Key points of this research result
"Core-to-core stacking," in which the central parts (cores) of polymer chains overlap, allows for charge mobility (easiness of electricity to flow) that is an order of magnitude higher than conventional polymer semiconductors, even in polymer semiconductors in which the polymer chains are not arranged regularly (low crystallinity).
In polymer semiconductors, the trade-off between ``making it easier to melt makes it harder for electricity to flow'' and ``making it easier for electricity to flow makes it harder to melt'' has been resolved.
Achieved the world's highest level of energy conversion efficiency in organic thin-film solar cells (OPV) using low-cost materials (fullerenes).
overview
A joint research team consisting of Professor Itaru Ozaka and Assistant Professor Tsubasa Mikie of the Graduate School of Advanced Science and Engineering at Hiroshima University, Professor Hideo Ohkita of the Graduate School of Engineering, Kyoto University, and Dr. Yusuke Nishiyama of JEOL Ltd. has revealed the origin of the high charge mobility [2] of PTNT2T, a polymer semiconductor [1] previously developed by a group at Hiroshima University, as well as organic thin-film solar cells (OPV). [3] achieved the world's highest level of energy conversion efficiency.
Conventionally, in polymer semiconductors, it has been thought that forming a crystal structure in which polymer main chains are regularly arranged is essential for improving charge transport properties. However, previous research has shown that PTNT2T exhibits high charge transport properties despite its low crystal structure order (crystallinity). This time, the joint research team discovered that "core-to-core stacking," in which the TNT skeletons that form the polymer main chain overlap each other at the molecular level, forms an efficient charge transport path with low crystallinity. Furthermore, we demonstrated that OPV using PTNT2T as a donor material in the power generation layer can maintain a high fill factor (FF) [4] even when the power generation layer is made thicker. In particular, with OPV that uses fullerene [5] derivative (PCBM) as an acceptor material, we were the first in the world to achieve an FF of over 80% in a thick film of over 300 nm, achieving the world's highest conversion efficiency of 12%. When increasing the area of OPV, it is essential to increase the thickness of the power generation layer due to manufacturing methods, so it is extremely important to be able to maintain performance even when the thickness is increased. Furthermore, the fact that we were able to achieve high conversion efficiency even using PCBM, which is a relatively inexpensive acceptor material, is important for reducing costs.
The results of this research provide new material design guidelines for high-mobility polymer semiconductors that do not depend on high crystallinity, and are expected to accelerate the practical application of OPV, which has been attracting increasing attention as a next-generation solar cell in recent years.
The results of this research were published online in the British Royal Society of Chemistry's comprehensive scientific journal "Chemical Science" on Monday, July 13, 2026 (6:00 p.m. Japan time).
Paper information
・Paper title: “Core-to-Core Overlap Promotes Interchain Charge Transport across Crystalline and Amorphous Regions in a Conjugated Polymer: High Fill Factors in Thick Organic Photovoltaic Cells”
・Authors: Tsubasa Mikie*, Tomokazu Morioku, Kodai Yamanaka, Momoka Hada, Hiroyuki Ishii, Yuki Sato, Jihun Jeon, Yutaka Ie, Kyohei Nakano, Keisuke Tajima, Yusuke Nishiyama*, Hyung Do Kim, Hideo Ohkita*, Itaru Osaka*
・Published magazine: Chemical Science
・DOI：10.1039/d6sc01947a
background
Polymer semiconductors are semiconductor materials that can be easily made into thin films through a coating process, and are expected to be applied to organic devices such as organic transistors and organic thin-film solar cells (OPVs). In order to improve the performance of these devices, the development of polymer semiconductors that exhibit high charge mobility is an important issue.
Until now, it has been thought that increasing the order (crystallinity) between polymer main chains is important in increasing the charge mobility of polymer semiconductors. On the other hand, there was a trade-off in that increasing crystallinity and achieving high mobility decreased solubility in solvents, making it difficult to form films through coating processes. Additionally, in OPVs, polymer semiconductors generally have low charge mobility, so when the thickness of the power generation layer exceeds 100 nm, performance deteriorates significantly. However, in order to create a large-area module, it is essential to have a thickness of 300 nm or more in order to make the power generation layer uniform. Therefore, the development of polymer semiconductors that achieve excellent charge transport properties while maintaining high solubility has become an extremely important issue for the practical application of OPV.
Contents of research results
This time, the joint research team focused on the polymer semiconductor PTNT2T (Figure 1), which was previously developed by a group at Hiroshima University. Unlike conventional polymer semiconductors, PTNT2T was found to have interesting properties such as high charge mobility despite low crystallinity and high solubility. Therefore, we investigated the charge transport mechanism of polymers in detail and considered its application to OPV devices.
First, we will examine the alignment direction of polymer main chains predicted from multi-incidence angle spectroscopic ellipsometry measurements [6] by team director Keisuke Tajima and senior researcher Kyohei Nakano of RIKEN, and the quantum direction predicted by Professor Hiroyuki Ishii of University of Tsukuba. Based on the effective mass in the main chain direction predicted from chemical calculations [7], we found that the reason PTNT2T exhibits high mobility despite low crystallinity is due to its excellent charge transport properties in the polymer main chain direction. On the other hand, when we investigated the aggregate structure of PTNT2T in more detail, we found that there was a certain strong interaction between the polymer main chains. As a result of solid-state NMR measurements [8] and crystal structure analysis of model compounds conducted by Dr. Yusuke Nishiyama of JEOL Ltd., it was discovered that in PTNT2T, core-to-core stacking is formed in which the TNT skeletons that form the polymer main chain largely overlap each other (Figure 2). Furthermore, quantum chemical calculations conducted by Professor Takashi Iehiro of the Institute of Industrial Science, Osaka University revealed that due to this core-to-core stacking, the overlap of π electrons is three times larger than normal, and it has become clear that PTNT2T is extremely advantageous for charge transport between main chains. This showed that PTNT2T forms an efficient charge transport path not only in the direction of the polymer main chain, but also between the polymer main chains due to this unique molecular-level interaction, and achieves high-speed charge transport (high mobility) despite low crystallinity. Additionally, electrical analysis conducted by Professor Hideo Ohkita's group at Kyoto University revealed that charge recombination [9] is significantly suppressed compared to charge recovery.
Therefore, when we fabricated an OPV device using PTNT2T as the donor material for the power generation layer, we found that, especially when PCBM was used as the acceptor material, the FF showed an extremely high value of over 80% even in the power generation layer, which was over 300 nm and three times the thickness of a normal power generation layer. Furthermore, the energy conversion efficiency was 12%, which is the highest level in the world despite such a thick film. Furthermore, even when non-fullerene was used as the acceptor material, the energy conversion efficiency was as high as 15.6%.
This research was carried out by Professor Itaru Ozaka, Assistant Professor Tsubasa Mikie, Specially Appointed Assistant Professor Kodai Yamanaka, Mr. Tomokazu Morioku (first year 2nd year doctoral student), Mr. Momoga Haneda (2nd year master's course), Professor Hideo Ohkita of Kyoto University Graduate School of Engineering, Assistant Professor Kim Hyung Do, Mr. Tomoki Sato (3rd year doctoral course graduate student), and Jeong Jihoon of the Graduate School of Advanced Science and Engineering, Hiroshima University. This is a joint research project by Professor Takashi Iehiro of the Institute of Industrial Science, Osaka University, team director Keisuke Tajima of RIKEN, senior researcher Kyohei Nakano, professor Hiroyuki Ishii of the Department of Mathematics and Materials at the University of Tsukuba, and Dr. Yusuke Nishiyama of JEOL Ltd.
The results of this research are supported by the Future Society Creation Project of the Japan Science and Technology Agency (JST) and ALCA-Next (Research and development title: ``Development of innovative organic semiconductors and the challenge of achieving 20% organic solar cell efficiency'', Research and development representative: Itaru Ozaka (Hiroshima University) (Professor), R&D period: November 2020 - March 2020, April 2020 - March 2021), and New Energy and Industrial Technology Development Organization (NEDO) Solar Power Generation Main Power Supply Technology Development Project (Research and development project title: "High efficiency and modularization technology development of see-through organic thin-film solar cells", Research and development representative: Itaru Ozaka (Hiroshima University) Professor), research and development period: July 2020 to March 2020).
Positioning of this result and future development
This research is an important result that overturns the conventional wisdom regarding polymer semiconductors that ``high charge transport properties can be achieved without increasing crystallinity.'' This makes it possible to achieve both the trade-off of high solubility and high charge mobility, and is expected to accelerate the development of high-performance, practical polymer semiconductors. Furthermore, the fact that we were able to achieve high conversion efficiency even with the use of an inexpensive acceptor material, PCBM, and a power generation layer as thick as 300 nm, can be said to be a major step forward toward the social implementation of OPV.
Reference materials
Figure 1 Chemical structure of the polymer semiconductor PTNT2T with a TNT skeleton (red part) previously developed by a group at Hiroshima University (Press release on November 18, 2024:Succeeded in improving the charge mobility of polymer semiconductors through structural expansion ~ Contributing significantly to improving the performance of organic thin-film solar cells ~).
Figure 2: Crystal structure of a model compound of PTNT2T: (top) the structure seen from directly above, and (bottom) the structure seen from the side, with molecular orbitals superimposed. A core-to-core stacking is formed in which the TNT skeletons largely overlap each other. This stacking structure allows the molecular orbitals to overlap three times more than normal, making it possible to efficiently transport charges between molecules.
Glossary
[1] Polymer semiconductor
An organic polymer compound (plastic) whose main chain has a π-conjugated structure consisting of alternating carbon-carbon single bonds and double bonds. Also called π-conjugated polymers, they originate from polyacetylene, which was developed by Hideki Shirakawa (2000 Nobel Prize in Chemistry winner) and others, and is a material that originated in Japan. Although it is plastic, it has semiconductor properties. Because it dissolves in organic solvents and forms thin films, it is used as a printable semiconductor in next-generation thin film devices.
[2] Charge mobility
An indicator of how easily electric charges (holes and electrons) flow in a substance. The higher this value is, the easier the current will flow.
[3] Organic thin film solar cell (OPV)
A general term for thin-film solar cells that use organic semiconductors as the power generation layer. In particular, an organic thin film solar cell manufactured by applying an organic semiconductor solution is called a coating type OPV. A power generation layer is made by mixing a p-type organic semiconductor (also called donor material) that transports holes and an n-type organic semiconductor (also called acceptor material) that transports electrons. A polymer semiconductor is generally used as a donor material, and a fullerene derivative or a π-conjugated molecule (non-fullerene) is generally used as an acceptor material. Unlike perovskite solar cells, these power generation materials do not contain heavy metals such as lead, so they are attracting attention as a new type of environmentally friendly solar cell.
[4] Fill factor (FF)
An indicator that shows that the charge generated by light absorption can be efficiently collected into the electrode.
[5] Fullerene
A general term for carbon allotropes with a molecular structure resembling a soccer ball made only of carbon atoms. Because it has the property of easily accepting electrons, it is widely used in organic thin-film solar cells as an "acceptor material" that receives electrons generated during power generation.
[6] Multi-incident angle spectroscopic ellipsometry measurement
An analysis method that back-calculates the optical structure of a thin film, such as its thickness and refractive index, from changes in the polarization state of light irradiated onto a thin film on a substrate. Various physical properties such as film thickness, dielectric function, refractive index, extinction coefficient, and orientation parameters can be determined.
[7] Effective mass
An indicator of how smoothly charge can move within a material. Based on quantum chemistry, the energy band structure of a compound can be determined and the effective mass can be determined based on that. The smaller the effective mass, the easier the charge will accelerate and move. Conversely, if the effective mass is large, the charge becomes difficult to move.
[8] Solid-state NMR measurement
Nuclear Magnetic Resonance (NMR) A type of spectroscopy that measures solid objects themselves. Structural information such as crystallinity, degree of crystallinity, degree of orientation, and degree of crosslinking in a solid state of a substance can be obtained.
[9] Charge recombination
A phenomenon in which holes and electrons generated in a substance meet, cancel each other out, and disappear. In a solar cell, if charge recombination occurs before the charge is recovered by the electrode, the generated charge cannot be extracted as electric power, resulting in a decrease in the performance of the solar cell.
[Contact information]
Hiroshima University Graduate School of Advanced Science and Engineering Professor Itaru Osaka
Tel: 082-424-7744 FAX: 082-424-5494
E-mail: iosaka＊hiroshima-u.ac.jp
Hiroshima University, Finance and General Affairs Office, General Affairs and Public Relations Department, Public Relations Group
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Kyoto University Public Relations Office International Public Relations Section
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University of Tsukuba
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Osaka University Institute of Scientific and Industrial Research Public Relations Office
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JEOL Ltd. Management Strategy Office
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