A\nDiphosphonic Acid-Based Interlayer for Highly Efficient and\nStable Inverted\nPerovskite Solar Cells

We\ninvestigate an interlayer of 6,6′-bis(4-(bis(4-methoxyphenyl)amino)phenyl)-[1,1′-binaphthalene]-(2,2′-diyl)bis(oxy)bis(propane-3,1-diyl)bis(phosphonic\nacid) (BINOL-PA) with undoped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]\n(PTAA) coverage. The incorporation of the 1,10-bi-2-naphthol central\ncore enhances π–π stacking and reduces charge recombination\nat the interface. Compared to PTAA alone (0.95 eV), BINOL-PA/PTAA\nexhibits a shorter distance from the Fermi energy (<i>E</i><sub>F</sub>) to the valence-band maximum (VBM) (0.36 eV). Two phosphoric\nacid units in BINOL-PA fine-tune the molecular dipoles. Theoretical\ncalculations reveal electrostatic surface potential differences between\nBINOL-PA and PTAA in their backbone structure. Open-circuit voltage\ndecay (OCVD) and electrochemical impedance spectroscopy (EIS) results\nsuggest suppressed interface recombination. The photovoltaic conversion\nefficiency (PCE), short-circuit current density (<i>J</i><sub>SC</sub>), open-circuit voltage (<i>V</i><sub>OC</sub>), and fill factor (FF) for the BINOL-PA/PTAA device are measured\nas 21.02%, 22.67 mA cm<sup>–2</sup>, 1.12 V, and 82.8%, respectively,\nall higher than those achieved by the PTAA device with a PCE of 18%.\nBINOL-PA/PTAA significantly elevates <i>V</i><sub>OC</sub> and FF values compared with dopant-free PTAA alone. The champion\ndevice retains over 89% of its initial PCE after being exposed to\nan ambient environment without encapsulation for more than 30 days.\nThe thermal aging test conducted under a nitrogen atmosphere demonstrates\nthat the efficiency retention rate for BINOL-PA/PTAA displays 60%\nof its initial efficiency after 1500 h.