Engineering Defects in 2D PdSe 2 to Tailor Photocarrier Dynamics for Enhanced Nonlinear Saturable Absorption and Optoelectronic Performance

ABSTRACT Engineering structural defects in 2D materials offers a powerful pathway to tailor their properties and functionalities, yet a comprehensive understanding of defect‐governed optical activity remained incomplete. Here, this study presents an in‐situ strategy to engineer defect concentration and type in 2D PdSe 2 through bottom‐up control of thickness and morphology, and elucidates the link between defect‐mediated photocarrier relaxation behaviors and boosted performance in intrinsic nonlinear optical saturation absorption (SA) and optoelectronics. Thinner rectangular PdSe 2 flakes were found to exhibit higher selenium vacancy (V Se ) concentration, while dendritic morphology further increased both V Se concentration and edge complexity. Moreover, defect‐regulated distinct photocarrier relaxation mechanisms were unveiled. Rapid shallow photocarriers trapping prevailing over slower deep trapping/recombination was amplified with rising V Se concentration and directly suppressed deep trapping via additional complex edge defects. Crucially, these shallow‐defect‐state‐dominated photocarrier dynamics enabled SA enhancement in thin rectangular PdSe 2 and further elevated in dendritic flakes (superior nonlinear absorption coefficient: − 5.05 × 10 5 cm GW −1 , modulation depth: ∼41.4%). Also, dendritic PdSe 2 exhibited stronger negative photoresponses at atmospheric environment, originating from more O 2 adsorption‐desorption and efficient photocarrier‐self‐recombination. This work provides effective routes for customizing 2D atomic crystals and engineering their structure‒property relationships, opening new possibilities for advanced photonic and optoelectronic applications.