Unraveling All‐Inorganic CsPbI₃ and CsPbI₂Br Perovskite Thin Films Formation – Black Phase Stabilization by Cs₂PbCl₂I₂ Addition and Flash‐Annealing

Abstract

Phase formation and transformation of 3D inorganic halide perovskites CsPbI3 and CsPbI2Br were investigated by in‐situ X‐ray diffraction (XRD) in nitrogen or air from 30 °C up to 320 °C. Thin films were obtained by spin‐coating of a liquid solution of salt precursors dissolved in DMSO under normal conditions. XRD data analysis, by using the Le Bail method, evidenced for the first time, to our knowledge, the spontaneous crystallization of black orthorhombic γ phase at ambient temperature, for both 3D perovskite phases. For once, this black γ phase could be studied upon heating. It transforms to the black tetragonal β and cubic α phases. The reverse transitions were also evidenced upon cooling. The influence of the all‐inorganic 2D Cs2PbI2Cl2 material addition over the black phase stability was unprecedently analyzed by in‐situ XRD. The mixture of the 3D and 2D phases, or mix [3D+2D], exhibits an improved stability of the black phase compared to the single 3D phase, and tends to induce highly oriented thin films and increased compactness, probably due to chemical insertion or anchoring. Finally, flash‐annealed thin films at various temperatures (RT<T<325 °C) of CsPbI2Br in air, alone and in the presence of the 2D phase, were studied. A more textured layer is evidenced for the mix [3D+2D] thin film, showing larger grains and more compact films above 200 °C. The mix [3D+2D] film flash‐annealed at 300 °C remained black for four days after air exposure and 50 % relative humidity.

Optoelectronic Devices Based on Scaffold Stabilized Black‐Phase CsPbI3Nanocrystals

“…Rietveld refinement, which could provide a more accurate analysis, cannot be reliably applied to thin films like the ones herein studied. [25] The source of this strain comes from the growth of crystallites within the voids of the scaffold and might prevent or hinder the formation of the unwanted yellow phase while helping the stabilization of the alpha phase at only 100 °C, as it has been proposed before. [26,27] Strain applied during crystal growth may alter the phase diagram of a determined system, and eventually modify the temperatures at which transitions occur.…”

Perovskite CsPbI2Br Thin Films Prepared under Nitrogen Flow and Black Phase Stabilization in the Presence of a Two‐Dimensional Inorganic Halide Material and Indium

“…In our previous work, we have shown that the perovskite crystallization process was slowed down in the presence of the 2D phase [10] . This crystallization process seems even further slowed down in the presence of indium.…”

“…[13,31] In our previous work, we have shown that the perovskite crystallization process was slowed down in the presence of the 2D phase. [10] This crystallization process seems even further slowed down in the presence of indium. On the SEM images of Figure 4, it seems that the grains discernible at this resolution for the 3D phase are reduced in size for the [3D + 2D] mixture.…”

“…[22] During our previous detailed structural study performed on inorganic halide perovskites we showed that it was possible to stabilise the black form of the 3D main structure longer than usual thanks to presence of Cs 2 PbCl 2 I 2 . [10] But the poor quality of the film obtained after in-situ XRD measurements could not be used for any type of electrical measurements. A flash thermal treatment just after spin-coating in air allowed us to form a more compact film.…”

Restructuring and Reshaping of CsPbX3 Perovskites by Lithium Salts

“…The CsPbI 3 delta phase was expected as the films were taken out to ambient atmosphere for the measurement and this material rapidly transitions to this phase when in contact with moisture. [ 17,18 ]…”