Plasmons fold up light in ultra-thin solar cells

Solar cells, integrated with a thin layer of metallic nanoparticles, can absorb sunlight much more strongly than conventional solar cells. As a result, solar cells can be made thinner, and thus cheaper, while still efficiently converting sunlight to electricity. Albert Polman of the FOM-Institute AMOLF in Amsterdam, The Netherlands, and his colleague Harry Atwater from the California Institute of Technology, Pasadena, CA, write this in a review article that appeared in the March issue of Nature Materials. In an earlier article, together with colleagues at Utrecht University and Philips Research, they provided experimental demonstrations of this new solar cell design.

The generation of electricity by solar cells is still relatively expensive. Reducing the cost of solar cells by a factor of 2-3 is needed to make solar electricity competitive with conventional energy sources. An important part of the cost is the cost of the material: in a standard silicon solar cell materials costs determine 75% of the price. The thickness of such a solar cell is 0.3 mm. Worldwide, research is going on to make solar cells much thinner and thus cheaper. A fundamental problem however is that the thinner solar cells poorly absorb sunlight. Especially the infrared light from the sun travels straight through a thin solar cell rather than it being absorbed (Figure 1a).

A solution of this problem is "to fold" the light inside the solar cell upon entrance into the cell. In this geometry, the incident light is scattered into the plane of the solar cell, from which it can not escape. In this way, even a very thin solar cell can capture a significant fraction of the infrared part of the solar spectrum. The team of Dutch/US collaborators shows that  this “folding of light” can be efficiently achieved by integrating a silver nanostructure in the backside of a solar cell. (Figure 1b)

In another design, also described in the Nature Materials review article, silver nanoparticles are deposited on the surface of a solar cell (Figure 1c).  Here too, light is scattered into the plane of the solar cell, and trapped inside and electric current is created. The silver nano-structures are strong light scatterers because the light generates so-called plasmons: resonant oscillations of the electrons in the silver.

Albert Polman, the leader of the Amsterdam team: "This new method allows the creation of solar cells on only one ten-thousandth of a millimeter thick that yet convert all colors of the sun into electricity. And the technique is applicable to all types of cells, whether they are made from silicon, polymers or other compounds. In fact, it may even become possible to realize large-scale fabrication of solar cells using materials that are scarce in the earth’s crust. For example, cadmium/tellurium solar cells are very efficient but require a thickness of 2 micrometer, and therefore suffer from the limited availability of tellurium. With our technology these cells can now be made 10-20 times thinner and large-scale fabrication becomes possible.

The research is part of an initiative by the Foundation for Fundamental Research on Matter (FOM) to strengthen research into renewable energy sources.

Reference
The article "Plasmonics for improved photovoltaic devices" by Harry A. Atwater and Albert Polman, is published in Nature Materials 9, 205 (2010). An earlier article with technical details was published in Applied Physics Letters 95, 183503 (2009).

Information
For additional infomation, contact:
Prof. Albert Polman, tel.  (020) 754 7100

Figure 1 (a) Blue and green light is well absorbed by a solar cell and converted to electricity, while light runs right through it. (b, c) Plasmonic solar cells: metal particles scatter the incoming sunlight that then remains trapped in the solar cell. ® AMOLF / Tremani

Artists’ impressions of plasmonic solar cells. ®AMOLF/Tremani