a route to eco-friendly solar cells*

Substituting fossil fuels with renewable sources is one of the main challenges of our society, and perovskite solar cells (PSCs) are among the most promising solutions for energy harvesting. However, the most efficient PSCs contain highly toxic lead. CsSnI3 could be one of the most promising eco-friendly alternatives for high-performance PSCs, thanks to the excellent optoelectronic properties of its black phase, but its deployment into general markets is hampered by the rapid phase changes of samples exposed to air.

Here, we simulate with first principles the finite-temperature phase diagram of CsSnI3, highlighting how it is exceptionally driven by the quantum and thermal anharmonic motion of ions. We find a remarkable agreement with experiments in the transition temperatures and lattice thermal expansion. Our results establish that the black perovskite is actually stable at room temperature and that its decomposition into the yellow phase, detrimental for photovoltaic applications, can be prevented with appropriate surface treatment and crystal growth. The simulations also disclose the almost unique decrease of the heat capacity upon heating and dispel the current assumption of the major role played by the rattling of the Cs+ cation on the phase diagram.

Our predictions, corroborated by an excellent agreement with experimental data, elucidate the origin of the decomposition of the perovskite phase and pave the way to its stabilization and the commercialization of eco-friendly CsSnI3 devices.