  Organic thermoelectric (TE) materials create a brand new perspective to
search for high-efficiency TE materials, due to their small thermal
conductivity. The overlap of pz orbitals, commonly existing in organic
{\pi}-stacking semiconductors, can potentially result in high electronic
mobility comparable to inorganic electronics. Here we propose a strategy to
utilize the overlap of pz orbitals to increase the TE efficiency of layered
polymeric carbon nitride (PCN). Through first-principles calculations and
classical molecular dynamics simulations, we find that A-A stacked PCN has
unexpectedly high cross-plane ZT up to 0.52 at 300 K, which can contribute to
n-type TE groups. The high ZT originates from its one-dimensional charge
transport and small thermal conductivity. The thermal contribution of the
overlap of pz orbitals is investigated, which noticeably enhances the thermal
transport when compared with the thermal conductivity without considering the
overlap effect. For a better understanding of its TE advantages, we find that
the low-dimensional charge transport results from strong pz-overlap
interactions and the in-plane electronic confinement, by comparing
{\pi}-stacking carbon nitride derivatives and graphite. This study can provide
a guidance to search for high cross-plane TE performance in layered materials.
