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. 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 × 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, 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 BF4−.

Efficient and Stable 3D/2D Perovskite Solar Cells through Vertical Heterostructures with (BA)4AgBiBr8 Nanosheets

“…Currently, most 3D/2D heterostructures are contrived via the modification of perovskite precursor solutions or post‐treatment of 3D perovskites. [ 5,27–34 ] For example, Snaith et al blended butylammonium‐based 2D perovskite precursor solution with the cesium formamidinium lead halide perovskite solution to form a mixture of 2D and 3D‐phase perovskite, which effectively inhibited nonradiative recombination in resultant devices. [ 21 ] Xu et al reported the construction of a 3D/2D heterostructure‐based PVSCs through the solvent‐assisted interfacial reaction between high concentration long‐chain organic molecules and the 3D perovskite surface, benefiting from the built‐in field between 3D and 2D perovskite, the generated holes in the 3D perovskite can be efficiently extracted into the 2D perovskite and future into the electrode without using additional hole‐transport layer (HTL) to achieve a PCE of 16.13%.…”

“…[21][22][23][24][25][26] Currently, most 3D/2D heterostructures are contrived via the modification of perovskite precursor solutions or post-treatment of 3D perovskites. [5,[27][28][29][30][31][32][33][34] For example, Snaith et al blended butylammonium-based 2D perovskite precursor solution with the cesium formamidinium lead halide perovskite solution to form a mixture of 2D and 3D-phase perovskite, which effectively inhibited nonradiative Perovskite solar cells (PVSCs) have drawn great attention due to their high processability and superior photovoltaic properties. However, their further development is often hindered by severe nonradiative recombination at interfaces that decreases power conversion efficiency (PCE).…”

Superhalogen Passivation for Efficient and Stable Perovskite Solar Cells

“…Interestingly, Stranks et al (2021) suggested a different passivation mechanism by the use of superhalogen. [ 35 ] The authors used MABF 4 for surface treatment of MAPbI 3 , and the introduction of BF 4 − significantly reduced trap density leading to improvement in PL characteristics with suppressed nonradiative recombination. Importantly, analysis of solid‐state nuclear magnetic resonance (NMR) revealed BF 4 − did not make an atomic‐level interaction with under‐coordinated Pb in the perovskite structure.…”

“…[ 27 ] In contrast, Stranks et al strongly corroborated no atomic‐scale interaction of superhalogen with the under‐coordinated Pb 2+ in the perovskite lattice by solid‐state NMR, which led to their conclusion that superhalogen rather scavenges unreacted MAI. [ 35 ]…”

Extracting Decay-Rate Ratios From Photoluminescence Quantum Efficiency Measurements in Optoelectronic Semiconductors

“…To test this fitting approach, and ascertain its reliability, we performed measurements on spincoated passivated MAPbI3 films of different thicknesses [22] (with samples increasing in thickness from sample 1 to sample 4, see methods). In the main text we present example data and fits, while all data and fits are presented in Figs.…”