Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.130
Publication date: 18th July 2019
A fully integrated monolithic device for solar water splitting consisting of two photo-absorber layers of different ideal band gaps has many benefits.1-4 We have investigated a series of tandem n-n heterojunctions of n-Si and BiVO4, which provide a photo-voltage sufficient for unassisted solar water splitting. We used a scalable deposition method for producing WO3 nano-rods via aerosol assisted chemical vapor deposition (AA-CVD), which serves as an electron-conducting scaffold for the subsequent formation of WO3/BiVO4 core shell nanostructures.5,6 Here we present a series of planar and nanostructured core-shell devices composed of n-Si/SiO2/TiO2 or SnO2(interface)/WO3(scaffold)/BiVO4(photo-absorber) heterojunctions. Solid-state electrical measurement have been used to probe the junction type and barrier height of the junction formed at the Si/metal oxide interfaces. Between n-Si/SiO2 and WO3 or BiVO4 we observe the formation of interfacial defects and unfavorable band alignment, which prevents charge transfer, diminishes the n-Si photovoltage, and induces recombination. This could be mitigated with the deposition of a thin TiO2 or SnO2 film onto the n-Si/SiOx interface, which act as effective passivation layers. Film thicknesses between 50 - 100 nm are optimum and result in the lowest onset potentials. Whilst the planar structured devices showed better onset potentials (lower than -0.2 VRHE) and thus higher n-Si photo-voltages, the nano-structured core shell devices exhibited larger photocurrents for water splitting due to light scattering. We find that surface etching of n-Si with HF to remove the intrinsic SiO2 can be detrimental to the effective n-Si photovoltage, whereas surface cleaning with NH4OH enhances the performance compared to untreated n-Si/SiO2 wafers. Optimized stand-alone tandem photoanodes consisting of Pt/Cu/In:Ga/n-Si/SiO2/TiO2(100 nm)/WO3(core-shell)/BiVO4/Fe(Ni)OOH achieved a stable photocurrent density of 0.25 mAcm-2 in 1.0 M KBi pH 9.3 buffer solution for up to 2 hours under simulated AM 1.5 G illumination without an applied bias. Differential electrochemical mass spectroscopy (DEMS) shows a faradaic efficiency of ~98% for hydrogen production. These results illustrate the importance of suitable interfacial layers in planar and nanostructured tandem devices.
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