Defect-Mediated Exciton Storage in Ag–In–Ga–S Nanocrystals

Colloidal Ag–In–Ga–S nanocrystals (NCs) represent a promising class of RoHS-compliant light emitters exhibiting narrow excitonic photoluminescence (PL). Here, we unveil a unique exciton storage mechanism in Ag–In–Ga–S NCs. Temperature-dependent PL and ultrafast transient absorption spectroscopy show that thermally activated back transfer from long-lived (∼1.8 μs) shallow defects repopulates the excitons, increasing both exciton lifetime and PL intensity. The thermally activated back transfer increases the excitonic PL lifetime systematically from a few nanoseconds at 6.5 K to about 100 ns at 300 K, a reverse trend compared to typical semiconductor NCs like CdSe. This reverse trend of Ag–In–Ga–S NCs mirrors dopant-mediated exciton dynamics in Mn-doped CdSe NCs but arises here from intrinsic defects of the undoped NCs. Our results establish a generalizable pathway for prolonging excitonic lifetime (exciton storage) with high PL intensity in semiconductor NCs (quantum dots), enabling potential applications in photocatalysis, photonic memory, and optoelectronic devices.