Bottom Interfacial Engineering for Methylammonium-Free Regular-Structure Planar Perovskite Solar Cells over 21%

Formamidinium cesium (FACs) perovskite solar cells (PSCs) with the exclusion of methylammonium (MA) cations often have greatly improved device stability; however, their inferior performance compared with MA-based devices has impeded the real application. Among various device engineering strategies, bottom interfacial engineering is a promising method to simultaneously achieve the passivation of interfacial defects and the crystallization control of perovskite. Herein, a simple and effective bottom interfacial design is presented to improve the efficiency and stability of FACs PSCs by capping o-phenanthroline derivatives on the ZnO electron transporting layer (ETL). The most efficient modifier, 4,7-Dichloro-1,10-phenanthroline (Cl-phen), can improve the crystallinity of the perovskite film by chlorinated surface and passivate the defects of ZnO by reducing surface hydroxyl groups and oxygen vacancies. In addition, Cl-phen modified ZnO shows better energy alignment with FACs perovskite and increases the built-in electric field cascade by 80 mV. As a result, a champion device efficiency of 21.15% is obtained using ZnO/Cl-phen bilayer ETL. The stability has also been improved using ZnO/Cl-phen bilayer ETL, in which 91.5% of initial PCE is retained after 1500 h of storage at ambient environment (RH: 40–50%) without encapsulation.