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Yeah, I know: every week, somebody somewhere announces some sort of solar cell "breakthrough" that turns out to be ... well, not much. I see them all the time too.

This one is different. Simple. Elegant. Easy to manufacture. And incredibly useful for solar cells of all types, especially thin-film cells. Follow below the arabesque to find out why.

The very first semiconductor ever made was a diode. It's a nifty crystalline device that allows current to flow through in one direction but not in the other direction. Since currently only "half" flows, it's a semi-conductor. (Get it? These guys are great with names.)

The simplest (and first) solar cell ever made was based on a simple diode. Normally a diode is made into a wire-shape. But imagine that you make it flat instead, and put a metal plate on one side, and a wire mesh on the other side, and make sure to orient the diode so that current only flows FROM the plate toward the mesh.

Solar Cell diagram
That's a solar cell. Why? Because when light hits the semiconductor, some electrons are knocked off the semiconductor and land on the wire mesh. Then the semiconductor is short electrons (it's positively charged) while the mesh has extra electrons (it's negatively charged). Now those extra electrons on the mesh want to get back to the positively charged semiconductor, but they can't, because the diode is oriented to prevent the flow of electrons in that direction. Instead, we attach an exit wire to the mesh, run it around to the plate on the back, and the electrons have to take that alternate pathway to get back to the semiconductor. And they do work along the way!

Now that's great, but there are a bunch of issues that make things less than ideal. First off, semiconductor material started out being only expensive stuff like silicon and gallium. Lately we've invented plastics that work as semiconductors, but their efficiency is lower than silicon. Also, you can use less semi-conductor by making it thinner (a so-called "thin film" cell), but you reach a conventional limit to thinness when you get down to the wavelength of light itself.

Sizing the mesh is tricky, because you'll capture more electrons with more mesh, but then that blocks more of the incoming light too. Some solar cells (especially thin film cells) get around that by using a transparent conductor called Indium-Tin-Oxide (ITO) instead of the mesh, but that's expensive stuff too.

And then some light just reflects off the surface of the cell and doesn't knock off any electrons. That's particularly true when the light comes in at sharp angles, which most of the light does on cloudy days.

In fact, a really good (and really cheap) thin-film solar cell is the holy grail of photovoltaics. Right now we can make thin film cells, but their cost per Watt is greater than conventional silicon, and the ITO layer is a big part of that. Plus the efficiency is low. So what if we could eliminate the ITO layer and boost efficiency at the same time? What if we could make PV cells in a continuous roll-to-roll process in a factory, instead of the current expensive batch process? And what if the price per Watt could be brought down to less than silicon, by using less-expensive plastic semiconductors?

Now here's the brilliant idea from Prof. Stephen Chou at Princeton University that addresses all of these issues. Replace the normal ITO (or wire mesh) on the front with a nano-mesh with openings sized smaller than the wavelength of light.

Subwavelength mesh for solar cell upper electrode
Chou tried several sizes of grid, but got good results with holes 175 nm in diameter, spaced 200 nm apart. Visible light has wavelengths between 380 and 700 nm, with sunlight (and human visual sensitivity) peaking at about 550 nm.

It turns out that light behaves in odd ways when it encounters a "subwavelength" grid. About a year ago, Chou noticed something very odd: when you block a hole of a subwavelength size, it's actually more likely that light will go though the hole than if the hole is unblocked! This is what led Chou to the subwavelength grid: the metal back of the cell acts to block the light, making it more likely that the light will make it through the grid and be trapped in the semiconductor layer.

Just as important, the subwavelength grid works even when light hits it at very large angles. This is of great importantance for a solar cell, because light usually doesn't hit it dead on. In the morning and evening the angles are greater, and on cloudy days the diffuse light hits from all directions at once. The subwavelength grid traps on average 90% of the incoming light, and in some cases as high as 96%. It's a quantum-mechanics roach motel for photons: they check in, but don't check out.

Another advantage: the grid has a lower electrical resistance than ITO, making the cell more efficient.

By eliminating the expensive ITO layer, a thin-film cell made this way is cheaper than current thin-film technology. (Chou used gold for his mesh, because it's easy to manipulate in the lab, but it could just as easily be made from aluminum.) Fabrication of the grid is extremely easy. You start by creating a cell-sized mold using light interference techniques, then use the mold to run off any number of grids you want. The entire thing can be done in a continuous roll manufacturing process, making it very cheap.

All together, Chou's results show that a thin-film cell using the subwavelength mesh performs about 175% better than a standard thin-film cell with ITO as the front conductor, a nearly three-fold improvement. (Because a 100% improvement is double ... you knew that, right?)

Standard disclaimers

Chou hasn't tested this grid with a silicon-based semiconductor, so far only as a thin-film organic (i.e., plastic) semiconductor. There is some reason to suspect there won't be as great a gain with silicon (because silicon is opaque, while thin-film plastic semiconductors are translucent.) But it still should be an improvement over existing large mesh structures. And plastic is a lot cheaper than various metal-based semiconductors. This could end up being as cheap as dye-sensitized solar cells, but without the degradation problems.

Yes, this is still just a lab demonstration, and no, we don't know what the final cost will be, and no, it won't be available any time soon (if by soon you mean in Home Depot next year). But it's still pretty awesome.


News story from PhysOrg

Paper in Optics Express by Stephen Y. Chou and Wei Ding

Wow, I wrote this late last night with a publish time of early this morning, got up late and I'm top of the rec list. You guys are awesome. And thanks for the commenters who pointed out my typos, hopefully they're all corrected now.

Originally posted to The Numerate Historian on Fri Dec 07, 2012 at 03:08 AM PST.

Also republished by Climate Change SOS, SciTech, and Kosowatt.

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