[Research results] Skillful molecular design improves the mobility of n-type polymer semiconductors by more than 5 times that of conventional devices - Contributing to the realization of IoT and a low-carbon society by improving the performance of printed devices -
Polymer semiconductors are organic materials (plastics) that have semiconductor properties. By making it into ink, it can be easily made into a thin film by a printing process, so it is expected to be applied to printed devices that have been attracting attention in recent years, such as organic transistor elements, organic thin-film solar cells, and organic thermoelectric conversion elements. The development of polymer semiconductors with excellent charge transport properties is one of the important issues in improving the performance of these devices. Polymer semiconductors include p-type semiconductors whose charges are holes and n-type semiconductors whose charges are electrons. However, although there are many reports on the development and research of p-type polymer semiconductors, there are few examples of research on n-type polymer semiconductors, and the current situation is that development is behind schedule. This is probably due to the fact that polymer semiconductors with π conjugated systems in their main chains are intrinsically electron-rich and have low electron-accepting properties. Until now, various polymer semiconductors using π electron-based skeletons substituted with imide groups, which have strong electron-withdrawing properties, have been developed in order to increase their electron-accepting properties. Polymers exhibiting high electron mobility were limited. The reason for this is that the imide group becomes a steric hindrance, destroying the planarity of the polymer main chain, which serves as an electron transport path, and disturbing the alignment structure of the polymer main chain. Therefore, the joint research team worked on the development of a π electron system skeleton with a novel imide group without steric hindrance. We found that polymer semiconductors synthesized using this skeleton as a building unit can form highly ordered structures and exhibit electron mobility five times higher than that of benchmark materials.
[Contents of research]
A research group at Hiroshima University had previously developed a π electron system scaffold called 'NPE' (Fig. 1a). NPE has a structure in which four ester groups are substituted on the π electron system skeleton called naphthobispyrazine. Although the electron-accepting property of polymer semiconductors with NPE is relatively high, it was insufficient for electron-transporting properties. Therefore, this time, in order to further increase the electron-accepting property, a research group at Hiroshima University developed an NPE called πWe have developed an electronic framework (Fig. 1a). Electrostatic potentials [4] of these frameworks were calculated by quantum chemical calculations, suggesting that NPIs are more electron-accepting than NPEs (Fig. 1b). In addition, a research group at the University of Tokyo used the first-principles calculation method[5] to calculate the band structure of a model compound of a polymer semiconductor with NPI and NPE. , suggesting that polymer semiconductors exhibit high electron mobility. Next, when the structure of the polymer "PNPI2T", which has NPI in the main chain structure, was examined by quantum chemical calculations, it was found that the planarity was greatly improved compared to the benchmark n-type polymer semiconductor N2200. Compared to the imide group in the NDI backbone used in N2200, the imide group in the NPI backbone is farther away from the adjacent thiophene ring, and steric hindrance is greatly reduced (Fig. 2a). In fact, when X-ray structural analysis of polymer thin films was performed at the beamline (BL46XU)[6] of the large synchrotron radiation facility SPring-8, the distance between the polymer main chains of PNPI2T was 3.4. Å, which is significantly smaller than the 3.9 Å of N2200. However, the electron mobility of the organic transistor device fabricated with PNPI2T as a semiconductor layer was 0.19 cm2/Vs, which was 0.14 cm2/Vs of the N2200 device fabricated under the same conditions. Vs) only showed a slightly higher value. This was thought to be due to the lower electron acceptability compared to N2200. Therefore, in order to further increase the electron-accepting property, we synthesized PNPI2T-iF2, in which the highly electronegative atom, fluorine, was substituted into the bithiophene moiety of PNPI2T so that the two molecules face each other (Fig. 3a). Substitution at this position is expected to induce non-bonded interactions between the fluorine atom and the sulfur of the thiophene ring, leading to the formation of a more ordered structure. However, PNPI2T-iF2 had an overly rigid backbone due to non-bonded interactions, resulting in lower solubility and poorer film-formability than PNPI2T, leading to the formation of inhomogeneous thin films. As a result, although PNPI2T-iF2 has an electron acceptability comparable to that of N2200, the electron mobility is about 0.1cm2/Vs, which is slightly lower than that of PNPI2T. Next, we synthesized 'PNPI2T-oF2' in which two fluorine atoms are substituted in opposite directions (Fig. 3a). Substitutions at this position do not allow non-bonded interactions such as those described above. As a result, PNPI2T-oF2 exhibited sufficient solubility and formed a uniform thin film. Furthermore, a research group at the National Institute for Materials Science performed photothermal polarization spectroscopy[7] of thin films, and found that PNPI2T-oF2 has a more ordered polymer main chain structure than PNPI2T and PNPI2T-iF2. I understand. In PNPI2T and PNPI2T-iF2, structure A and structure B, which have different orientations of thiophene with respect to NPI, are equally stable in terms of energy at the binding site of NPI and thiophene, and either structure can be included in the polymer backbone. In contrast, in PNPI2T-oF2, structure B is energetically unstable due to steric hindrance between fluorine and NPI, and structure A is preferentially included in the polymer backbone. In other words, it can be inferred that PNPI2T-oF2 exhibited a more highly ordered backbone structure due to the higher stereoregularity (Fig. 3b). As a result, the electron mobility of the device using PNPI2T-oF2 was 0.7cm2/Vs, which was significantly higher than that of the PNPI2T device. This is more than five times higher than the N2200 element and comparable to amorphous silicon. This time, we have created a new imide-substituted π electron system skeleton called NPI as a building unit that can simultaneously bring "high electron acceptability" and "highly ordered arrangement" to polymer semiconductors. This research is a very important result for the development of high-performance n-type polymer semiconductors.
![[Research results] Skillful molecular design Improves the mobility of n-type polymer semiconductors by more than five times that of conventional ones -Contributing to the realization of IoT and a low-carbon society by improving the performance of printed devices-](https://website-google-hk.oss-cn-hongkong.aliyuncs.com/drawing/article_results_9/2022/3/9/4564e4d624f99661ac7f43e2697ffa4f_1.jpeg "[Research results] Skillful molecular design of n-type polymers More than Five Times the Mobility of Semiconductors Compared to Conventional Devices - Contributing to the realization of IoT and a low-carbon society by improving the performance of printed devices -")
This research was conducted by Professor Itaru Osaka, Assistant Professor Tsubasa Mikie, and Yuka Iwasaki (Master's course) from the Graduate School of Advanced Science and Engineering, Hiroshima University, and Associate Professor Toshihiro Okamoto from the Graduate School of Frontier Sciences, the University of Tokyo. This is the result of joint research by Professor Masatomo Kakutani, Senior Researcher at the National Institute for Materials Science, and Tomoyuki Koganezawa, Senior Researcher at the Synchrotron Radiation Research Center. This research was funded by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research B (Research Project Number: 16H04196, 21H01916), Research Activity Startup Support (Research Project Number: 20K22535), and Strategic Creative Research by the Japan Science and Technology Agency (JST). Promotion Project CREST Research Area ``Creation of Innovative Energy Harvesting Technology Using Microscopic Energy'' (Research Supervisor: Kenji Taniguchi) Research Subject ``Development of Hybrid Energy Harvesting Devices Using Band Conductive Organic Semiconductors'' : Toshihiro Okamoto (Associate Professor, Graduate School of Frontier Sciences, The University of Tokyo), etc.
[Future development]
This time, by developing a polymer semiconductor using a new π electron system skeleton with substituted imide groups, we obtained electron mobility five times higher than that of conventional semiconductors. It is a level that can be sufficiently applied to devices that can be driven by amorphous silicon. In the future, it is expected that the electron mobility will be further improved by optimizing the chemical structure. We are also currently investigating the application of the developed polymer semiconductors to organic thin-film solar cells and organic thermoelectric conversion elements. In this way, we can contribute to the realization of an IoT society and a low-carbon society.
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