Fluorination-Dependent Phosphonic Acid Additives for Enhanced Perovskite Film Quality, Stability, and Photovoltaic Performance in Inverted Solar Cells

Defect passivation is essential for suppressing non-radiative recombination and improving the stability of perovskite solar cells. In this work, we examine how the degree of fluorination in benzylphosphonic acid additives affects film quality and device performance. Two fluorinated analogs are compared, and 2,3,4,5,6-pentafluorobenzylphosphonic acid (pFBPA) is found to adsorb preferentially at iodine vacancy sites through stable Pb-O coordination, as confirmed by binding energy analysis. This passivation effect produces better film morphology, more favorable energy level alignment, and improved charge carrier dynamics. In addition, the high electronegativity of fluorine atoms gives the film marked hydrophobicity, which enhances moisture resistance. Consequently, pFBPA-modified inverted MAPbI3 solar cells reach a power conversion efficiency of 18.5%, compared with 16.1% for the control devices, and show improved stability. These results indicate that tuning the fluorination level of phosphonic acid additives offers a practical route to boost both efficiency and stability in inverted perovskite photovoltaics.