Electronic and optical properties of two-dimensional covalent organic frameworks

Two-dimensional covalent organic films with a macrocyclic network were recently synthesized experimentally under simple solvothermal conditions [J. W. Colson et al., Science, 2011, 332, 228], which offers immense potential 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 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-PBBA COF presents a type-I heterojunction alignment where the band-edged wave functions are localized in the same region, achieving 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.