Mechanisms for Engineering Highly Anisotropic Conductivity in a Layered Covalent-Organic Framework
2016
Er, Dequan | Dong, Liang | Shenoy, Vivek B.
Two-dimensional (2D) covalent-organic framework (COF) materials provide a promising solution to the lightweight, durable, and flexible electronic applications such as organic photovoltaics and organic light-emitting diodes. In this paper we report a theoretical study based on density functional theory calculations on a recently synthesized covalent-organic framework that has been experimentally demonstrated to possess excellent carrier mobility and photoconductivity along the vertical direction. Our calculations reveal the dependence of the carrier mobility on the number of layers and the stacking order, and show that the conduction is achieved by electron hopping between adjacent layers along the vertical pathways that are composed of aligned donor or acceptor groups. We find that the direct band gap in a monolayer shifts to an indirect band gap in bulk (multiple layers), with decreased carrier effective masses along the vertical direction. The vertical interlayer interaction further enhances the in-plane charge transfer from the donor to the acceptor parts, and hence the probability of electron hopping between adjacent layers. Our results not only explain conductivity enhancement mechanism in COFs, but also provide guidelines in designing highly conductive 2D polymer optoelectronic devices.
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