Researchers have made stable, lead-free perovskite solar cells with a brand new titanium-based perovskite. The material, cesium titanium bromide, should be especially suitable for tandem solar cells, in which perovskite cells are combined with silicon cells to boost efficiency.

The solar cells, reported recently in the journal Joule, have an efficiency of 3.3%. They retained 94% of this efficiency after 14 days of exposure to heat, humidity, and ambient light without any encapsulation. That is a good start for first-ever devices made from a new material, says Nitin Padture, a professor of engineering at Brown University, who led the new work. For comparison, solar cells made with the well-studied methylammonium lead halide perovskites have efficiencies of around 22%, but the first device made in 2009 was 3.8% efficient and barely stable.

Despite the soaring efficiencies of traditional perovskite solar cells, their use of toxic lead is a concern. Researchers have recently made lead-free perovskites based on tin and antimony. But it is rare to have materials that are easy to synthesize, have suitable bandgaps, good optoelectronic properties, and are highly stable.

Last year, Padture and colleagues at the University of Nebraska-Lincoln used first-principles density functional theory calculations to theoretically predict a promising new family of perovskites, Cs₂TiX₆ (X = I or/and Br), which showed the best combination of traits for photovoltaic devices.

The team has now made high-quality thin films and solar cells with this material. They exposed a silicon substrate to CsBr vapors to deposit a thin CsBr film, which they heated in the presence of TiBr₄ vapors at 200°C to give a uniform Cs₂TiBr₆ film. To make solar cells, they sandwiched the film between a titanium dioxide electron-transporting layer and a poly(3-hexylthiophene) hole-transporting layer. “This was a true integration of computational materials science chemistry and experimental effort,” Padture says. “It shows that true integration gives you really good results. Otherwise you’d be shooting in the dark.”

The material has a bandgap of 1.8 eV, which is ideal for tandem cells because the titanium perovskite can absorb higher energy photons that silicon cannot absorb with its smaller bandgap. The solar cells display an open circuit voltage—a key parameter of photovoltaic potential—of over 1 V, when cells made from other lead-free perovskites have given less than 0.6 V.

The researchers are now investigating ways to deposit the films from solutions at lower temperatures, which would be more cost-effective. The challenge, says Padture, is to find good solvents that evaporate without causing damage. Increasing the efficiency of the solar cells is also a priority. The group plans to do that by finding the right combination of materials to conduct electrons and holes, he says. “We can also tailor the interfaces between the device layers to avoid traps and charge-recombination centers.”

Despite the low frequency of the reported solar cells, “the future looks promising” for this new class of solar cell materials, says Pabitra Nayak, a materials physicist at Oxford University. Not much is known about the material’s defect chemistry, which may limit its progress, he says. “It is very early to say if it can challenge the state-of-the-art perovskite cell, but it opens up a new unexplored area for solar cell material development.”

Read the article in Joule.