Mechanochemically Synthesized SnS Nanocrystals: Impact of Nonstoichiometry on Phase Purity and Solar Cell Performance

We demonstrate nontoxic, earth-abundant light-absorbing SnS thin films fabricated by a low-cost, environmentally friendly nonvacuum process. SnS nanocrystals (NCs) are mechanochemically synthesized from elemental powders without the use of any other additives or solvents. To investigate the effect of the Sn-to-S stoichiometric ratio on the crystalline phase of the SnS NCs, the nonstoichiometry is systematically controlled from 0.95 (Sn_0.95S) to 1.05 (Sn_1.05S) by adjusting the mixing ratio of the Sn and S powders. The crystallographic evolution with the milling time signifies that the formation of the SnS phase follows a mechanochemically driven self-propagation reaction mechanism. The as-synthesized SnS NCs with a stoichiometric composition (i.e., Sn_1.00S) are found to contain a Sn₂S₃ impurity phase in a non-negligible amount, which can be subsequently eliminated by a post heat treatment at 500 °C in a reducing atmosphere. Interestingly, however, the formation of Sn₂S₃ during the mechanochemical synthesis process is greatly alleviated by introducing a Sn-excess composition (e.g., Sn_1.05S). In addition, the solar cell with a Sn_1.05S absorber exhibits a much higher efficiency as compared to the Sn_0.95S- or Sn_1.00S-based devices, which is likely attributed to the improved phase purity of Sn-excess SnS as well as to its better microstructure with higher crystallinity than the other compositions.