SUNNYVALE, Calif., Oct. 11, 2011 — A special ink that can suspend silicon nanoparticles evenly in a solution is proving useful in solar cell production. The silicon ink can lay down precise microns-thick lines that are needed to dope the silicon emitter exactly under the front metal contacts, which make the solar cell work.
Dopants, or impurities, are used to change the conductivity of silicon and to create the internal electric fields that are needed to turn photons into electrons and thus into electricity. One of the great challenges is to distribute the exact concentrations of dopants in precisely the correct locations throughout the device. The silicon nanoparticles in the ink contain dopant atoms that can be driven into silicon solar cells to form a selective emitter.
NREL’s Yanfa Yan, left, principal scientist, and Kim Jones, staff scientist, work with a focused ion beam electron microscope to analyze the topography of materials such as Innovalight’s silicon ink. (Images: Dennis Schroeder/NREL)
Solar cell technology company Innovalight, with the help of researchers at the Department of Energy’s National Renewable Energy Laboratory (NREL), developed the silicon ink that they said can deliver high concentrations of dopants to extremely localized regions of the emitter, increasing the solar cell’s efficiency. Innovalight was acquired by DuPont this summer. (See: DuPont Acquires Innovalight, Expands Solar Energy)
“The question was, can you print this ink in very well defined lines and drive in dopants only in the material underneath the lines to create a well-defined selective emitter,” said Kristin Alberi, NREL senior scientist. If so, the increased concentration of dopants right under the contacts would lower the resistance at the metal contact, while the rest of the cell contained low-doped silicon. The team said that would mean jumps in efficiency and savings of huge amounts of money.
“On some level, you want the emitter to be highly doped so it makes a better contact with the metal,” Alberi said. “But if it’s too heavily doped elsewhere, that’s bad.”
That’s why a selective emitter that is heavily doped only in precise portions of a solar cell is such a promising technology.
The next question the team tackled was whether, using a screen-printing approach, silicon ink can lay down differently doped lines without having the ink spread all over the place. If the ink spreads, the spots where silicon is supposed to be lightly doped get the overflow from the spots where silicon is highly doped.
The inside of the specimen/vacuum chamber of NREL’s FEI Nova 200 dual-beam electron microscope used to analyze the topography of materials such as Innovalight’s silicon ink. The instrument is used to produce site-specific sections for high spatial microstructural analysis.
They found that when used in a low-cost screen-printing process, the silicon ink delivered a 1 percentage point absolute increase in efficiency of the solar cells.
If that doesn’t sound like much, consider that a typical silicon solar cell array in the field may convert 15 percent of the photons that hit it into usable electricity. That 1 percentage point increase actually represents a 7 percent increase in power output for a cell that is typically 15 percent efficientl — at a cost that is so low that it basically goes unnoticed at large solar-cell manufacturing plants.
“That’s a huge impact for almost nothing,” said Richard Mitchell, NREL’s lead investigator on the project.
In the manufacturing process, the silicon ink spills onto a screen, a squeegee pushes it one way as the silicon wafers pass through, then pushes the ink the other way as new wafers appear below the screen. The ink reaches the cell only at the precise points where a tiny slit in the screen’s mask lets it get through. The slits are narrower than a human hair.
Every once in a while, a syringe adds some more silicon ink to the screen to ensure the spread is even and that the liquid doesn’t run out.
Once the silicon and the dopants are where they should be on the unfinished cell, they are heated — not enough to melt them, but just enough to drive the dopants contained within the silicon ink into the solar cell.
“This is the first technology that showed that exactly where you print is exactly where the cell gets doped — to a precision of a micron,” Mitchell said.
NREL and Innovalight shared an R&D 100 award for 2011 for the silicon ink technology, given by R&D 100 Magazine.