Fluorinated Oligomer Wrapped Perovskite Crystals for Inverted MAPbI Solar Cells with 21% Efficiency and Enhanced Stability.

Defects at the grain boundary provide sites for nonradiative recombination in halide perovskite solar cells (PSCs). Here, by polymerization and fluorination of a Lewis acid of 4,4-bis(4-hydroxyphenyl)pentanoic acid, a fluorinated oligomer (FO-19) is synthesized and applied to passivate these defects in methlyammonium lead iodide (MAPbI). It is demonstrated that the carboxyl bond of FO-19 was coordinated with Pb ions in the perovskite films to achieve a wrapping effect on the perovskite crystals. The defects of perovskite film are effectively passivated, and the undesirable nonradiative recombination is greatly inhibited. As a result, FO-19 gives a power conversion efficiency of 21.23% for the inverted MAPbI-based PSCs, which is among the highest reported values in the literature. Meanwhile, the corresponding device with FO-19 exhibits significantly improved humidity and thermal stability. Therefore, this work offers insights into the realization of high-efficiency and stable PSCs through fluorinated additive engineering.