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Ligand-engineered bandgap stability in mixed-halide perovskite LEDs

Abstract

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent—approaching those of commercial organic light-emitting diodes—have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to ‘clean’ the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications—such as photovoltaics—for which bandgap stability is required.

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Fig. 1: Nanocrystal synthesis.
Fig. 2: Effect of ligand treatment on the solution photoluminescence and the structural properties of the nanocrystals.
Fig. 3: Characterization of mixed-halide MAPb(I1−xBrx)3 NC-LED devices.
Fig. 4: Characterization of interactions between the ligands and the nanocrystal surface using NMR spectroscopy.
Fig. 5: Optimized structures of interacting surface-adsorbed ligands.

Data availability

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

Code availability

Computational codes used in this work are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was partially funded by the Engineering and Physical Sciences Research Council (EPSRC) UK through grants EP/M005143/1 and EP/M015254/2. This work is part of the PEROCUBE project, which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 861985. Y.H., A.S., R.S., H.J.S. and R.H.F. acknowledge funding and support from the SUNRISE project (EP/P032591/1), funded by the EPSRC. A.S. and R.H.F. acknowledge support from the UKIERI project. A.S. acknowledges funding and support from DST, Pratiksha Trust, IISc and MHRD. R.S. acknowledges a Newton International Fellowship from The Royal Society. M.L.C. acknowledges financial support from the Achievement Rewards for College Scientists (ARCS) Foundation, Oregon Chapter. This study was partially supported by the National Research Foundation of the Republic of Korea (NRF-2018R1C1B6005778, 2018R1A2B2006198, 2020R1A4A1018163 and 2019R1A6A1A10073437) and the Materials Innovation Project (NRF-2020M3H4A3081793) funded by the National Research Foundation of Korea. NMR data were acquired on a 400-MHz solid-state NMR spectrometer (AVANCE III HD, Bruker) at KBSI Western Seoul Center. The HR-TEM, XPS, NMR, confocal photoluminescence and SEM experiments were supported by UNIST Central Research Facilities (UCRF). J.H.P. thanks A. Lee for assistance with TEM. Y.H. thanks M. Danie for assistance with TEM; S. M. Rabea, T. S. Ibrahim, A. M. Ali and T. Janes for assistance with NMR analysis; and M. N. Ahmed for advice on Fig. 1. We thank B. Wenger for discussions concerning the use of EDTA as a ligand in perovskite nanocrystals. Y.H. thanks A. Marshall and J. Sahmsi for discussions concerning the FTIR and N. Sakai for discussing XRD results. We thank V. Burlakov and S. Mahesh for discussions concerning the halide-segregation mechanism. Y.H. acknowledges funding and support from Linguistix Tank Inc. (LXT AI), Canada.

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Authors

Contributions

Y.H. initiated the project, synthesized the nanocrystals, conceived the multidentate-ligand approach, developed and performed the ligand treatment process and, with J.H.P., performed the TEM, UV–vis absorption, photoluminescence and XRD measurements. Y.H., A.S. and H.J.S. planned the experiments and overall project targets. Y.H. coordinated the collaboration efforts and was assisted by A.S. J.H.P. fabricated LED devices. A.S. assisted with the synthesis of the nanocrystals and performed the PLQY and photothermal deflection spectroscopy measurements. R.S. assisted with the PLQY and photothermal deflection spectroscopy measurements. Y.H., M.L.C., M.J., C.Y. and J.L. performed the NMR experiments and analysed the data. J.H.P. performed STEM experiments and Y.H. analysed the data. J.H.P. and B.R.L. assisted with the characterization of the materials and the LEDs. M.H.S., H.C., S.H.P. and B.R.L. provided support for the characterization of the materials and devices. J.H.P. and B.R.L. carried out the device-stability tests. M.L.C. and J.C.S. designed and executed the TAS experiments and M.L.C. performed data analysis. E.M., E.R. and F.D.A. conducted the computational simulations. C.Y.W., B.R.L., R.H.F. and H.J.S. supervised the work undertaken in their laboratories. Y.H. drafted the first version of the manuscript, with assistance from J.C.S., C.Y.W. and H.J.S. All authors have read and commented upon, or contributed to the writing of, the manuscript.

Corresponding authors

Correspondence to Yasser Hassan, Cathy Y. Wong, Bo Ram Lee or Henry J. Snaith.

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Competing interests

H.J.S. is a co-founder of Oxford PV, which is commercializing perovskite-based photovoltaics. H.J.S. and R.H.F. are co-founders of Helio Display Materials, which is commercializing perovskite materials for light-emitting applications.

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Peer review information Nature thanks Chih-Jen Shih and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Characteristics of neat nanocrystals and nanocrystals with ligand treatment.

a, Photothermal deflection spectra of neat perovskite nanocrystal films and E+G-treated nanocrystal films deposited on quartz substrates. The inset shows the calculation of the average Urbach energy for these two samples. b, c, Time-resolved photoluminescence decay of neat nanocrystals and E+G-treated nanocrystals, as a solution dispersed in toluene (b) and as films (c). d, The corresponding time-resolved photoluminescence lifetimes of the neat nanocrystals and the E+G-treated nanocrystals. e, FTIR spectra of drop-cast films from nanocrystals synthesized in oleic acid. neat and after treatment with EDTA, glutathione or E+G. f, XPS of nanocrystals before and after ligand treatment, showing that the ligand-treated nanocrystals approximately correspond to MAPb(I0.4Br0.6)3, whereas the neat nanocrystals were synthesized using the composition of MAPb(I0.67Br0.33)3.

Extended Data Fig. 2 HR-TEM images of nanocrystals.

a, b, HR-TEM images of neat nanocrystals at different magnifications. c, Fast Fourier transformation of the selected region in b, in which the interplanar lattice spacing of the cubic phase is 0.60 and 0.42 nm for the {001} and {110} family of planes. d, e, HR-TEM images of E+G-treated nanocrystals at different magnifications. f, Fast Fourier transformation of the selected region in e showing a similar cubic structure to neat nanocrystals.

Extended Data Fig. 3 Effect of ligands on nanocrystal excited-state dynamics.

a, Schematic of TAS procedure. Coloured circles represent photoluminescence measurements before (black) and after (red) illumination with a 405-nm laser. Periodic re-exposure of the sample to the 405-nm laser maintained a stable photoluminescence wavelength during TAS scans without causing additional segregation. The duration of the re-exposure, T, was 10 s and 3 s for neat and E+G treated nanocrystals, respectively. For neat nanocrystals, the photoluminescence wavelength was initially 649 nm and was 669 ± 1 nm after exposure; the photoluminescence of E+G-treated nanocrystals was initially 628 nm and was 632 ± 1 nm after exposure. be, TAS of neat (b, c) and E+G-treated (d, e) nanocrystals before (b, d) and after (c, e) illumination. f–m, Transients of neat (f–i) and E+G-treated (j–m) nanocrystals before (f, g, j, k) and after (h, i, l, m) illumination. Traces are average signal in wavelength ranges chosen to highlight wavelength-dependent dynamics. Blue, green, yellow and red traces for the neat nanocrystals and the E+G-treated nanocrystals correspond to 560–600 and 550–580 nm, 600–640 and 580–610 nm, 640–680 and 610–640 nm, 680–720 and 640–670 nm, respectively. Black dashed lines are a global fit to a tri-exponential function. n, Time constants before (after) illumination of each nanocrystal sample, found by a global fit of transient absorption spectra. Colours refer to those in Fig. 2g, h.

Extended Data Fig. 4 Halide segregation in diluted neat nanocrystals.

a, b, Normalized photoluminescence spectra of spin-cast films of neat nanocrystals and polymethyl methacrylate (PMMA), in a nanocrystal:PMMA mass ratio of 1.03 (a) and 0.006 (b). A 405-nm continuous-wave laser was used as an excitation source and caused halide segregation, with the duration of irradiation indicated in the legend. The redshifted shoulder that developed during irradiation is ascribed to the recombination from within an iodide-enriched minority phase with a smaller bandgap. This shoulder showed no dependence on the nanocrystal concentration in the film, indicating that segregation can occur in isolated nanocrystals. c, d, Confocal photoluminescence images (c) and normalized photoluminescence spectra (d) of spin-cast films of neat nanocrystals over time under constant excitation with a 405-nm continuous-wave laser. The photoluminescence spectra were obtained from the highlighted region of c. e, f, Confocal photoluminescence images (e) and normalized photoluminescence spectra (f) of spin-cast films of neat nanocrystals and PMMA, in a nanocrystal:PMMA mass ratio of 0.001 over time under constant excitation with a 405-nm diode laser. The photoluminescence spectrum was obtained from the highlighted region of e.

Extended Data Fig. 5 Stability of the electroluminescence spectra of the mixed-halide MAPb(IxBr1−x)3 NC-LEDs with different ligand treatments.

ac, Current density–voltage (JV) curves (a), luminance–voltage (LV) curves (b) and EQE–current density curves (c) of NC-LEDs with different ligands treatment. df, Electroluminescence spectra of EDTA-treated (d), glutathione-treated (e) and E+G-treated (f) NC-LEDs at different bias voltages. gi, Electroluminescence spectra of EDTA-treated (g), glutathione-treated (h) and E+G-treated (i) NC-LEDs over time at a constant current density of 10 mA cm−2. jl, Electroluminescence spectra after treatment of the EDTA-treated (j), glutathione-treated (k) and E+G-treated (l) NC-LEDs with 1-adamantanecarboxylic acid (ADAC), measured over time at a constant current density of 10 mA cm−2.

Extended Data Fig. 6 Characteristics of nanocrystal films with different charge-transporting layers.

a, Time-resolved photoluminescence decays of E+G-treated nanocrystal films with various charge-transporting layers (excitation at 450 nm). PNCs, perovskite NCs; PD, PEDOT:PSS; P-TPD, poly-TPD. b, Photoluminescence intensities of E+G-treated nanocrystal films with various charge-transporting layers (excitation at 350 nm). Photoluminescence decays considerably faster, and photoluminescence intensity is reduced, in the presence of a poly-TPD HTL, whereas little change is observed with a TPBi ETL. This shows that exciton quenching markedly affects the interface between poly-TPD and the nanocrystals, resulting in a deterioration of device efficiency. c, Time-resolved photoluminescence decays of E+G-treated nanocrystal films with various HTLs (excitation at 450 nm). d, Photoluminescence intensities of E+G-treated nanocrystal films with various HTLs (excitation at 450 nm). TFB HTLs have longer photoluminescence decays and larger photoluminescence intensity compared to the poly-TPD HTLs, indicating that there is less exciton quenching at the interface between the nanocrystals and the TFB HTLs. e, f, Photoemission cutoff energy (e) and the valence-band region (f) of neat and E+G-treated nanocrystals from ultraviolet photoemission spectra. g, h, Optical bandgaps of neat nanocrystals (g) and E+G-treated nanocrystals (h).

Extended Data Fig. 7 Device characteristics and stability of the electroluminescence spectra of the mixed-halide MAPb(IxBr1−x)3 NC-LEDs.

a, Schematic illustrations of the NC-LED configuration showing the device architecture: ITO/PEDOT:PSS/poly-TPD/MAPb(I1−xBrx)3 nanocrystals/TPBi/LiF/Al. b, Current density–voltage (JV) and luminance–voltage (LV) curves of NC-LEDs. c, Luminous efficiency plotted against current density for NC-LEDs. d, EQE–current density curves of NC-LEDs. e, f, Electroluminescence spectra of neat (e) and E+G-treated (f) NC-LEDs at different bias voltages. g, Peak wavelength of electroluminescence of neat and E+G-treated NC-LEDs at different current densities. h, i, Peak wavelength of electroluminescence of neat (h) and E+G-treated (i) NC-LEDs at a constant current of 0.5 mA cm−2 for one hour, followed by measurements of the same device after resting in the glove box for the indicated times. The electroluminescence peak shifts under the constant current density of 0.5 mA cm−2 but recovers to its initial position after resting in the glove box for 5 h, indicating the reversibility of halide segregation.

Extended Data Fig. 8 Histogram of maximum EQE values and operating stability of mixed-halide MAPb(IxBr1−x)3 NC-LEDs.

a, Histogram of maximum EQE values for E+G-treated NC-LEDs, collected from 25 devices. b, Operational stability of E+G-treated NC-LEDs measured in air at a constant current of 0.1 mA cm−2 (initial luminance (L0) = 22 cd m−2), 1 mA cm−2 (L0 = 141 cd m−2) and 10 mA cm−2 (L0 = 585 cd m−2). cf, Electroluminescence spectra of NCs-LEDs with neat (c, e) and E+G ligand-treated (d, f) nanocrystal layers at a different lifetimes and with constant current densities of 1 mA cm−2 (c, d) and 10 mA cm−2 (e, f).

Extended Data Fig. 9 1H NMR spectra of solutions.

a, MAPb(IxBr1−x)3 nanocrystals before and after treatment with E+G mixture in d8-toluene. b, Magnification of the circled region of the 1H NMR spectra in a (between 5.4 and 5.6 ppm), showing details of the proton bound to C=C of oleic acid/oleylamine. Fine structure can be seen on top of the broad resonance after ligand treatment, which is indicative of free oleic acid and oleylamine ligands. c, Systematic addition of E+G to PbI2 in d6-DMSO solutions. d, e, EDTA, glutathione and E+G in d6-DMSO (d) and D2O (e). When both ligands are combined in the E+G mixture, the individual peak positions are retained, indicating that there is no chemical interaction between the two ligands. f, Glutathione in d6-DMSO and in a mixture of d6-DMSO with D2O, showing the disappearance of the amide NH peaks (corresponding to exchangeable protons) in the presence of D2O. g, h, Systematic addition of EDTA (g) and glutathione (h) to PbI2 in d6-DMSO solutions.

Extended Data Fig. 10 DFT-optimized structures of surface-adsorbed ligands.

a, Structure of glutathione (left) and EDTA (right), with fragments that were used to study their interaction with the perovskite highlighted. bf, DFT-optimized structures of surface-absorbed glutathione (b), EDTA (c) one glutathione molecule and one EDTA molecule (d), two EDTA molecules (e) and two glutathione molecules (f). gi, The optimized structure of an iodine Frenkel defect pair (defective iodine atoms in yellow) on the bare PbI2-terminated perovskite surface (g), in the presence of adsorbed glutathione (h) and in the presence of adsorbed E+G (i). The iodine vacancy is highlighted with a dashed circle. j, Binding energies of fragments and complete molecules to the PbI2-terminated perovskite surface. Data in parenthesis is the value per adsorbed molecule. k, DFT-optimized structures of the Pb2+ complexes with EDTA and with glutathione. The relative binding energies showing that EDTA binds more strongly to Pb2+ than does glutathione by 0.80 eV.

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Hassan, Y., Park, J.H., Crawford, M.L. et al. Ligand-engineered bandgap stability in mixed-halide perovskite LEDs. Nature 591, 72–77 (2021). https://doi.org/10.1038/s41586-021-03217-8

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