Array of sunshine

Arrays of silicon microwires grown by a vapour–liquid–solid method point the way to more efficient photocathodes.

Credit: © 2010 AAAS

Crystalline silicon wafers can be used as semiconductor photovoltaics, and also show promise in the role of photocathodes in the light-driven production of hydrogen from water. High-purity silicon is normally required in order to exploit its optical absorption depth, but one way of reducing this cost is to use vertical arrays of silicon microwires. Although theory predicts that the efficiency of array devices should reach 15%, in practice only 3.4% efficiency has been reached for a single wire and just ∼0.1% for macroscopic arrays.

Now, Harry Atwater, Nathan Lewis and colleagues from the California Institute of Technology have improved¹ silicon-wire-array device efficiency through greater control of impurities, defects and doping. They used a copper-catalysed vapour–liquid–solid-growth method to grow the arrays from a stream of SiCl₄ over a silicon surface. The wires covered around 4% of the surface.

From the voltage obtained in a photochemical cell, they estimate that the wires were free from defects or impurities. The internal quantum yields were very high, offering hope that either improving the packing fraction or introducing scattering elements between the wires will significantly improve the overall efficiency.