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 CsPbI, while the phase instability issue of the tin-based analogue of CsSnI has not been resolved yet. This highlight article summarizes the empirical advances in stabilizing the metastable phases of CsPbI, aiming to provide useful guidelines to accelerate the development of phase-stable CsSnI 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.