Another major breakthrough for solar energy

By Alex Kimani

During America’s last election cycle, one of President Trump’s key campaign promises was to revive the dying coal sector and bring back coal jobs. But even intense lobbying by the president has done little to stem the tide as coal plants continue to drop out at a steady clip.

The recent retirement of TVA’s giant 1,150 megawatt-Paradise 3 coal plant, despite Trump’s impassioned pleas, serves as a grim reminder that coal’s best days are behind it, with the U.S. Department of Energy acknowledging as much in its latest report.

The so-called natural gas bridge has lately become the bane of coal. Now, next-generation perovskite solar cells are likely to not only put the final nail in the coal’s coffin but also to twist the knife into a suffering oil and gas industry.

Back in May, we reported that the U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) had forged a public-private consortium dubbed the US-MAP for U.S. Manufacturing of Advanced Perovskites Consortium, which aims to fast-track the development of low-cost perovskite solar cells for the global marketplace.

That partnership appears to be bearing fruit, with the consortium recently announcing highly encouraging advancements in perovskite technology that could boost the efficiency of perovskite solar cells from the current ceiling of ~25% to a dreamy 66%.

High-Performance Perovskite PV Coming
Silicon panels pretty much rule the solar energy sector, with more than 90% of panels manufactured using the versatile element.

Silicon PV cells have their advantages: They’re quite robust and relatively easy to install. Thanks to advances in manufacturing methods, they’ve also become less expensive, especially over the past decade, particularly the polycrystalline panels constructed in Chinese factories.

However, they still come with a significant drawback: Silicon PV panels are quite inefficient, with the most affordable models managing only 7%-16% energy efficiency depending on factors such as placement, orientation, and weather conditions. Indeed, solar cells have been around for more than six decades, yet commercial silicon has barely scraped into the 25% range, maxing out at a theoretical 30%. This sad state of affairs is due to the fact that Si panels are wafer-based rather than thin-film, which makes them sturdier and more durable. The trade-off, however, is efficiency.

To meet the world’s rapidly growing energy appetite—and achieve the kind of de-carbonization goals that would help slow the impact of climate change—it would take hundreds of years to build and install enough silicon PV panels. This is way too slow to be practical for our purpose, considering that we have a mere 10-year window to act to avert irreversible and catastrophic climate change.

For years, scientists have experimented with alternative crystal formations that would allow panels of similar size to capture more energy. Until now, few designs emerged that were commercially viable, particularly thin-film cells that could theoretically achieve much higher levels of efficiency.

Thin-film PV panels can absorb more light and thus can produce more energy. These panels can be manufactured cheaply and quickly, meeting more energy demand in less time. There are a few different types of thin-film out there, all of them a little different from standard crystalline silicon (c-si) PV panels.

Amorphous silicon (a-Si) panels are the oldest form of thin-film: a chemical vapor deposits a thin layer of silicon onto glass or plastic, producing a low weight panel that isn’t very energy efficient, managing 13.6%. Then there are cadmium telluride (CdTe) panels, which uses the cadmium particle on glass to produce a high-efficiency panel. The drawback there is the metal cadmium, which is toxic and difficult to produce in large quantities.

These panels are usually produced using evaporation technology: the particles are superheated, and the vapor is sprayed onto a hard surface, such as glass. They are thin, but not as dependable or durable as c-si panels, which currently dominate the market. Perovskite has so far proven to be the most promising and has now managed to break the efficiency glass ceiling.

Perovskites are a family of crystals named after Russian geologist Leo Perovski. They share a set of characteristics that make them potential building blocks for solar cells: high superconductivity, magnetoresistance, and ferroelectricity. Perovskite thin-film PV panels can absorb light from a wider variety of wave-lengths, producing more electricity from the same solar intensity.

In 2012, scientists finally succeeded in manufacturing thin-film perovskite solar cells, which achieved efficiencies over 10%. But since then, efficiencies in new perovskite cell designs have skyrocketed: recent models can reach 20%+, all from a thin-film cell that is (in theory) much easier and cheaper to manufacture than a thick-film silicon panel.

At Oxford University, researchers reached 25% efficiency; a German research team has achieved 21.6%, while a new record was set in December 2018, when an Oxford lab reached 28% efficiency.

The National Renewable Energy Laboratory NREL built composite Silicon-Perovskite cells by putting perovskites atop a silicon solar cell to create a multijunction solar cell, with the new cell boasting an efficiency of 27% compared to just 21% when only silicon is used.

And now the most significant breakthrough yet: The Oak Ridge National Lab, the Department of Energy’s largest science and energy laboratory, has announced the discovery of novel hot-carrier perovskite solar cells that could achieve a conversion efficiency approaching 66%.

According to ORNL, “The discovery could improve novel hot-carrier solar cells, which convert sunlight to electricity more efficiently than conventional solar cells by harnessing photogenerated charge carriers before they lose energy to heat.”

The big trick here is to prevent the solar cells from wasting energy in the form of heat.

“When sunlight strikes a solar cell, photons create charge carriers–electrons and holes–in an absorber material. Hot-carrier solar cells quickly convert the energy of the charge carriers to electricity before it is lost as waste heat.

Preventing heat loss is a grand challenge for these solar cells, which have the potential to be twice as efficient as conventional solar cells. The conversion efficiency of conventional perovskite solar cells has improved from 3% in 2009 to more than 25% in 2020. A well-designed hot-carrier device could achieve a theoretical conversion efficiency approaching 66%,” ORNL adds.

Solar’s moment in the sun
There’s no word yet regarding the price points or when this product might hit the markets. Still, ORNL says it’s close to becoming a commercial reality and could be deployed in other real-world applications such as solid-state lighting, dynamic sensing and actuation, advanced radiation detection, quantum information science, and photo-catalysis in the near future.

The timing appears perfect, too, with solar tipped to dominate the global electricity scene over the next couple of decades.


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