Jan 18, 2021
High-Mobility Semiconducting Two-Dimensional Conjugated Covalent Organic Frameworks with p-Type Doping
Two-dimensional conjugated covalent organic frameworks (2D c-COFs) have recently emerged as a unique class of 2D conjugated polymers that display high in-plane π-conjugation and weak out-of-plane interactions. Because of their tailorable architectures, abundant active sites, well-defined structures, inherent porosity, chemical stability, and (opto)-electronic properties, these materials are promising for chemiresistors, logic and memory devices, and energy storage. For many of these applications, long-range charge transport is required. Therefore, much effort has been devoted over the last years to interrogate the nature of the conductivity in 2D c-COFs. Recent studies have demonstrated charge carrier mobilities ranging from 5 to 8 cm2/(Vs). Although these mobilities are encouraging, their conductivities have remained rather low (typically <10-6 S/cm). To further improve the conductivity, doping strategies have been employed by incorporating guest molecules that act as dopants, e.g., linear conducting polymers, C60, iodine (I2), etc. For instance, I2-doping has demonstrated improvement of up to 3 orders of magnitude in the conductivities, but this approach was often associated with amorphization/irreversible structural changes of 2D c-COFs. A fundamental understanding of the doping interactions within the lattice in COFs remains largely unexplored.
In order to investigate the role of dopant for 2D c-COFs, researchers from Technical University of Dresden (Chair for Molecular Functional Materials) and collaborators have demonstrated a doping-defined polycrystalline 2D c-COF (ZnPc-pz-I2) through molecular I2-doping of metal-phthalocyanine-based pyrazine-linked 2D c-COF ZnPc-pz. I2-molecules sit preferentially in the COF pores and near the skeleton. Hall effect measurements reveal that doping improves the conductivity and carrier density by approximately 3 and 2 orders of magnitude respectively in ZnPc-pz-I2. Notably, doping also leads to an unprecedented improvement in the Hall charge mobility from ~5 to ~22 cm2/(Vs). Density functional theory (DFT) and time-resolved terahertz spectroscopy (TRTS) show that this record mobility is related to an increase in the scattering time after doping, likely related to the formation of ordered pathways for charge carrier migration between the electron donor (ZnPc-pz) and acceptor (I2) within the framework. Such unique phenomena have never been reported among doped COF materials. This work highlights the potential of developing structurally-defined, doped 2D c-COFs with high conductivity and high mobility, which provides insight on a fundamental understanding of the role of the dopant and the host-dopant interplay necessary to elucidate structure-electronic property relationships.
This work is financially supported from EU Graphene Flagship (GrapheneCore3, No. 881603), ERC Grants (T2DCP and FC2DMOF (No. 852909)), H2020-MSCA-ITN (ULTIMATE, No. 813036), DFG projects (COORNETs, SPP 1928 and CRC 1415, No. 417590517), the German Science Council, Center for Advancing Electronics Dresden (EXC1056), the regional government of Comunidad de Madrid under projects 2017-T1/AMB-5207 & P2018/NMT-4511, as well as the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, Grant No. SEV-2016-0686).
Reference:
Mingchao Wang, Mao Wang, Hung-Hsuan Lin, Marco Ballabio, Haixia Zhong, Mischa Bonn, Shengqiang Zhou, Thomas Heine, Enrique Cánovas*, Renhao Dong*, Xinliang Feng*. High-Mobility Semiconducting Two-Dimensional Conjugated Covalent Organic Frameworks with p-Type Doping. J. Am. Chem. Soc. 2020, 142, 52, 21622–21627.