Impact of a Thin Sacrificial Mo Layer on the Formation of the Wide Band Gap ACIGSe Absorber/ITO Thin-Film Solar Cell Interface

The effect of a thin sacrificial Mo interlayer on the device performance of a thin-film solar cell based on a wide band gap (1.46 eV) (Ag, Cu)(In, Ga)Se₂ (ACIGSe)/indium tin oxide (ITO) layer stack and its influence on the chemical composition at the absorber/transparent back contact (TBC) interface are investigated. The solar cell without a Mo interlayer exhibits an efficiency of less than 1%, whereas the inclusion of an approximately 10 nm thick Mo interlayer produces a cell with an efficiency of 10.1%, with the fill factor being more than tripled. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy line scans show strong GaO_<i>x</i> formation at the absorber/transparent conductive oxide (TCO) interface in a Mo-free sample and a significant decrease in the amount of GaO_<i>x</i> in a sample with the Mo interlayer included. Cleaving the absorber/TBC interface allowed us to study the absorber back side and the exposed ITO TCO using hard X-ray photoelectron spectroscopy. The nearly complete conversion of the metallic Mo interlayer into MoSe₂ and MoO_<i>x</i> is revealed. It is suggested that the formation of MoO_<i>x</i>, which may act as a hole-selective contact, together with inhibited GaO_<i>x</i> formation might be responsible for the pronounced performance enhancement for the cell with the thin Mo interlayer.