Perovskite Solar Cells Are Greener Than Silicon
Tom Lombardo posted on August 18, 2020 |
Perovskite solar cells have lower carbon footprints and shorter energy payback periods than silicon.
Six inch perovskite on Si wafer. (Image courtesy of Oxford PV.)
Six inch perovskite on Si wafer. (Image courtesy of Oxford PV.)

It’s no secret that solar panel manufacturing is a dirty business, largely due to the intense heat that’s required to purify silicon. The amount of CO2 emitted during that process is more than negated by the fact that once operational, the panels will generate lots of carbon-free electricity. Nonetheless, scientists and engineers are still looking for ways to reduce the carbon footprint of photovoltaic (PV) panels. 

One promising silicon alternative is perovskite, whose PV properties have been improving so rapidly that the material has been the subject of thousands of research papers over the past decade. The U.S. Department of Energy (DoE) also thinks the mineral has potential, as it recently earmarked $20 million to advance perovskite stability, efficiency, manufacturing and testing. (Interested in applying for a DoE grant? The agency is hosting a webinar on August 21 to discuss funding opportunities.)

Flexible perovskite solar cell. (Image courtesy of National Renewable Energy Laboratory [NREL].)
Flexible perovskite solar cell. (Image courtesy of National Renewable Energy Laboratory [NREL].)

A recent life-cycle analysis of various PV technologies found that manufacturing multilayer perovskite cells has a lower carbon footprint than fabricating silicon cells or perovskite-on-silicon tandem cells. Even better, perovskite panels are less expensive to manufacture and easier to recycle. Oh, and they’re slightly more efficient than their silicon counterparts. But there are trade-offs—most notably, their lack of stability.

Perovskite solar cells have three enemies: air, moisture and heat. The first two are easily defeated by encapsulation, but it’s difficult to keep something cool when it sits in direct sunlight for hours on end. As a result, even the most stable perovskite cells have only survived about 4,000 hours of continuous light in laboratory testing. (That’s roughly equivalent to two years at five peak-sun-hours per day.) But given that the material’s photovoltaic efficiency has increased from less than 4 percent to over 25 percent in a mere decade, and that stackable tandem cells can inexpensively improve that number, it’s easy to see why researchers are focusing their efforts on making this technology practical. 

Life Cycle Analysis

In Life Cycle Energy Use and Environmental Implications of High-Performance Perovskite Tandem Solar Cells, published in the July 2020 issue of Science Advances, researchers examined three types of solar panels—state-of-the-art silicon, perovskite-silicon tandems (silicon cells coated with a perovskite layer), and perovskite-perovskite tandems—to compare their carbon footprints and energy payback periods. They also considered the environmental impact of the additional materials needed to make a panel along with manufacturing scalability. 

Given the number of unknowns regarding perovskite’s field performance, the scientists made certain assumptions. First, they conceded that the first generation of commercial perovskite panels may only last 15 years, compared to 30 years for silicon. Second, they didn’t include the additional carbon footprint associated with replacing the perovskite panels in the field. 

Carbon Footprint 

What most of us call “carbon footprint” is scientifically known as the “greenhouse gas emission factor” (GGEF), which represents the CO2 emitted per unit of energy generated. The lower the number, the “greener” the technology. In the electrical world, the kilowatt-hour (kWh) is the standard unit of energy, so the GGEF is measured in grams of CO2-equivalent per kilowatt-hour (g CO2-eq/kWh). For renewable technologies like solar and wind, the only emissions are those related to mining, transporting, manufacturing, and end-of-life, so that number gets spread out over the lifetime of the device. For fossil fuels, the GGEF takes into account drilling/mining, transporting, refining, and burning. 

In this study, the researchers calculated that the GGEF of state-of-the-art silicon panels—the kind used on most utility-scale PV farms—is 24.6 g CO2-eq/kWh. Perovskite-on-perovskite tandem cells came in at 10.7 g CO2-eq/kWh, and perovskite-on-silicon tandems fared worst of all at 46.8 g CO2-eq/kWh. 

Comparison of greenhouse gas emission factors. (Image courtesy of Science Advances.)
Comparison of greenhouse gas emission factors. (Image courtesy of Science Advances.)

(Side note: How green are renewable energy sources compared to fossil fuels? Silicon solar panels have a GGEF of ~25 g CO2-eq/kWh, wind power is 10, whereas natural gas and coal tip the scales at 488 and 1,000, respectively. In other words, today’s solar panels are nearly 20x more eco-friendly than natural gas and 40x greener than coal.)  

Energy Payback Period

Critics of renewable energy claim that it takes more energy to produce a solar panel than the unit will produce in its lifetime. That may have been true 60 years ago, but it’s patently false today. Countless studies have shown that PV’s energy payback period—the amount of time the panel has to work in the field in order to recoup the energy that went into making it—is less than two years. Perovskite will make it even better.

Perovskite-perovskite tandem cells need 78 percent less energy to manufacture than perovskite-silicon tandems, as 90 percent of the energy needed to produce a silicon panel is used to purify the silicon. In contrast, Perovskite-only cells, can be made with a low-temperature screen-printing process that’s not only less energy-intensive but also less expensive. As a result, the energy payback period for silicon is 1.52 years but only 0.35 years (about four months) for perovskite-perovskite tandems. Although the perovskite-silicon tandems have a higher carbon footprint than the other PV technologies, their energy payback period is 1.44 years—slightly less than with silicon—due to the fact that they’re more efficient than silicon-only.

Energy payback periods of different perovskite-silicon combinations. (Image courtesy of Science Advances.)
Energy payback periods of different perovskite-silicon combinations. (Image courtesy of Science Advances.)

Commercializing Perovskite Solar Panels

With long-term stability being such a roadblock, why is the industry putting time and money into developing perovskite panels? Perovskite cells offer many advantages over silicon beyond just a small carbon footprint. First, perovskite is “tunable” to a particular wavelength of light. This makes it easy to stack multiple layers of the material, enabling the cell to absorb and convert more of the solar spectrum into electricity. Currently, this is done with multi-junction solar cells, but those are so expensive that they’re mainly used in the space program, where electrical efficiency overrules panel cost. Thanks to the simple screen-printing process, multilayered perovskite cells are inexpensive to produce. Perovskite can also be made on a low-cost, light, flexible substrate, and doesn’t require a glass front, making it suitable for building-integrated photovoltaics, solar-assisted electric vehicles, and portable electronics. 

To improve the material’s stability, engineers have investigated carbon nanotube coatings to shield the perovskite from the elements. Early results have shown that the coating not only protects the perovskite but also increases its efficiency. Additional research on long-term stability is still needed, but scientists are extrapolating on the known properties of other thin-film materials in order to expedite the work.

With commercial entities and government agencies in Europe, Asia, and North America racing to commercialize perovskite solar panels, it seems inevitable that we’ll see the first generation of these PV modules in the near future. Still, it’ll take a long time for these panels to replace the silicon standard, so don’t expect to see a downturn in that market anytime soon. 

Additional Resources

Perovskite Solar Cells | Department of Energy

The Reality Behind Solar Power's Next Star Material

The Path to Perovskite on Silicon PV | Professor Henry Snaith

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