Nature of the excited state in lead iodide perovskite materials: Time-dependent charge density response and the role of the monovalent cation

Abstract

Charge density response is responsible for the excited-state properties of lead iodide perovskites and is related to both the light absorption properties as well as subsequent electronic and lattice relaxation in the system, important for the working conditions of the material in solar cell applications. Here we investigate the nature of the excited state and its relation to pathways for electronic and lattice relaxations by performing time-dependent density-functional theory (TDDFT). Charge density response upon photoexcitation close to the band edge and deeper into the absorption spectra are investigated for three lead perovskite compounds with different $A$-site monovalent cations ${\mathrm{CsPbI}}_{3},\mathrm{C}{\mathrm{H}}_{2}{(\mathrm{N}{\mathrm{H}}_{2})}_{2}\mathrm{Pb}{\mathrm{I}}_{3}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.28em}{0ex}}(\mathrm{FAPb}{\mathrm{I}}_{3})$, and $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}(\mathrm{MAPb}{\mathrm{I}}_{3})$. The carrier cooling mechanism is analyzed and shows that the initial force acting on the nuclei follows the symmetry of the ground-state electronic structure upon photoexcitation with a force parallel to the polarization of the incoming light. This effect is investigated for the three different compounds and shows an initial force for induced ionic movement that depends on both the underlying symmetry of the inorganic lattice as well as on the type and orientation of the organic cation. The excess energy after thermalization under blue-light illumination is large enough for overcoming the activation energy for iodide migration and can thus trigger vacancy formation. Iodide vacancies are seen to be dipole-field compensated by the organic cation, with a shielding of the local field, and thus form an explanation for the defect tolerance found in these systems under photovoltaic operation. A partial charge transfer from the inorganic cage to the monovalent organic cation is predicted with TDDFT calculations for blue- and UV-light illumination with a population of antibinding orbitals in the N--H bond in both ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}$ (MA) and ${\mathrm{CH}}_{2}{({\mathrm{NH}}_{2})}_{2}$ (FA), where the implication for this is discussed in terms of the intrinsic photostability of organic cation containing lead perovskites. The results show the importance of a fundamental understanding of the excited-state properties of perovskite material to reveal the underlying mechanism for the defect tolerance and thus high photovoltaic performance when using organic dipolar cations as well as a rationale for using mixed halide perovskites to decrease the halide migration, effect of vacancy formation, and stability issues under blue- and UV-light illumination.