Anionic diffusion in a soft crystal lattice of hybrid halide perovskites affects their stability, optoelectronic properties and the resulting device performance. The use of two-dimensional (2D) halide perovskites improves the chemical stability of perovskites and suppresses the intrinsic anionic diffusion in solid-state devices. Based on this strategy, devices with an enhanced stability and reduced hysteresis have been achieved. However, a fundamental understanding of the role of organic cations in inhibiting anionic diffusion across the perovskite–ligand interface is missing. Here we demonstrate the first quantitative investigation of the anionic interdiffusion across atomically flat 2D vertical heterojunctions. Interestingly, the halide diffusion does not follow the classical diffusion process. Instead, a ‘quantized’ layer-by-layer diffusion model is proposed to describe the behaviour of the anionic migration in 2D halide perovskites. Our results provide important insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform with an enhanced stability for heterostructure integration.
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This work is supported by the Office of Naval Research (Grant no N00014-19-1-2296, Program Managers, J. Parker and P. Armistead). Akriti acknowledges support from the Frederick N. Andrews Fellowship. B.P.F. acknowledges support from the Purdue Process Safety & Assurance Center. B.M.S. acknowledges the Air Force Office of Scientific Research (AFOSR) for their support (Grant no. FA9550-18-S-0003, Program Manager, K. Caster). The DFT computational work by P.L. and J.Y. is supported in part through computational resources provided by the Information Technology Department at Purdue University. The transmission electron microscopy work is supported by funding from the National Natural Science Foundation of China (Grant no. 21805184), the Natural Science Foundation of Shanghai (Grant no. 18ZR1425200) and the Center for High-Resolution Electron Microscopy (CћEM) at ShanghaiTech University (Grant no. EM02161943). We thank Q. Zhao, L. Huang, R. Agrawal, B. W. Boudouris and C. Davis for helpful discussions.
Authors declare no competing interests.
Peer review information Nature Nanotechnology thanks Dehui Li, Biwu Ma and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended Data Fig. 1 Cross-sectional characterization of (BA)2PbBr4-(BA)2(MA)2Pb3I10 vertical heterostructure.
a, SEM image of the vertical heterostructure (top view). The dashed rectangle highlights the area used for cross-sectional characterization. b, Carbon coating of the rectangular area of interest. Preparation of the cross-sectional lamella by c, digging a hole and d, thinning by Ga ions. e, Prepared cross-section of the vertical heterostructure. Scale bars in a–e are 10 µm. f, Low-magnification STEM image of the vertical heterostructure. The scale bar is 1 µm. g, Enlarged STEM image of the vertical heterostructure. STEM images show a clear interface between (BA)2PbBr4 and (BA)2(MA)2Pb3I10. EDS elemental mapping of h, Pb, i, Br, and j, I. k, Overlaid bromine and iodine EDS mapping. Scale bars in g–k are 200 nm. The brighter bromine and iodine signals in the EDS elemental mappings are mainly concentrated in the pure phase (BA)2PbBr4 and (BA)2(MA)2Pb3I10 regions, respectively. The dimmer Br signal from (BA)2(MA)2Pb3I10 region and iodine signal from (BA)2PbBr4 region can be attributed to diffusion, which is likely caused by the harsh sputtering and focused ion-beam milling during the cross-sectional sample preparation. These results suggest that electron microscopy is not an ideal tool to study the anion inter-diffusion kinetics for 2D halide perovskites due to sample damage.
Extended Data Fig. 2 Analysis of origin of halide alloy PL peak in (BA)2PbBr4-(BA)2(MA)2Pb3I10 vertical heterostructure after heating at 80 °C.
a, Bright-field image of vertical heterostructure. b, c, PL image of vertical heterostructure at room temperature and after heating at 80 °C for 40 hrs, respectively. The circled regions denote the area used for PL emission measurement of the heterostructure, bromide perovskite and iodide perovskite. Dashed lines are added in all the bright-field and PL images for the ease of distinction of bromide (light blue) and iodide (yellow) domains. Scale bars in a–c are 10 µm. d, PL spectrum of the vertical heterostructure before and after heating at 80 °C for 40 hrs. Presence of halide alloy PL peak at ~ 511 nm (dashed blue curve) is evident in bromide perovskite layer. No new PL peak (dashed red curve) is observed in iodide perovskite layer after the heating process. e, f, Bright-field and PL image of (BA)2PbBr4, respectively. g, PL image of the (BA)2PbBr4xI4(1−x) alloy formed after placing bulk crystal of (BA)2(MA)2Pb3I10 on top of (BA)2PbBr4 nanocrystal and heating it for 4 hrs at 80 °C. The bulk crystal of (BA)2(MA)2Pb3I10 is removed after the heating process to analyze the formation of (BA)2PbBr4xI4(1-x) perovskite in the bottom layer. Scale bars in e–g are 5 µm. h, PL spectrum of (BA)2PbBr4 nanocrystal at room temperature and (BA)2PbBr4xI4(1-x) alloy formed in the bottom layer after heating (BA)2PbBr4-(BA)2(MA)2Pb3I10 vertical heterostructure at 80 °C for 4 hrs.
SEM images of (BA)2PbBr4xI4(1-x) alloy formed after placing bulk crystal of (BA)2(MA)2Pb3I10 on top of (BA)2PbBr4 nanocrystal and heating it for a, 4 hrs and e, 10 hrs at 80 °C. EDS elemental mappings of b, f, Pb, c, g, Br, and d, h, I for the SEM images in a and e, respectively. All scale bars are 3 µm. i, j, EDS spectrum corresponding to the SEM images in a and e, respectively. The insets show the zoomed-in spectrum to highlight the bromine and iodine counts. The weak iodine signal in EDS elemental mapping and spectrum after 4 hrs of heating suggest that the iodine ions diffused to only top few layers of the bromide perovskite after 4 hrs of heating. After 10 hrs of heating, almost all the bromide perovskite layers show dominant iodine composition as evident from the EDS mapping and spectrum.
Extended Data Fig. 4 Analysis of new phase formation in (BA)2PbX4-(BA)2(MA)2Pb3X10 (X = I, Br) and alloy formation in (BA)2(MA)2Pb3Br10-(BA)2PbI4 vertical heterostructures after heating at 80 °C.
a, Bright-field image of (BA)2PbI4-(BA)2(MA)2Pb3I10 vertical heterostructure. b, c, PL images of (BA)2PbI4-(BA)2(MA)2Pb3I10 vertical heterostructure at room temperature and after heating at 80 °C for 4 hrs, respectively. d, PL spectrum of (BA)2PbI4-(BA)2(MA)2Pb3I10 vertical heterostructure before and after heating at 80 °C for 4 hrs. No PL shift and no new PL emission from any new halide perovskite phases is observed after the heating process. e, Bright-field image of (BA)2PbBr4-(BA)2(MA)2Pb3Br10 vertical heterostructure. f, g, PL images of (BA)2PbBr4-(BA)2(MA)2Pb3Br10 vertical heterostructure at room temperature and after heating at 80 °C for 4 hrs, respectively. h, PL spectrum of (BA)2PbBr4-(BA)2(MA)2Pb3Br10 vertical heterostructure before and after heating at 80 °C for 4 hrs. No PL shift and no new PL emission from any new halide perovskite phases is observed after the heating process. i, Bright-field image of (BA)2(MA)2Pb3Br10-(BA)2PbI4 vertical heterostructure. j, k, PL images of (BA)2(MA)2Pb3Br10-(BA)2PbI4 vertical heterostructure at room temperature and after heating at 80 °C for 4 hrs, respectively. l, PL spectrum of (BA)2(MA)2Pb3Br10-(BA)2PbI4 vertical heterostructure before and after heating at 80 °C. Alloy peak centered ~ 605 nm observed after heating indicates formation of iodide dominant (BA)2(MA)2Pb3Br10xI10(1-x) (n = 3) alloy perovskite rather than (BA)2PbBr4xI4(1-x) (n = 1) alloy perovskite. Dashed lines are added in all the bright-field and PL images for the ease of distinction of bottom (light blue) and top (yellow) perovskite domains. All scale bars are 5 µm.
Extended Data Fig. 5 Investigation of halide inter-diffusion in (2T)2PbBr4-(BA)2(MA)2Pb3I10 vertical heterostructure.
a, SEM image of bottom bromide perovskite layer after placing bulk crystal of (BA)2(MA)2Pb3I10 on top of (2 T)2PbBr4 nanocrystal and heating it for 10 hrs at 80 °C. b, c, d, EDS elemental mappings of Pb, Br, and I, respectively, for the SEM image in a. Scale bars in a–d are 10 µm. e, EDS spectrum corresponding to the SEM image in a. f, g, h, i, j, PL images of thin films of (2T)2PbBr4xI4(1-x) alloy perovskite with xBr = 0.4, 0.3, 0.2, 0.1, and 0, respectively. All scale bars are 20 µm. k, PL emission spectrum of alloy perovskites with xBr = 1,0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, and 0. The legend texts for xBr > 0.4 have been shown in grey color to highlight that no PL is observed for these compositions. Note that (2T)2PbBr4 perovskite has no PL emission due to type II band alignment between the inorganic [PbBr4]2− and organic ligand 2T+ layers. The red PL emission in (2T)2PbBr4-(BA)2(MA)2Pb3I10 vertical heterostructure comes from the top iodide perovskite layer. However, (2T)2PbI4 has type I band alignment with green PL emission. As all other heterostructures show an alloy PL emission peak which is iodide dominant, we expected the halide inter-diffusion in 2T system to be thermodynamically driven towards formation of an iodine dominated (2T)2PbBr4xI4(1-x) perovskite with green PL emission. To ensure that absence of green alloy PL emission in 2T system indicates absence of iodide diffusion rather than quenched PL of iodide dominant alloy perovskite, it is important to estimate the bromide concentration at which the (2T)2PbBr4xI4(1-x) alloy perovskite transitions from type II to type I band alignment. Extended Data Fig. 5f–k shows the PL emission spectrum of (2T)2PbBr4xI4(1-x) alloy perovskites obtained by spin coating the bromide and iodide precursor solutions mixed in the desired stoichiometric ratios. It is evident from Extended Data Fig. 5k that for xBr ≤ 0.4, (2T)2PbBr4xI4(1-x) perovskite has type I band alignment and for xBr > 0.4, it transitions to type II band alignment. Therefore, the absence of alloy PL emission in the diffusion studies is indicative of the inhibiting role of 2T organic cation rather than type II band alignment of iodide dominant alloy perovskites.
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Akriti, Shi, E., Shiring, S.B. et al. Layer-by-layer anionic diffusion in two-dimensional halide perovskite vertical heterostructures. Nat. Nanotechnol. (2021). https://doi.org/10.1038/s41565-021-00848-w