Lithium Thiostannate Spinel Thin Films as Potential Photovoltaic Absorbers

Quaternary lithium thiostannate spinels represent a promising class of non-toxic earth-abundant semiconductor absorbers for photovoltaic applications, reported to be air-stable in bulk form. To the best of our knowledge, no attempts to prepare their thin films and evaluate their photoresponse have been reported. Here, we demonstrate the first successful deposition of nanocrystalline Li_1.66CuSn_3.33S₈ spinel films (∼350 nm thickness) via pulsed laser deposition (PLD). We observe that the crystal structure of the PLD target governs the structure and optoelectronic properties of the resulting film. A phase-pure spinel target yields a homogeneous layer with a high degree of crystallinity, strong optical absorptance with a clear absorption edge at ∼1.5 eV and low sub-bandgap values, as determined by photothermal deflection spectroscopy. The photoluminescence peak centred at 836 nm, consistent with the absorption band edge, together with measurable photoconductivity, confirms semiconducting character. Therefore, we identify the PLD-deposited Li_1.66CuSn_3.33S₈ films as a candidate for a photo-absorber material, while noting that further improvements in film crystallinity, sulphur stoichiometry and Urbach energy are required before photovoltaic device integration can be assessed.