conjugated polymers," which are composed of linked π-conjugated molecules, can absorb visible light and have efficient luminescence properties, and are used in organic electronics materials such as OLEDs and polymer solar cells.  Since photophysical processes based on excitons generated by photoabsorption proceed on a time scale of femtoseconds (10−15 s) to microseconds (10−6 s), we have been studying photophysical and photochemical phenomena on a molecular time scale by using time-resolved spectroscopy with ultrashort laser pulses.  Our laboratory uses time-resolved spectroscopy with ultrashort laser pulses to elucidate the mechanisms of photophysical and photochemical phenomena on the molecular time scale.

2. Optoelectronic properties of conjugated polymer

Phase-separated structure and electronic properties of molecules on spatial scales using current-measuring atomic force microscopy

Blending multiple conjugated molecular materials can create new functions.  For example, by blending an electron-donating polymer (D) with an electron-accepting polymer (A), charge carriers are generated by electron transfer from the D material to the A material.  Since the physical properties of these charge carriers are governed by the phase-separated structure on the nanometer scale, we observe and control the phase-separated structure on the molecular spatial scale and study the relationship between the structure and the optolectronic properties and functions.

3. Development of high-efficiency polymer solar cells

Colorful and lightweight next-generation solar cells fabricated from polymeric materials in solution at room temperature

We are developing polymer solar cells, which are expected to be the next generation of solar cells, using conjugated polymer blend films.  We aim to improve power conversion efficiency by developing new materials and devising new charge generation principles, and by making full use of time-resolved spectroscopy to answer questions such as "How is photocurrent generated?" and "How are charge carriers transported?"  On the basis on such molecular-level analysis, we are working on improvements to realize highly efficient polymer solar cells.