Dependence of Electron-Hole Recombination Rates on Charge Carrier Concentration: A Case Study of Nonadiabatic Molecular Dynamics in Graphitic Carbon Nitride Monolayers

Through systematic nonadiabatic molecular dynamics (NA-MD) calculations in a prototypical graphitic carbon nitride (C₃N₄) monolayer, we demonstrate a strong dependence of electron-hole recombination time scales on the size of the simulation supercell and hence on the formal concentration of charge carriers. Using our recently developed NA-MD methodology with extended tight-binding electronic structure calculations, we have been able to conduct such calculations in C₃N₄ monolayers containing up to 5600 atoms. The predicted time scales vary from 7 ps in the smallest (2 × 2) systems to 23 ns in the largest (8 × 8, 10 × 10) ones and depend on the NA-MD methodology and the basis of electronic excitations used. The concentration dependence becomes negligible only in very large systems, such as 8 × 8 or 10 × 10 supercells. In this limit, the time scales also become insensitive to the number of electronic excitations used in NA-MD calculations and to the inclusion of decoherence effects. We propose an effective state reduction model to explain this result.