If only an hour's worth of the sun's energy pouring down on
the Earth could be harvested, it would power the entire planet for a year,
according to David Mitzi, who leads the team at IBM Research (Yorktown Heights,
N.Y.) which developed its world's record holding thin-film solar cell.
A scanning electronic
microscope (SEC) image of a thin-film photovoltaic structure consisting of
spin-coated kesterite compounds atop a glass substrate.
"Solar cells currently contribute less than 0.1 percent to our
electricity supply, due to their high cost and the fact that they contain
elements which limit production capacity," says Mitzi. "Our team has
now moved us closer to overcoming these problems by developing a solar cell
that can not only compete on a cost-per-watt basis with conventional
electricity generation, but which can also be deployed at the terawatt levels
we need."
Thin-film solar cells convert light into electricity using
several layers of photovoltaic materials on a substrate. For traditional solar
cells, those layers are semiconductors deposited at high temperatures in a
vacuum atop an expensive pure crystalline silicon wafer. The trend in thin-film
solar cells, however, is to cut costs by using room temperature, solution-based
coating methods -- spraying, dipping and spinning -- on inexpensive plastic
substrates.
Today, one class of these less expensive thin-film solar
cells is called CIGS (copper-indium-gallium-selenium). Unfortunately the
"I" in CIGS stands for "indium," which is not only
expensive, but also is in short supply due to its widespread use to make
transparent thin-film transistors (TFTs) used in flat-panel displays.
IBM Research set out to best the thin-film CIGS solar cells
by substituting tin and zinc for indium, thereby lowering their cost. Secondly,
IBM perfected a "spray-on" manufacturing technique that merely
spin-coats an inexpensive glass substrate with the necessary compounds of
"kesterite" (copper, tin, zinc, sulfur and selenium). This
solution-based manufacturing technique works by dissolving the copper and tin
in a solvent into which it pours nanoparticles of zinc. The slurry is then
spin-coated onto a glass substrate in the presence of sulfur or selenium
vapors.
IBM's low-cost manufacturing technique nevertheless results
in a new world's record for kesterite thin-film solar cells, 9.6 percent efficiency – 40 percent
higher than the previous world record of 6.8 percent set last year by a Japanese
research group at Nagaoka National College of Technology.
Next, the IBM Research team aims to optimize its
architecture further to boost the efficiency into the 11 percent range that rivals
current commercial CIGS solar cells. The team also plans to experiment with
even less expensive manufacturing techniques including dip coating, spray
coating and slit casting.
