Emergent Self‐Trapped Excitonic Behavior Arising From the Interplay of Size, Solvent, and Dopants in Black Phosphorus Quantum Dots

ABSTRACT Self‐trapped excitons (STEs) enable broadband emission and robustness against environmental perturbations, yet tunable and biocompatible STE platforms remain elusive. Here we demonstrate that black phosphorus quantum dots (BPQDs) constitute an exceptional platform for STEs. Using optimally tuned range‐separated hybrid functionals, we accurately capture the excitation‐induced structural distortions and electronic localization in BPQDs. Quantum confinement primarily drives STE formation, as size reduction forces electron–hole localization into anisotropic antibonding states to amplify bond elongation. Conversely, solvation counteracts this localization, driving the system toward a free‐exciton regime. Edge doping exhibits a pronounced concentration‐dependent duality, offering additional degrees of freedom for STE regulation. Notably, heavy oxygen doping most effectively stabilizes deep STEs, whereas low‐concentration elemental doping consistently attenuates trapping strength. Consequently, strongly confined STE states exhibit remarkable robustness against environmental perturbations. These findings are further extended to puckered quantum dots of other group VA elements (As, Sb, Bi), providing general design principles for STE engineering in optoelectronic and biomedical applications.