Unexpectedly high cross-plane thermoelectric performance of layered carbon nitrides

Abstract

Organic thermoelectric (TE) materials provide a brand new perspective to search for high-efficiency TE materials, due to their low thermal conductivity. The overlap of p_z orbitals, commonly existing in organic π-stacking semiconductors, can potentially result in high electron mobility comparable to that of inorganic materials. Here we propose a strategy to utilize the overlap of p_z orbitals to increase the TE efficiency of a layered polymeric carbon nitride (PCN). Through first-principles calculations and classical molecular dynamics simulations, we find that an A–A stacked PCN has unexpectedly high cross-plane ZT up to 0.52 at 300 K, and can contribute to n-type TE groups. The high ZT originates from its one-dimensional charge transport and low thermal conductivity. The thermal contribution of the overlap of p_z orbitals is investigated, which slightly enhances the thermal transport when compared with that without considering the overlap effect. That is, there is a limited influence of orbital overlap to thermal conductivity. To explore the physical insight behind its TE advantages, we find that the low-dimensional charge transport results from strong p_z-overlap interactions and the in-plane electron confinement, by comparing π-stacking carbon nitride derivatives and graphite. This study can provide guidance to search for layered materials with high cross-plane TE performance.