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Perovskite solar cells (PSCs) have attracted great attentions due to their excellent photovoltaic performance. Further improvement of device efficiency is limited by the recombination of the charge carriers even a heterojunction-based architecture is employed. Herein, we discovered that a simple surface modification of the perovskite film by incorporating isopropyl alcohol (IPA) post-treatment could convert the top surface to be more n-type, which spontaneously formed an n/n+ homojunction between the bulk (weak n type (n)) and the surface (more n-type (n+)) in a single perovskite film. A reconstruction toward a PbI 2 -rich top surface was induced by the remove of excess formamidinium iodide dissolved in a polar solvent IPA. This reconstructed n/n+ homojunction lead to the enlarged built-in electric field for efficient charge separation and transport of the photogenerated carriers. The inverted PSCs with the reconstructed n/n+ homojunction contribute to improve efficiency and narrow their efficiency gap with conventional n-i-p type devices. Our result suggests that the homojunction structure is an effective approach to reduce carrier recombination loss and achieve highly efficient PSCs. [Display omitted] • A facile surface homojunction reconstruction is achieved by solvent treatment. • A PbI 2 -rich perovskite surface is induced by the remove of excess FAI. • The reconstructed n/n+ homojunction brings about the enlarged built-in electric field. • The reconstructed n/n+ homojunction contributes to improved photovoltaic efficiency. [ABSTRACT FROM AUTHOR]

• The fluorine atom has induced electrostatic potential and dipole moment change. • Cl- doping can enhance crystallization and prevent phase change. • The champion device achieves a PCE of 16.29% and the humidity stability is improved. [Display omitted] All-inorganic CsPbI 2 Br material has attracted considerable attention due to its good balance between phase stability and absorption capacity. However, the power conversion efficiency (PCE) and device stability of CsPbI 2 Br-based perovskite solar cells (PSCs) still lag far behind those of their counterparts, mainly deriving from the notoriously poor moisture stability under humid environment and large serious charge recombination. Herein, a typical phenethylammonium chlorine (PEACl) and its fluorinated derivative 4-fluoro-phenethylammonium chlorine (F-PEACl) are individually incorporated to modify the surface of CsPbI 2 Br film to ameliorate the above issues. Benefiting from the comprehensive passivation effect of ammonium halogen, the trap states of treated CsPbI 2 Br are vastly suppressed. In particular, fluorination significantly increased the dipole moment and hydrophobic property of F-PEACl. Thus, the present CsPbI 2 Br PSC achieves a champion PCE of 16.29% with high open-circuit voltage of 1.272 V, while the reference device and PEACl-treated device yield PCEs of 14.30% and 15.64%, respectively. More importantly, the PSC with F-PEACl capping layer retains 87.2% of its initial PCE after 1000 h storage under 20% RH ambient without encapsulation. Our results reveal the underlying mechanism of performance improvement by surface passivation strategy and provide an effective approach to further boost the CsPbI 2 Br PSC efficiency and stability. [ABSTRACT FROM AUTHOR]