Crystal Phases and Thermal Stability of Co-evaporated CsPbX₃ (X = I, Br) Thin Films

Abstract

We present the growth, phase transitions, and thermal decomposition of CsPbX (X = I, Br) thin films monitored by in situ X-ray diffraction (XRD). The perovskite films are prepared in vacuum via co-evaporation of PbX and CsX (X = I, Br) onto glass substrates. In situ X-ray diffraction allows the observation of phase transitions and decomposition while the samples are heated with a linear temperature ramp. Our experiments reveal the decomposition route for the CsPbX perovskites in high vacuum, with a much higher stability than their hybrid organic-inorganic MAPbX counterparts. We also observe the response of a black CsPbI thin film to exposure to ambient air at room temperature using the same XRD system. Exposing the black CsPbI to ambient air leads to the formation of yellow orthorhombic δ-CsPbI, whose crystal structure could be identified by its X-ray diffraction pattern. Additionally, the linear coefficients of expansion are determined for δ-CsPbI and the (020)-orientation of CsPbBr.

Low Temperature Synthesis of Stable γ‐CsPbI3 Perovskite Layers for Solar Cells Obtained by High Throughput Experimentation

“…Comparison with reference peak positions shows that the observed phases can only be assigned to the distorted perovskite γ‐CsPbI 3 (space group Pbnm ) in the brown region and the non‐perovskite phase δ‐CsPbI 3 (space group Pnma ) in the yellow region . This confirms that the stable perovskite at room temperature is the γ‐CsPbI 3 and not the cubic α‐phase, in agreement with some recent studies . Le Bail analysis was performed for the patterns acquired in the laboratory for both phases taking as starting models the δ‐ and γ‐phases reported by Marronnier et al The analysis resulted in the δ‐phase with lattice parameters a = 10.471 ± 0.002 Å, b = 4.790 ± 0.001 Å, and c = 17.781 ± 0.003 Å, and the γ‐phase with a = 8.629 ± 0.001 Å, b = 8.834 ± 0.001 Å, and c = 12.472 ± 0.002 Å (see Figure S1 and Table S1 in the Supporting Information).…”

All‐Inorganic CsPbX3 Perovskite Solar Cells: Progress and Prospects

“…[25] As the temperature decreases from where the "high"-temperature cubic phase is stable,t he PbX 6 octahedra in the cubic phase will increasingly tilt and the crystal structure will transform from the cubic to the lower-symmetry tetragonal and then orthorhombic forms. [26,27] Thes tructural (phase) stability of ABX 3 (the generic perovskite composition) is determined largely by the volumetric ratio between the BX 6 octahedra and the Acation. Thef ormation of the perovskite structure can be predicted from the Goldschmidt tolerance factor, t,w here t is given by Equation (1): [28,29]…”

Review on Recent Progress of All‐Inorganic Metal Halide Perovskites and Solar Cells

“…High‐quality perovskite films characterized by a pinhole‐free morphology are critical for the construction of high‐performance inorganic PSCs as pinholes lead to direct contact between charge selective layers, well known as shunts, which severely lowers the value of the V oc and overall device performance. The coevaporation technique, usually involving the sublimation of two kinds of precursors in a vacuum chamber, has been demonstrated to be effective in achieving uniform perovskite films over a large‐area substrate. In addition, this process does not use solvents, and is also suitable for insoluble or poor‐soluble materials deposition, i.e., the bromide precursors.…”