Trapping the light fantastic
New research at UNSW is increasing the amount of light that can be trapped in thin-film photovoltaics.
Thin-film PV is seen to be one of the major ways to reduce the cost of PV production, as it is about 100 times thinner than traditional silicon wafers. Thin film semiconductor materials, such as silicon, is usually deposited on a substrate such as glass, and is just 2–3 microns thick, as opposed to traditional silicon cells, which are about 300 microns thick.
But as Dr Supriya Pillai, of the ARC Photovoltaic Centre of Excellence in the School of Photovoltaics and Renewable Energy Engineering, notes, thin-film technology has difficulties to overcome. “The light that is actually being absorbed is being dramatically reduced because the thickness is so low,” she says. “It’s 100 times less thick than a usual cell, so you reduce the amount of light absorbed significantly.”
One of her challenges has been to increase the amount of light entering the thin film, so that as much of the available light can be caught and used. One of the ways to do that is by texturing the cells, but that can lead to reduced performance and is difficult to achieve in thin films. Supriya is therefore adding a layer of carefully constructed silver nanoparticles to a finished cell in an emerging field called plasmonics. “The metal nanoparticles act like antennas absorbing the incident radiation and scattering it into the underlying semiconductor,” she says. “When you texture a cell, you are increasing the surface area. When you use plasmonics, it’s a totally different layer put on once the device is fabricated. It’s not increasing the surface area of the silicon in any way, and it doesn’t negatively affect the electrical properties.”
Supriya says the nanoparticles are designed to not only maximise the scattering of light, but to do it at large angles, “so they are basically scattering beyond the escape cone, so it has a chance to be caught again because it keeps bouncing within the cell. Larger particles tend to scatter more than smaller particles – something in the 100–300 nanometre range seems to be ideal. The main challenge of the work at the moment is to incorporate it with other layers in the cell with minimal losses.”
When nanoparticles form on the rear of the cell, significant light is still lost. “We have to minimise the loss,” she says. “This means we have to use the nanoparticles with additional layers without affecting the scattering properties.”
The next step will be to see whether the nanoparticle layer can be incorporated into conventional reflective layers that are currently used at the back of PV cells, such as white paint or a reflective metal. Promising results have been achieved so far.