Abstract:

Black phosphorus (BP) is a promising candidate for photovoltaic and optoelectronic applications owing to its excellent electronic and optical properties. It is believed that defects generally accelerate non-radiative electron-hole recombination in BP and hinder improvement of device performance. Experiments defy this expectation. Using state-of-the-art ab initio time-dependent density functional theory combined with non-adiabatic molecular dynamics, we investigate the non-radiative electron-hole recombination in monolayer (MBP) and MBP containing nanopore defects (MBP-ND). We demonstrate that non-radiative electron-hole recombination is promoted by the P-P stretching vibrations, and the recombination time of MBP-ND is approximately 5.5 times longer than that of the MBP system. This is mainly attributed to the following three factors: First, the nanopore creates no mid-gap state when increasing the bandgap by 0.22 eV owing to the downshift of the valence band maximum, caused by the decrease in the inter-layer P-P bond length, thereby weakening the antibonding interaction. Second, the nanopore reduces the overlap of electron and hole wave functions by diminishing the charge densities near the defect. Simultaneously, the nanopore significantly inhibits the thermal-driven atomic fluctuations. The increased bandgap correlated with the decreased wave function overlap and slowed thermal motions of the nuclei in the MBP-ND system reduces the non-adiabatic coupling by a factor of approximately 2 with respect to the pristine system. Third, the slow atomic motions weaken the electron-vibrational interaction and decrease the intensity of the major vibration mode at 440 cm⁻¹, which is the main source for creating non-adiabatic coupling, leading to loss of coherence formed between a pair of electronic states via non-adiabatic coupling and causing electron-hole recombination that results in a 1.5-fold increase in the coherence time in the MBP-ND system with respect to the MBP system. Consequently, the increased bandgap and decreased non-adiabatic coupling compete successfully with the prolonged coherence time, extending the excited-state lifetime to 2.74 ns in the system containing nanopore defects, which is only 480 ps in the pristine system. These phenomena arise owing to a complex interplay of the unusual chemical, structural, electrostatic, and quantum properties of BP with and without nanopore defects. This study is of great significance for understanding the excited-state properties of BP. The detailed mechanistic understanding of the prolonged charge carriers lifetime of MBP decorated with nanopore defects provides key insights for defect engineering in BP and other 2-dimensional materials for a broad range of solar and electro-optic applications by reducing the non-radiative charge and energy losses.

Key words: Monolayer black phosphorus, Nanopore defect, Non-radiative electron-hole recombination, Time-dependent density functional theory, Non-adiabatic molecular dynamics