An improved process for making solar cells could allow manufacturers to cut the amount of silicon needed in half. Since silicon can account for about three-quarters of the cost of conventional solar cells, this could significantly lower the price of solar power. The technique can reduce the amount of other materials used and improve solar-cell performance.
The process uses ink-jet printing to make electrical connections within a solar cell, replacing the existing screen-printing process. Because the ink-jet method is more precise, it can use less material for these connections. Also, because the printheads don’t make contact with the silicon, the method works with thinner silicon wafers. The process uses an ink-jet printer built by iTi Solar, based in Boulder, CO, that was originally designed for printing electronics, such as the contacts on touch screens.
The National Renewable Energy Laboratory (NREL), in Golden, CO, which helped direct the design of the device, is now starting to produce solar-cell prototypes using the technology. Since the ink-jet printer can be dropped into existing solar-cell manufacturing lines, it could be used in commercial production within a year, estimates Maikel van Hest, a scientist at NREL.
One of the first applications could be in the manufacture of silicon solar cells, the most common type of solar cell sold today. Silicon absorbs light and converts it into electrons, and then an array of silver lines printed on the silicon collects these electrons, creating an electrical current. In conventional manufacturing, these silver lines are printed using screen printing. The silver ink used in the new process is much more conductive than the silver paste used in screen printing, and ink-jet printing is more precise. As a result, much thinner lines can be printed–35 to 40 micrometers wide, compared with 100 to 125 micrometers wide with screen printing, van Hest says. Using less silver saves money. It also improves the performance of the solar cell since the thinner lines shade less of the active material.
The biggest advantage, van Hest says, is that ink-jet printing, unlike screen printing, does not involve applying pressure to the silicon wafer. That makes it possible to use far thinner wafers, he says. In conventional solar cells, the wafers are about 200 micrometers thick. “If you go any thinner than that, most of them will break” during manufacturing, van Hest says. “If you go to a noncontact method, you don’t have to worry about that. You can use cells as thin as a hundred micrometers, or even thinner. That means you can save 50 percent on the cost of silicon.” NREL decided to use iTi Solar’s printers, as opposed to those from other manufacturers, for two main reasons. The first is the accuracy of the system: the printers can apply ink with an accuracy of within 1 micrometer, as opposed to 10 to 15 micrometers with other systems, van Hest says. It’s also easy to adapt to working with different inks and different solar-cell technologies. The system could, for example, be used to manufacture thin-film solar cells made of semiconductors such as copper indium gallium (di)selenide (CIGS), employing inks that NREL has developed.
Ink-jet printing has been considered before for solar-cell manufacturing. “There initially was a lot of buzz about it–two years ago, everyone was talking about it,” says Bruce Morgan, iTi Solar’s CEO. “Then as the practical results were seen, people got discouraged.” He says the problem was that the printheads didn’t have high enough resolution, or companies didn’t understand the distortions in the materials that were being printed on. The initial results of the iTi Solar system suggest that it has solved these problems. “We’ve seen what it can do, and so far, it’s very impressive,” van Hest says.
The NREL system is designed to produce only prototype cells. Van Hest says that the technology can easily be scaled up, such as by increasing the number of printheads, to make many cells at once for commercial production.