diffusion length, and high light absorption coefficient. [7][8][9] These research efforts have witnessed the steady promotion of their power conversion efficiencies (PCEs) from 3.8% to 25.5%. [10] Nonetheless, the instability of the organic components against light, humidity, and heat remains the most pernicious problem jeopardizing their commercial applications. [11][12][13] It is therefore recommended to replace the organic components with inorganic cations, one example being α-CsPbI 2 Br, [14][15][16][17][18][19][20] which demonstrates outstanding optoelectronic properties. While the stability is being improved, the PCE (17.46%) [21] of α-phase CsPbI 2 Br is still far below the Shockley-Queisser (SQ) limit (24.75%) [22] and thus inferior to its hybrid organic-inorganic counterparts.Recently, it was observed that surface defect passivation can effectively enhance the PCE of the CsPbI 2 Br PSC. [23][24][25][26][27][28][29] Annihilation of surface I and Br vacancies via the incorporation of external anions was proposed to be the root cause of the improved performance, [30] but the underlying mechanism is still under debate. Some researchers suggested that the interactions with external anions could suppress the migration of surface ions in CsPbI 2 Br, thus alleviating the hysteresis effect, [31] while others ascribed the improvement to the reduction of parasitic nonradiative charge-carrier recombination centers formed by surface point defects. [32] One such point defect in CsPbI 2 Br is the Pb Br (Pb replacing Br) antisite. It has been shown in the literature that Pb Br is occasionally formed when a Br vacancy is generated in the lattice with an additional Pb captured at this vacant site. [33][34][35][36] This defect can induce detrimental deep-level defect states in the bandgap. [23,24,28,[37][38][39][40] Several reports have indicated that alkali metal halide can electronically passivate the Pb Br defect at the surface and prolong the durability of the solar cells. [37][38][39][40] In the passivation process, the ions are adsorbed at the surface vacant sites with sufficiently low kinetic barriers, which is similar to ion batteries, but different in that the adsorption process is irreversible while ionic intercalation in batteries is reversible. [41] Despite the abundance of experimental evidence, the atomistic understanding of the passivation mechanisms has not yet been achieved.Considering the fact that the substantive key to passivation lies in the incorporation of external anions, we conduct a Despite the excellent optoelectronic properties, organic-inorganic hybrid perovskite solar cells (PSCs) still present significant challenges in terms of ambient stability. CsPbI 2 Br, a member of all-inorganic perovskites, may respond to this challenge because of its inherent high stability against light, moisture, and heat, and therefore has gained tremendous attraction recently. However, the practical application of CsPbI 2 Br is still impeded by the notorious phenomenon of photoinduced halide segregation. Herein, by...
