How Effective Are Sub-Bandgap States in AgInS₂ Quantum Dots for Electron Transfer?

Abstract

Ternary I−III−VI 2 semiconductors, such as CuInS 2 and AgInS 2 (compliant with RoHS, restriction of hazardous substances), are useful as light-harvesting materials. However, the presence of sub-bandgap states (donor−acceptor pair or DAP) introduces complexity during their activation through photoexcitation. When photoirradiated, the photogenerated charge carriers in AgInS 2 quantum dots undergo rapid relaxation to populate intrinsic DAP states while competing with charge carrier recombination. Interestingly, these defect-related DAP states can be activated through sub-bandgap excitation and, thus, extend the absorption range to the near-infrared region. We have now employed time-resolved absorption and emission techniques to glean mechanistic insights into the photophysical properties of intragap states of AgInS 2 quantum dots (QDs) and their participation in interfacial electron transfer. When the AgInS 2 QDs are excited with above bandgap excitation (400 nm), we observe a prompt formation (<1 ps) of the bleach at wavelengths closer to the bandgap, indicating the formation of a charge-separated pair. This transient bleach shifts to lower energies with time (∼5 ps), indicating population of sub-bandgap states via relaxation of electrons and holes from the conduction and valence bands, respectively. These sub-bandgap states which can also be populated via direct excitation using low energy (λ < E g ) excitation exhibit prompt bleach (<1 ps) in contrast to bandgap excitation. The excited DAP states are long-lived (∼1 μs) and can participate in the electron transfer process. We have elucidated the electron transfer dynamics from these midgap states of AgInS 2 by employing ethyl viologen (EV 2+ ) as a probe molecule. The role of surface-anchored viologen as an electron shuttle was further exploited by using free-floating benzoquinone (BQ) as a secondary electron acceptor. The sub-bandgap response of AgInS 2 to promote electron transfer paves the way to extend the photoresponse of ternary I−III−VI 2 semiconductor-based photocatalytic systems.