New insights on the chemistry of plasma-enhanced atomic layer deposition of indium oxysulfide thin films and their use as buffer layers in Cu(In,Ga)Se₂ thin film solar cell

A comparative chemical analysis of In_xS_y and In₂(S,O)₃ thin films grown by atomic layer deposition (ALD) and plasma-enhanced ALD, respectively, was performed to understand the challenges and issues related to the assistance of plasma, especially for the implementation of these films as ultrathin (<50 nm) interfacial buffer layers in copper indium gallium diselenide (CIGS) solar cells. The films were synthesized using indium acetylacetonate [In(acac)₃], hydrogen sulfide, and an Ar/O₂ plasma as indium, sulfur, and oxygen precursors. Film growth mechanisms and chemistries were studied using gas phase measurements by quadrupole mass spectrometry and x-ray photoelectron spectroscopy for surface and in-depth characterizations. Distinctive signatures of thermal and plasma processes on the overall compositions of the films were evidenced, which were further discussed and explained. Added to this, the impact of the plasma on the underlying substrate, using silicon as a reference, was further investigated to identify its modification. This extensive study has led to a readjustment of the deposition conditions of In₂(O,S)₃ thin films and allowed promising implementation as buffer layers in CIGS solar cells.