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Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells

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Abstract

Metal halide perovskites of the general formula ABX3—where A is a monovalent cation such as caesium, methylammonium or formamidinium; B is divalent lead, tin or germanium; and X is a halide anion—have shown great potential as light harvesters for thin-film photovoltaics1,2,3,4,5. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI3) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells6,7,8,9, and maximizing the performance of this material in such devices is of vital importance for the perovskite research community. Here we introduce an anion engineering concept that uses the pseudo-halide anion formate (HCOO) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. The resulting solar cell devices attain a power conversion efficiency of 25.6 per cent (certified 25.2 per cent), have long-term operational stability (450 hours) and show intense electroluminescence with external quantum efficiencies of more than 10 per cent. Our findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing a facile access to solution-processable films with improved optoelectronic performance.

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Fig. 1: Characterization of the FAPbI3 films.
Fig. 2: Solid-state NMR spectra and molecular dynamics simulations.
Fig. 3: Characterization of the photovoltaic performance of the FAPbI3 PSCs.
Fig. 4: Stability of the FAPbI3 PSCs.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Code availability

The code used for this study is available from the corresponding author upon reasonable request.

Change history

  • 08 April 2021

    This Article was amended to correct the Peer review information.

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Acknowledgements

We thank W. R. Tress for discussions, and the staff at beamlines BL17B1, BL14B1, BL11B, BL08U and BL01B1 of the SSRF for providing the beamline, and the Swiss National Supercomputing Centre (CSCS) and EPFL computing center (SCITAS) for their support. This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2020M1A2A2080746). This work was also supported by ‘The Research Project Funded by U-K Brand’ (1.200030.01) of Ulsan National Institute of Science & Technology (UNIST). D.S.K. acknowledges the Development Program of the Korea Institute of Energy Research (KIER) (C0-2401 and C0-2402). L.E. acknowledges support from the Swiss National Science Foundation, grant number 200020_178860. U.R. acknowledges funding from the Swiss National Science Foundation via individual grant number 200020_185092 and the NCCR MUST. A.H. acknowledges the Swiss National Science Foundation, project ‘Fundamental studies of dye-sensitized and perovskite solar cells’, project number 200020_185041. M.G. acknowledges financial support from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 881603, and the King Abdulaziz City for Science and Technology (KACST).

Author information

Authors and Affiliations

Authors

Contributions

J.J., B.W. and J.Y.K. conceived the project. J.J., Minjin Kim and H.L. prepared the samples, performed the relevant photovoltaic measurements, analysed the data and wrote the manuscript. J.S. synthesised the FAHCOO material. Minjin Kim and D.S.K. certified the efficiency of the PSCs. Y.J.Y. carried out photoluminescence and UV–vis absorption spectroscopy. S.J.C. and I.W.C. performed the time-resolved photoluminescence, SEM and XRD measurements. Y.J. and H.L. collected the light-intensity-dependent JV data. P.A. and U.R. designed and performed all the DFT calculations and molecular dynamics simulations. Maengsuk Kim and J.H.L contributed to the DFT calculations. A.M., M.A.H. and L.E. conducted the solid-state NMR measurements and analysis. B.P.D. performed the atomic force microscopy measurements. H.L. conducted the long-term operational stability measurements, EQEEL measurements and analysed the data. Y.Y. performed the two-dimensional grazing-incidence XRD measurements. F.T.E contributed to the analysis of the time-resolved photoluminescence data. S.M.Z. coordinated the project. A.H. and M.G. proposed experiments and M.G. wrote the final version of the manuscript. A.H., D.S.K., M.G. and J.Y.K. directed the work. All authors analysed the data and contributed to the discussions.

Corresponding authors

Correspondence to Anders Hagfeldt, Dong Suk Kim, Michael Grätzel or Jin Young Kim.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Characterization of the perovskite films with and without FAFo.

a, The Tauc plot of the 2% Fo-FAPbI3 perovskite film. b, A full photoluminescence decay of the reference, 2% Fo-FAPbI3 and 4% Fo-FAPbI3 perovskite films. c, The distribution of the grain sizes of the reference and 2% Fo-FAPbI3 films. The box + whisker plots show the distribution of the grain sizes for both reference and 2% Fo-FAPbI3 perovskite films. The distribution is based on 22 data points each. d, e, The top-view SEM image (d) and the cross-sectional SEM image (e) of the 4% Fo-FAPbI3 perovskite film. f, g, AFM images of the reference (f) and the 2% Fo-FAPbI3 (g) perovskite films. h, The XRD patterns of the reference, 2% Fo-FAPbI3 and 4% Fo-FAPbI3 perovskite films. Peaks labelled with an asterisk are assigned to the FTO substrates, which can be seen for the 4% sample owing to the lower intensity of the perovskite reflections. i, Integrated one-dimensional grazing-incidence XRD pattern of the reference and 2% Fo-FAPbI3 films.

Extended Data Fig. 2 The composition of the Fo-FAPbI3 perovskite film.

a, b, 1H–13C cross-polarization spectra of mechanosynthesized FAPbI3 with 5% FAHCOO (a) and a scraped thin film of 2% Fo-FAPbI3 (b), recorded at 12 kHz MAS and 100 K. In b the formate signal can be seen as a minor shoulder on the FAPbI3 peak. A minor signal arising from the PTFE that is used to seal the rotor is also visible. c, d, TOF-SIMS measurements of the reference (c) and the 2% Fo-FAPbI3 (d) films. e, Quantitative, directly detected 13C solid-state NMR measurement of 2% Fo-FAPbI3 scraped thin film at 12 kHz MAS and 100 K.

Extended Data Fig. 3 Ab initio molecular dynamics simulations.

a, Molecular dynamics snapshot showing the coordination of Pb2+ ions with HCOO anions in the perovskite precursor solution. As a guide to the eye, we highlight only Pb2+ and HCOO ions; the remaining ions and solvent molecules are shown as transparent. b, The radial distribution function g(r) between the oxygen atoms of HCOO and Pb2+ over the full ab initio molecular dynamics trajectory of around 11 ps. c, Initial configuration of FAPbI3 with surface iodide replaced by HCOO anions. d, The top view of surface atoms on the FA+-terminated side. e, The top view of the surface atoms on the Pb2+-terminated side. Pb2+–HCOO and FA+–HCOO bonding and hydrogen-bonding networks are illustrated with magenta dashed lines. All ions are shown in ball-and-stick representation. Pb2+ ions, yellow; iodide, light pink; oxygen, red; carbon, light blue; nitrogen, dark blue; sulfur, light yellow; hydrogen, white.

Extended Data Fig. 4 DFT-relaxed slabs of FAPbI3 with different anions adsorbed at an iodide-vacancy site on the surface.

a, Structure of a pure FAPbI3 slab with a Pb–I terminated surface on the top and an FA–I terminated surface on the bottom side. be, Front view of the Cl (b), Br (c), BF4 (d) and HCOO (e) passivated surface. f, An illustration of iodide-vacancy passivation by HCOO. g, h, DFT-relaxed FAPbI3 slab with HCOO adsorbed at the iodide-vacancy site on the Pb–I (g) and the FA–I (h) terminated surface. All chemical species are shown in ball-and-stick representation. Pb2+, grey; iodide, violet; oxygen, red; carbon, dark brown; nitrogen, light blue; bromide, red-brown; chloride, light green; boron atoms, dark green; fluoride, yellow; hydrogen atoms, white.

Extended Data Fig. 5 Bonding between formamidinium and different anions on the surface of FAPbI3.

a, Structure of a pure FAPbI3 slab with FA–I termination on the top and Pb–I termination on the bottom side. b, c, The front view (b) and the side view (c) of the HCOO passivated surface. df, Cl (d), Br (e) and BF4 (f) passivated surface. All chemical species are shown in ball-and-stick representation. Pb2+, grey; iodide, violet; oxygen, red; carbon, dark brown; nitrogen, light blue; bromide, red-brown; chloride, light green; boron atoms, dark green; fluoride, yellow; hydrogen, white. g, Relative desorption strength of FA+ cations on different passivated surfaces.

Extended Data Fig. 6 Photovoltaic performance of the PSCs under different conditions.

a, J–V curve of the target PSC measured without a metal mask. b, J–V curves of the reference PSC and the PSC with 2% formamidinium acetate. c, J–V curves of the reference and 2% Fo-FAPbI3 PSCs without the MACl additive. d, J–V curves of the reference and the 2% Fo-FAPbI3 PSCs without using octylammonium iodide passivation. FF, fill factor.

Extended Data Fig. 7 J–V metrics of the reference and target PSCs during the operational stability test.

ac, The change in Jsc (a), Voc (b) and fill factor (c) of the reference and target cells over the 450-h MPP tracking measurement.

Extended Data Table 1 Detailed J–V parameters of the reference and target PSCs under both reverse and forward voltage scans
Extended Data Table 2 Detailed J–V parameters of the reference and target PSCs under different light intensities

Supplementary information

Supplementary Information

This file contains Supplementary Notes 1-6, Supplementary Figs 1-3 and Supplementary References.

Video 1

The coordination of HCOO- anions with Pb2+cations.

Video 2

MD of HCOO- passivated FA-I terminated interface of FAPbI3.

Video 3

MD of HCOO- passivated Pb-I terminated interface of FAPbI3.

Video 4

Perovskite fabrication process.

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Jeong, J., Kim, M., Seo, J. et al. Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells. Nature 592, 381–385 (2021). https://doi.org/10.1038/s41586-021-03406-5

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