Excited-State Dynamics in Monolayer Black Phosphorus: Exciton Relaxation Modulated by Defects

Black phosphorus (BP) is a promising candidate for diverse optoelectronic and solar energy applications. The efficiency of these devices, however, is affected by the intrinsic defects of BP. In this study, we employ static electronic structure calculations combined with the linear-response time-dependent density functional theory (LR-TDDFT)-based nonadiabatic dynamics simulation method, which explicitly incorporates excitonic effects, to systematically investigate the regulatory effects of six distinct defect types on the excited-state dynamics in monolayer BP. Defect engineering in pristine BP drastically modifies both ground-state (e.g., density of states) and excited-state properties (e.g., absorption spectrum and exciton size). Such changes result in pronounced differences in exciton relaxation dynamics across various defect configurations. Among all defect models, the DV-(5|8|5)-2 configuration manifests unparalleled characteristics: (i) narrowest band gap (1.43 eV), (ii) maximum exciton size variation across excited-states, (iii) two absorption peaks exhibiting a substantial intensity contrast, and (iv) shortest exciton relaxation time scale. This study offers valuable insights into the influence of defects on the electronic and excitonic properties of BP, laying a foundation for the rational design of BP-based optoelectronic materials.