Hole transport mechanism of MoO x /a-Si: H(i)/n-Si heterojunction photovoltaic device: source of “S-Shape” behavior

For the purpose of introducing a high work-function layer to improve the implied voltage in asymmetric silicon-based heterojunction (SHJ) photovoltaic device, molybdenum oxide (MoOx, 0≤x≤3) has been applied to the system. However, besides the role of extracting hole on one side, another singularity behavior presented in the nonequilibrium state, i.e., worsen photovoltaic feature ("S-shape") of current density-voltage (J-V) was revealed with an inappropriate chemical state of the MoOx film. The source of "S-shape" behavior of MoOx/a-Si: H(i)/n-Si heterojunction device was co-analyzed by X-ray photoelectron spectroscopy (XPS) with depth profiling, ultraviolet photoelectron spectroscopy (UPS), current density-voltage representation, minority carrier lifetime survey and AFORS-HET software simulation. It was found that an amorphous SiOx interlayer was spontaneously formed during deposition of MoOx film onto a-Si: H(i)/n-Si substrate, blocking transport of holes. The decreases in work-function of MoOx layer are attributed to oxidation reaction at MoOx/a-Si: H(i) boundary zone, which results in the decline of holes selectivity. A rising O/Si ratio of SiOx interlayer induces an augment of valence band offsets, which could be used to mainly interpret the "S-shape" response because of a barrier that hinders thermionic emission of holes. Meanwhile, the thicker (>4 nm) SiOx layer leads to a lower tunneling probability of holes. The characteristic analysis of the MoOx/a-Si: H(i)/n-Si heterojunction device deepens understanding for hole transport mechanism of the device.