Watching Hybrid Perovskites Fall Apart – A Journey Across Physical Space

Hybrid perovskites constitute a promising platform for next-generation photovoltaics. But they are intrinsically unstable. For the first time, a combination of state-of-the-art radiation-scattering experiments and ab initio calculations across temperature and pressure identifies the mechanisms underpinning phase transformations, all the way up to the onset of structural collapse and eventual amorphization.

Original publication: Methylammonium Lead Iodide across Physical Space: Phase Boundaries and Structural Collapse.

Hybrid organic-inorganic perovskites (HOIPs) continue to attract substantial attention owing to their remarkable photophysical response, of direct relevance to the deployment of next-generation photovoltaics. In spite of this promise, HOIPs exhibit hard-to-tame intrinsic and extrinsic instabilities that hamper further progress. Exploring how HOIPs may be stabilized or destabilized upon the application of external stimuli constitutes a key step in the journey. The use of physical pressure represents a means of achieving this goal, and the present work has capitalized from ongoing advances in radiation-scattering methodologies, along with extensive Ab-Initio Molecular Dynamics (AIMD) simulations. The figure presented below summarizes our experimental and computational results for the paradigmatic HOIP MethylAmmonium Lead Iodide (hereafter MAPI). The first thing to note is the richness of the phase diagram for this (seemingly simple!) HOIP, exhibiting five distinct phases below 350 K and 20 kbar.

Figure. The left panel shows the phase diagram of MAPI as a function of temperature and up to 25 kbar, obtained from the neutron and synchrotron X-ray data. The two figures on the right show the corresponding AIMD phase diagrams for the two structural models presented in the main text – low-symmetry (top) and high-symmetry (bottom). The heat maps in these figures give the corresponding formula-unit densities across the P-T plane. Grey (blue) symbols correspond to the specific points explored in our (previous) experiments. Greek letters denote the different phases of MAPI explored in this work.

Previous high-pressure studies had been restricted to near-to-ambient conditions, where it is practically impossible to discern the static or dynamical nature of the disorder at the atomic and molecular scales. In particular, one is to note the marked negative slope of the phase boundary between the low-temperature (ordered) g-phase and the two high-pressure (disordered) phases – that is, MAPI undergoes a clear contraction upon heating above the low-temperature triple point at 1 kbar. This result alone is already indicative of intrinsic instabilities at relatively modest pressures. The variation of the Formula-Unit Density (FUD) across the P-T diagram shown in the figure provides us with a model-independent assessment and validation of different structural models and the simulations. In particular, we find that the (high-symmetry) Pnma model inferred from previous studies at ambient pressure cannot give rise to the abrupt structural contraction above 5 kbar observed in the experiments, marking the onset of structural collapse. Instead, relaxing the symmetry of the unit cell by avoiding the end-to-end ordering of the MA cations leads to an overall weakening of the N-H···I hydrogen bonds between organic and inorganic sub-lattices, in quantitative agreement with experiment. AIMD shows that eventual and irreversible amorphization is then driven by further and quite pronounced octahedral distortions of the inorganic framework, which becomes possible only under these circumstances.