Zwitterion-Functionalized SnO 2 Substrate Induced Sequential Deposition of Black-Phase FAPbI 3 with Rearranged PbI 2 Residue.

Black-phase formamidinium lead iodide (FAPbI 3 ) 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 (PbI 2 ) residue, most sequential deposition methods of FAPbI 3 -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 (SnO 2 ) is introduced as the electron-transport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI 3 . The SnO 2 ETL treated with the zwitterion of formamidine sulfinic acid (FSA) can help rearrange the stack direction, orientation, and distribution of residual PbI 2 in the perovskite layer, which reduces the side effect of the residual PbI 2 . Besides, the FSA functionalization also modifies SnO 2 ETL to suppress deep-level defects at the perovskite/SnO 2 interface. As a result, the FSA-FAPbI 3 -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 FAPbI 3 PSCs with improved optoelectronic performance.