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Solar Power

Turning down the heat turns up better perovskites

Lower-temperature process improves the efficiency of single-crystal solar cells

by Neil Savage, special to C&EN
February 26, 2020

20200226lnp2-perov.jpg
Credit: ACS Energy Lett.
A single-crystal perovskite retains its methylammonium (blue dots) at the surface when crystallized at 90 °C. This leads to fewer defects and higher solar capture efficiency than perovskites made at higher temperatures.

A method to make perovskite crystals at lower temperatures could point the way toward lower-cost solar cells by eliminating defects that reduce perovskites’ efficiency (ACS Energy Lett. 2020, DOI: 10.1021/acsenergylett.9b02787).

Most solar cells are made from silicon or gallium arsenide, which can convert roughly 30% of the light striking them to electricity. Those more established technologies can be expensive to make, relying on vacuum deposition and requiring temperatures of around 1000 °C. But methylammonium lead iodide can be made into the crystal structure known as a perovskite in a cheaper, easier, solution-based process.

Most current work on perovskite solar cells focuses on polycrystalline versions, where the efficiency record is 25%, say Omar Mohammed and Osman Bakr, materials scientists at King Abdullah University of Science and Technology. But polycrystalline films are only a few hundred nm thick, whereas single crystals can be grown to approximately 20 µm. The thicker films can absorb more light so single-crystal solar cells could prove to be superior. The only problem is that so far, single-crystal lead perovskite solar cells don’t reach 20% efficiency.

In the new work, Bakr and colleagues lowered the solution processing temperature from above 120°C to below 90°C to create single-crystal perovskite solar cells with an efficiency of 21.9%. Perovskite solar cells have made it this far in only about 10 years of research, much faster than other materials reached similar efficiencies, Bakr says. “We can expect probably by next year or so for perovskites to match the best silicon or best gallium arsenide single crystals,” he says.

Perovskites are grown using so-called inverse temperature solvents, in which solubility drops when temperature increases, the reverse of how most solvents work. The standard process relies on the solvent gamma-butyrolactone (GBL). Methylammonium lead iodide’s solubility in this solvent drops at approximately 120°C, leading to crystal formation. But at that temperature the methylammonium tends to evaporate, changing the crystal structure that forms at the surface relative to the material below. This surface crystal has less desirable electronic characteristics and creates a lattice mismatch between the surface crystal and the buried perovskite, causing defects that further cut efficiency.

To avoid these problems, the team went searching for an inverse temperature solvent in which methylammonium lead iodide would crystallize at a lower temperature. It also had to be a solvent that wouldn’t react with other solar cell components if trace amounts remained. “There aren’t many of them,” Bakr says. Propylene carbonate was promising, but only dissolves a small concentration of the perovskite. Eventually, the team found that a mixture of 35% propylene carbonate and 65% GBL dissolved enough perovskite to be practical while lowering the crystallization temperature to 90 °C, avoiding the methylammonium evaporation and allowing their single crystal perovskite to achieve its record efficiency.

With these results, “I believe the single crystal perovskite solar cells will definitely attract increasing attention,” says Rui Zhu, a professor of physics at Peking University who works with perovskites. He expects this advance will lead to further increases in performance. If perovskites can match or surpass the performance of silicon and gallium arsenide, Bakr says, “they should really drive down the cost of solar cells quite a bit.”

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