One way to improve the efficiency of solar cells is to allow light to bounce around inside them, increasing the chances that it will be absorbed. One way to do this is to roughen the surface of a silicon cell, so that photons that enter the material tend to be reflected inside it. But by how much does this light trapping improve performance?
In the 1980s, various physicists calculated that any increase cannot be greater than 4n^2, where n is the refractive index of the material. That’s a decent improvement.
But things have changed since the 1980s, not least because it is now possible to make layers of silicon much thinner than the wavelength of the light they are expected to absorb and to carve intricate patterns in these layers. How does this nanophotonic technology change the effect of light trapping?
Today, Zongfu Yu and buddies at Stanford University in California, tackle this question and say that nanophotonics dramatically changes the game.
That’s basically because light trapping works in a different way on these scales. Instead of total internal reflection, light becomes trapped on the surface of nanolayers, which act like waveguides. This increases the amount of time the photons spend in the material and so also improves the chances of absorption.
Because of the geometry of the layers, some wavelengths are trapped better than others and this gives rise to resonances at certain frequencies.
What Yu and co show is that by designing the layers in a way that traps light effectively, it is possible to beat the old limit by a substantial margin.
They study, in particular, a patterned cell designed to trap light on the surface and calculate that this can enhance absorption by 12*4*n^2, a significant improvement.
That’s important because it places a new fundamental limit on the amount of light that can trapped in a nanophotonic cell.
Physicists have long known that thinner solar cells are better in a number of ways: they use less material and so are cheaper to make and the electrons they produce are easier to collect making them potentially more efficient. Now they know that light trapping is more effective in thinner layers too.
Handy to know because when it comes to making photovoltaics better, every little helps.
Ref: arxiv.org/abs/1004.2902: Fundamental Limit of Nanophotonic Light-trapping in Solar Cells