Thermodynamics and the Intrinsic Stability of Lead Halide Perovskites CH3NH3PbX3

The role of thermodynamics in assessing the intrinsic instability of the CH 3 NH 3 PbX 3 perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH 3 NH 3 X(s) and PbX 2 ( s) is first analyzed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH 3 NH 3 PbI 3 , the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH 3 (g) + CH 3 X(g), CH 3 NH 2 (g) + HX(g) or CH 3 NH 3 X(g) is analyzed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized.