Orientational disorder and molecular correlations in hybrid organic-inorganic perovskites: From fundamental insights to technological applications

TL;DR

This study uses molecular dynamics simulations to analyze orientational disorder and molecular correlations in hybrid perovskites, revealing how molecular rotations and couplings influence phase transitions and local ordering, with implications for optoelectronic applications.

Contribution

It provides a detailed quantitative analysis of molecular dynamics and entropy contributions in hybrid perovskites, highlighting the role of molecular couplings and local order in phase transitions.

Findings

Rigid molecular rotations and correlations significantly contribute to entropy changes.

Molecular conformational changes have negligible effect on entropy across phases.

Local orientational order inhibits formation of large polar nanodomains.

Abstract

Hybrid organic-inorganic perovskites (HOIP) have emerged in recent years as highly promising semiconducting materials for a wide range of optoelectronic and energy applications. Nevertheless, the rotational dynamics of the organic components and many molecule interdependencies, which may strongly impact the functional properties of HOIP, are not yet fully understood. In this study, we quantitatively analyze the orientational disorder and molecular correlations in the archetypal perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ (MAPI) by performing comprehensive molecular dynamics simulations and entropy calculations. We found that, in addition to the usual vibrational and orientational contributions, rigid molecular rotations around the C-N axis and correlations between neighboring molecules noticeably contribute to the entropy increment associated with the temperature-induced order-disorder phase transition in MAPI, $\Delta S_{t}$. Molecular conformational changes are equally infrequent in the low-$T$ ordered and high-$T$ disordered phases and have a null effect on $\Delta S_{t}$. Conversely, the couplings between the angular and vibrational degrees of freedom are substantially reinforced in the high-$T$ disordered phase and significantly counteract the phase-transition entropy increase resulting from other factors. Furthermore, the tendency for neighboring molecules to be orientationally ordered is markedly local, consequently inhibiting the formation of extensive polar nanodomains both at low and high temperatures. This theoretical investigation not only advances the fundamental knowledge of HOIP but also establishes physically insightful connections with contemporary technological applications like photovoltaics and solid-state cooling.