Ag₈SnS₆: a new IR solar absorber material with a near optimal bandgap

We report the synthesis and photovoltaic properties of a new ternary solar absorber - Ag₈SnS₆ nanocrystals prepared by successive ionic layer adsorption reaction (SILAR) technique. The synthesized Ag₈SnS₆ nanocrystals have a bandgap E _g of 1.24-1.41 eV as revealed from UV-Vis and external quantum efficiency (EQE) measurements. Its photovoltaic properties were characterized by assembling a liquid-junction Ag₈SnS₆ sensitized solar cell for the first time. The best cell yielded a J _sc of 9.29 mA cm^-2, a V _oc of 0.23 V, an FF of 31.3% and a power conversion efficiency (PCE) of 0.64% under 100% incident light illumination using polysulfide electrolyte and Au counter electrode. The efficiency improved to 1.43% at a reduced light intensity of 10% sun. When the polysulfide was replaced by a cobalt electrolyte with a lower redox level, the V _oc increased to 0.54 V and PCE increased to 2.29% under 0.1 sun, a respectable efficiency for a new solar material. The EQE spectrum covers the spectral range of 300-1000 nm with a maximum EQE of 77% at λ = 600 nm. The near optimal E _g and the respectable photovoltaic performance suggest that Ag₈SnS₆ nanocrystals have potential to be an efficient IR solar absorber.