Thermal stability and miscibility of co-evaporated methyl ammonium lead halide (MAPbX₃, X = I, Br, Cl) thin films analysed by in situ X-ray diffraction

Abstract

We present the identification of crystalline phases by in situ X-ray diffraction during growth and monitor the phase evolution during subsequent thermal treatment of CH3NH3PbX3 (X = I, Br, Cl) perovskite thin films.

Characterization of Thin MAPb(I1–xBrx)3 Alloy Halide Perovskite Films Prepared by Sequential Physical Vapor Deposition

“…The thin MAPb(I 1−x Br x ) 3 film diffractograms (Figures 2A,B) show the presence of all the prominent peaks of cubic MAPbBr 3 and the (001) peak corresponding to PbI 2, but do not contain the principal (110) peak characterizing tetragonal MAPbI 3 . This points to the fact that the tetragonal phase of MAPbI 3 is transformed to the cubic phase of MAPbBr 3 as MAPb(I 1−x Br x ) 3 is formed, consistent with previous reports (Noh et al, 2013;Gilescrig et al, 2015;Pistor and Burwig, 2018). The value of bromine mole ratio x was determined using an empirical relationship between the bandgaps of the MAPb(I 1−x Br x ) 3 alloy and those of its MAPbI 3 and MAPbBr 3 components, given in Equation 1,…”

“…Previous studies have shown that MAPb(I 1−x Br x ) 3 with such high amounts of x exists in the cubic phase (Noh et al, 2013;Zhang et al, 2016). Pistor and Burwig (2018) gave an elaborate explanation for the formation of MAPb(I 1−x Br x ) 3 by exposing MAPbI 3 to MABr vapor and MAPbBr 3 to MAI vapor. They proved that it was due to reversible halide exchange at high substrate temperatures and added that it is much easier to replace I with Br than to replaces Br with I.…”

“…They were able to prove that I-rich MAPb(I 1−x Br x ) 3 had a tetragonal phase, and showed higher PCE and charge carrier lifetimes. Pistor and Burwig (2018) synthesized MAPb(I 1−x Br x ) 3 by vacuum-based two source co-evaporation method and demonstrated a total and reversible halide exchange in MAPbI 3 and MAPbBr 3 upon exposure to the opposite MAX (X is I or Br) at a substrate temperature of 120 • C. However, the preparation of MAPb(I 1−x Br x ) 3 by co-evaporation is not reproducible since it involves rate monitoring which fluctuates. Contrary to the results of Zhang et al who illustrated complete solubility with x ranging from 0.0 to1.0, Lehmann et al (2019) showed that there exists a miscibility gap for x in the range 0.29 to 0.92 for a solution synthesized powders of MAPb(I 1−x Br x ) 3 and strong phase separation for values of x ranging from 0.29 to 0.62.…”

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI3 films

“…32 Other characterization methods can be undertaken in situ, using bespoke deposition chambers to take measurements in real time, or ex situ, by growing films of increasing thickness under the same conditions. In situ X-ray diffraction (XRD) measurements have been carried out during co-evaporation of several metal halide perovskites, 49,50 however the time resolution is limited by the amplitude of the signal, and so these measurements were not able to capture the early stages of growth where the signal is low. Ex situ XRD 51 and XPS [51][52][53] measurements on MAPbI 3 show the formation of additional species at the perovskite-substrate interface such as PbI 2 51,53 and decomposition products of MAI, 52 which prevents the growth of pure MAPbI 3 thin films.…”

Room-Temperature Vacuum Deposition of CsPbI2Br Perovskite Films from Multiple Sources and Mixed Halide Precursors

“…deposited perovskite films, 93,94 might be related with the porosity observed by SEM (Figure 2b), suggesting a limited connectivity and hence uncorrelated growth of the grains. It is interesting to point out that phasepure CsPbI2Br can be readily obtained through vacuum deposition, while it cannot be formed by mechanochemical synthesis even after thermal treatment at 650 ℃.…”