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.
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