Intrinsic and Extrinsic Exciton Recombination Pathways in AgInS₂ Colloidal Nanocrystals

Ternary I-III-VI₂ nanocrystals (NCs), such as AgInS₂ and CuInS₂, are garnering interest as heavy-metal-free materials for photovoltaics, luminescent solar concentrators, LEDs, and bioimaging. The origin of the emission and absorption properties in this class of NCs is still a subject of debate. Recent theoretical and experimental studies revealed that the characteristic Stokes-shifted and long-lived luminescence of stoichiometric CuInS₂ NCs arises from the detailed structure of the valence band featuring two sublevels with different parity. The same valence band substructure is predicted to occur in AgInS₂ NCs, yet no experimental confirmation is available to date. Here, we use complementary spectroscopic, spectro-electrochemical, and magneto-optical investigations as a function of temperature to investigate the band structure and the excitonic recombination mechanisms in stoichiometric AgInS₂ NCs. Transient transmission measurements reveal the signatures of two subbands with opposite parity, and photoluminescence studies at cryogenic temperatures evidence a dark state emission due to enhanced exchange interaction, consistent with the behavior of stoichiometric CuInS₂ NCs. Lowering the temperature as well as applying reducing electrochemical potentials further suppress electron trapping, which represents the main nonradiative channel for exciton decay, leading to nearly 100% emission efficiency.