Tetrafluoroborate-Induced Reduction in Defect Density in Hybrid Perovskites through Halide Management.

Hybrid-perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF , introduced via methylammonium tetrafluoroborate (MABF ) in a surface treatment for MAPbI perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge-carrier recombination with a reduction in first-order recombination rate from 6.5 × 10 to 2.5 × 10 s in BF -treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). F, B, and N solid-state NMR is used to elucidate the atomic-level mechanism of the BF additive-induced improvements, revealing that the BF acts as a scavenger of excess MAI by forming MAI-MABF cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF .