Defect-assisted Auger recombination in graphitic carbon nitride revealed by excitation-density-dependent transient absorption spectroscopy

Defect-assisted Auger recombination in graphitic carbon nitride (CN) is systematically elucidated using excitation-density-dependent femtosecond transient absorption spectroscopy (fs-TAS). The band-edge state of CN is demonstrated to exhibit a twofold degeneracy, originating from electronic transitions between symmetry-equivalent exciton states formed by conjugated π-orbitals. Quantitative analysis of the π-orbital exciton recombination kinetics by a rate equation resolves three distinct photocarrier relaxation pathways: monomolecular trapping, bimolecular recombination, and defect-assisted Auger recombination. The photocarrier kinetics accelerates nonlinearly as the excitation densities increase, revealing the dominance of the Auger recombination under intense photoexcitation. Notably, the extracted Auger recombination coefficient on thermally treated CN (TTCN) is significantly reduced by an order of magnitude compared to that of pristine CN (PCN), mainly attributing to the deactivation of the defect states by crystallinity enhancement. These findings establish a comprehensive kinetic framework for defect-mediated Auger recombination in graphitic CN, offering critical guidance to advance its applications for photoelectric conversion.