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 (N 2 H 5 ) 2 S, [Cu 6 S 4 ] 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 (N 2 H 5 ) 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 [Cu 6 S 4 ] 2– complexes. This difference in sulfur incorporation efficiency may be due to the relatively weak Cu–S bonds present in the [Cu 6 S 4 ] 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.