Recently, Zhang Fapei’s research team from the High Magnetic Field Science Center of the Hefei Institute of Material Science, Chinese Academy of Sciences proposed a new strategy for the growth of organic materials induced by strong magnetic fields to achieve structural regulation of high-performance semiconductor polymer films and improve their charge transport capabilities. ACS Applied Materials & Interface, Journal of Materials Chemistry C and Applied Physics Letters.
Effective control of molecular orientation and film order in organic semiconductor films is conducive to the realization of high-performance organic field-effect transistors (OFETs) and solar cells. The development of high-efficiency and high-universal solution phase film formation technology is an important way to achieve this goal. The use of a magnetic field to induce the molecular orientation of the film on the macro-scale can be used as a direct and "clean" method to grow large-area oriented organic films, which has attracted the attention of the academic community. In the previous research, the research group used the solution coating method under a strong magnetic field to realize the control of the structure and carrier transport characteristics of multiple crystalline (and semi-crystalline) semiconductor polymer films for the first time, and proposed the magnetic orientation of the film. Growth mechanism (Adv. Funct. Mater. 2015, 25, 5126). However, the organic film prepared by this method has problems such as poor morphology and thickness uniformity, uncontrollable film thickness, etc., which affect the repeatability of the photoelectric performance of the film device.
In response to the above problems, Zhang Fapei's research team proposed for the first time a new strategy of solvent vapor annealing (SVA-HMF) under a strong magnetic field. The researchers deposited a "wet film" with a uniform thickness through solution spin coating, placed it in a closed container containing a saturated organic solvent, and "annealed" it under a strong magnetic field to obtain a donor-acceptor (DA) copolymer P (NDI2OD-T2) film with a large area (centimeter scale) and highly oriented film texture. Researchers found through microstructure characterization that the morphology and thickness uniformity of the prepared polymer film are improved, and the degree of polymer backbone chain orientation and film crystallinity are better than those prepared by solution coating (ACS Appl. Mater. Interfaces 2020 , 12, 29487); by studying the effect of SVA-HMF conditions on the structure and morphology of the film, the researchers proposed the dynamic mechanism of magnetically induced solvent annealing to regulate the structure of the polymer film; through the preparation of OFET devices, it was found that P(NDI2OD- T2) The mobility anisotropy of the device prepared by the oriented film reaches 102, and its electron mobility is more than one order of magnitude higher than that of the device prepared by the unoriented film.
In addition, for another DA copolymer PDPP2TBT with a different molecular structure, the SVA-HMF method can also achieve a large area and highly oriented film, which shows that the method has universal applicability in regulating the structure of semiconductor polymer films. The magnetically oriented PDPP2TBT film exhibits a hole mobility as high as 1.56 cm2/Vs (J. Mater. Chem. C 2020, 8, 4477). The researchers measured the carrier mobility at varying temperatures and found that the thermal activation energy EA of carrier jumping in P(NDI2OD-T2) and PDPP2TBT oriented films is lower than that of unoriented films. This is due to the magnetically induced skeleton chain orientation It leads to the formation of a fast intra-chain charge conduction path, which enhances the delocalization of carrier jumping motion. Studies have shown that adding a small amount (2.0wt%) of graphene nanosheets to the semiconductor polymer matrix can further improve the molecular chain orientation of the polymer film prepared by SVA-HMF and enhance the carrier anisotropy of OFET devices (Appl . Phys. Lett. 2020, 117, 063301).
This research is helpful to deepen the understanding of the interaction mechanism between the magnetic field and organic semiconductor molecules, the relationship between the structure of organic semiconductor thin film and the performance of related devices. The magnetically induced thin film growth method proposed by the research team provides a way to develop new high-performance organic semiconductor materials and improve the photoelectric performance of devices. The research work is supported by the National Natural Science Foundation of China and national key research and development projects.
Figure 1. (a) Molecular structure of semiconducting polymer P (NDI2OD-T2) and PDPP2TBT; (b) Schematic diagram of the process of growing oriented film by strong magnetic field induced solvent vapor annealing (SVA-HMF)
Figure 2. (a) Transfer curves of TG/BC and BG/BC type OFET devices based on magneto-oriented P (NDI2OD-T2) film; (bc) P (NDI2OD-T2) (b) and PDPP2TBT (c) orientation The relationship of carrier mobility of thin film OFET with temperature
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