Solar cells made with sunlight-absorbing metallo-organic perovskite materials have basked in the photovoltaic (PV) spotlight in recent years. The reason is that perovskite materials such as methyl ammonium lead trihalides can be much less expensive to make and process than crystalline silicon, a conventional PV material, yet they offer comparable performance. But perovskite cells are not stable: the photoactive layer decomposes when exposed to heat and bright sunlight, which erodes device performance and impedes commercialization. A team led by Sai Bai, Feng Gao, and Henry J. Snaith of the University of Oxford now report that incorporating an ionic liquid in the perovskite layer stabilizes the device’s performance (Nature 2019, DOI: 10.1038/s41586-019-1357-2). Specifically, the team added an imidazolium tetrafluoroborate to the perovskite layer and sandwiched it between layers of nickel oxide and a polymer known as PCBM, materials that extract positive and negative charges, respectively. The researchers found that maintaining the cell at 70 °C while illuminating it continuously for more than 1,800 h caused the peak conversion efficiency (roughly 20%) to fall by just 5%, less than half of commonly reported degradation rates. They attribute the performance to the ionic liquid, which leads to a better-formed interface between the perovskite and nickel oxide films and enhances interaction between those layers.