Unraveling the photovoltaic properties of gallium nitride nanorods for solar water splitting

Researchers have increasingly focused on converting solar energy directly to hydrogen fuel via the water-splitting reaction as a way to reduce use of fossil fuels. Hydrogen can be burned without producing carbon dioxide and consequently does not contribute to climate change. One area of recent focus has been gallium nitride (GaN) nanowire arrays on silicon that promote the solar water splitting reaction with 3.3% solar-to-hydrogen efficiency.

Doughty et al used surface photovoltage spectroscopy (SPS) to directly observe the function of the GaN nanowire arrays.

“SPS is a non-destructive technique that allows us to measure charge separation in photoactive materials as a function of light intensity and wavelength,” said author Frank Osterloh. Typically, to measure the properties of a p-n semiconductor junction, two electrical contacts are required.

“SPS extracts this information in a contactless manner using a Kelvin probe that hovers 1 millimeter above the sample,” said Osterloh. “This provides data about the bandgap of the materials and their junction properties”

The results aid the understanding of photochemical charge transfer and separation in group III-V semiconductor nanostructures. Visible absorbers for the water splitting process must be cheap, abundant, stable, and possess appropriate band positions for water photoelectrolysis.

“There are infinite possibilities to fabricate inexpensive light-absorbing materials. The important task is to identify those with suitable photovoltaic properties and sufficient stability for solar energy conversion, and SPS can help with that,” said Osterloh.

Source: “Surface photovoltage spectroscopy observes junctions and carrier separation in gallium nitride nanowire arrays for overall water-splitting,” by Rachel M. Doughty, Faqrul A. Chowdhury, Zetian Mi, and Frank E. Osterloh, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0021273 .