Self-stabilizing molecular solution for Cu2SnS3 thin film: An insight into the oxidation inhibitor of bivalent tin ion

Molecular solution with the metal-thiourea complex is a facile and effective route for Cu2SnS3 (CTS) thin film deposition. However, its stability is still challenging due to flexible metal salts precursors. Here we report a strategy to enhance the stability of the Cu–Sn–S molecular solution that use bivalent Sn to suppress the oxidation of the S2- of thiourea via the redox reaction. The self-stabilizing mechanism has been carefully revealed by the systematic experiments and confirmed by the spontaneous reaction process of metal-thiourea system. It demonstrates that Sn with a low valence state weakens the transfer of lone pair electrons from thiourea to Cu2+ and Sn4+ ions, alleviating the sulfur precipitates from thiourea oxidation because of the easily reducible character of Sn2+ ion. The solution with Sn2+ antioxidant additive can be stored for a long time under air ambient, which is then spin-coated and sulfurized with Sn powder compensation. The uniform CTS thin film with composition control is finally achieved and delivers a device efficiency of 1.15%. This work provides a guide on stabilizing the solution precursor for chalcogenide thin film deposition.