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Protecting semiconductors for solar-fuel generators

Editorial
article image Caltech researchers at the Joint Center for Artificial Photosynthesis (JCAP) have found a way to protect the semiconductors from corrosion, while still allowing the materials to continue absorbing light efficiently.

RESEARCHERS at Caltech have devised a method to protect the semiconductors in solar-fuel generators.

Solar-fuel generators use sunlight to split water, yielding hydrogen gas that can then be used as a clean fuel. Water is oxidated to produce oxygen gas, then reduced for hydrogen.

These devices are currently in the development stage, but would require efficient light-absorbing materials that attract and hold sunlight to serve as the photoelectrode and a catalyst drive the chemical reactions needed to split water into its constituent components.

Historically, it has been particularly difficult to come up with a light-absorbing material that will robustly carry out the oxidation half-reaction. Semiconductors like silicon and gallium arsenide are excellent light absorbers, but they corrode when submerged in the special water solutions used in solar-fuel generators.

Previous efforts involved putting a protective layer on the semiconductors, but an overly thin layer offered little by way of protection. A thick layer prevents corrosion, but affects the ability of the semiconductor to absorb light, and prevents electrons from passing through to reach the catalyst.

Caltech researchers at the Joint Center for Artificial Photosynthesis (JCAP) have found a way to protect the semiconductors from corrosion, while still allowing the materials to continue absorbing light efficiently.

This means common semiconductors like silicon and gallium arsenide can now be used in solar-fuel generators.

The researchers used atomic layer deposition to form a layer of titanium dioxide (TiO2) on single crystals of silicon, gallium arsenide, or gallium phosphide. Specifically, they had to use a variant of TiO2 called "leaky TiO2", which leaks electricity.

When this leaky TiO2 is deposited as a film between 4 and 143nm thick, it is optically transparent on the semiconductor crystals—allowing them to absorb light—and protected them from corrosion but allowed electrons to pass through with minimal resistance.

On top of the TiO2, the researchers deposited 100-nanometer-thick "islands" of an abundant, inexpensive nickel oxide material that successfully catalysed the oxidation of water to form molecular oxygen.

The researchers are now looking at whether the protective coating would work as well if applied using an inexpensive, less-controlled application technique, such as painting or spraying the TiO2 onto a semiconductor.

They will also need to test the protective capabilities of the TiO2 over longer periods of time.

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