RESUMO
The electronic structure of halide perovskites is central to their carrier dynamics, enabling the excellent optoelectronic performance. However, the experimentally resolved transient absorption spectra exhibit large discrepancies from the commonly computed electronic structure by density functional theory. Using pseudocubic CsPbI3 as a prototype example, here, it is unveiled with both ab initio molecular dynamics simulations and transmission electron microscopy that there exists pronounced dynamical lattice distortion in the form of disordered instantaneous octahedral tilting. Rigorous first-principles calculations reveal that the lattice distortion substantially alters the electronic band structure through renormalizing the band dispersions and the interband transition energies. Most notably, the electron and hole effective masses increase by 65% and 88%, respectively; the transition energy between the two highest valence bands decreases by about one half, agreeing remarkably well with supercontinuum transient-absorption measurements. This study further demonstrates how the resulting electronic structure modulates various aspects of the carrier dynamics such as carrier transport, hot-carrier relaxation, Auger recombination, and carrier multiplication in halide perovskites. The insights provide a pathway to engineer carrier transport and relaxation via lattice distortion, enabling the promise to achieve ultrahigh-efficiency photovoltaic devices.
RESUMO
The organic molecules in hybrid perovskites can easily rotate within the inorganic lattice at room temperature, leading to a crystal-liquid duality. The liquid-like behavior of the organic molecules is commonly believed to play a critical role in the dynamical stability, but the microscopic mechanism remains unclear. Furthermore, the presence of dynamically rotating molecules raises concerns regarding the reliability of assessing the stability of hybrid perovskites based on simple yet commonly used descriptors such as the Goldschmidt tolerance factor. Here we assess the finite-temperature phonons of hybrid perovskites by mapping ab initio molecular dynamics configurations onto an equivalent dynamical pseudo-inorganic lattice and extracting the effective force constants. We find that as compared to the formamidinium or cesium cations, stronger anisotropy and wider range of the thermal motion of the methylammonium molecule are essential for enhancing the dynamical stability of hybrid perovskites. The cation radius that determines the tolerance factor is, in fact, less important. This work not only enables a pathway to further improve the stability of hybrid perovskites, but also provides a general scheme to assess the stability of hybrid materials with dynamical disorder.
