The search for less expensive solar cells drives many lines of research these days, with trends toward smaller collectors and less expensive materials leading the way.
Many solar cells use gold and other pricey metals to provide junctions within the cell structure. Gold closed Friday at $1,204.00 per troy ounce on the London Metal Exchange, and nickel at $10.01 per pound. That would make gold worth $17,558 per avoirdupois pound (14.583 troy ounces per pound), or 1,754 times more expensive than nickel.
According to Gizmag, University of Toronto investigators found that substituting nickel for the previously used gold as collection contacts in their colloidal quantum dot solar cells provided equal performance, at a 40 to 80-percent drop in solar cell prices. Following that math, current pricing of solar cells such as Ascent’s thin film units at $6.00 per Watt could drop to $2.40 to $1.20 per Watt; near the $1.00 per Watt goal many cell makers have long sought. Disappointingly, the price doesn’t drop 175,400 percent because of the small amount of precious metal used in the cells.
Their paper, “Depleted-Heterojunction Colloidal Quantum Dot Photovoltaics Employing Low-cost Electrical Contacts,” was published online in the July 15 Applied Physics Letters of the American Institute of Physics. Researchers had some problems with the nickel mixing with the quantum dots, otherwise known as nanocrystals, but the addition of a nanometer thick layer of lithium fluoride provided separation and allowed current flow from the quantum dots to the nickel. Quantum dots have the added advantage of being created through low-cost chemical reactions.
In a similar move to use more mundane materials to cut cell costs, Berkeley Lab’s Solar Materials Energy Research Group found that embedding a nine-percent concentration of selenium in zinc oxide increased the oxide’s ability to absorb light. Mary Mayer, a fourth-year doctoral student at UC, Berkeley, sees an even more exciting application for her work, splitting water into hydrogen and oxygen. This reaction, at some undefined future time could make hydrogen-powered cars a possibility, although she dismisses the idea of any “meaningful numbers” soon.
As reported in Science News, Greg Nielson of Sandia National Laboratories in Albuquerque is working with his research group on “lint-sized” solar cells that will not only produce electricity, but also cut the amount of silicon needed for them to deliver a Watt of power. Sandia’s cells are one-tenth the thickness of ordinary silicon cells, and small laterally. Nielson’s group is embedding tiny cells into glass or plastic plates, with each lens concentrating sunlight onto a “pinpoint” sized solar cell below. Since silicon is needed only at the focal point of each lens, about one percent of the material required for commercial cells, “silicon is no longer the dominant cost, but a negligible one,” according to Nielson.
The 250 micrometer to 10 millimeter cells, shaped like hexagons, are called “glitter” by the lab personnel, and have an energy conversion efficiency of about 15 percent. Nielson predicts “getting over 20 percent.”
Two other approaches use an array of solar cells with a vertical orientation, much like the pile of a carpet.
John Rogers, at the University of Illinois at Urbana-Champaign, fashions hair-thin strands (10 to 15 micrometers thick) of silicon, which “flexes like a strand of hair” according to Science News. These strands are arranged like carpet pile, and are so thin that much light passes through without bouncing off a strand.
“By backing the cells with a reflective material, however, photons that initially evaded the silicon will bounce back for a second chance at collection. ‘We found that 15 micrometers is just about the right thickness for that kind of double-pass configuration,’ Rogers says. ‘It will collect about 90 percent of the light.’ And the efficiency of these cells is already good, he says, on the order of 12 percent.”
As with the Sandia Labs cells, the Illinois team uses concentrators to focus light on the strands. The article describes the clever way in which the team etches thousands of cells from a block of pure silicon, then lifts and transfers and attaches these onto a “soft piece of slightly tacky rubber, which acts as the base for the cells.
“’We can lift up thousands of these cells at a time and then simply rubber-stamp them down onto a surface’” coated with a thin-film adhesive, Rogers says. “’Our throughputs correspond to millions of devices per hour — much, much higher than can be achieved with even the most sophisticated tools for doing that [by] pick-and-place.’”
In a similar effort by Caltech scientists the fibers make do without a concentrator, first reported here in a February 24 entry. The April issue of Nature Materials “describes a prototype that resembles a sparse carpet of tiny fibers that stretch up toward the light. In the latest designs the fibers are 100 micrometers long and 1 or 2 percent as wide.”
As with the other cell types, this carpet-like arrangement uses far less silicon, but gets a high efficiency from bouncing the light the length of the fibers, which are surrounded by a clear silicon caulk-like material.
As described in Science News, “Some photons entering the carpet will immediately hit a semiconductor fiber. Many more will miss the wires, which cover only 1 to 5 percent of the carpet’s footprint. But by making the wires effectively long and the carpet’s bottom reflective, photons not initially collected will ricochet repeatedly within the carpet until the silicon collects most of them, explains team leader Harry Atwater.”
Ricocheting photons produce the “same light absorption” as “a sheet that’s 100-percent silicon,” with only one percent of the high-cost material. The ricochet effect also obviates the need for concentrators, which Caltech researchers claim don’t work well when the sun is not at the right angle in the sky or when obscured by clouds.
Both articles note that none of the approaches, though promising, is yet ready for commercial production, but as Mary Mayer says of solar research, “if you can dream it, someone is trying to research it.”