By adding large amine molecules to perovskites, researchers have made highly efficient lead-free perovskite solar cells. The devices, reported in the journal Joule, are stable and boast the ability to self-heal. Lead-free solar cells are notorious for degrading as they operate in sunlight, but these new devices can recover their performance when they are placed in an inert atmosphere for a few hours.

Even as efficient, low-cost perovskite solar cells get close to reaching the market, their potential toxicity has raised concerns. The best current perovskite materials contain lead, and researchers have been trying to find lead-free alternatives for years.

A Northwestern University research team led by professor of chemistry Mercouri Kanatzidis first reported lead-free perovskite solar cells five years ago by replacing lead with tin. The best organic-inorganic tin halide perovskite devices have since reached power conversion efficiencies close to 10%. But the biggest drawback of tin-based perovskites is that the tin oxidizes easily, making the devices unstable.

Kanatzidis, Zhaoxin Wu of Xi’an Jiaotong University, and their colleagues have now found a way to overcome this instability. They added the large amine molecule 3-phenyl-2-propen-1-amine (PPA) to the formamidinium tin iodide (FASnI3) perovskite film. The PPA cations partly replace some of the FA in the perovskite.

This gave polycrystalline materials with larger, more oriented crystals. The effect was positive only up to a PPA content of 10–15%. After that, the grain size decreased again.

Solar cells made with the PPA additive had an efficiency of 9.6%. They retained 92% of this capacity after 1400 hours (60 days) of being stored indoors. When the cells are heated or exposed to air, their efficiency drops but they recover most of this efficiency drop after being placed in a nitrogen atmosphere for 24 hours. Such self-recovery has been seen before in lead-based perovskite solar cells.

The PPA forms a two-dimensional (2D) PPA-tin perovskite structure at the boundaries of the three-dimensional (3D) crystals, Kanatzidis says. This 2D layer passivates defects that can trap charges, improving efficiency. And it keeps iodide and other small ions from migrating too far, so that when the material is rested in an inert environment they can return more easily to their original positions, helping the material recover its structure. “So the 2D phase makes the 3D phase better,” he says. The additives should work for lead-based cells too, he hypothesizes.

These results are important and impressive, says Eugene Katz, a professor of solar energy and environmental physics at Ben-Gurion University of the Negev, who was not involved in the work. The researchers need to test the operation of these devices under sunlight for a more practical assessment of the method, he says. But nonetheless, the works shows a path for increasing the efficiency and stability of lead-free solar cells.

Read the abstract in Joule.