Two-step growth of CsPbI3-xBrx films employing dynamic CsBr treatment: toward all-inorganic perovskite photovoltaics with enhanced stability

Recently, CsPbI3-xBrx (x = 1-2) all-inorganic perovskites have been studied to overcome the phase instability of CsPbI3. However, the addition of Br inevitably leads to a bandgap that is too wide (1.93-2.03 eV) compared to that of CsPbI3 (1.73 eV). To avoid this, the present study focuses on developing stable alpha-CsPbI3-xBrx (x < 0.7) perovskite films with lower Br levels. Our dynamic CsBr treatment incorporates Br- into the CsPbI3 lattice, greatly enhancing the morphology, optical properties, stability, and photovoltaic performance of the resultant CsPbI3-xBrx (x < 0.7) perovskite films. At x = 0.66, the perovskite film with the relatively low bandgap of 1.84 eV contributes to achieving an impressive power conversion efficiency (PCE) of 14.08% and, after 1200 h storage under a nitrogen atmosphere without encapsulation, the PCE remains at around 70% of this initial level. This two-step growth using dynamic CsBr treatment opens up promising routes for the feasible fabrication of inorganic perovskite photovoltaics with exceptional stability.