Interfacial Energy Level Modulation by Tuning the Electronic Character of Covalent Organic Frameworks: A Linker Functionalization Strategy

Covalent organic frameworks (COFs) are optoelectronic functional materials. Exploring reliable ways to modulate the interfacial energy level alignment between COFs and inorganic substrates, one of the key factors determining the charge transfer efficiency and stability, is critical for practical applications in organic electronic devices. Here, we aim to tune the electronic properties of TPB-TP-COF and PPy-CC-Ph by linker functionalization. In both cases, it is observed that the energy levels of the frontier orbitals are decreased by linker functionalization with electron-withdrawing groups and increased by linker functionalization with electron-donating groups. Similar effects of linker functionalization are found for the COFs on inorganic substrates. Consequently, the electron injection barrier from the valence band maximum of the COF to the Fermi level of the inorganic substrate is reduced remarkably by incorporation of electron-donating groups into the linker, and the electron injection barrier from the Fermi level of the inorganic substrate to the conduction band minimum of the COF is reduced similarly by incorporation of electron-withdrawing groups into the linker. Moreover, we show that the stability of the COF/graphene and COF/Au(111) interfaces is improved by delocalization of the electron lone pairs of the functional groups. Linker functionalization enables efficient modulation of the interfacial energy level alignment between COFs and inorganic substrates to improve the interface stability and, thus, optimize the device implementation.