A novel way to make thin, uniform coatings developed at Rice University could reduce the cost of making conventional silicon solar cells, and could open the way for new kinds of solar cells that are far more efficient or cheaper than conventional ones.
The technology, which deposits coatings in a low-temperature, liquid-based process rather than the high-temperature gas-based process used now, is being commercialized by Natcore Technology, a startup in Red Bank, New Jersey. The company plans to use the technology to replace a standard step in conventional solar cell manufacturing—adding an antireflective coating to silicon wafers to help them to absorb more light. It will also offer a more advanced antireflection technology, called black silicon.
At the same time, Natcore is developing more advanced applications of the process, including fabricating solar cells made of carbon nanotubes or nanoscale crystals called quantum dots. Such solar cells will probably take years to commercialize, but could far outperform conventional solar cells. Nano solar cells have been attempted before, but the company thinks its new manufacturing technology could make them affordable.
As a replacement for high-temperature processes on a conventional manufacturing line, the liquid-based process can lower manufacturing costs. Natcore’s CEO, Charles Provini, estimates that replacing a conventional coating machine with one of his company’s could save a solar manufacturer about $1 million in electricity costs per year.
Manufacturers don’t currently use liquid-based processes for antireflection coatings in part because it’s been difficult to make the coating uniform enough for solar cells. The problem arises from the way a liquid process typically works. The coating forms as reactants in the liquid interact with a surface. As the reactants are used up, the rates of deposition change, resulting in variations in the thickness of the coating. Researchers at Rice addressed this problem by developing a system for continuously replenishing the reactants while also closely monitoring the thickness of the films.
One of Natcore’s advanced nano solar cell designs involves depositing layers of quantum dots on a silicon solar cell. The quantum dots are designed to absorb colors that silicon doesn’t, potentially doubling the efficiency of solar cells. This has been tried before, but forming a layer of quantum dots has required expensive processing technology, and it has proven difficult to space the quantum dots to avoid unwanted electrical discharges between them. The Natcore process is inexpensive, and it provides a means for controlling the arrangement of the quantum dots by coating them with a layer of silicon dioxide that acts as a spacer. The company has decided to start by coating conventional silicon solar cells to make it easier for the industry to adopt the technology, but could eventually do away with silicon wafers for an entirely quantum-dot-based solar cell that uses more than one type of quantum dot to efficiently absorb the entire range of wavelengths in sunlight.
Another design, which Natcore is developing together with Kodak, involves using the liquid deposition process to coat a network of carbon nanotubes with a solar semiconductor material to produce thin, flexible solar cells. Natcore says solar cells using this design could be about as efficient as conventional silicon solar cells, but cost roughly half as much to make, in large part because they could be made with the same equipment that Kodak has used to make photographic film. Because the solar cells would be light and flexible, they would also be easier to install, cutting installation costs in half, Natcore estimates.
Andrew Barron, the professor of chemistry and materials science at Rice University who developed the liquid deposition technology, says the carbon nanotube design is closer to commercialization than the quantum dot one. He says researchers have made small prototype solar cells—the remaining development work has to do with working out the details of manufacturing. The quantum dot solar cells are still at an early stage—the researchers have only so far used the liquid process to show that it’s possible to distribute the quantum dots as needed. They haven’t built solar cells yet.
Natcore has raised about $6 million through a public offering on a Canadian stock exchange. It has also signed joint venture agreements with companies in China and Italy. The company plans to license its technology to others, rather than manufacture solar cells itself. It is currently testing a prototype version of a commercial-scale liquid deposition machine, and Provini says the company has four solar cell manufacturers lined up to buy the commercial version of the machine, if the company meets certain technical milestones.