Development of organic semiconductors with ultrafast electrons

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Professors Kimoon Kim and Ji Hoon Shim together with Dr. Yeonsang Lee of the Department of Chemistry of Pohang University of Science and Technology (POSTECH) and Professor Jun Sung Kim of the Department of Physics of POSTECH and the Center for Artificial Low-Dimensional Electronic Systems of the Institute for Basic Science created conductive two-dimensional polymers that exhibit electron mobility which is comparable to graphene. Their research is included in the online edition of Cheman international chemical journal.

Called a “dream material,” graphene exhibits electron mobility 140 times faster than silicon and strength 200 times that of steel. However, the lack of a band gap, which is essential for regulating the electric current, prevents its use as a semiconductor.

Researchers have been actively exploring various approaches to develop a semiconductor that exhibits the exceptional properties of graphene. A promising approach is the development of conducting polymers. Researchers are investigating conductive polymers with a fused aromatic backbone, which mimic the chemical structure of graphene, with the aim of achieving exceptional properties. Yet, challenges arise during synthesis due to interlayer stacking between the growth media, which hinders proper polymer growth.

In this study, the team used triazacoronene, which possesses a chemical structure similar to that of graphene, and introduced bulky, pendant functional groups into its periphery.

By introducing steric hindrance from these pendant groups, the team successfully suppressed the stacking of two-dimensional polymer intermediates during the polymerization of triazacoronene monomers. This led to increased solubility of the intermediates and facilitated the synthesis of two-dimensional polymers with a higher degree of polymerization and fewer defects, resulting in excellent electrical conductivity after p-type doping.

Remarkably, magnetotransport measurements revealed that coherent multi-carrier transport with finite n-type carriers exhibit exceptionally high mobility above 3,200 cm2 V−1 s−1 and a long phase coherence length of more than 100 nm, in stark contrast to hole carrier transport with 25,000 times lower mobility at low temperatures. This dramatic disparity between electron and hole carrier transport is attributed to spatially separated electronic states near the Fermi level, which consists of dispersive and flat bands.

Professor Kimoon Kim from POSTECH expressed the importance of the research saying: “We have made a breakthrough in addressing low electron mobility, a major challenge in organic semiconductors, and in controlling the conduction pathways for electrons and holes at molecular level.” He added: “This research sheds light on improving material performance in various industrial applications, including batteries and catalysts.”

Reference: Lee Y, Choi M, Park I, et al. Observation of ultrafast electrons in hanging embedded conducting two-dimensional polymers. Chem. 2024;10(4):1160-1174. doi: 10.1016/j.chempr.2023.12.007

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