Cation Exchange in Zn–Ag–In–Se Core/Alloyed Shell Quantum Dots and Their Applications in Photovoltaics and Water Photolysis

Quantum dot-sensitized solar cells (QDSSCs), especially those based on ternary AgInX2 (X = S, Se) QDs, suffer from very low power conversion efficiencies (PCEs) owing to presence of high density of trap states and imperfect surface coverage by surfactants. This has a similar adverse effect on water photolysis when the solar cells are integrated with the electrolyzer. Passivation of the QD surface with an inorganic shell is vital and temporally regulated cation exchange can come to the rescue, whereby an alloyed shell can minimize the interfacial trap states which are mostly abundant in core/shell QDs with sharp interfaces. Herein, Zn–Ag–In–Se (ZAISE) core QDs were cation-exchanged with Cd2+ ions to create an alloyed shell of Ag–In–Zn–Cd–Se. When the QDs were used as absorbers, the PCE was enhanced from 3.01% for core ZAISE QDs to 4.71% for core/alloyed shell QDs by optimizing the shell thickness; 4.71% is also one of the superior PCEs in AgIn(S/Se)2-based photovoltaic systems. The density of states investigated via density functional theory formalism shows that the feasibility of charge transportation varies with the thickness of the alloyed shell. Furthermore, a solar-to-hydrogen efficiency (ηSTH) of 2.66 ± 0.06% was achieved when four QDSSC devices were integrated with a water-splitting electrolyzer constructed with earth-abundant catalysts.