Thinner wafers will slash solar cells costs, say MIT researchers
Indeed, researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs further, this time by slimming down the silicon cells themselves.
Currently, 90 percent of the world’s solar panels are made from crystalline silicon, and the industry continues to grow at a rate of about 30 percent per year, the researchers say. Today’s silicon photovoltaic cells are made from wafers of silicon that are 160 micrometers thick, but with improved handling methods, the researchers propose this could be shaved down to 100 micrometers — and eventually as little as 40 micrometers or less, which would only require one-fourth as much silicon for a given size of panel.
That could not only reduce the cost of the individual panels, they say, but even more importantly it could allow for rapid expansion of solar panel manufacturing capacity. That’s because the expansion can be constrained by limits on how fast new plants can be built to produce the silicon crystal ingots that are then sliced like salami to make the wafers. These plants, which are generally separate from the solar cell manufacturing plants themselves, tend to be capital-intensive and time-consuming to build, which could lead to a bottleneck in the rate of expansion of solar panel production. Reducing wafer thickness could potentially alleviate that problem, the researchers say.
The new findings are detailed in a paper in the journal Energy and Environmental Science, co-authored by MIT postdoc Zhe Liu, professor of mechanical engineering Tonio Buonassisi, and five others at MIT and NREL. The researchers describe their approach as “technoeconomic,” stressing that at this point economic considerations are as crucial as the technological ones in achieving further improvements in affordability of solar panels.
The study looked at the efficiency levels of four variations of solar cell architecture, including PERC (passivated emitter and rear contact) cells and other advanced high-efficiency technologies, comparing their outputs at different thickness levels. The team found there was in fact little decline in performance down to thicknesses as low as 40 micrometers, using today’s improved manufacturing processes.
The analysis shows that, although changing over the huge panel-manufacturing plants to adapt to the thinner wafers will be a time-consuming and expensive process, the benefits could far outweigh the costs. In the future, the thickness could potentially be reduced to as little as 15 micrometers, thanks to technologies that grow thin wafers of silicon crystal directly rather than slicing them from a larger cylinder. Development of thin silicon has received little attention in recent years because the price of silicon has declined from its earlier peak. But, because of cost reductions that have already taken place in solar cell efficiency and other parts of the solar panel manufacturing process and supply chain, the cost of the silicon is once again a factor that can make a difference, the authors highlight.
“Efficiency can only go up by a few percent. So if you want to get further improvements, thickness is the way to go,” Buonassisi says. But the conversion will require large capital investments for full-scale deployment.
The purpose of this study, he says, is to provide a roadmap for those who may be planning expansion in solar manufacturing technologies. By making the path “concrete and tangible,” he says, it may help companies incorporate this in their planning. “There is a path,” he says. “It’s not easy, but there is a path. And for the first movers, the advantage is significant.”
MIT – www.mit.edu
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