Degradation of MoOx Thin-Films Properties in Excessive Oxygen Environments and Its Influence on Dopant-Free Silicon Solar Cells

Transition metal oxides such as molybdenum oxide (MoO_x) demonstrate significant potential as efficient hole-selective passivating contacts in silicon heterojunction solar cells. Achieving efficient hole collection necessitates precise control over the optical and electrical properties of MoO_x films. In this study, the effects of oxygen flow rate ($F_{O_2}$) on the growth, optical properties, and electrical properties of thermally evaporated MoO_x films are investigated. In the Kelvin probe force microscopy results, it is indicated that MoO_x thin-film deposition followed an island-to-layer growth model. X-ray photoelectron spectroscopy shows that MoO_x films exhibit stoichiometric composition with fully oxidized Mo⁶⁺ ions, without additional oxygen. Notably, the O 1s orbital peak shifts toward higher binding energy with increased $F_{O_2}$, indicating defect introduction. Consequently, the work function of MoO_x films decreases from 5.93 to 5.51 eV as $F_{O_2}$ increases from 0 to 8 sccm. The maximum optical bandgap of the MoO_x films exceeds 3.60 eV. As a proof of concept, $F_{O_2}$'s impact on MoO_x as a front buffer layer for dopant-free silicon solar cells is analyzed. An efficiency of 20.8% was achieved for dopant-free silicon solar cells after optimized MoO_x film deposition, with an open-circuit voltage of 713.7 mV, short-circuit current density of 39.1 mA cm⁻², and fill factor of 74.6%.