Optical Absorption and Exciton Dispersion of Bulk Corrugated g-C3N4

Graphitic carbon nitride (g-C3N4) is a promising layered metal-free photocatalyst whose optical properties are strongly affected by structural corrugation induced by nitrogen lone-pair repulsion. However, calculations for the bulk have largely focused on the ideal planar structure. Here, we investigate the structural, electronic, and excitonic properties of corrugated bulk g-C3N4 using density functional theory, many-body perturbation theory, and the Bethe–Salpeter equation. The absorption spectrum is dominated by bound excitons below the quasiparticle gap, with the first bright exciton at 2.75 eV (dark in the ideal flat structure) in good agreement with experiment. Bulk excitons arise from symmetric and antisymmetric combinations of monolayer excitations mixed by interlayer hopping, leading to a Davydov splitting and to states with mixed Frenkel and charge-transfer character. Their bandwidths are only a few to a few tens of meV, indicating inefficient exciton propagation both in-plane and out-of-plane.