Valence band offset at MoO3/CdTe interface probed by X-ray photoelectron spectroscopy

CdTe-based thin-film solar cells fabricated with Cu-free MoO
₃
/Au back contacts have achieved cell efficiencies as high as 14%. The incorporation of MoO
₃
buffer layers improves the open circuit voltage but reduces the fill factor (FF) of the CdTe cells compared to the cells using Au-only contacts. X-ray photoelectron spectroscopy measurements suggest a 2.75±0.20 eV valence band offset (VBO) between CdTe and MoO
₃
. Such a large VBO explains the decrease in FF with the increase of the thickness of the MoO
₃
buffer layer.