Tin(II) sulfide is a chemical compound of tin and sulfur. The chemical formula is SnS. Its natural occurrence concerns herzenbergite (α-SnS), a rare mineral. At elevated temperatures above 905 K, SnS undergoes a second order phase transition to β-SnS (space group: Cmcm, No. 63).[3] In recent years, it has become evident that a new polymorph of SnS exists based upon the cubic crystal system, known as π-SnS (space group: P213, No. 198).[4][5]
In principle, tin(II) sulfide can be prepared directly from the elements:
However, the high-temperature reaction vaporizes sulfur. Consequently effecting the proper stoichiometry is difficult.[6] At near-room temperature in liquid ammonia solution, the same reaction still gives a mixture of 60% stannous and 35% stannic sulfide.[7] Likewise metallic tin contaminates triaryltin sulfide pyrolysates.[7]
At cryogenic temperatures, stannous chloride dissolves in liquid hydrogen sulfide. It then decomposes to the sulfide, but only slowly.[8]
Instead, molten, pure, anhydrous potassium thiocyanate reliably reduces stannic oxide to SnS at 450 °C. An aqueous rinse then removes the potassium sulfide coproduct:[6]
Tin(II) sulfide is a dark brown or black solid, insoluble in water, but soluble in concentrated hydrochloric acid. Tin(II) sulfide is insoluble in (NH4)2S. It has a layer structure similar to that of black phosphorus.[9] As per black phosphorus, tin(II) sulfide can be ultrasonically exfoliated in liquids to produce atomically thin semiconducting SnS sheets that have a wider optical band gap (>1.5 eV) compared to the bulk crystal.[10]
Tin(II) sulfide is an interesting potential candidate for next generation thin-film solar cells. Currently, both cadmium telluride and CIGS (copper indium gallium selenide) are used as p-type absorber layers, but they are formulated from toxic, scarce constituents.[11] Tin(II) sulfide, by contrast, is formed from cheap, earth abundant elements, and is nontoxic. This material also has a high optical absorption coefficient, p-type conductivity, and a mid range direct band gap of 1.3-1.4 eV, required electronic properties for this type of absorber layer.[12] Based on the a detailed balance calculation using the material bandgap, the power conversion efficiency of a solar cell utilizing a tin(II) sulfide absorber layer could be as high as 32%, which is comparable to crystalline silicon.[13] Finally, Tin(II) sulfide is stable in both alkaline and acidic conditions.[14] All aforementioned characteristics suggest tin(II) sulfide as an interesting material to be used as a solar cell absorber layer.
At present, tin(II) sulfide thin films for use in photovoltaic cells are still in the research phase of development with power conversion efficiencies currently less than 5%.[15] Barriers for use include a low open circuit voltage and an inability to realize many of the above properties due to challenges in fabrication, but tin(II) sulfide still remains a promising material if these technical challenges are overcome.[13]
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