Bifunctional Hybrid a-SiO_x(Mo) Layer for Hole-Selective and Interface Passivation of Highly Efficient MoO_x/a-SiO_x(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 structure and a planar rear a-SiO/poly(Si(n)) structure severally. The simple MoO /n-Si heterojunction device obtains an efficiency of 16.7%. It is found that the accompanying ternary hybrid SiO(Mo) interlayer (3.5-4.0 nm) is formed at the MoO /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 film indicates that the gradient distribution of SiO 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(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 /n-Si heterojunction device is well-described by the tunnel-recombination model. Meanwhile, the a-SiO/poly(Si(n)) has been assembled on the rear of the device for direct tunneling of photoinduced electrons and blocking photoinduced holes.

Towards electrostatic doping approaches in ultra-thin body semiconductor materials and devices

“…At the interface with the Si, an amorphous interlayer of about 2 nm in thickness is observed in both cases. Such a layer was also reported in other studies [175] where energy-filtered TEM (EF-TEM) analysis revealed a composition of Si, O and Mo atoms, referred to as a hybrid a-SiO x (Mo) layer. The accompanying SiO x (Mo) interlayer, which is possibly formed during the deposition of MoO x on Si, could have multiple roles in the charge carrier transport in our devices such as passivating the Si surface, hence suppressing the electron current and inducing a hole inversion layer.…”

“…The accompanying SiO x (Mo) interlayer, which is possibly formed during the deposition of MoO x on Si, could have multiple roles in the charge carrier transport in our devices such as passivating the Si surface, hence suppressing the electron current and inducing a hole inversion layer. Further, the sub-stiochiometric nature of the e-beam deposited MoO x thin films was confirmed by X-ray photoelectron spectroscopy (XPS) analysis (not shown) which indicates x=2.42±0.24 that is in close agreement with other reports [162,175]. The bipolar-mode diode characteristics of the MoO x based MnIS diodes as studied by electrical and light emission measurements will be treated in the following sections.…”

“…The observed high minority carrier injection ratio (γ) in the Pd/MoO x /Si Si diodes indicates the role of the high-barrier and associated hole inversion layer at the MoO x /Si interface. In addition, the passivating nature of MoO x based contact on Si [175,180], possibly originating from the observed SiO x (Mo) interlayer, with a low surface recombination velocity, could also play a role in suppressing the TE further as confirmed by TCAD simulations. Fig.…”

“…First, the MoO x is a high work function material, and the simulations performed here substantiate that for barrier heights higher than about 0.9 eV, a significant inversion layer will be formed. Second, the passivating nature of MoO x based contact on Si [175,180], possibly originating from the observed SiO x (Mo) interlayer that may be partly insulating and is represented by a low surface recombination velocity, could also play a role in suppressing the electron injection even further. Thirdly, a significant amount of negative fixed charge may also be present in the deposited layer.…”

“…If chemical reactions at the interface with the Si do not play a decisive role [176], such a high φ m could be instrumental in the formation of a hole inversion (accumulation) layer on n-type (p-type) Si substrates. The presence of a hole inversion layer at the MoO x /n-Si interface was previously investigated using material characterization approaches such as photoelectron spectroscopy, in conjugation with capacitance-voltage (C−V) measurements [175,177]. Also, I−V measurements along the Si interface of Pd/MoO x /n-Si devices were recently performed using dedicated electrical test structures [169] to directly monitor any inversion, but there it was only concluded that if an inversion layer was present it had a sheet resistance higher than ∼ 50 kΩ/ .…”