A joint research team including Professor Itaru Osaka and Assistant Professor Tsubasa Mikie of the Applied Chemistry Program, Graduate School of Advanced Science and Engineering at Hiroshima University and Professor Hideo Ohkita of the Graduate School of Engineering at Kyoto University announced that they developed a new π-electron-system skeleton for use in polymer semiconductors. Polymer semiconductors synthesized using this skeleton exhibit high charge mobility. The researchers succeeded in substantially improving the charge mobility of organic field-effect transistors (OFETs) and energy conversion efficiency of organic photovoltaics (OPVs). This study is expected to contribute to realizing an IoT society and a low-carbon society and the results are published in the international journal Chemical Science on November 18.
Because polymer semiconductors can be easily prepared as thin films by printing, they are expected to be used in next-generation electronic devices, such as OFETs and OPVs. One challenge for this is increasing the coplanarity and rigidity of the polymer main chain to improve the charge transport properties of polymer semiconductors. Steric hindrance with adjacent units can be reduced by introducing a fused polycyclic (fused ring) π-electron-system skeleton as a building unit that constitutes the main chain of the polymer, in particular by fusing thiophene to the end of the structure to expand the π-electron-system skeleton.
The research group previously developed various polymer semiconductors with naphthobisthiadiazole (NTz) as the π-electron-system skeleton. To further improve polymer semiconductor performance, they now developed dithienonaphthobisthiadiazole (TNT), a π-electron-system skeleton comprising thiophene fused to the ends of NTz. Using microwaves in the thiophene fusion reaction that had already been developed, the reaction of fusing thiophene to the electron-deficient π-electron system skeleton, which is difficult to perform from the reaction mechanism standpoint, was effectively performed.
TNT-based polymers, PTNT2T and PTNT1-F with TNT, were subsequently synthesized and analyzed. The analysis results show that these polymers have improved main chain rigidity and interaction compared to the NTz-based polymers. When OFETs were fabricated using these polymer semiconductors in the active layer, the TNT-based polymers exhibited considerably higher charge mobility than the NTz-based polymers. Furthermore, OPV cells that used PTNT1-F in the power-generation layer showed an energy conversion efficiency of 17.4%, which is 1.3 times higher than that of OPV cells employing PNTz1-F and comparable to the world's highest level for OPVs.
In the future, the researchers plan to use the prepared TNT to synthesize new organic semiconductor materials and to develop applications in various organic devices. Mikie said, "The key to the success of this research was improving the synthetic yield of the new π-electron-system skeleton, which was initially 1% but ultimately increased to 60%. We also focused on the purification of the skeleton and could synthesize it at nearly 100% purity. As a result, high-quality polymer materials could be obtained with good reproducibility, resulting in high device performance. This finding is a result of the students' perseverance in conducting experiments. We hope to further improve the efficiency of solar cells in the future."
Journal Information
Publication: Chemical Science
Title: Dithienonaphthobisthiadiazole synthesized by thienannulation of electron-deficient rings: an acceptor building unit for high-performance π-conjugated polymers
DOI: 10.1039/d4sc05793g
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