Molybdenum oxide: A superior hole extraction layer for replacing p-type hydrogenated amorphous silicon with high efficiency heterojunction Si solar cells

Transition metal oxides (TMO) are extensively applied as a surface passivation and carrier-selective contact layer through replacing boron/phosphorus doped emitter layers in silicon heterojunction (SHJ) solar cell applications. In this regard, molybdenum oxide (MoO₃) has drawn a significant attention as a hole extraction layer owing properties such as wide bandgap (∼3 eV), high work function (>6 eV) and low temperature deposition. Thus, we fabricated SHJ solar cells with a dopant-free MoO_x applied at the front surface contact layer. Thermally evaporated MoO_x films were exhibited optical characteristics such as high transmittance, high bandgap and low absorption coefficient as compared to a-Si:H(p) and μc-SiO_x:H (p) layers. X-ray photoelectron spectroscopy (XPS) analysis confirmed the stoichiometric and oxidation deficiency states of the of the MoO_x layers. Whereas, MoO_x films undergoing long-term air exposure showed an increase in Mo⁵⁺ cations due to the increased oxygen vacancy. The fabricated MoO_x/c-Si heterojunction solar cells achieved a significant power conversion efficiency (η) of 20%, best open circuit voltage (V_oc) of 695 mV, high short circuit current density (J_sc) of 38.88 mA/cm² and a fill factor (FF) of 74.0%. These results implying that MoO_x is as an excellent dopant-free material for alternate p-doped a-Si:H emitter layers in SHJ solar cell applications.