Electron-Volt Fluctuation of Defect Levels in Metal Halide Perovskites on a 100 ps Time Scale

Abstract

Metal halide perovskites (MHPs) have gained considerable attention due to their excellent optoelectronic performance, which is often attributed to unusual defect properties. We demonstrate that midgap defect levels can exhibit very large and slow energy fluctuations associated with anharmonic acoustic motions. Therefore, care should be taken classifying MHP defects as deep or shallow, since shallow defects may become deep and vice versa. As a consequence, charges from deep levels can escape into bands, and light absorption can be extended to longer wavelengths, improving material performance. The phenomenon, demonstrated with iodine vacancy in CH 3 NH 3 PbI 3 using a machine learning force field, can be expected for a variety of defects and dopants in many MHPs and other soft inorganic semiconductors. Since large-scale anharmonic motions can be precursors to chemical decomposition, a known problem with MHPs, we propose that materials that are stiffer than MHPs but softer than traditional inorganic semiconductors, such as Si and TiO 2 , may simultaneously exhibit excellent performance and stability.

Fluctuations at Metal Halide Perovskite Grain Boundaries Create Transient Trap States: Machine Learning Assisted Ab Initio Analysis

“…However, in the traditional semiconductors the trap states form stable configurations, allowing one to consider a static description of defect properties. In contrast, perovskites are much softer and undergo significant thermal structural fluctuations. , The midgap traps appear only transiently, as discussed in detail below, and should have much weaker influence on charge carrier dynamics.…”

“…This GB has been studied previously. 44,45 Because perovskites are soft and are known to undergo large thermal fluctuations, 38,46 we expect that other grain boundaries should exhibit similar properties. The planewave basis energy cutoff is set to 350 eV for geometry optimization, electronic structure calculation, and ab initio molecular dynamics calculations.…”

Interplay of structural fluctuations and charge carrier dynamics is key for high performance of hybrid lead halide perovskites

“…Such significant anharmonic effects have been detected in MHPs previously and correlated with several important optoelectronic properties. 64,66,67…”

“…Such significant anharmonic effects have been detected in MHPs previously and correlated with several important optoelectronic properties. 64,66,67 As temperature increases, the anharmonic scores of the inorganic atoms reduce. Anharmonicity is associated with largeamplitude atomic displacements.…”

Anion Doping Delays Nonradiative Electron–Hole Recombination in Cs-Based All-Inorganic Perovskites: Time Domain ab Initio Analysis

“…The decoherence time, estimated as the pure-dephasing time of the optical response theory, is much shorter than the electron–hole recombination time, and therefore, decoherence effects should be included in the calculations. The method has been applied to investigate excited state dynamics in a broad range of perovskites, − including perovskite quantum dots, two-dimensional perovskites, ,, perovskite containing dopants and grain boundaries, − ,, perovskite heterojunctions, , and other systems. − …”