Solar cells fabricated from metal sulfide quantum dots require facile and reproducible synthetic methods that determine the particle size, distribution, shape and faceting of the quantum dots. These properties will affect the energetic landscape of the nanocrystal thin films, by modifying the bandgap, trap density and surface composition. So far, an established hot injection synthesis route has been adapted for most metal sulfide quantum dots, resulting in solar cells with record efficiencies such as 15.5% for PbS dots (Advanced Energy Materials 2022, 12 (35), 2201676) and 9.2% for environmentally-friendly AgBiS₂ (Nature Photonics 2022, 16, 235). The synthesis of most metal sulfide quantum dot relies on the use of a strongly reactive sulfur source: bis(trimethylsilyl) sulfide (TMS)₂S. This sulfur compound is known to hydrolyze in the presence of water, creating toxic and foul smelling H₂S. Consequently, its storage and handling are problematic even for a small-scale laboratory facility. Moreover, this hydrolyzation causes the synthesis to be irreproducible, since seemingly the same synthetic conditions end up leading to large variations in the quantum dot average size and distribution. These disadvantages, together with its high cost, make (TMS)₂S unsuitable for large scale applications and support the motivation to find alternative sulfur precursors for metal sulfide quantum dot synthesis.

In this study we demonstrate that bis(stearoyl) sulfide (St₂S) is an excellent alternative to (TMS)₂S and illustrate this via the synthesis of PbS and AgBiS₂ quantum dots and their application in photovoltaics. St₂S is a solid, odor-free, air-stable sulfur compound with a low melting point of 60°C, which can be used instead of (TMS)₂S with minimal changes to the synthesis procedure. We characterize the quantum dots by transmission electron microscopy, and confirm the chemical composition by diverse spectroscopic techniques. Furthermore, we prove that without further optimization, PbS quantum dot solar cells made using St₂S are equally efficient as those made using (TMS)₂S. In the case of AgBiS₂, the performance with St₂S is slightly lower than that of the referenced devices, due to a smaller overall size of the nanoparticles. Importantly, photovoltaic cells fabricated using St₂S precursors are more stable because the dots are less prone to surface oxidation. To summarize, the substitution of the unstable (TMS)₂S with the air-stable St₂S is highly promising for the synthesis of metal sulfide quantum dots for a broad range of applications.