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Copper oxide photocathodes: Laser experiment reveals location of efficiency loss

Copper oxide (Cu2O) is a very promising candidate for future solar energy conversion: as a photocathode, the copper oxide (a semiconductor) might be able to use sunlight to electrolytically split water and thus generate hydrogen, a fuel that can chemically store the energy of sunlight.

Copper oxide has a band gap of 2 electron volts, which matches up very well with the energy spectrum of sunlight. Perfect copper oxide crystals should theoretically be able to provide a voltage close to 1,5 volts when illuminated with light. The material would thus be perfect as the top-most absorber in a photoelectrochemical tandem cell for water splitting. A solar-to-hydrogen energy conversion efficiency of up to 18 per cent should be achievable. However, the actual values for the photovoltage lie considerably below that value, insufficient to make copper oxide an efficient photocathode in a tandem cell for water splitting. Up to now, loss processes near the surface or at boundary layers have been mainly held responsible for this.

A team at the HZB Institute for Solar Fuels has now taken a closer look at these processes. The group received high-quality Cu2O single crystals from colleagues at the California Institute of Technology (Caltech), then vapour-deposited an extremely thin, transparent layer of platinum on them. This platinum layer acts as a catalyst and increases the efficiency of water splitting. They examined these samples in the femtosecond laser laboratory (1 fs = 10-15 s) at the HZB to learn what processes lead to the loss of charge carriers and in particular whether these losses occur in the interior of the single crystals or at the interface with the platinum.

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