Constructing Unit Modification To Regulate The Electronic Energy Level Of COFs And The Mechanism

Covalent organic frameworks（COFs）are a class of emerging optoelectronic functional materials that have gained increasing attention due to their potential applications in charge transfer,light absorption/emission,charge injection/extraction/capture within and between molecules,as well as their potential applications in gas storage and separation,energy storage,catalysis,sensing,and optoelectronics.Furthermore,their potential applications as electron or hole injection layers in ultrathin organic optoelectronic devices,such as organic light-emitting diodes（OLEDs）or organic field-effect transistors（OFETs）,have sparked research interest in their interactions with other two-dimensional materials.One important factor in such applications is the alignment of energy levels at the interface between the electrode material and the electron（or hole）injection/transport material.Exploring reliable methods to modulate the interface energy level alignment between COFs and inorganic substrates is a key factor in determining charge transfer efficiency and stability.Another important factor is the energy and orbital distribution of the frontier orbitals of the molecular orbitals.The exploration and systematic study of two-dimensional covalent organic framework materials and in-depth investigation of related molecular modification mechanisms have significant application value.Based on density functional theory,the main research content and conclusions of this paper are as follows:（1）In this work,we theoretically investigate a modification strategy that combines electron-donating/electron-withdrawing groups with the connecting units in COFs.We extensively study TPB-TP-COF,PPy-CC-Ph,and other substituted COFs.The interlayer interaction energy of the double-layer COFs substituted by electron-donating groups is stronger than that of the COFs substituted by electron-withdrawing groups due to the increased interlayer stacking of electrons.Additionally,functional groups with lone pairs of electrons increase the interlayer interaction energy.It is observed that the energy levels of the valence band maximum（VBM）and conduction band minimum（CBM）of the isolated single-layer COFs are reduced by substituting electron-withdrawing groups in the connecting units and increased by substituting electron-donating groups.（2）Using density functional theory,we investigate the doping effects of electron acceptor molecules TCNQ,F₄TCNQ,and TCNE on the frontier orbital energy levels of sp²c-COF and COF366 single layers and bilayers.Due to charge transfer from COFs to the dopant,the Fermi level is shifted to a lower energy and lowered to the top of sp²c-COF and COF366.The VBM/CBM energy levels of COFs are also reduced.Moreover,the increase in electron density shifts the LUMO energy level of the dopant to a higher energy.Subsequently,two electronic states,namely,the top of the valence band of COFs and the LUMO of the p-type dopant,are pinned around the Fermi level.This implies that electron acceptor molecules can serve as effective p-type dopants for COFs.Furthermore,it is confirmed that the stronger the electron-withdrawing ability of the p-type dopant or the higher the surface density of the dopant,the greater the change in the frontier orbital energy levels of the single-layer COF.（3）The influence of functionalized connecting units on the frontier orbital energy levels of COF single layers deposited on inorganic matrices is similar,resulting in changes in the interface energy level alignment of the composite system.Therefore,incorporating electron-donating groups into COF connecting units significantly reduces the electron injection potential from the VBM of the COF single layer to the Fermi level of the inorganic substrate.Conversely,the introduction of electron-withdrawing groups leads to a reduction in theelectron injection potential from the inorganic substrate to the COF single layer.Furthermore,the interface stability of COF/Au（111）or COF/graphene is improved due to the presence of induced functional groups with lone pairs of electrons,which strengthens the energy of the interface interaction.