Extended Data Fig. 5: Investigation of halide inter-diffusion in (2T)2PbBr4-(BA)2(MA)2Pb3I10 vertical heterostructure.
From: Layer-by-layer anionic diffusion in two-dimensional halide perovskite vertical heterostructures
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.
