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

Abstract

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 BF4-, introduced via methylammonium tetrafluoroborate (MABF4) in a surface treatment for MAPbI3 perovskite is reported. The optical spectroscopic characterisations 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 × 106 to 2.5 × 105 s-1 in BF4--treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5% to 10.4%). 19F, 11B and 14N solid-state NMR is used to elucidate the atomic-level mechanism of the BF4- additive-induced improvements, revealing that the BF4- acts as a scavenger of excess MAI by forming MAI–MABF4 cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase is presumably due to the formation of MAI-MABF4 cocrystal, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us, for the first time, to elucidate the microscopic mechanism of action of BF4-. ; S.N. would like to acknowledge Royal Society-SERB Newton International Fellowship for funding. S.D.S. acknowledges the Royal Society and Tata Group (UF150033) and the EPSRC (EP/R023980/1). This work has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 841136. M.A.H. acknowledges support from the Royal Society (RP/R1/180147). S.M. thanks the EPRSC for funding. J.L.M-D. and W.-W. L. thank the UK Royal Academy of Engineering, grant CiET1819_24, EPSRC grants EP/N004272/1, EP/P007767/1, the Winton Programme for the Physics of Sustainability, and Bill Welland.