Formation Thermodynamics, Stability, and Decomposition Pathways of CsPbX₃ (X = Cl, Br, I) Photovoltaic Materials

Abstract

Standard enthalpies of formation of CsPbX3 (X = Cl, Br, I) perovskites from halides and from elements at 298 K were measured using solution calorimetry. Intrinsic and extrinsic stabilities of CsPbX3 halides were analyzed and compared with those of CH3NH3PbX3. The main difference between the stabilities of CsPbX3 and CH3NH3PbX3 halides was found to stem from the different chemical natures of cesium and methylammonium cations. Indeed, the enthalpies of formation of CsPbX3 from binary constituent halides, Δf H°hal, are only slightly more negative than those of CH3NH3PbX3. Small values of Δf H°hal imply that the entropic contribution to the Gibbs free energy of the formation of CsPbX3 and CH3NH3PbX3 is significant and, hence, of utmost importance for understanding the intrinsic stability of these compounds and their analogues. Regarding the extrinsic stability, the presence of gaseous O2, H2O, and CO2 was shown to be crucial for the stability of the iodide, CsPbI3, for which several decomposition reactions, exergonic at 298 K, were identified. At the same time, chloride, CsPbCl3, and bromide, CsPbBr3, are much less sensitive to these chemical agents. However, liquid water should degrade all of the CsPbX3 halides.

Thermal properties of metal-halide perovskites

“…114 The main difference between the stability of hybrid and all-inorganic perovskite halides was found to be due to the different chemical properties of the A-site cations, because their entropic contribution to free Gibbs energy is of utmost importance to understand the intrinsic stability of these compounds. 132 While a number of stability issues may be solved with proper encapsulation, 133 it has to be noted that a critical intrinsic stability problem arises from the behavior under thermal mechanical stress. To make matters worse, it has long been known that perovskites have phase transitions at elevated temperatures up to 85 1C.…”

Thermodynamic and Kinetic Aspects of Formamidinium Lead Iodide Thermal Decomposition

“…Coming now to consider the stability issues, while a number of papers have been devoted to the intrinsic stability of MAPI − and CsPbI 3 , − few data are currently available on FAPI. In particular, data about the thermodynamics of its decomposition, as well as a detailed kinetic analysis of the process, are totally lacking in the literature.…”

CsPbI3 nanocrystal films: towards higher stability and efficiency

“…This indicates that the functional black α-CsPbI 3 phase is entropically favored. [33][34][35][36] Theoretical calculations further prove that vibrational instabilities exist, induced by octahedral tilting in their high-temperature cubic phase from lattice-dynamics calculations. 32 Investigation of single-crystal γ-CsPbI 3 indicates that the rattling of the Cs cation, lower coordination of the Cs site, and local octahedral distortion contribute to the thermodynamic instability at room temperature.…”