Metal halide perovskites are often made by mixing cations or halides with formamidinium (FAPbI3), to get high power-conversion efficiency in perovskite solar cells. But at the same time, the most stable phase of FAPbI3 is photoinactive, meaning that it does not react to light—the opposite of what a solar power harvester should do. In addition, solar cells made with FAPbI3 show long-term stability issues. Now, researchers led by Michael Grätzel and Anders Hafgeldt at EPFL, have developed a deposition method that overcomes the formamidinium issues while maintaining the high conversion of perovskite solar cells.

In the new method, the materials are first treated with a vapor of methylammonium thiocyanate (MASCN) or formamidinium thiocyanate FASCN. This innovative tweak turns the photoinactive FAPbI3 perovskite films to the desired photosensitive ones.

The scientists used the new FAPbI3 films to make perovskite solar cells. The cells showed more than 23% power-conversion efficiency and long-term operational and thermal stability. They also featured low (330 mV) open-circuit voltage loss and a low (0.75 V) turn-on voltage of electroluminescence.