Regulating surface potential maximizes voltage in all-perovskite tandems

The open circuit voltage (VOC) deficit in perovskite solar cells (PSCs) is greater in wide bandgap (>1.7 eV) cells than in ~1.5 eV perovskites.1,2 Quasi-Fermi level splitting (QFLS) measurements reveal VOC-limiting recombination at the electron transport layer (ETL) contact.3-5 This, we find, stems from inhomogeneous surface potential and poor perovskite-ETL energetic alignment. Common monoammonium surface treatments fail to address this; instead we introduce diammonium molecules to modify the perovskite surface states and achieve a more uniform spatial distribution of surface potential. Using 1,3-propane diammonium (PDA), QFLS increases by 90 meV, enabling 1.79 eV PSCs with a certified 1.33 V VOC, and > 19% power conversion efficiency (PCE). Incorporating this layer into a monolithic all-perovskite tandem, we report a record VOC of 2.19 V (89% of the detailed balance VOC limit) and > 27% PCE (26.3% certified quasi-steady-state). These tandems retain more than 86% of their initial PCE after 500 hrs operation.