The defect properties at surfaces or grain boundaries of metal halide perovskites are largely unexplored due to the complexity of surface structures stirred by the rotational A-site cations with varied dipole moments. Using a combination of density functional theory (DFT) and time-dependent DFT methods, we study the nature of iodine vacancies at the surfaces of lead iodide perovskites (APbI3) with A-site cations including methyl­ammonium (MA = CH3NH3 +), formamidinium, and cesium. It is found that the light-induced charge distributions are sensitively dependent on MA orientation at the MAI-terminated surfaces with vacancies at the apical position while the electronic excitation is marginally affected by A-site species at both the AI- and PbI-terminated surfaces with vacancies at the equatorial site. Such variations of electronic excitation are rationalized by analyzing the electrostatic interactions between the A-site cations and charged defects as well as the projected p orbitals of Pb atoms at the bottom of the conduction band.

Structural variations of lead halide perovskites (LHPs) upon light illumination play an important role in their photovoltaic applications. However, density functional theory (DFT)-based simulations have often been performed to unveil the nature of defects in LHPs without light illumination. So far, the nature of traps in LHPs triggered by the light remains largely unexplored. In this work, hole traps induced by the halogen interstitial in LHPs are studied by combining DFT and time-dependent DFT approaches, the latter of which treats electron−hole and electron−nuclei interactions on the same footing. Both a semilocal exchange functional and hybrid functional are adopted to relax the ground-state and excitedstate geometries followed by the calculations of energy levels of hole traps. The effect of the self-interaction corrections on the light-triggered geometric deformation and the electronic structure of hole traps is analyzed. Relaxation energies that correspond to the light-triggered geometric deformation are also calculated with different functionals. The relationship between the hole traps and light-triggered geometric variations are then explored.

The role of organic cations in A-sites of lead halide perovskites (LHPs) on carrier dynamics has been debated in an effort to understand the remarkable properties in these materials. However, the change of A-site species in LHPs often leads to the variation of crystalline phases at room temperature. Herein, we combine density functional theory (DFT) and time-dependent DFT methods to study electron traps in CH 3 NH 3 PbI 3 which exhibits different structural phases with temperature and in APbBr 3 [A = CH 3 NH 3 , CH(NH 2 ) 2 , or Cs] with their crystalline phases at room temperature. Regardless of halide species, electron traps arising from halide vacancies at surfaces are spatially localized in tetragonal phase and turn to be rather delocalized in orthorhombic and cubic phases. The reason is revealed by analyzing the projected p orbitals of Pb atoms at conduction band edges, providing a novel strategy of healing surface defects to improving the performances of the LHP solar cells.

The Rashba effect induced by the presence of spin− orbit coupling and the breaking of inversion symmetry has been proposed as a plausible cause for reduced radiative recombination rates in lead halide perovskites (LHPs). However, it is unclear how the crystalline phases impact the Rashba effect which has been experimentally observed with the difficulty on differentiating the factors from the bulk and surfaces. Here, we investigate the Rashba effect from the bulk and surfaces of MAPbX 3 (MA = CH 3 NH 3 and X = I and Br) in different phases by using the density functional theory approach. Our simulations disclose that the Rashba effect extracted from the bulk and surfaces with the ordered arrangement of oriented MA cations is progressively quenched in MAPbX 3 from hightemperature to low-temperature phases largely due to the diminished distortion of inorganic octahedrons. More importantly, the angle-resolved Rashba splitting is not significantly affected when going from the bulk to surfaces of MAPbI 3 in the room-temperature phase. This is highly different from the case of MAPbBr 3 in the roomtemperature phase where the Rashba splitting exhibiting a three-dimensional feature in the bulk collapses into anisotropic Rashba splitting at the surfaces. These findings would provide insights for manipulating spin-polarized carrier dynamics in photovoltaic devices and spintronic devices with LHPs in the presence of ferroelectricity arising from the order of the MA species.

The TiO2/MAPbI3 (MA=CH3NH3) interfaces have manifested correlation with current‐voltage hysteresis in perovskite solar cells (PSCs) under light illumination conditions, but the relations between the photo‐induced charge transfer and the collective polarization response of the dipolar MA cations are largely unexplored. In this work, we adopt density functional theory (DFT) and time‐dependent DFT approach to study the light‐triggered charge transfer across the TiO2/MAPbI3 interfaces with MAI‐ and PbI‐exposed terminations. It is found that regardless of the surface exposure of the MAPbI3, the photo‐induced charge transfer varies when going from the ground‐state geometries to the excited‐state configurations. Besides, thanks to the electrostatic interactions between the ends of MA cations and the photogenerated electrons, the photo‐induced charge transfer across the interfaces is enhanced (weakened) by the negatively (positively) charged CH3 (NH3) moieties of the MA species. Resultantly, the positively charged iodine vacancies at the TiO2/MAPbI3 interfaces tend to inhibit the charge transfer induced by light. Combining with the energy level alignment which is significantly modulated by the orientation of the MA species at the interfaces, the dipolar MA cations might be a double‐edge sword for the hysteresis in PSCs with the TiO2/MAPbI3 interfaces.