Mechanism of Sulfur Incorporation into Solution Processed CuIn(Se,S)2 Films

We have investigated the incorporation of sulfur into CuIn(Se,S)(2) thin films from different bonding environments in hydrazine-based precursor solutions. Sulfur is present in the form of (N2H5)(2)S, [Cu6S4](2-), and [In-2(Se,S)(4)](2-) complexes in mixed CuIn(Se,S)(2) precursor solutions. On the basis of compositional information from the precursor solutions and annealed films, we find that the incorporation efficiency of sulfur from (N2H5)(2)S into the final film is extremely low as a result of the high volatility of this compound and its weak interaction with other species while in solution. Using the same methodology, we additionally report that approximately 80% of the sulfur from [In-2(Se,S)(4)](2-) is incorporated into the final material, compared to approximately 40% of the sulfur from [Cu6S4](2-) complexes. This difference in sulfur incorporation efficiency may be due to the relatively weak Cu-S bonds present in the [Cu6S4](2-) structure, which are somewhat unstable compared to the In-S bonds in the [In-2(Se,S)(4)](2-) complex. This method makes it possible to precisely control the sulfur content in CuIn(Se,S)(2) films by adjusting the S/Se ratio of the [In-2(Se,S)(4)](2-) ions in the final precursor solution. These results will enable the precise adjustment and optimization of the energy band gap in solution-processed CuIn(Se,S)(2) absorber layers for the future fabrication of improved photovoltaic devices.