Capturing the sun
Researchers seek safer, cheaper solar technologies
Parag Vasekar says, "What appeals to me is the philosophy of developing a thin-film solar cell using Earth-abundant materials."
When research scientists Tara Dhakal and Parag Vasekar joined the Center for Autonomous Solar Power (CASP) two years ago, the lab was empty save for one necessary raw material — sunlight shining through the windows.
Today, the lab is packed with equipment that students and researchers use to test and retest their ideas about how to best capture the sun’s energy and store it for later use.
Capturing and storing are why the word “autonomous” is in the center’s name; being able to use sun-generated electricity on a cloudy day is key to making solar power a first choice, not an alternative, among consumers. Dhakal and Vasekar are focused on one piece of that process: thin-film solar cells.
Thin-film solar cells aren’t new, but the CASP’s approach to making them is. Most commercial solar cells are made with semiconductor materials such as cadmium telluride, silicon or CIGS (copper, indium, gallium and selenide). But cadmium is toxic and tellurium and indium are rare. Being able to use easy-to-find materials such as iron, sulfur, zinc, phosphorous, copper and tin could lead to safer, more efficient and less expensive solar cells. Using combinations of these environmentally benign and Earth-abundant elements, Dhakal and Vasekar are focusing on three sunlight absorbers: copper-zinc-tin-sulphide (CZTS), iron sulphide and zinc phosphide.
The quest for the two men is to determine which materials, in what combinations and in what amounts, will produce the most electricity.
They started with a short list of three or four candidates. Already Dhakal has found something he’s excited about. It’s iron disulfide, also known as pyrite or fool’s gold. It’s plentiful and cheap.
“I have a little pack of pyrite I bought in a toy store for $2,” Dhakal says. But the pyrite needed for a thin-film solar cell must be grown in the laboratory so it will be pure and smooth. It’s a tricky, exacting process that must be repeated for each mineral they try.
Typical solar cells are assembled into rigid solar panels and are commonly seen on roofs of houses. The upfront cost of manufacturing and installation (reinforcing a roof to support the weight) is not always attractive to consumers.
To make a thin-film solar cell, layers of light-absorbing semiconductor material are deposited over a larger area onto a bendable substrate, typically plastic. They are lightweight and easy to transport and install, and if the CASP can make them safer and cheaper, it could change the way people use solar power.
“So many people are in solar-cell research, but we want to do something different. Not only different for the sake of difference, but to make a difference,” Dhakal says.
While the two researchers take small but calculated steps in the lab, Charles R. Westgate, director of the CASP, has his eye on the big picture, and that’s the ability for Binghamton University to provide the research that energy businesses can use to generate jobs. The thin-film solar cell may be the first to have a technology available for potential commercial development, he says.
“After 2011, the number of films being deposited and the number of cells being made and tested have gone way up. We’ve already had good cells, but we want a great one,” Westgate says.
Dhakal is hopeful that iron disulfide might make a great one. “We are in a testing phase right now. We may have a surprise in a couple of months. If it doesn’t work, we’ll go back and see what went wrong. That’s the fun part of it.”