Researchers at the University of York in the UK, in collaboration with the NOVA University of Lisbon, say they have found a way to boost the output of solar cells by up to 125%. Regular readers know I am math challenged, so I am not going to get caught up in a discussion of what that claim actually means mathematically. Instead, I am going to let the abstract of their study, published recently by the journal Optica, speak for itself. [Note: the study is made available pursuant to a Creative Commons Attribution 4.0 License.]
There is a lot of technical information contained in that report, so if you are interested in the nuts and bolts of this discovery, feel free to delve deeper. The article is entitled “Light trapping in solar cells: simple design rules to maximize absorption” and is written by Kezheng Li, Sirazul Haque, Augusto Martins, Elvira Fortunato, Rodrigo Martins, Manuel J. Mendes, and Christian S. Schuster.
Solar cells can strongly benefit from optical strategies capable of providing the desired broadband absorption of sunlight and consequent high conversion efficiency. While many diffractive light-trapping structures prove high absorption enhancements, their industrial application rather depends on simplicity concerning the integration to the solar cell concept and the process technology. Here, we show how simple grating lines can perform as well as advanced light-trapping designs. We use a shallow and periodic grating as the basic element of a quasi-random structure, which is highly suitable for industrial mass production. Its checkerboard arrangement breaks the mirror symmetry and is shown, for instance, to enhance the bulk current of a 1 µm slab of crystalline silicon by 125%. We explain its excellent performance by drawing a direct link between a structure’s Fourier series and the implied photocurrent, derived from a large and diverse set of structures. Our design rule thus meets all relevant aspects of light-trapping for solar cells, clearing the way for simple, practical, and yet outstanding diffractive structures, with a potential impact beyond photonic applications.
The importance of this research is that it permits the use of much thinner slices of photovoltaic silicon to produce the same amount of electricity as the thicker silicon cells used in solar panel production today. Dr Christian Schuster from the University of York department of physics says, “We found a simple trick for boosting the absorption of slim solar cells. Our investigations show that our idea actually rivals the absorption enhancement of more sophisticated designs while also absorbing more light deep in the plane and less light near the surface structure itself.
“Our design rule meets all relevant aspects of light-trapping for solar cells, clearing the way for simple, practical, and yet outstanding diffractive structures, with a potential impact beyond photonic applications. This design offers potential to further integrate solar cells into thinner, flexible materials and therefore create more opportunity to use solar power in more products.
“In principle, we would deploy ten times more solar power with the same amount of absorber material. Ten times thinner solar cells could enable a rapid expansion of photovoltaics, increase solar electricity production, and greatly reduce our carbon footprint. In fact, as refining the silicon raw material is such an energy intensive process, ten times thinner silicon cells would not only reduce the need for refineries but also cost less, hence empowering our transition to a greener economy.”
The graphic below shows some of the arrangements the researchers considered. In the end, the checkerboard arrangement proved the easiest to manufacture and the most effective in terms of boosting the output of solar cells.
For those of you who like to drill down into the nitty gritty details, the graphic below illustrates how the checkerboard arrangement hits the sweet spot for enhancing the amount of electricity produced from a solar cell. For the sake of clarity, the caption of this graphic is presented here in full.
“(a) Representation of the checkerboard’s photonic domain and computational unit cell. (b) The parameter map shows the computed maximum achievable photocurrent density 𝐽max as a function of the grating period and domain size. The inset shows the test cell with the checkerboard structure over it. The linewidth is here kept at half the grating period. The red dot marks the optimal parameter set that maximizes the broadband absorption in the 1 µm c-Si layer.”
I have seen some headlines pertaining to this story that suggest the researchers have figured out how to get ten times more electricity from a photovoltaic cell. That is clearly wrong. What they have done is find a way to use one tenth the amount of photovoltaic silicon to make solar panels. Thinner cells mean the silicon is used more efficiently, which could reduce the cost of solar panels. They may also allow lighter, more flexible solar panels. No matter how you slice it, this research is good news for the transition away from fossil fuels and toward zero emission electricity.