PHYSICAL REVIEW B 100, 045203 (2019) Nature of the excited state in lead iodide perovskite materials: Time-dependent charge density response and the role of the monovalent cation Meysam Pazoki * and Tomas Edvinsson † Department of Engineering Sciences – Solid State Physics, Uppsala University, Box 534, 75121 Uppsala, Sweden (Received 21 December 2018; revised manuscript received 20 June 2019; published 12 July 2019) 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 CsPbI 3 , CH 2 (NH 2 ) 2 PbI 3 (FAPbI 3 ), and CH 3 NH 3 PbI 3 (MAPbI 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 CH 3 NH 3 (MA) and CH 2 (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. DOI: 10.1103/PhysRevB.100.045203 I. INTRODUCTION Hybrid lead iodide perovskites represent a new class of solar cell materials that has been introduced very recently and attracted a great deal of research attention in the solar cell field [1]. Their potentially low fabrication cost together with high device efficiencies and tunable band gap make the lead halide perovskites promising for future single- or multiple-junction solar cell devices [2]. The ongoing research is in progress to modify the material composition [3] and synthesis procedure [4–9] in order to obtain improved stability and better polycrystalline quality with less detrimental trap states [10]. Stable devices are crucial to be able to produce tandem devices together with commercially available silicon solar cells passing the standard solar cell device stability tests [11]. However, under the umbrella of promising low- cost and high-efficiency devices, there are quite many diverse physical properties of the hybrid perovskites that make them unique. First, they belong to the perovskite family and, as * meysam.pazoki@angstrom.uu.se † tomas.edvinsson@angstrom.uu.se such represent a quite open structure where both electron and ion diffusion can occur. As for conventional fully inorganic perovskites, the choice of elements and variation in anion and cation size allow tuning of lattice symmetry, electron density, orbital types, and their overlap, resulting in diverse physical properties. The inclusion of a dipolar organic cation in the A site introduces further degrees of freedom for Coulomb interactions and may lead to ferroelectric domains, dipolar stacking, or dipolar charge screening by orientation of the cation as well as dipole-vacancy interactions [12]. Many in- triguing phenomena have been reported such as a giant dielec- tric constant [13], a switchable photovoltaic effect [14,15], an anomalous photovoltaic effect [16], Stark effects [17,18], anomalous hysteresis effect [19,20] in current-voltage scans, photoinduced traps [21], polarizability/ferroelectric domains [22], ferroelasticity [23], and photoinduced ionic movement [24]. In direct relevance and significance to device efficiency in solar cells are the interaction with light, electron-hole interaction, carrier cooling, vacancy formation and electronic interaction with the vacancies, and transport properties. The dipolar nature of the crystal together with soft mechan- ical properties contribute to make the vacancy sites mobile [25–27] where, especially under long-term illuminations or 2469-9950/2019/100(4)/045203(10) 045203-1 ©2019 American Physical Society