Electron–Rotor Interaction in Organic–Inorganic Lead Iodide Perovskites Discovered by Isotope Effects

We report on the carrier–rotor coupling effect in perovskite organic–inorganic hybrid lead iodide (CH₃NH₃PbI₃) compounds discovered by isotope effects. Deuterated organic–inorganic perovskite compounds including CH₃ND₃PbI₃, CD₃NH₃PbI₃, and CD₃ND₃PbI₃ were synthesized. Devices made from regular CH₃NH₃PbI₃ and deuterated CH₃ND₃PbI₃ exhibit comparable performance in band gap, current–voltage, carrier mobility, and power conversion efficiency. However, a time-resolved photoluminescence (TRPL) study reveals that CH₃NH₃PbI₃ exhibits notably longer carrier lifetime than that of CH₃ND₃PbI₃, in both thin-film and single-crystal formats. Furthermore, the comparison in carrier lifetime between CD₃NH₃PbI₃ and CH₃ND₃PbI₃ single crystals suggests that vibrational modes in methylammonium (MA⁺) have little impact on carrier lifetime. In contrast, the fully deuterated compound CD₃ND₃PbI₃ reconfirmed the trend of decreasing carrier lifetime upon the increasing moment of inertia of cationic MA⁺. Polaron model elucidates the electron–rotor interaction

Polar Fluctuations in Metal Halide Perovskites Uncovered by Acoustic Phonon Anomalies

Solution-processable metal halide perovskites (MHPs) offer great promise for efficient light-harvesting and -emitting devices because of their long carrier lifetime and superior carrier transport properties. Similar to traditional oxide perovskites, MHPs exhibit strong dynamic disorders that may further impact their electronic properties. Here we investigate the coherent acoustic phonons of inorganic and organic–inorganic (hybrid) MHP single crystals (CsPbCl₃, MAPbCl₃, CsPbBr₃, MAPbBr₃, FAPbBr₃, MAPbI₃, and FAPbI₃) using pump–probe reflection spectroscopy. We show significant phonon softening in the cubic phase of all compositions close to the cubic-to-tetragonal phase transition temperature. Such phonon softening in conjunction with strong acoustic damping is attributed to pretransitional polar fluctuations. Comparison of MHPs with different compositions show that the degree of pretransitional fluctuations is correlated with the size rather than the dipole moment of the A-site cations, and further with the size of the anions