A mixture of CsPbI3 and FAPbI3 is thermodynamically stabilized in the perovskite phase with respect to the pure δ phases.

Organic-inorganic lead halide perovskites have unusual, temperature-dependent emission characteristics.

We present a reliable simulation strategy for estimating the surface tension, the work of adhesion, and all related macroscopic work functions of fluid/vacuum and fluid/solid interfaces, directly from the atomic-level stresses in the system. Our methodology employs efficient algorithms (developed here and from the literature) for fast and reliable simulations of high molar mass polymer melts and is applied to the well-tested molten polyethylene/graphite interface, as well to the free surface of molten polyethylene, using a united atom model for the polymer. The surface thermodynamic properties are obtained for a broad range of molar masses and temperatures and are compared to experimental data, theoretical models, and earlier simulation studies. The individual components of the stress tensor are isolated, and their profiles along the aperiodic dimension are correlated to the orientational and structural features of the polymer chains near the interfaces. The distributions of end segments in free and capped polymer films are obtained for various temperatures and molar masses. The simulation procedure, the adequacy of the models employed for the stress tensor, and the tail corrections to surface thermodynamic properties as well as subtle issues arising in simulations of polymer/solid interfaces are discussed in detail.

Mixed cation perovskites, [HC(NH 2 ) 2 ] x Cs (1−x) PbI 3 , (FA x Cs (1−x) PbI 3 ) with x = 0.8, achieve high solar cell power conversion efficiencies while exhibiting long-term stability under ambient conditions. In this work, we perform density functional theory calculations, first-principles molecular dynamics simulations, solid-state nuclear magnetic resonance (NMR), and X-ray powder diffraction (XRD) measurements aimed at investigating the possible phase stability of Cs + -rich FA x Cs (1−x) PbI 3 , (0 ≤ x ≤ 0.5) mixed-cation materials as potential candidates for tandem solar cell applications. Estimations of the free energy of the mixtures with respect to the pure compounds together with calculations of the relative phase stability at 0 K and at finite temperature show that although the mixtures can form, the δ phase remains the thermodynamically most stable phase at room temperature. This is fully corroborated by solid-state NMR and XRD measurements and is in contrast to FA + -rich Cs/FA mixtures, where small additions of Cs + are sufficient to stabilize the perovskite phase at ambient conditions. The atomistic origin for this contrasting behavior arises from an energetic destabilization of the perovskite phase on the one hand caused by the incorporation of a large cation (FA + ) into the relatively small host lattice of γ-CsPbI 3 and on the other hand is induced by the lower degree of distortion of the host lattice. These observations allow us to propose a new design principle for the preferential stabilization of the perovskite phase over the competing δ phase.