Toward durable all-inorganic perovskite solar cells: from lead-based to lead-free

Organic–inorganic metal halide perovskite solar cells (PSCs) have attracted extensive attention from the photovoltaic (PV) community due to their fast-growing power conversion efficiency from 3.8% to 26.7% in only 15 years. However, these organic–inorganic hybrid PSCs suffer from inferior long-term operational stability under thermal and light stress, due to the fragile hydrogen bonds between organic cations and inorganic slabs. This motivates the exploration of more robust all-inorganic alternatives against external stimuli, by substituting inorganic cesium (Cs) cations for volatile organic cations. Despite reinforced ionic interaction between Cs cations and metal halide frameworks, these Cs-based all-inorganic perovskites tend to undergo spontaneous phase transition from photoactive black phases to non-perovskite yellow phases at room temperature, significantly deteriorating their optoelectronic performance. Thus, tremendous efforts have been made to stabilize the black phase of CsPbI₃, while the phase instability issue of the tin-based analogue of CsSnI₃ has not been resolved yet. This highlight article summarizes the empirical advances in stabilizing the metastable phases of CsPbI₃, aiming to provide useful guidelines to accelerate the development of phase-stable CsSnI₃ for durable lead-free PV applications. Finally, the remaining challenges and future research opportunities are outlined, providing a road map to realize efficient and durable all-inorganic perovskite solar cells towards practical applications.