Surface and Interfacial Passivations for FASnI₃ Solar Cells with Co-Cations

Seven cations, methylammonium (MA), ethylammonium (EA), aziridinium (AZ), dimethylammonium (DMA), 2-hydroxyethylammonium (HEA), imidazolium (IM), and guanidinium (GA), were used to cocrystallize with formamidinium (FA) to form cocationic tin perovskite solar cells (TPSCs) using a two-step fabrication procedure. Time-of-flight second ion mass spectrometry results indicate that the pristine FA-based TPSC involved both vacancies on the perovskite surface and perovskite/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interface, and GA has the ability to passivate both surface and interfacial vacancy defects to give impressive device performance (PCE = 11.2%) and stability (>5000 h) with a negligible effect of hysteresis.