Solar energy is a hot topic these days, with frequent reports of new innovations that drive cost down and efficiency up. Solar modules are designed to absorb sunlight and convert a portion of the light energy into electric power. Therefore, one thing a solar module shouldn‘t do is reflect sunlight, because every photon that bounces off is one less electron than can be converted from light to electricity inside a given cell inside the photovoltaic (PV) solar module.
So, one challenge for PV solar module manufacturers has been, how to make that module surfaces less reflective. Currently this is accomplished on crystalline silicon solar modules through the use of surface treatments that add texture to the silicon surface and non-reflective coatings to the exterior glass surface. These processes have led to a dramatic decrease in reflectivity from crystalline silicon solar modules over time, such that current products reflect very little light.
One measure of the reflectivity is albedo — the ratio of solar radiation across the visible and invisible light spectrum reflected by a surface. Albedo varies between 0, a surface that reflects no light, and 1, a mirror-like surface that reflects all incoming light. Solar modules with a single anti-reflective coating have a reflectivity of around 0.1. By comparison, sand has an albedo between 0.15 and 0.45 and agricultural vegetation (leafy crops, etc.) has an albedo between 0.18 and 0.25. In other words, solar modules have a lower reflectivity than our area’s prevailing ground cover: vegetation (trees, plants), grass, sand, soil, snow (in wintertime), etc.
Solar modules crafted using thin-film technologies rather than crystalline silicon have historically exhibited lower efficiencies (9-11%) than are available with crystalline silicon (15-17%), thus resulting in lower energy production, making it difficult for them to compete with traditional crystalline silicon-based modules with substantially higher efficiencies. This, together with other challenges they face in the area of manufacturing processes, have caused thin-film silicon companies to struggle, and many have gone out of business over the last few years (Solyndra, Abound, etc). However, if thin-film technology could “turn the corner” by reducing the manufacturing complexities and costs they currently face (due to their gas film deposition approach) and could also increase efficiency so that their energy production was comparable to or better than that of crystalline silicon modules, that could lead to new market growth for modules using thin film technology.
One approach that has been experimented with in labs is the use of multiple thin-film layers of silicon, each “tuned” to absorb different wavelengths of light, thus generating more electric power per given amount of solar energy striking the cells.
Another innovation may be coupled with this “tandem” silicon layer approach to decrease reflectivity further and thus increase energy production. Researchers at the Golden-based National Renewable Energy Laboratory (NREL) have come up with a solution: peppering a solar cell with trillions of tiny holes. NREL’s black-silicon process uses an acid bath and a small amount of silver nitrate as a catalyst to oxidize the silicon and thus create these nano-holes.
In just three minutes at room temperature, the process can put a trillion microscopic holes in a 6-by-6-inch solar cell. Each of those tiny holes traps a bit of sunlight, such that only 2 percent of the light is reflected, according to NREL researchers. That extra light boosts the solar cell’s power.
This “holes” technique, dubbed “dark solar,” has been licensed to Red Bank, N.J.-based Natcore Technology Inc., which plans to combine it with the company’s own low-cost solar-cell-manufacturing process that uses liquid deposition rather than gas deposition. The combination of these new processes may lead to the successful fabrication of tandem thin-film “dark solar” cells that have lower reflectivity, higher efficiency, and lower manufacturing costs than are typical today.
“It’s a perfect coupling of technologies,” says Natcore CEO Chuck Provini. Natcore is a research and development lab, which seeks to marry these various new technologies and fabrication techniques and bring them to the point where they are reproducible at low cost, and thus can be duplicated in a manufacturing production environment. Then it will seek to license the combined technologies to solar module manufacturers who would produce modules based on the new “dark solar” tandem cells.
Arise Energy’s Take: The challenge for Natcore will be getting the engineering of the process right…and that will take time. So, watch for dark solar and tandem cells to potentially emerge in production solar modules sometime within the next decade. If and when that happens, we will see lower module costs as well as increased performance of the arrays built using these modules. And that will take us closer to (or beyond) grid parity with PV solar.