As the price of solar panels has plunged, a strange thing has happened. The panels have gone from being a large fraction of installation costs to a relatively minor component. That means all the other things—permits, labor, supporting hardware, and so on—make up the bulk of the cost of putting panels on your roof. While these costs are going down as well, they're not falling at nearly the same rate as the panels themselves.

All of which raises an intriguing question: if there's a large fixed cost involved in getting panels on your roof, does it make sense to install more efficient panels, despite their higher costs? A collaboration between MIT researchers and people at solar power companies have answered this question with a very qualified "yes." Critically, one of the qualifications is that they assume availability of a technology we haven't developed yet.

Efficiency limits

Currently, thin-film solar panels have efficiency percentages in the teens, while silicon has reached the low 20s. While there's some room for improvements in both of these technologies, progress is probably going to be incremental. There also exist some alternative technologies that have high material costs that aren't likely to drop substantially any time soon. Beyond those, physics sets a hard cap on the maximum efficiency possible at 33 percent.

But there is a way to get around the expensive materials and physical limits: stacking multiple cells. The materials we use for solar power typically absorb photons within a specific range of energies. Outside that range, photons will pass through the material as if it were transparent (which, at those wavelengths, it is). By placing a different material, operating on different wavelengths, beneath the first solar cell, it's possible to have the two cells grab different chunks of the spectrum. In such a system, both cells operate at close to their normal efficiencies.

There have even been three-layer (or "triple junction") solar cells created. While the cells at the bottom of the stack don't operate at their full efficiency, they're still better than throwing the photons away.

The problem has been that it's far cheaper to manufacture a simple one-layer design. While multi-junction designs have their places where weight or size are critical, they're a relatively niche product.

But, as we noted before, the economics of solar have also changed in a way that could make expensive hardware more economically viable. If putting solar panels on roofs gets expensive enough, then it can make economic sense to try to get as much value—in terms of electricity generated—out of that expense. The new study attempts to determine if we've crossed that threshold.

Thin is in

The researchers focused on thin-film technology, which is dominated by two materials: cadmium telluride (often called cadtel) and CIGS, or copper indium gallium selenide. While these have lower efficiencies than silicon, they're very cheap to make and have therefore remained competitive for utility-scale installations where space isn't at a premium. At the moment, nobody is manufacturing multi-junction panels using these two technologies, but the processing involved in existing manufacturing is compatible, meaning there's no obvious reasons why it couldn't be done.

The team behind the new paper considered two different ways of making this happen. The first essentially involves layering two complete panels on top of each other, with separate wiring for each material. The second method they considered is to simply layer the two materials on top of each other and have a single set of wiring capture charges from both. The former is more efficient, but the latter is cheaper to build. While these panels haven't been made, data from manufacturers of existing panels based on these technologies provides reasonable cost estimates. Existing thin-film panels were also included in the analysis.

The researchers then performed a levelized cost of electricity analysis (LCOE), which considers the total lifetime system costs vs. the amount of electricity produced. This was done for three climates: dry, humid, and temperate. The climate differences are important because water vapor absorbs part of the spectrum of one of the thin-film materials, altering its efficiency.

When it comes to utility-scale installations, the results are very clear: multi-junction solar panels offer no clear advantages. That's in part because space isn't an issue and in part because large numbers of panels can share the same infrastructure, making the panels a greater percentage of the total costs. As a result, whatever technology has the lowest dollars-per-Watt comes out on top.

Space saver

For homes, however, space is at a premium, and all costs are shared by a relatively small number of panels—and panels may account for as little as 20 percent of the total price. And here, the most efficient panel comes out on top—in this case, the one with two separate panels stacked on top of each other. It was ahead in LCOE by anywhere from five to eight percent, depending on the climate.

This analysis is sensitive to the total cost of installation, which is dropping and has been targeted for improvement by the US Department of Energy. But the authors of the new paper estimate that these costs would have to be cut by half before a single-layer thin-film panel would make more sense.

Of course, that analysis ignores an additional option: silicon panels, which have higher efficiencies and very competitive pricing. The authors note that this same analysis could be applied to competing technologies, which include silicon and dual-layer panes with silicon powering one of the layers. But it's somewhat strange that the technology that is most frequently used for rooftop installs wasn't even considered here. That said, thin-film technologies are relative newcomers to solar power, and there may be significant manufacturing improvements that can be achieved with them; the same's unlikely to be true for silicon.

Overall, these numbers will be sensitive to any changes in costs, whether they're due to improved manufacturing, lower installation costs, or electricity prices. But they do give us a sense of where things are now and what will have to change in order for the economics to end up different. And, if something looks likely to be stable for long enough, it can be used to direct policy. For example, if the DOE is confident that installation costs will continue to soak up more than 70 percent of the price of rooftop solar, then it could work to promote multi-layer solar manufacturing.

Nature Energy, 2018. DOI: 10.1038/s41560-018-0126-z (About DOIs).