PNNL

Abstract

Two-dimensional covalent organic film with macrocyclic network under simple solvothermal conditions was recently synthesized experimentally [Colson, J. W.; et al. Science 2011, 332, 228], which offers immense potentials for optoelectronic applications as in the case of graphene. Here we systematically investigate the electronic and optical properties of such novel covalent organic frameworks (COF-5, TP-COF and NiPc PBBA-COF) as free-standing sheets using density-functional theory. The results shed considerable light on the nature of spatial carrier confinement with band offset. COF-5 exhibits a type-II heterojunction alignment with the significant valence and conduction band offsets, suggesting an effective spatial carrier separation of electrons and holes. In TP-COF, the valence offset is close to zero related to the dispersed distribution of photoexcited holes over the entire structure, while the conduction-band offset is still remarkable, indicating the effective confinement of photoexcited electrons. NiPc PBBACOF presents a type-I heterojunction alignment where the band-edged wave functions are localized in the same region, achieving the effective spatial carrier congregation. The calculated absorption peaks of the optical absorption of TP-COF and NiPc-PBBA COF frameworks are in agreement with experimental measurements, thus providing theoretical insights into experimental observed transmission spectra of these frameworks.