IBM photovoltaic scientists Teodor Todorov and David Mitzi on Friday detailed the findings of a paper that showed the highest efficiency to date for solar cells made from a combination of copper, zinc, tin, and selenium (CZTS). Published in Advanced Energy Materials, the technical paper described a CZTS solar cell able to convert 11.1 percent of solar energy to electricity.
That level of efficiency is a significant jump from the 10.1 percent efficiency Mitzi and colleagues showed last year. (See, Efficiency Solar Cells from Cheaper Materials). The paper also argues that CZTS solar cells could achieve efficiencies high enough to make them commercially viable.
The benchmark CZTS the IBM-led research group is pursuing is CIGS, solar cells made from copper, indium, gallium, and selenium. In commercial products, the efficiency of these cells is about 12 percent in solar panels and 20 percent in labs. If CZTS could get to 15 percent efficiency, it could start to displace CIGS, says Mitzi.
“There’s no fundamental reason that we know of why we cannot get up to the same efficiencies with CZTS as we do with CIGS,” he says. “It’s one of the key goals for the field for the next year or two.”
Developing an alternative to CIGS is driven not so much by cost as availability of the raw material to make cells, Mitzi explained. Gallium and indium, which is used in flat-panel production, could become constrained as solar volumes increased. Much of the production of those elements is centered in China, he added.
Replacing those elements with more abundant copper and tin in CZTS cells has the potential to supply 500 gigawatts of solar power, many times more than CIGS, according to the paper. The researchers have sought to replace selenium with more abundant sulfur as well to address material availability.
Having CZTS match CIGS on efficiency within five years is a reasonable time horizon, Mitzi said. IBM is working with solar companies in the research, including Japan-based Solar Frontier, which installed 577 megawatts worth of CIGS panels last year, according to GTM Research.
Solar manufacturers have developed thin-film solar panels because they promise to be cheaper. But the costs of the incumbent silicon solar processes have dropped precipitously over the past three years. (See, The Dog Days of Solar).
That has made thin-film’s cost advantage dwindle. But there are a number of research groups working with CZTS with an eye towards the long term. “We’re building an understanding of what we need to fix and address to get the higher performance,” says Mitzi.