Working to specifications from the university’s Nanoscale Systems Integration (NSI) group, the spin-out R&D company Innos has used equipment at Philips’
The patterns are based on the structures on the surface of the eyes of night-flying moths, explained NSI researcher Darren Bagnall. ‘These prevent reflections, which serves two purposes: it stops predators spotting the moths, and increases the amount of light their eyes collect.’ Current solar cells tend to reflect light when the sun is low in the sky, but applying patterns like those on the moths’ eyes would reduce much of this reflection and increase light collection by around 10 per cent, he added.
The challenge is that the features of the pattern are tens of nanometres across, and patterning at this scale is very difficult. Innos used electron-beam (e-beam) direct writing, which utilises a focused beam of high-energy electrons to cure sections of a layer of a polymer known as a resist, applied to the unpatterned silicon surface. Once the uncured resist is washed away, the exposed silicon can be etched to produce the desired pattern.
‘It’s like a photographic negative,’ explained Innos marketing director Alec Reader. ‘You need the resist to cure on the areas of silicon you want to be unetched.’
Bagnall’s team is now studying the properties of its patterned silicon surface. The next phase will be to analyse how the surface features produce their effects, and alter the design to further decrease light reflection. ‘The refractive index of the materials has a big effect, and we need to discover how to take account of that.’