Bifunctional Ytterbium (III) Chloride Driven Low‐Temperature Synthesis of Stable α‐CsPbI3 for High‐Efficiency Inorganic Perovskite Solar Cells

Abstract Inorganic perovskite CsPbI 3 has been studied as a promising alternative light‐absorbing material for photovoltaic application due to its suitable band gap for converting solar light and enhanced stability toward ambient conditions compared to the organic–inorganic halide perovskite. However, the photoactive α‐phase of CsPbI 3 can only be stabilized at a high temperature and the phase transition from α‐phase to δ‐phase easily occurs at room temperature. Herein, ytterbium (III) chloride (YbCl 3 ) as a bifunctional additive is introduced into the perovskite precursor to stabilize the black α‐phase, and high‐quality CsPbI 3 films are obtained at a temperature as low as 80 °C. Yb 3+ partly replaces Pb 2+ to enhance the tolerance factor and favors the formation of α‐phase. The Lewis adduct complex of YbCl 3 ·DMSO in the perovskite film can passivate the perovskite film with reduced defects and help enhance the stability of the α‐phase. The YbCl 3 ‐modified planar‐type α‐CsPbI 3 perovskite solar cells show a champion power conversion efficiency of 12.4% with an impressive stability at ambient conditions. The additive induced controlled crystallization provides a simple and promising way to improve the photovoltaic performance of inorganic perovskite solar cells.