A recent study demonstrates an innovative solar radiation nano-concentrator which uses a graphene metamaterial. It consists of an array (~1.5 × 1.5 μm2) of gold plasmonic concentrator cells which can nano-focus the solar radiation down to 60nm spot size on a solar cell active area. It can operate over the entire solar energy bandwidth up to 2400nm wavelength, an extension on the conventional solar cells operating wavelength range of up to 1800nm. This novel nano-concentrator can collect and make use of the solar weak radiations as well that occur between 1800nm and 2400nm. This nano-concentrator array has potential applications in photovoltaic energy harvesting within rural areas, outer space, biomedical technology, self-sustained sensors, and displays.

Graphene as a material

Graphene is an emerging material in nanotechnology. It has several unique physical properties, such as optical transparency, high electron mobility, very small electron effective mass, high electrical conductivity, broadband optical spectrum, electrostatic doping ability, and high thermal conductivity. These properties make it a perfect candidate for various energy applications including clean sustainable energy generation and harvesting. For example, graphene can be used in solar cells as an anti-reflection coating to maximise solar power absorption. Alternatively, it can be used to enhance solar energy conversion efficiency as well as making solar cells lightweight, flexible, and more cost-effective .

The production of graphene mono- and multi-layers has advanced in recent years, with the processing, synthesis, and fabrication of these materials becoming ever easier and more cost effective, a promising development for the increased practical use of graphene. However, it is worth mentioning that graphene still has a well-known disadvantage – it has low optical absorption at  ~2.3%.

Plasmonics

Plasmonics refers to the collective oscillations of free electrons that are found at the surface of noble metals such as gold or platinum. They are usually accompanied by sub-wavelength electromagnetic fields and concentrate at high intensities at the edges of the metal. Therefore, combining a noble metal with graphene in a nano-material structure can overcome graphene’s low optical absorption and increase the energy absorption. This is due to the plasmonic concentration of the optical field within the nano-scale graphene layers, as shown in the nano-concentrator structure described below.

Nano-concentrator cells

As shown in the image above, a one-concentrator unit cell consists of two waveguide sections that work together to focus light along a graphene metamaterial. Light enters via the coaxial waveguide – one that shares the same axis – and goes into a tapered section that can plasmonically nano-focus the solar radiation in the light. This is followed by a straight waveguide where the graphene metamaterial is integrated with in the 60nm gap. This means that the nano-focused optical intensity will interact with several stacked graphene levels which form a mesh-like structure. By integrating the graphene metamaterial in this section, it will maximise the length that the light will interact with the graphene and exposes both sides of the graphene layers to the propagating light.

The graphene metamaterial consists of 10 concentric graphene shell layers, which are cross-connected by five horizontal graphene annuli (ring-shaped structures). The concentrator design along with the metamaterial design maximises the solar optical absorption efficiency within the graphene metamaterial. The measured graphene optical absorption is enhanced up to 27 times the original value, with a maximum value of 62% and an almost flat response covering the ultra-wide solar bandwidth. The enhanced optical absorption of the weak solar radiation received within the extended band (1800–2400nm) alone can reach up to 50%.

An improved concentrator?

The tested device shows polarisation insensitivity operation, which allows to collection all the polarised and unpolarised solar radiation. It also has a good field-of-view of 120°, which allows it to collect solar radiation without the need for two-axis mechanical trackers. It is also more compact in size, meaning that it can easily be included in solar cell modules at a reasonable cost.

In conclusion, the graphene-integrated nano-concentrator has the advantages of a high degree of concentration, no need for a mechanical tracking system, extended ultra-broadband operation, almost flat optical response, compact size, polarization insensitivity, and large field-of-view.