Abstract
Mixed halide (I/Br) complex organic/inorganic hybrid perovskite materials have attracted much attention recently because of their excellent photovoltaic properties. Although it has been proposed that their stability is linked to the chemical inhomogeneity of I/Br, no direct proof has been offered to date. Here, we report a new method, secondary electron hyperspectral imaging (SEHI), which allows direct imaging of the local variation in Br concentration in mixed halide (I/Br) organic/inorganic hybrid perovskites on a nanometric scale. We confirm the presence of a nonuniform Br distribution with variation in concentration within the grain interiors and boundaries and demonstrate how SEHI in conjunction with low-voltage scanning electron microscopy can enhance the understanding of the fundamental physics and materials science of organic/inorganic hybrid photovoltaics, illustrating its potential for research and development in “real-world” applications.
Highlights
Metal halide organic/inorganic perovskite photovoltaic materials that crystallize with an ABX3 structure have attracted intense academic and industrial attention for optoelectronic applications.[1−4] In this ABX3 structure, A, B, and X are monovalent cations [methylammonium (MA), formamidinium (FA), and cesium (Cs)], divalent cations (Pb and Sn), and halide anions (I, Cl, and Br), respectively.[5−7] Of the various organic/ inorganic perovskite materials, methylammonium lead triiodide (MAPbI3) is the most studied material for photovoltaic devices.[8]
Because we integrate the SE spectroscopy with SE imaging, we refer to this method as secondary electron hyperspectral imaging (SEHI) in which carefully selected spectral window(s) within the overall SE signal is used to reveal the presence of nanoscale inhomogeneities in Br composition in the lateral direction and across the film thickness with a sub-10 nm resolution
We first identify the origin of the SE emission spectra, with the aim of mapping halide distributions in mixed cation [Cs and formamidinium iodide (FAI)]-based and halide (I/ Br)-based organic/inorganic hybrid perovskites
Summary
Metal halide organic/inorganic perovskite photovoltaic materials that crystallize with an ABX3 structure have attracted intense academic and industrial attention for optoelectronic applications.[1−4] In this ABX3 structure, A, B, and X are monovalent cations [methylammonium (MA), formamidinium (FA), and cesium (Cs)], divalent cations (Pb and Sn), and halide anions (I, Cl, and Br), respectively.[5−7] Of the various organic/ inorganic perovskite materials, methylammonium lead triiodide (MAPbI3) is the most studied material for photovoltaic devices.[8]. As it has been established that the structure and composition of heavily mixed hybrid perovskites (HPs) is very complex because of the occurrence of spontaneous spinodal decomposition, amorphous impurities, and related nanoscale phase segregation,[15] a novel, fast microscopy technique that can reveal such variations in films and real devices is urgently called for.[16] This is to enable further advances in the device performance. SEHI enables the observation and identification of nanosized local regions with excess organic material in complex organolead mixed-halide perovskite films that resemble device structures. We find that SEHI enables direct observation of several nanoscale features in organolead mixed-halide perovskites, which cannot be revealed using conventional scanning electron microscopy (SEM), LVSEM, or any other analytical technique because of the different
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