Zwitterion‐Functionalized SnO₂ Substrate Induced Sequential Deposition of Black‐Phase FAPbI₃ with Rearranged PbI₂ Residue

Black-phase formamidinium lead iodide (FAPbI₃ ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI₂ ) residue, most sequential deposition methods of FAPbI₃ -based perovskite will introduce external ions like methylammonium (MA⁺ ), cesium (Cs⁺ ), and bromide (Br⁻ ) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO₂ ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI₃ . The SnO₂ ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI₂ in the perovskite layer, which reduces the side effect of the residual PbI₂ . Besides, the FSA functionalization also modifies SnO₂ ETL to suppress deep-level defects at the perovskite/SnO₂ interface. As a result, the FSA-FAPbI₃ -based perovskite solar cells (PSCs) exhibit an excellent power conversion efficiency of up to 24.1% with 1000 h long-term operational stability. These findings provide a new interface engineering strategy on the sequential fabrication of black-phase FAPbI₃ PSCs with improved optoelectronic performance.