The dynamics of organic cations in metal halide hybrid perovskites (MHPs) have been investigated using numerous experimental and computational techniques because of their suspected effects on the properties of MHPs. In this Perspective, we summarize and reconcile key findings and present new data to synthesize a unified understanding of the dynamics of the cations. We conclude that theory and experiment collectively paint a relatively complete picture of rotational dynamics within MHPs. This picture is then used to discuss the consequences of structural dynamics for electron−phonon interactions and their effect on material properties by providing a brief account of key studies that correlate cation dynamics with the dynamics of the inorganic sublattice and overall device properties.M etal halide perovskites (MHPs) are enjoying considerable academic and industrial interest due to their high photovoltaic power conversion efficiencies (22% as of July 2017), 1 ease and low cost of production, and broad material tuneability. 2 Besides applications in photovoltaics, MHPs may find use as solar thermoelectric materials, 3 LEDs, lasers, 4 and nonvolatile memory. 5 The macroscopic properties of MHPs emerge from diverse microscopic phenomena, including crystal structure, 6 defects, cation disorder, 7 ion migration, 8 and spin−orbit coupling. 9 Linking these phenomena to macroscopic performance is challenging because of the complex interplay between them. In this Perspective, we will address cation dynamics, their interactions with lattice vibrations (phonons), and their hypothesized effects on device performance.MHPs share a common ABX 3 (perovskite) structure as shown in Figure 1. This consists of (A) an organic molecule (e.g., methylammonium (MA) or formamidinium (FA)) or a large inorganic atom (e.g., caesium), (B) a metal dication (e.g., Pb (II) or Sn (II) ) and, (X) halide anions (Cl, Br, I, or some combination thereof). The M and X species form a cornersharing octahedral framework. The charge-balancing A-site cation occupies the central cavity generated by this framework and has a strong effect on the MHP structure, quantified by the Goldschmidt tolerance factor. 10 A-site ions just above or below the optimum size may induce tilting of the octahedra within the BX 3 sublattice away from a cubic perovskite. Mixing of ions will lead to inhomogeneity in the local structure, which may result in coupling of cation motion and equilibrium distribution to more complex local lattice dynamics.

We examine the effect of temperature on three viscosity-sensitive fluorophores termed ‘molecular rotors’. In the case of the conjugated porphyrin dimer, it can be used for measuring both viscosity and temperature concurrently.

We present a new framework to study organic photovoltaic devices in which a model that integrates device physics with excited state dynamics is applied to explain transient and steady-state spectroscopic and optoelectronic measurements.

Using vibrational sum frequency generation (VSFG) spectroscopy, we investigate the behaviour of organic cations at the surface of a series of multilayer lead halide perovskite systems, finding that the behaviour of the organic cations changes dramatically close to the interface.

The power conversion efficiency of ternary organic solar cells (TOSCs), consisting of one host binary blend and one guest component, remains limited by large voltage losses. The fundamental understanding of the open-circuit voltage (VOC) in TOSCs is controversial, limiting rational design of the guest component. In this study, we systematically investigate how the guest component affects the radiative and non-radiative related parts of VOC of a series of TOSCs using the detailed balanced principle. We highlight that the thermal population of charge-transfer and local exciton states provided by the guest binary blend (that is, the guest-component-based binary blend) has a significant influence on the non-radiative voltage losses. Ultimately, we provide two design rules for enhancing the VOC in TOSCs: high emission yield for the guest binary blend and similar charge-transfer-state energies for host/guest binary blends; high miscibility of the guest component with the low gap component in the host binary blend.

Hybrid inorganic-organic perovskites have attracted considerable attention over recent years as promising processable electronics materials. In particular, the rich structural dynamics of these ‘soft’ materials became a subject of investigation...

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