Nanoscale light trap


Plasmonics is an emerging technology for capturing, guiding, and concentrating light at the nanoscale. Applied to photovoltaics, it offers a path to trap light more efficiently into the photoactive material of various types of solar cells. This technique may become one of the breakthroughs that will push the efficiency curve of PV above what is possible with today’s evolutionary, slowly improving techniques.
The ability to increase the light absorption of semiconductors with an important factor will enable conventional solar cells to be made with much less semiconductor material, reducing costs and manufacturing time, and improving the sustainability of solar cell production from scarce materials. With the technique, it will also be possible to design nanostructured absorbers to collect appreciable quantities of light, leading to much greater flexibility in cell structure and design.
Plasmonic-enhanced PV has already proven to work in the lab, and looks promising for use in industrial-scale production. However, there is need for further systematic research into the fundamentals of the plasmonic effects, and into their application in industrial production processes. This effort is now underway within the project PRIMA!, sponsored by the European Union. The project is coordinated by imec (Belgium). The project partners are a consortium of universities, research institutes and PV companies: Imperial College London (UK), Chalmers University of Technology (Sweden), Photovoltech (Belgium), Quantasol (UK) and Australian National University (Australia).
PRIMA! targets plasmonic enhancement of various solar cell technologies: c-Si solar cells, multi-c-Si solar cells, organic solar cells, dye sensitized solar cells and III-V quantum well solar cells. As these technologies are intrinsically very different in terms of materials, processing conditions, and active wavelengths, the enhancement strategies should be fine-tuned for each of them.

Nanoconcentration of light

The existence of surface plasmons has been known for decades. But they only started to attract attention in the late 80s, when researchers experimentally confirmed the plasmonic effect on nanopatterned substrates. In 1989, for example, Thomas Ebbesen, working at the NEC Research Institute, discovered that when he illuminated a thin gold film imprinted with millions of microscopic holes, the foil transmitted more light than the total light impinging on the area taken up by the holes.
What happens is that light waves directed at the interface between a metal and a dielectric, under the right circumstances can start to resonate with the mobile electrons at the surface of the metal.
These resonances have been called “surface plasmons”, density waves of electrons that propagate along the metal’s surface much like the ripples on the water’s surface after you throw in a stone. In the case of Ebbesen’s gold film, this resonance and the resulting surface plasmons amplifies the light that shines through the gold film.
The field of research was termed plasmonics, and a race started to use this effect in a whole array of applications. Examples are the use of surface plasmons in integrated circuits (ICs), where they could travel along nanowire interconnects, much smaller than is possible with conventional optical waveguides. Other possible applications are improving the sensitivity of biomedical sensors, the efficiency of light emitting diodes, or, as we discuss in this article, the efficiency of solar cells.

Plasmonics applied to PV

For solar cells, the idea is to use the plasmonic effect to trap more light in the cell – to have a concentrator built into the cell itself. There are essentially three ways in which the plasmonic effect works this magic.
One way is through scattering the incoming light. If you place a nanometallic particle in a homogeneous medium, the light that hits it will scatter in all directions, with no preference. But when a nanometallic particle is placed close to the interface between two dielectrics (e.g. air and silicon), the light scatters preferentially into the material with the higher permittivity. Also, all light that is scattered beyond the critical angle for reflection will remain trapped in the cell. For the silicon-air interface, the critical angle is 16 degrees.
The shape, size, and material of the metal nanoparticles determine the efficiency of the scattering effect. Also, the effect is most pronounced for wavelengths close to the peak of the plasmon resonance spectrum. This can be tuned by engineering the dielectric of the surrounding matrix. In addition, when the nanoparticles resonate, their scattering cross-sections grow, up to ten times the geometrical cross-section. A ten percent random coverage of a cell with metal nanoparticles would thus be enough to scatter all the incident sunlight.
Up to now, the plasmonic structures used in PV experiments were usually quite simple (spheres, rods). What structures were used was more dictated by fabrication methods than by understanding the mechanism. Part of the effort of our new project will be to determine the optimal plasmonic structures for different types of PV cells and applications.
There is a second way to use the plasmon excitation to increase the efficiency of solar cells. Light hitting metallic nanoparticles causes a strong enhancement of the local field, which increases the light absorption in the surrounding photoactive material. With this technique, nanoparticles buried in the material effectively behave as small antennas, capturing and amplifying the incident light. This effect seems to work especially well for small particles (five to twenty nanometers), for which the albedo is low.
Last, a third light-trapping technique is the conversion of light into so called surface plasmon polaritons (SPPs). SPPs are the surface electromagnetic waves that are caused by incident light on a metal/dielectric interface. They propagate in a direction parallel with a metal/dielectric interface. Thus this property folds the incident light, effectively changing its direction. It can now be absorbed along the lateral, much longer, direction of the solar cell. As metal back- and front-contacts are standard elements of solar cells, this plasmonic concept can be integrated quite naturally.

The road to industrial use

To date the application of plasmonics to photovoltaic solar cells has resulted, among others, in several demonstrations of scattering in silicon solar cells, evidence of field enhanced absorption in III-V solar cells and organic solar cells and a demonstration of scattering in III-V waveguide solar cells, amorphous Si solar cells and in dye sensitized solar cells. The common theme in all these reports has been that they have been limited to the proof-of-concepts. There has been little effort to see how plasmonic amplification could be integrated in industrial production processes, and whether the efficiency gains outweigh the added fabrication cost and complexity.
Although there certainly has been progress made in the understanding of the different types of plasmonic interactions with solar cells – scattering and/or light concentration –, there is still no detailed and definitive physical understanding of the enhancement processes and their contributions in a given solar cell design.
Within the project PRIMA!, we look into the plasmonic enhancement of various solar cell technologies. Specifically, we will investigate plasmonic enhancement strategies for c-Si solar cells, multi-c-Si solar cells, organic solar cells, dye sensitized solar cells and III-V quantum well solar cells. These technologies are intrinsically very different in terms of materials, processing conditions, and active wavelengths. This calls for separate and fine-tuned enhancement strategies for each of them.
For each cell technology, we determine the optimum plasmonic structures to achieve plasmonic enhancement, taking into account the specificities of each concept. We do this using rigorous computational techniques, state-of-the-art nanofabrication, taking advantage of the partnership with industrial leaders in photovoltaic manufacturing in terms of manufacturing facility, but also knowledge and know-how of the industrial fabrication of solar cells.
With these results, we then study how these structures can best be integrated into the cell production. For this, the solar cell companies that are participating in the project will fabricate and test a number of structures, benchmarking them against state-of-the-art solar cells.
For each cell technology and test structure, we can then set off the production cost against the efficiency gain. If these numbers are beneficial, the technique is a candidate for inclusion in the industrial production process.