It really was a bargain from the bargain bin. Graduate student Alex Smith wanted to make a solar cell work better, and he found his solution in the most unlikely of places: a Blu-ray copy of Jackie Chan’s Super Cop.
Now picture a solar cell. It’s probably blue or some other dark color, glassy, arrayed into rows upon rows—and, depending on your taste, aesthetically pleasing. That doesn’t matter, though. Looks aren’t central to saving the environment. Making sunlight into electricity that can power your home is.
Most of the solar cells that look like the picture above are made of crystalline silicon, the dominant material currently used in solar cells. Even though they work well, silicon cells can be heavy, rigid, and expensive to produce. These and other factors have led to the development of thin film solar cells—which are much less expensive, lightweight, and flexible.
Even with all these desirable characteristics, they’re not as efficient at converting sunlight into electricity. Thin film solar cells derive their name from the thinness of their photoactive layer -- the part of the cell that absorbs light. This layer can be thousands of times thinner than the ones in conventional, crystalline silicon solar cells. Because there’s not as much stuff to absorb light, less light is absorbed, which generates substantially less electricity.
One way to increase the efficiency of a solar cell is to increase the absorption rate of the photoactive layer. No solar cell absorbs all the light which hits it. Light can, and mostly does, fly through the photoactive layer without being absorbed.
In order to increase the energy output of a solar cell, scientists are trying to make the light stay within the photoactive layer longer. One method takes advantage of light’s “bounciness.” Imagine a beam of light is like the path of a ball moving across a pool table. The ball can be made to bounce against the pool table’s edge at a variety of angles. A direct hit on a flat surface, for example, rebounds the ball straight back from whence it came. Hitting an edge at an angle would change the ball’s trajectory, making it bounce off in another direction. Making use of multiple edges to make multiple bounces can precisely control where the ball ultimately lands. (Provided, of course, one’s billiards technique lands the ball in the intended location—certainly mine wouldn’t.)
Light can behave similarly. A surface patterned with ridges and edges changes how light travels and the right pattern would bend the light exactly how it was most needed. In solar cell design, the goal is to keep light bouncing against the photoactive layer, so that the solar cell has more opportunities to absorb light.
To do this, we etch or stamp ridges onto the other key part of the solar cell: the reflective layer. This layer, usually a strip of material sandwiched right up against the photoactive layer, functions like the edges of the pool table. If the reflective layer is smooth, light bounces straight back into the photoactive layer, and out the other side. But if the reflective layer has ridges and bumps, there are additional edges where the light can angle off in different directions. Much like how careful planning will land a billiards ball in the desired hole if the proper pattern is in place, the reflective layer makes it so the light stays in the photoactive layer longer. This involves nudging the light just a bit so that it stays bouncing in the photoactive layer for a much longer period of time.
The best pattern
Choosing and making the right pattern for an optimal solar cell is pretty difficult. For starters, the patterns that work best aren’t visible to the eye. They’re on the nanoscale, about a million times smaller than an ant—a nanopattern–and they should be quasi-random. Quasi-random means literally partly-random, or a value randomly picked from a constrained set of possibilities, leading to a pattern that’s not completely ordered, but not quite disordered.
Imagine that you’re at a fast food chain and they’re holding a small lottery. The announcer uses a smartphone app to randomly pick what the next prize will be: either a small, medium, or large drink. This lottery is quasi-random. Even though the individual choices are random, the app picks a size from the only three options available, rather than from an infinite variety of different sizes. Now let’s say the app drew 20 prizes. If you lined up all the cups in the order selected—say small, small, large, medium, large, small—the resulting pattern would be quasi-random.
Quasi-random patterns are ideal for bouncing light, but at the nano-level precision needed for solar cells, they are very expensive. Even though it’s possible to achieve your desired pattern, a sample that’s the size of a postage stamp may take weeks to make and thousands of dollars to fabricate.
Enter the Blu-ray
This was the conundrum faced by my colleague, materials engineer Alex Smith. He had experience in the solar cell industry before, and had already been working on nanopatterned solar cells for research, but he wanted some practice before undertaking an exciting new collaboration. You see, a small, quasi-random patterned sample cost several thousand dollars. One mistake, and it could cost not—quite everything, but it would certainly feel that way.
His method for practicing came one day when he saw pictures of CD-recordables (CD-Rs)—specifically, their data layer. The physical imprint of a CD-R consists of a series of alternating hills and valleys, making parallel lines. They were patterned and two-dimensional – surprisingly close to the quasi-random patterns he wanted to produce for solar cells -- but not quite right.
After some searching, he found that Blu-ray disc patterns were just the right size range for his solar cell design. He first guinea pig disc was Jackie Chan’s Super Cop (a whopping five dollars). He stamped the disc’s nanopattern onto the photoactive layer of a solar cell, creating ridges on the adjacent reflective layer. It worked! The solar cell could bounce and absorb light much more efficiently than before. Jackie Chan improved the solar cell’s performance by an average of about twenty percent.
The punchline: standardization
The pattern of ones and zeroes making up the data on a Blu-ray disc is quasi-random. Similar to Morse code or Braille, the “pits,” or the depressions of the pattern, and the “lands” making up the raised parts have lengths that fall within a specific range. This is a side effect of making entertainment mass-producible. The disc data on any Blu-ray or DVD is standardized by error compression and modulation algorithms. Among other things, these algorithms allow a disc reader to differentiate the disc content from, for example, a scratch or a piece of dust, and introduce fail-safes that let the disc last longer. Most importantly for our hack, the algorithms impose constraints, which ultimately make the disc pattern quasi-random.
So next time you're looking to throw out a copy of a movie you don't really like, or really, anything which seems to have a dedicated purpose, take a moment. What else could you use this for? How else can you think about it and its constituent parts? You might discover the next life hack or the next big discovery. Or in the case of our lab, show that Jackie Chan isn't just one of the greatest action heroes of all time; he's great at pumping up solar cells, too.