Wringing more solar electricity from low-cost materials is a major focus for scientists and those who want the world to get away from using fossil fuels for electricity. Researchers at IBM recently announced that they were able to do just that with a new type of compound that uses cheaper ingredients than what goes into some of the solar panels today.
The researchers reported in Advanced Energy Materials that they fabricated solar cells with copper, zinc, tin and sulfur (CZTS) that for the first time could convert 11.1 percent of the sunlight that falls on it into electricity. That’s a 10 percent improvement from the 10.1 percent efficiency IBM achieved last year and published in Progress in Photovoltaics.
The more electricity you can squeeze from the same set of materials, the lower the generation cost. So increasing the efficiency of solar cells is important to lower the price of solar electricity, which is currently more expensive than generating electricity from coal or natural gas. Coal and natural gas technologies have already benefited from decades of improvements, so replacing them with equally cheap power will take time.
Researchers at IBM and elsewhere are exploring the use of the CZTS compound partly because zinc and tin are more abundant and found in more diverse regions in the world than indium and gallium, which are used to make some of the solar cells today. Recent conflicts over rare earth elements have shown that the path to building a world of clean energy technologies, such as batteries for electric cars and wind turbines, can have major geopolitical roadblocks.
I caught up with IBM’s solar researcher David Mitzi, who also is a co-author of the new paper on CZTS, earlier this year to find out why IBM is investing in CZTS research. Aside from the benefit of cheaper and more available materials, the CZTS compound, because of its crystal structure, also could lead to thinner solar cells than those made with silicon, which is the most common material for making solar cells today. Thinner cells means using less materials and doing so, presumably, at lower costs.
The pace of efficiency improvement in CZTS cell research since the mid-1990s also is a big draw for IBM, Mitzi told me. In 2008, the most efficient CZTS cell achieved 6.7 percent efficiency, and IBM reported a record 9.7 percent efficiency in 2010. A year after that, the company pushed that figure to 10.1 percent.
Mitzi and his fellow researchers are gunning for 15 percent efficiency in order to position CZTS cells competitively against other solar cell technologies. Several companies that produce copper-indium-gallium-selenide (CIGS) solar cells are getting efficiencies in the mid-teens. First Solar is making solar cells in the lower end of the mid-teen range but claims to be on its way to increase that to 17.3 percent some day. Most of the silicon solar cells can get a bit higher efficiency except for SunPower, which is rolling out cells that can hit 24 percent at times.
The goal is to get to 15 percent in about two years, Mitzi said. Achieving that milestone won’t be easy, he acknowledged. Being able to get higher efficiency isn’t enough. Figuring out how to make those high-efficient cells cheaply is key, and that will involve a lot of trial and error in designing and running factory equipment.
IBM isn’t interested in making and selling CTZS cells though. The company is working with partners that might one day commercialize IBM’s research. The partners include Solar Frontier (a CIGS solar cell maker), Tokyo Ohka Kogyo (a semiconductor equipment supplier) and DelSolar (a silicon solar cell maker).
Photo courtesy of IBM Research, and Agua Caliente.
