Thinner and Lighter Solar Cells
Solar cells have been changing for years and gradually becoming less expensive, more compact and more efficient. Most efforts at improving solar cells have focused on increasing the efficiency of their energy conversion, or on lowering the cost of manufacturing. But now MIT researchers are opening another avenue for improvement, aiming to produce the thinnest and most lightweight solar panels possible which will reduce manufacturing and installation costs.
Such panels, which have the potential to surpass any substance other than reactor-grade uranium in terms of energy produced per pound of material, could be made from stacked sheets of one-molecule-thick materials such as graphene or molybdenum disulfide.
Graphene is an allotrope of carbon. In this material, carbon atoms are arranged in a regular hexagonal pattern. Graphene can be described as a one-atom thick layer of the mineral graphite, (many layers of graphene stacked together effectively form crystalline flake graphite). Among its other well-publicized superlative properties, it is very light, with a 1-square-meter sheet weighing only 0.77 milligrams.
Graphene has a unique combination of high electrical conductivity and optical transparency, which make it a good candidate for use in solar cells. Because this material only absorbs 2.3% of visible light, it is a candidate for applications as a transparent conductor.
Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering at MIT, says the new approach "pushes towards the ultimate power conversion possible from a material" for solar power. Grossman is the senior author of a new paper describing this approach, published in the journal Nano Letters.
Although scientists have devoted considerable attention in recent years to the potential of two-dimensional materials such as graphene, Grossman says, there has been little study of their potential for solar applications. It turns out, he says, "they’re not only OK, but it’s amazing how well they do."
A solar cell (also called a photovoltaic cell) is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell (in that its electrical characteristics—e.g. current, voltage, or resistance—vary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source.
By far, the most prevalent bulk material for solar cells is crystalline silicon, also known as solar grade silicon. Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.
Using two layers of such atom-thick graphene materials, Grossman says, his team has predicted solar cells with 1 to 2 percent efficiency in converting sunlight to electricity, That’s low compared to the 15 to 20 percent efficiency of standard silicon solar cells, he says, but it’s achieved using material that is thousands of times thinner and lighter than tissue paper. The two-layer solar cell is only 1 nanometer thick, while typical silicon solar cells can be hundreds of thousands of times that. The stacking of several of these two-dimensional layers could boost the efficiency significantly.
"Stacking a few layers could allow for higher efficiency, one that competes with other well-established solar cell technologies," says Marco Bernardi who was the lead author of the paper.
Pound for pound, he says, the new solar cells produce up to 1,000 times more power than conventional photovoltaics. At about one nanometer (billionth of a meter) in thickness, "It’s 20 to 50 times thinner than the thinnest solar cell that can be made today," Grossman adds. "You couldn’t make a solar cell any thinner."
This slenderness is not only advantageous in shipping, but also in ease of mounting solar panels. About half the cost of today’s panels is in support structures, installation, wiring and control systems, expenses that could be reduced through the use of lighter structures.
Molybdenum disulfide and molybdenum diselenide, the materials used in this work, are just two of many 2-D materials whose potential could be studied, to say nothing of different combinations of materials sandwiched together. "There’s a whole zoo of these materials that can be explored, "Grossman says.
For further information see Graphene Solar Cells.
Graphic image by Jeffrey Grossman and Marco Bernardi.