Defect Suppression in Ag–In–S Quantum Dot Solar Cells via Gallium Alloying and GaS y Shelling

Ternary semiconductor quantum dots (QDs) such as silver indium sulfide (AgInS2) represent an emerging class of heavy-metal-free nanomaterials with strong potential for light-harvesting applications. However, their photophysical properties are strongly influenced by intrinsic and surface defects, which act as donor–acceptor pair (DAP) states. Incorporating gallium into the crystal lattice is an effective way to reduce the number of defect states. We have alloyed the AgInS2 lattice with gallium to make AgInxGa1–xS2 QDs, subsequently growing a GaSy shell around the gallium-alloyed core to make AgInxGa1–xS2–GaSy core/shell QDs. These QDs are stable for several weeks when stored in the dark. The photovoltaic performance of these QDs in liquid-junction quantum dot-sensitized solar cells (QDSSCs) is evaluated to investigate the effect of gallium insertion in these ternary semiconductors. The electron injection from excited AgInS2 and its two gallium derivatives into mesoporous TiO2, probed using femtosecond transient absorption spectroscopy, indicated rate constants on the order of 109–1010 s–1. Insertion of gallium into the AgInS2 QDs enhances the photovoltaic performance, as evidenced by the increase in open-circuit voltage (Voc), short-circuit current density (JSC), and overall power conversion efficiency (PCE). The gallium insertion strategy presented in this study demonstrates the feasibility of improving the photoconversion efficiency of group I–III–VI ternary QD semiconductors by mitigating interstitial defect states.