Phase Diagram and Dielectric Response of Hybrid Lead Halide Hollow Perovskites: A Universal Behavior of Molecular Cation Mixing

Mixing of molecular cations in hybrid lead halide perovskites is used to effectively tune the stability and performance of the photovoltaic devices based on these compounds. Upon introduction of bulky molecular cations such as ethylenediammonium (EN), the perovskite framework becomes locally broken resulting in so-called hollow perovskites. Here, we use a set of different experimental techniques to probe the structural phase transitions, molecular cation dynamics, and dielectric response in methylammonium-based MA1–xENxPb1–0.7xI3–0.4x hollow perovskites containing different amounts of EN molecular cations (x ≤ 0.26). We determine the temperature–composition phase diagram of this system and show that the introduction of EN results in stabilization of the desirable high-symmetry cubic phase, as the structural phase transitions become partially suppressed. Broadband dielectric spectroscopy experiments are used to study the dynamics of molecular cations for different fractions of EN. For high levels of EN, we observe signatures of a dipolar glass phase formation. Our findings indicate a universal trend of the dielectric response upon molecular cation mixing independent of the size and charge of the incorporated guest cations in MAPbI3.