Holding the sun with plasmonic black metals - improving solar absorption
Gold, silver and aluminium assume a black appearance when they are nanostructured in a certain way – a phenomenon that could pave the way for black silver engagement rings and black gold Olympic medals. Well, the second part isn’t true, but the first part may prove extremely useful to scientists, especially those seeking to develop photovoltaic solar cells.
You see, researchers at Lawrence Livermore National Laboratory in California, USA, are taking advantage of this effect by modifying metal surfaces to increase the solar absorption efficiency of photovoltaics. Some photovoltaics incorporate randomly distributed nanoparticles or roughened metal surfaces, but their spectral performance cannot be controlled and they cannot be tuned. Lead researcher Dr Tiziana Bond claims to have achieved these desired properties using surface plasmon technology.
She says, ‘With the advent of plasmonics, the metallic interfaces can be exploited to enhance the light collection efficiency, either by scattering, total internal reflection or absorbance. In our case, the metal interfaces provide a double functionality – as metal contacts to provide the electrical controls, and as strong absorbers to more efficiently focus the solar radiation within the semiconductor.’
The array template is patterned over a 10cm wafer using laser interference lithography, which is then transferred onto a silicon (or silica) substrate using directional deep reactive ion etching. According to Bond, the height and profile of the nanowires are controlled by etch time, etchant composition, and other parameters such as base pressure and bias power.
Bond adds, ‘We control and improve the absorption efficiency of the nanoresonator arrays by manipulating the quality factor. For instance, we can modify the finesse of the cavities as well as their spectral dynamic range by adjusting the geometry of the nanostructures, such as the gap and length of the resonator cavities and the optical properties of the dielectric metal interface.’
The team has tried using gold, aluminium and silver for the fabrication process. Each material has its advantages. Aluminium is abundant, cheap and offers large spectral coverage. Silver is efficient at transferring energy from the plasmon modes into the absorptive dielectric material, and gold nanoarrays have shown significant biological sensing potential.
Bond, and her collaborator Dr Mihail Bora, claim the process is repeatable and scalable, though the technology is not commercially available yet. Dr Bora adds, ‘We have to address fundamental phenomena such as plasmonic-exciton formation, charge splitting, diffusion and, eventually, more practical manufacturing issues. In order to exploit the full spectrum absorbance feature, we will also investigate how to integrate with multi-junction devices.’
The nanopillar arrays and their associated modifications may also be useful in other devices, such as tunable filters, spectral shields and highly sensitive detectors.