Effective Fermi-Level Modulation of Two-Dimensional Conjugated Covalent Organic Frameworks Achieved by Introducing p-Type Organic Dopants

Covalent organic frameworks (COFs) are a new emerging class of two-dimensional functional materials, and their unique electronic character plays a key role in practical applications. Here, to provide a useful strategy to tune the electronic character of COFs, doping effects with organic molecular acceptors, TCNQ, F4TCNQ, and TCNE, on the frontier orbital energy levels of sp2c-COF and COF366 mono- and bilayers are explored. Due to the charge transfer from COFs to dopants, the Fermi level is shifted to a lower energy and drops into the top valence band of sp2c-COF and COF366, and the VBM/CBM energy level of COFs is also decreased. Moreover, the increased electron density shifts the LUMO energy level of dopants to a higher energy. Then, two electronic states, the top valence band of COFs and the LUMO of the p-type dopants, are pinned around the Fermi level. It means that the organic molecular acceptor serves as an effective p-type dopant for COFs. In addition, it is confirmed that the stronger the electron-accepting ability of p-type dopants or the higher the surface density of the dopants, the larger the variation of the frontier energy levels of COF monolayers will be. Thereby, an overall linear correlation between the electronic property variations of COFs and the charge transfer amount from COFs to p-type dopants is observed. Our results proved that surface doping with organic molecular acceptors is a reliable approach to modulate the frontier energy level of COFs, which provides an effective strategy to optimize the performance of COF-based devices.