Europium-Doped CsPbI2Br for Stable and Highly Efficient Inorganic Perovskite Solar Cells

All-inorganic perovskite films hold promise for improving the stability of perovskite solar cells (PSCs). However, the 3D α phase of narrow-bandgap inorganic perovskites is thermodynamically unstable at room temperature, limiting the development of high-performance inorganic PSCs. Here, we show that europium doping of CsPbI2Br stabilizes the α phase of this inorganic perovskite at room temperature. We rationalize it by using solid-state nuclear magnetic resonance and high-angle annular dark-field scanning transmission electron microscopy, which show that europium is incorporated into the perovskite lattice. We demonstrate a maximum power-conversion efficiency of 13.71% for an inorganic PSC with the CsPb0.95Eu0.05I2Br perovskite and a stable power output of 13.34%. Using electroluminescence we show that incorporation of europium reduces non-radiative recombination, resulting in high open-circuit voltage of 1.27 V. The devices retain 93% of the initial efficiency after 370 hr under 100 mW cm−2 continuous white light illumination under maximum-power point-tracking measurement.