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Thinner really is better for cheaper solar cells

Slimming down silicon wafers could lead to lower cost of solar panels and faster industry expansion
by TR Pakistan

Researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs of solar panels further, this time by slimming down the silicon cells themselves.

Thinner silicon cells have been explored before, especially around a dozen years ago when the cost of silicon peaked because of supply shortages. But this approach suffered from some difficulties: The thin silicon wafers were too brittle and fragile, leading to unacceptable levels of losses during the manufacturing process, and they had lower efficiency. The researchers say there are now ways to begin addressing these challenges through the use of better handling equipment and some recent developments in solar cell architecture.

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.

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, the heart of these solar panels, 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, which would only require one-fourth as much silicon for a given size of panel.

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Researchers claim that this could not only reduce the cost of the individual panels, but 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. The researchers say that reducing wafer thickness could potentially alleviate this problem.

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 little decline in performance down to thicknesses as low as 40 micrometers, using today’s improved manufacturing processes.

“We see that there’s this area (of the graphs of efficiency versus thickness) where the efficiency is flat and so that’s the region where you could potentially save some money,” notes Liu.

Changing over the huge panel-manufacturing plants to adapt to the thinner wafers will be a time-consuming and expensive process, but the analysis shows the benefits can far outweigh the costs, Liu says. It will take time to develop the necessary equipment and procedures to allow for the thinner material, but with existing technology it should be relatively simple to go down to 100 micrometers, which would already provide significant savings. Further improvements in technology such as better detection of microcracks before they grow, could help reduce thicknesses further.

The authors of the study believe that in the future, the thickness could potentially be reduced to as little as 15 micrometers. New technologies that grow thin wafers of silicon crystal directly rather than slicing them from a larger cylinder could help enable such further thinning.

Development of thin silicon has received little attention in recent years because the price of silicon has declined from its earlier peak. However, due to 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 say.

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“Efficiency can only go up by a few percent. So if you want to get further improvements, thickness is the way to go,” says Buonassisi. 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,” Buonassisi says. “It’s not easy, but there is a path. And for the first movers, the advantage is significant.”

What may be required is for the different key players in the industry to get together and lay out a specific set of steps forward and agreed-upon standards, as the integrated circuit industry did early on to enable the explosive growth of that industry. “That would be truly transformative,” he says.

Andre Augusto, an associate research scientist at Arizona State University who was not connected with this research, says “refining silicon and wafer manufacturing is the most capital-expense (capex) demanding part of the process of manufacturing solar panels. So in a scenario of fast expansion, the wafer supply can become an issue. Going thin solves this problem in part as you can manufacture more wafers per machine without significantly increasing the capex.” He adds that “thinner wafers may deliver performance advantages in certain climates,” performing better in warmer conditions.

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