Bifunctional Hybrid a-SiO<i>_x</i>(Mo) Layer for Hole-Selective and Interface Passivation of Highly Efficient MoO<i>_x</i>/a-SiO<i>_x</i>(Mo)/n-Si Heterojunction Photovoltaic Device

Abstract

The promising n-Si-based solar cell is constructed for the purpose of realizing hole- and electron-selective passivating contact, using a textured front indium tin oxide/MoO _x structure and a planar rear a-SiO _x/poly(Si(n⁺)) structure severally. The simple MoO _x/n-Si heterojunction device obtains an efficiency of 16.7%. It is found that the accompanying ternary hybrid SiO _x(Mo) interlayer (3.5-4.0 nm) is formed at the MoO _x/n-Si boundary zone without preoxidation and is of amorphous structure, which is determined by a high-resolution transmission electron microscope with energy-dispersive X-ray spectroscopy mapping. The creation of lower-oxidation states in MoO _x film indicates that the gradient distribution of SiO _x with Mo element occurs within the interlayer, acting as a passivation of silicon substrate, which is revealed by X-ray photoelectron spectroscopy with depth etching. Specifically, calculations by density functional theory manifest that there are two half-filled levels (localized states) and three unoccupied levels (extended states) relating to Mo component in the ternary hybrid a-SiO _x(Mo) interlayer, which play the roles of defect-assisted tunneling and direct tunneling for photogenerated holes, respectively. The transport process of photogenerated holes in the MoO _x/n-Si heterojunction device is well-described by the tunnel-recombination model. Meanwhile, the a-SiO _x/poly(Si(n⁺)) has been assembled on the rear of the device for direct tunneling of photoinduced electrons and blocking photoinduced holes.