GLOBAL CHALLENGES

Smarter Solar Cells Made Cheaper, More Plentiful

R. Colin Johnson | Date: 02-11-10 | Comments

IBM Research recently broke the world's efficiency record for inexpensive, thin-film solar cells by substituting widely available elements for the rare elements that make photovoltaic devices so expensive today.

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.