We investigate the hypothesis of dynamic tetragonal domains occuring in cubic CH$_3$NH$_3$PbI$_3$ using high-resolution neutron spectroscopy to study a fully deuterated single crystal. The $\textit{R}$-point scattering above the 327.5 K tetragonal-cubic phase transition is always resolution-limited in energy and therefore inconsistent with dynamic domain predictions. This behavior is instead consistent with the central peak phenomenon observed in other perovskites. The scattering may originate from small, static, tetragonal-phase domains nucleating about crystal defects, and the temperature dependence demonstrates the transition is first-order.
Using cross-correlation current noise spectroscopy, we have investigated carrier dynamics in methylammonium lead triiodide solar cells. This method provides a space selectivity for devices with planar multi-layered structure, effectively amplifying current noise contributions coming from the most resistive element of the stack. In the studied solar cells, we observe near full-scale shot noise, indicating the dominance of noise generation by a single source, likely the interface between the perovskite and the spiro-OMeTAD hole-transport layer. We argue that the strong 1/f noise term has contrib…
This article reports on studies of chlorine-substituted MAPbI3 using combined temperature-dependent XRD synchrotron and Pb-L3 edge EXAFS to analyze the anharmonicity of the lead halide bond. The EXAFS parameters were described in the orthorhombic phase by an Einstein or T2 type behavior, which was then compared with the experimental EXAFS parameters of the tetragonal/cubic phase. In the orthorhombic phase, it was observed that the asymmetry of the pair distribution function (cumulant C3) in MAPbCl3 is much lower than in MAPbI3. Compared with the behavior in the orthorhombic phase, the anharmon…
Metal halide perovskites (MHPs) are known to undergo several structural phase transitions, from lower to higher symmetry, upon heating. While structural phase transitions have been investigated by a wide range of optical, thermal and electrical methods, most measurements are quasi-static and hence do not provide direct information regarding the fundamental timescale of phase transitions in this emerging class of semiconductors. Here we investigate the timescale of the orthorhombic-to-tetragonal phase transition in the prototypical metal halide perovskite, methylammonium lead triiodide (CH3NH3P…
Methylammonium lead triiodide (CH3NH3PbI3) perovskite solar cell is a gem in the list of photovoltaic semiconductors. Although there are numerous fundamental and technological questions yet to be addressed covering various aspects of this system for its commercialization, this study has employed first-principles DFT to model the [PbI6(CH3NH3)n]m zero-dimensional nanoclusters. Using the calculated binding energy landscapes, it has answered the question: why the corner-sharing PbI6 octahedron is surrounded by eight units of the organic cation in the large-scale supramolecular structures of the C…
Lead halide perovskites such as methylammonium lead triiodide (MAPI) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution processable material works so well is currently missing. Previous research has revealed that MAPI possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and find direct evidence for another form of disorder in single c…
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of…
The defect tolerance nature of organic-inorganic hybrid perovskite is reflected from its stupendous growth in photovoltaic performances. The presence of lattice defect can manipulate or even gives rise to some exceptional properties which otherwise would have remained unseen. One of such properties reported in this article is the experimental observation of defect mediated room temperature ferromagnetism in methylammonium lead halide perovskite for the very first time, ably supported by ab-initio calculations. Theoretical analysis predicts the ferromagnetism principally arises from the iodide …
The (001) surface of the emerging photovoltaic material cesium lead triiodide (CsPbI3 ) is studied. Using first-principles methods, we investigate the atomic and electronic structure of cubic (α) and orthorhombic (γ) CsPbI3 . For both phases, we find that CsI-termination is more stable than PbI2-termination. For the CsI-terminated surface, we then compute and analyse the surface phase diagram. We observe that surfaces with added or removed units of nonpolar CsI and PbI2 are most stable. The corresponding band structures reveal that the α phase exhibits surface states that derive from the condu…
We study the effects of bismuth doping on the crystal structure and phase transitions in single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr3. By measuring temperature-dependent specific heat capacity (Cp) we find that, as Bi doping increases, the phase transition assigned to the cubic to tetragonal phase boundary decreases in temperature. Furthermore, after doping we observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to previously reported effects …
Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI$_3$) perovskite, is perceived to be a promising material for optoelectronics and spintronics. However, lead toxicity and instability under air and moisture restrict its practical uses. Hence, it is essential to reduce lead extent by substituting appropriate alternatives. Here, we substitute Sn and Ge in cubic MAPbI$_3$ and compare various properties of hybrid perovskites by employing state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with semilocal and hybrid func…
Methylammonium lead tribromide perovskite (CH3NH3PbBr3, or MAPbBr3) as a photovoltaic material has attracted a great deal of recent interest. Factors that are important in their application in optoelectronic devices include their fractional contribution of the composition of the materials as well as their microscopic arrangement that is responsible for the formation of well-defined macroscopic structures. CH3NH3PbBr3 assumes different polymorphs (orthorhombic, tetragonal and cubic) depending on the evolution temperature of the bulk material. Density functional theory calculations have been per…
Organic-inorganic lead halide perovskites containing a mixture of iodide and bromide anions consistently perform better in donor-acceptor heterojunction solar cells than the standard methylammonium lead triiodide material. This observation is counterintuitive, as it is generally expected for photovoltaic materials that heterogeneities and compositional disorder cause reduced carrier diffusion length and conversion efficiency. Here, we combine ultrafast photoinduced electroabsorption and broadband fluorescence up-conversion spectroscopy measurements to scrutinize the carrier dynamics in mixed-c…
Organolead halide perovskites are a family of hybrid organic-inorganic compounds whose remarkable optoelectronic properties have been under intensive scrutiny in recent years. Here we use inelastic X-ray scattering to study low-energy lattice excitations in single crystals of methylammonium lead iodide and bromide perovskites. Our findings confirm the displacive nature of the cubic-to-tetragonal phase transition, which is further shown, using neutron and x-ray diffraction, to be close to a tricritical point. Lastly, we detect quasistatic symmetry-breaking nanodomains persisting well into the h…
Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Here we specifically address the energy level order of optical transitions and demonstrate photon echos in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times $T_2$ for excitons and biexcitons at cryogenic temperature to be 0.79 ps and 0.67 ps, respectively. Most importantly, we have developed a…
Metal halide perovskites APbX3 (A+ = FA+ (formamidinium), MA+ (methylammonium) or Cs+, X- = I-, Br-) are considered as prominent innovative components in nowadays perovskite solar cells. Crystallization of these materials is often complicated by the formation of various phases with the same stoichiometry but structural types deviating from perovskites such as well-known the hexagonal delta FAPbI3 polytype. Such phases are rarely placed in the focus of device engineering due to their unattractive optoelectronic properties while they are, indeed, highly important because they influence on the op…
Owing to the increasing popularity of lead-based hybrid perovskites for photovoltaic (PV) applications, it is crucial to understand their defect physics and its influence on their optoelectronic properties. In this work, we simulate various point defects in pseudo-cubic structures of mixed iodide-bromide and bromide-chloride methylammonium lead perovskites with the general formula MAPbI_{3-y}Br_{y} or MAPbBr_{3-y}Cl_{y} (where y is between 0 and 3), and use first principles based density functional theory computations to study their relative formation energies and charge transition levels. We …
We present an \textit{ab initio} molecular dynamics study of the temperature-induced phases of methylammonium lead bromide (MAPbBr$_3$). We confirm that the low-temperature phase is not ferroelectric, and rule out the existence of any overall polarization arising from the motion of the individual sub-lattices. Our simulations at room temperature resulted in a cubic \textit{Pm-3m} phase with no discernible local orthorhombic distortions. At low temperatures, such distortions are shown to originate from octahedral scissoring modes, but they vanish at room temperature. The predicted timescales of…
Thermal history controls the optoelectronic response of lead halide perovskites through structure and dynamics
University of Cambridge（剑桥大学）；Imperial College London（伦敦帝国理工学院）；Ulsan National Institute of Science and Technology (UNIST)（蔚山科学技术院）；Linköping University（林雪平大学）；Chalmers University of Technology（查尔姆斯理工大学）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）；University of Oxford（牛津大学）
Second-order Raman and anharmonic lattice dynamics in lead-halide perovskites
Institute of Science and Technology Austria（科学技术奥地利研究所）；King Fahd University of Petroleum and Minerals（法赫德国王石油与矿产大学）；Interdisciplinary Research Center (IRC) for Advanced Materials, King Fahd University of Petroleum and Minerals（法赫德国王石油与矿产大学先进材料跨学科研究中心）；King Abdullah University of Science and Technology（阿卜杜拉国王科技大学）；Center for Renewable Energy and Storage Technologies, Division of Physical Science and Engineering, King Abdullah University of Science and Technology（阿卜杜拉国王科技大学物理科学与工程学部可再生能源与储能技术中心）；Aarhus University（奥胡斯大学）
