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

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.