Silicon is the second most abundant element on Earth (behind oxygen). It makes up 90% of the planet’s crust. So, from a materials cost point of view, making photovoltaic (PV) solar cells from crystalline silicon has always made sense. Indeed, the cost of crystalline silicon solar cells has fallen from $1.80/watt in 2010 to just $0.35/watt last year.
However, silicon must be highly refined and processed at over 1450°C to be made into silicon wafers that can then be turned into PV cells. According to a 2017 report by the National Renewable Energy Laboratory (NREL), about $0.10/watt of the $0.35/watt cost of a solar cell is related to the preparation of the silicon. To reduce the costs of producing solar energy, PV researchers are constantly trying to find new materials that don’t require as much energy to process as silicon does. The hunt has led them to perovskites.
Perovskite is a calcium titanium oxide (CaTiO3) mineral that was first discovered in Russia in 1839. Naturally occurring perovskite has been mined in Arkansas in the US, the Urals in Russia, and Switzerland, Sweden, and Germany. The ability of perovskites to absorb other atoms into their basic crystal structure made them ideal for a variety of electronics applications. It wasn’t until 2009, however, that researchers discovered that perovskite produced electricity when exposed to light.
Methyl ammonium lead triiodide (CH3NH3) Pb I3 is one of the perovskites that is under investigation for PV solar cells. (Image source: NREL)
At first, the perovskite solar cells only showed a 3% efficiency. Within two years, that number had doubled. By 2012, the efficiency had more than tripled. Although the material looked promising, researchers were finding that it posed many challenges. “Perovskites were unstable and there were all kinds of problems—but you could see in all the measurements that these were high-quality semiconductors,” said Jao van de Lagemaat, director of NREL’s Chemistry and Nanoscience Center in a 2018 NREL report.
Unlike the high temperature process required to make silicon solar panels, perovskite processing is much simpler. A solution of chemicals is applied to a substrate—either by spray deposition, printing with a roller, or even with a paintbrush. Inkjet printing of perovskite solar cells has also been successfully demonstrated. When the solution dries, it leaves crystals that serve as semiconductors. When light strikes the crystals, photons knock loose electrons that are collected to generate electricity. The thin layer of perovskite crystals can be made into flexible solar panels that are more adaptable to a variety of applications than traditional silicon solar cells.
By 2014, synthetic perovskites were being made in the laboratory, allowing novel formulations to be examined and the material challenges to be addressed. Perovskite structures become unstable when exposed to moisture, so control of humidity is critical. UV light also has detrimental effects over time. A solution to both problems under examination is the use of a hydrophobic photopolymer coating that can exclude moisture and reduce the UV effects.
Presently, the highest efficiencies of commercially available crystalline silicon PV solar panels is between 22 and 25 percent. The current efficiency record for a perovskite cell, certified by NREL in 2017, stands at 22.7 percent. NREL charts the evolution of PV solar cell efficiency. “Perovskite has only had one slope and it’s been very steep,” David Moore, an NREL scientist, said in the 2018 report. “Its first entry on the NREL chart was higher than most of the other technologies had after 10 years. It started high and its slope has also been high. That’s why people are so excited,” noted Moore.
If perovskite PV cells are so promising, why are they not yet on the market? NREL scientists admit that, right now, growing the crystals on the substrate to produce high efficiencies is almost an art. It is easy to create perovskite crystals that either have defects or are non-uniform and non-continuous. Scalability is another issue. As a perovskite solar cell’s area increases, its efficiency decreases, making it difficult to build large cells. Lastly, silicon PV solar cells are a known quantity. They can produce electricity for well over 30 years without significant degradation. Researchers just don’t have enough experience with perovskite to know if they can solve its instability challenges. Still, the potential for PV cells that can provide electricity for just pennies per watt is attractive.
“Although there are still many obstacles, many challenges, if you wait until all challenges are cleared up and then start to work on it, by that time it will be too late,” Kai Zhu, an NREL researcher, said in the 2018 perovskite report. “That’s a risk factor you have to consider when you invest in a new technology. The next three to five years will be very exciting,” he added.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.