Impact of Structural Defects on the Electronic Properties of Two-Dimensional Covalent Organic Frameworks

While structural defects appear to be unavoidable in two-dimensional covalent organic frameworks (2D COFs), little is known of their influence over the material electronic properties. Here, we investigate the impact of typical defects such as single vacancies, double vacancies, and ring defects on the electronic structures of 2D COFs, taking COF-C4N and TANG-COF as representative examples. To facilitate the modeling of extensive systems with defect densities ranging from 1.25% to 25%, i.e., values on the order of those found experimentally, we rely on efficient density functional-based tight binding methods. In the presence of vacancies and ring defects, the 2D COFs still exhibit semiconducting characteristics with bandgap values that remain close to those of the corresponding perfect configurations. However, the different types of defects bring different modifications to the electronic structure: Vacancy defect levels are found to be primarily located near the top of the valence bands and occupied, while ring defect levels are situated just below the bottom of the conduction bands and unoccupied. Our study of how defects impact the electronic properties of 2D COFs provides valuable insight for future explorations of defect engineering in COFs.