Shifting the Thermodynamics of Polymorph Transitions in Metal-Halide Perovskites

We demonstrate nanoconfinement as a strategy to stabilize high-temperature cubic phases of metal-halide perovskites and prevent humidity-induced degradation. A combination of temperature-dependent x-ray diffraction and photoluminescence experiments were employed to track polymorph transitions in the range of 4 – 373 K. For methylammonium lead triiodide (MAPbI₃) confined within the nanopores of anodized aluminum oxide templates, the tetragonal-to-cubic phase transition shifted from T = 300 K to T = 170 K. These crystals were stable for a period of at least two years of storage in air, compared to unconfined crystals that degrade into perovskite precursors within two weeks of air exposure. For nanoconfined cesium lead triodide (CsPbI₃), the tetragonal-to-cubic phase transition, which typically occurs at 583 K, was absent to temperatures as low as 4 K. We hypothesize that nanoconfinement introduces lattice strain into the crystals, shifting the relative free energies of the respective polymorphs and increasing energy barrier to polymorph transitions. In both systems, the cubic phase represents the smallest band gap polymorph, with important implications for solar cell device operation and efficiency.