The past several years has witnessed a surge of interest in organometallic trihalide perovskites, which are at the heart of the new generation of solid-state solar cells. Here, we calculated the static conductivity of charged domain walls in n- and p- doped organometallic uniaxial ferroelectric semiconductor perovskite CH3NH3PbI3 using the Landau-Ginzburg-Devonshire (LGD) theory. We find that due to the charge carrier accumulation, the static conductivity may drastically increase at the domain wall by 3-4 orders of magnitude in comparison with conductivity through the bulk of the material. Also, a two-dimensional degenerated gas of highly mobile charge carriers could be formed at the wall. The high values of conductivity at domain walls and interfaces explain high efficiency in organometallic solution-processed perovskite films which contains lots of different point and extended defects. These results could suggest new routes to enhance the performance of this promising class of novel photovoltaic materials.
Methylammonium lead trihalide perovskite solar cells (CH3NH3PbY3, where Y = I(3-x)Brx=1-3, I(3-x)Clx=1-3, Br(3-x)Cl x=1-3, and IBrCl) are photonic semiconductors. Researches on various fundamental and technological aspects of these materials are extensively on-going to make them stable environmentally and for commercialization. Research studies addressing these materials as organometallic are massively and repeatedly appearing in very reputable, highly cited and high impact peer-reviewed research publications, including, for example, Energy and Environmental Science, Nature Chemistry, Nature C…
In the development of highly efficient photovoltaic cells, solid perovskite systems have demonstrated unprecedented promise, with the figure of merit exceeding nineteen percent of efficiency. In this paper, we investigate the optical and vibrational properties of organometallic cubic perovskite CH3NH3PbI3 using first-principles calculations. For accurate theoretical description, we go beyond conventional density functional theory (DFT), and calculated optical conductivity using relativist quasi-particle (GW) correction. Incorporating these many-body effects, we further solve Bethe-Salpeter equ…
Perovskite-based optoelectronic devices have shown great promise for solar conversion and other optoelectronic applications, but their long-term performance instability is regarded as a major obstacle to their widespread deployment. Previous works have shown that the ultralow thermal conductivity and inefficient heat spreading might put an intrinsic limit on the lifetime of perovskite devices. Here, we report the observation of a remarkably efficient thermal conductance, with conductivity of 11.2 +/- 0.8 W m^-1 K^-1 at room temperature, in densely-packed perovskite CH3NH3PbI3 films, via noncon…
Organometallic lead halide perovskites are highly efficient materials for solar cells and other optoelectronic applications due to their high quantum efficiency and exceptional semiconducting properties. A peculiarity of these perovskites is the substantial ionic motion under external forces. Here, we reveal that electric field-and light-induced ionic motion in MAPbX3 crystals (X=Cl, Br, I and MA=CH3NH3) leads to unexpected piezoelectric-like response, an order of magnitude larger than in ferroelectric perovskite oxides. The nominal macroscopic symmetry of the crystals is broken by redistribut…
Understanding the type, formation energy and capture cross section of defects is one of the challenges in the field of organometallic halide perovskite (OMHP) devices. Currently, such understanding is limited, restricting the power conversion efficiencies of OMHPs solar cells from reaching their Shockley Queisser limit. Here, we report on deep level (DL) defects and their effect on free charge transport properties of single crystalline methylammonium lead bromide perovskite (MAPB). In order to determine DL activation energy and capture cross section we used photo-Hall effect spectroscopy (PHES…
We report on the temperature dependence of thermal conductivity of single crystalline and polycrystalline organometallic perovskite CH3NH3PbI3. The comparable absolute values and temperature dependence of the two sample's morphologies indicate the minor role of the grain boundaries on the heat transport. Theoretical modelling demonstrates the importance of the resonant scattering in both specimens. The interaction between phonon waves and rotational degrees of freedom of CH3NH3+ sub-lattice emerges as the dominant mechanism for attenuation of heat transport and for ultralow thermal conductivit…
Great success has been achieved in improving the photovoltaic energy conversion efficiency of the organic-inorganic perovskite-based solar cells, but with very limited knowledge on the thermal transport in hybrid perovskites, which would affect the device lifetime and stability. Based on the potential developed from the density functional theory calculations, we studied the lattice thermal conductivity of the hybrid halide perovskite CH3NH3PbI3 using equilibrium molecular dynamics simulations. Temperature-dependent thermal conductivity is reported from 160 K to 400 K, which covers the tetragon…
The low thermal conductivity of piezoelectric perovskites is a challenge for high power transducer applications. We report first principles calculations of the thermal conductivity of ferroelectric PbTiO$_3$ and the cubic nearly ferroelectric perovskite KTaO$_3$. The calculated thermal conductivity of PbTiO$_3$ is much lower than that of KTaO$_3$ in accord with experiment. Analysis of the results shows that the reason for the low thermal conductivity of PbTiO$_3$ is the presence of low frequency optical phonons associated with the polar modes. These are less dispersive in PbTiO$_3$, leading to…
ASnO3 (A=Ba, Sr) are unique perovskite oxides in that they have superior electron conductivity despite their wide optical band gaps. Using first-principles band structure calculations, we show that the small electron effective masses, thus, good electron conductivity of ASnO3 can be attributed to the large size of Sn in this system that gives the conduction band edge with antibonding Sn and O s characters. Moreover, we show that ASnO3 can be easily doped by La with shallow LaA(+/0) donor level. Our results, therefore, explained why the perovskite BaSnO3, SrSnO3, and their alloys are promising …
Organometallic perovskite is one of the potential materials in the various optoelectronic research fields. This research work demonstrates the synthesis of CH3NH3PbI3-ZnS microrods via one-step spin coating for optoelectronic applications. Incorporation of ZnS in the perovskite material caused bandgap variation in the visible wavelength range.
The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs$_2$AgBiBr$_6$ , the prototypical compound in this class, and show that the lattice dynamics of Cs$_2$AgBiBr$_6$ is highly anharmonic, largely in regards to tilting of AgBr$_6$ and BiBr$_6$ octahedra. Using an energy and temperature dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal ph…
Lattice thermal conductivity ($κ_{lat}$) of MgSiO$_3$ postperovskite (MgPPv) under the Earth's lower mantle high pressure-temperature conditions is studied using the phonon quasiparticle approach by combing \textit{ab initio} molecular dynamics and lattice dynamics simulations. Phonon lifetimes are extracted from the phonon quasiparticle calculations, and the phonon group velocities are computed from the anharmonic phonon dispersions, which in principle capture full anharmonicity. It is found that throughout the lowermost mantle, including the D" region, $κ_{lat}$ of MgPPv is ~25% larger than …
The pursuit for an appropriate universal scaling factor to satisfy the time-temperature superposition principle for grain and grain boundary responses has been explored in the ac conductivity domain for polycrystalline double perovskite oxides A2HoRuO6 (AHR; A = Ba, Sr, Ca). The samples show different structural phases ranging from cubic to monoclinic with decreasing ionic radii. The degree of distortion in the materials is correlated to the strength of bonding through the bond valence sum (BVS). The conductivity spectra for all the samples obey the power law behaviour. The contribution of dif…
MASnI$_3$, an organometallic halide, has great potential in the field of lead-free perovskite solar cells. Ultraviolet photons have been shown to generate deep trapping electronic defects in mesoporous TiO$_2$-based perovskite, affecting its performance and stability. In this study, the structure, electronic properties, and optical properties of the cubic, tetragonal, and hexagonal phases of MASnI$_3$ were studied using first-principles calculations. The results indicate that the hexagonal phase of MASnI$_3$ possesses a larger indirect band gap and larger carrier effective mass along the \emph…
We report lattice thermal conductivities of MgO and MgSiO3 in the perovskite and post-perovskite structures at conditions of the Earth's lower mantle, obtained from equilibrium molecular dynamics simulations. Using an advanced ionic interaction potential, the full conductivity tensor was calculated by means of the Green-Kubo method, and the conductivity of MgSiO3 post-perovskite was found to be significantly anisotropic. The thermal conductivities of all three phases were parameterized as a function of density and temperature. Assuming a Fe-free lower-mantle composition with mole fractions xMg…
Oxide materials exhibit several novel structural, magnetic, and electronic properties. Their stability under ambient conditions, easy synthesis, and high transition temperatures provide such systems with an ideal ground for realizing topological properties and real-life technological applications. However, experimental evidence of topological states in oxide materials is rare. In this study, we have synthesized single crystals of oxide double perovskite Sr2FeMoO6 and revealed its topological nature by investigating its structural, magnetic, and electronic properties. We observed that the syste…
Understanding thermal transport at the micro- to nanoscale is crucially important for a wide range of technologies ranging from device thermal management and protection systems to thermal-energy regulation and harvesting. In the past decades, non-contact optical methods such as time-domain and frequency-domain thermoreflectance (TDTR and FDTR) have emerged as extremely powerful and versatile thermal metrological techniques for the measurement of material thermal conductivities. Here, we report the measurement of thermal conductivity of thin films of CH3NH3PbI3 (MAPbI3), a prototypical metal-ha…
Confinement-induced Ultrafast Conductivity in 2D Perovskites resolved by correlative Terahertz-NIR Spectroscopy
University of Bayreuth（拜罗伊特大学）；Ulm University（乌尔姆大学）；Bayreuth Institute of Macromolecular Research (BIMF)（拜罗伊特大分子研究所）；Bavarian Polymer Institute (BPI)（巴伐利亚聚合物研究所）；The University of Melbourne（墨尔本大学）
Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO), is the first member of a new class of organometallic hybrids which adopts the structural pattern and physical properties of classical perovskites in two dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be tailored by intercalation of organic donor molecules, such as tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF), and by the inorganic acceptor SbF$_3$. Integration of donor molecules leads to a more insulating behavior of poly-MTO, whereas SbF$_3$ insertion does…
We propose to insert TiO layers to perovskite ATiO$_{3}$ to form a superlattice and use first-principles calculations to investigate its basic properties. Our computational analysis shows that the structure, which consists of repeated ATiO_{3} and TiO layers, has strong anisotropic conductivity. The structure immediately suggests a possible control of its conductivity by ion displacements related to its intrinsic ferroelectricity. In addition, we have obtained the structural information of its low-energy phases with the aid of phonon calculation and examined their evolution with epitaxial stra…
The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitised and organic photovoltaics. High power conversion efficiency can be realised in both mesoporous and thin-film device architectures. We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses)…
Using finite-temperature phonon calculations and machine-learning methods, we calculate the mechanical stability of about 400 semiconducting oxides and fluorides with cubic perovskite structures at 0 K, 300 K and 1000 K. We find 92 mechanically stable compounds at high temperatures -- including 36 not mentioned in the literature so far -- for which we calculate the thermal conductivity. We demonstrate that the thermal conductivity is generally smaller in fluorides than in oxides, largely due to a lower ionic charge, and describe simple structural descriptors that are correlated with its magnit…
We predict high thermoelectric efficiency in the layered perovskite La$_2$Ti$_2$O$_7$, based on calculations (mostly ab-initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit $ZT$ computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around 10$^{20}$ cm$^{-3}$. The Seebeck thermopower coefficient is between 200 and 300 $μ$V/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the …
We present a thorough dielectric investigation of the hexagonal perovskites La1.2Sr2.7IrO7.33 and La1.2Sr2.7RuO7.33 in a broad frequency and temperature range, supplemented by additional infrared measurements. The occurrence of giant dielectric constants up to 10^5 is revealed to be due to electrode polarization. Aside of dc and ac conductivity contributions, we detect two intrinsic relaxation processes that can be ascribed to ionic hopping between different off-center positions. In both materials we find evidence for charge transport via hopping of localized charge carriers. In the infrared r…
Germanium-based oxides such as rutile GeO2 are garnering attention owing to their wide band gaps and the prospects for ambipolar doping for application in high-power devices. Here, we present the use of germanium tetraisopropoxide (GTIP) (an organometallic chemical precursor) as a source of Ge for the demonstration of hybrid molecular beam epitaxy (MBE) for Ge-containing compounds. We use Sn1-xGexO2 and SrSn1-xGexO3 as model systems to demonstrate this new synthesis method. A combination of high-resolution X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron sp…
Perovskites are mixed oxides with the general formula ABO3. Many different compositions have been found to be possible and their number is growing rapidly due to their characteristics that make them ideal for various applications, such as fabrication of cathodes for solid oxide fuel cells (SOFCs). Oxide powders of LaxSr1-xCo1-y-zFeyMnzO3-δ (x=0.4, 0.6, 0.8, y=0.4, 0.8 and z=0.1, 0.2), named LSCFM, were prepared by mixing commercial powders. The powders have been investigated as a function of the stoichiometry and the crystal structure parameters have been determined using Rietveld analysis. Th…
We have prepared polycrystalline samples of (Sr$_{1-x}$La$_x$)$_2$ErRuO$_6$ and (Sr$_{1-x}$La$_x$)$_2$YRuO$_6$, and have measured the resistivity, Seebeck coefficient, thermal conductivity, susceptibility and x-ray absorption in order to evaluate the electronic states and thermoelectric properties of the doped double-perovskite ruthenates. We have observed a large Seebeck coefficient of -160 $μ$V/K and a low thermal conductivity of 7 mW/cmK for $x$=0.1 at 800 K in air. These two values are suitable for efficient oxide thermoelectrics, although the resistivity is still as high as 1 $Ω$cm. From …
The intrinsic d.c. electrical resistivity ($ρ$) - measurable on single crystals only - is often the quantity first revealing the properties of a given material. In the case of CH$_3$NH$_3$PbI$_3$ perovskite measuring $ρ$ under white light illumination provides insight into the coexistence of extended and shallow localized states (0.1 eV below the conduction band). The former ones dominate the electrical conduction while the latter, coming from neutral defects, serve as a long-lifetime charge carrier reservoir accessible for charge transport by thermal excitation. Remarkably, in the best crysta…
A single crystal monolith of La0.9Sr0.1FeO3 and thin pulsed laser deposited film of La0.8Sr0.2Fe0.8Ni0.2O3 were subject to angle integrated valence band photoemission spectroscopy in ultra high vacuum and conductivity experiments in ambient air at temperatures from 300 K to 800 K. Except for several sputtering and annealing cycles, the specimen were not prepared in-situ.. Peculiar changes in the temperature dependent, bulk representative conductivity profile as a result of reversible phase transitions, and irreversible chemical changes are semi-quantitatively reflected by the intensity variati…
Thermal conductivity ($κ$) of mantle minerals is a fundamental property in geodynamic modeling. It controls the style of mantle convection and the time scale of the mantle and core cooling. Cubic CaSiO$_3$ perovskite (CaPv) is the third most abundant mineral in the lower mantle (LM) (7 vol%). However, despite its importance, no theoretical or experimental estimate of CaPv's $κ$ exists. Theoretical investigations of its properties are challenging because of its strong anharmonicity. Experimental measurements at relevant high pressures and temperatures are equally challenging. Here we present $a…
Films of (FA$_{0.79}$MA$_{0.16}$Cs$_{0.05}$)$_{0.97}$Pb(I$_{0.84}$Br$_{0.16}$)$_{2.97}$ were grown over TiO$_{2}$, SnO$_{2}$, ITO, and NiO. Film conductivity was interrogated by measuring the in-phase and out-of-phase forces acting between the film and a charged microcantilever. We followed the films' conductivity vs. time, frequency, light intensity, and temperature (233 to 312 K). Perovskite conductivity was high and light-independent over ITO and NiO. Over TiO$_{2}$ and SnO$_{2}$, the conductivity was low in the dark, increased with light intensity, and persisted for 10's of seconds after t…
In this communication we suggest a formulation of the optical conductivity as a convolution of an energy resolved joint density of states and an energy-frequency labelled transition rate. Our final aim is to develop a scheme based on the augmented space recursion for random systems. In order to gain confidence in our formulation, we apply the formulation to three alkaline earth titanates CaTiO_3, SrTiO_3 and BaTiO_3 and compare our results with available data on optical properties of these systems.
Exploring the microstructural origins of conductivity and hysteresis in metal halide perovskites via active learning driven automated scanning probe microscopy
We report measurements of the heat capacity and cross-plane thermal conductivity of 2D (CxH2x+1NH3)2[MAPbBr3]n-1PbBr4 (MA = methylammonium) lead bromide perovskites (2D LHPs) at room temperature as a function of both the octahedral layer thickness (n = 1,2,3) and the organic spacer chain length (x=4,5,6,7,8) using differential scanning calorimetry (DSC) and frequency domain thermoreflectance (FDTR) respectively. We observe ultralow thermal conductivities (0.18-0.51 W/m K) for all 2D LHPs studied, but surprisingly minimal suppression of thermal conductivity with respect to bulk MAPbBr3 (0.5 W/m…
Halide perovskites show unusual thermalisation kinetics for above bandgap photo-excitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The cross-over between low-fluence and high-fluence `phonon bottleneck' cooling is due to a Mott transition where the polarons overlap ($n \ge 10^{18}/\mathrm{cm}^3$) and the phonon sub-populations are shared. We calculate the initial rate of cooling (thermalisation) from the scattering time in the Fröhlich polaron model to be 78 meVps$^{-1}$ for $\mathrm{CH}_3\mathrm{NH}_3\mathrm{PbI}_3$. This rapid in…
Perovskite compounds have the potential to harvest solar energy as well as exploit the thermoelectric potential of a number of available materials. Here, we present the electronic, structural, thermoelectric, and optical properties of Cs2AgSbX6 (X = Cl, Br, I) perovskite with the help of the density functional theory (DFT). The WC-GGA approximation was used to calculate the structural parameters. All these compounds crystalize in a cubic unit cell with lattice constant increasing from 10.65 Å (Cl) to 11.14 Å (Br) to 11.86 Å (I). The mBJ-functional shows a semiconducting nature for these compou…
Low-temperature transport in high-conductivity correlated metals: a density-functional plus dynamical mean-field study of cubic perovskites
Center for Computational Quantum Physics, Flatiron Institute（平顿研究所计算量子物理中心）；Department of Physics and Astronomy, Stony Brook University（石溪大学物理与天文学系）；Institute for Theoretical Physics, University of Cologne（科隆大学理论物理研究所）；Department of Physics, Massachusetts Institute of Technology（麻省理工学院物理系）；Center for Computational Mathematics, Flatiron Institute（平顿研究所计算数学中心）；Collège de France（法兰西公学院）
Much recent attention has been devoted towards unravelling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites (HOP). Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr$_3$, FAPbBr$_3$, MAPbI$_3$ and $α$-FAPbI$_3$ (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants caracterized by a very soft shear modulus C$_{44}$. Further, a tendency towards an incipient ferroelastic transition is obs…
The hybrid halide perovskites, the very performant compounds in photovoltaic applications, possess large Seebeck coefficient and low thermal conductivity making them potentially interesting high figure of merit ($ZT$) materials. For this purpose one needs to tune the electrical conductivity of these semiconductors to higher values. We have studied the CH$_3$NH$_3$MI$_3$ (M=Pb,Sn) samples in pristine form showing very low $ZT$ values for both materials; however, photoinduced doping (in M=Pb) and chemical doping (in M=Sn) indicate that, by further doping optimization, $ZT$ can be enhanced toward…
The empty perovskite ScF3 exhibits negative thermal expansion up to 1100 K. We demonstrate that ab-initio calculations of temperature-dependent effective phonon spectra allow to quantitatively describe the behavior of this compound and the suppression of negative thermal expansion. Based on this result, we predict an anomalous temperature dependence of the thermal conductivity and interpret it as a general feature of the perovskite class. Finally, we comment on the fact that the suppression of negative thermal expansion at such a high temperature is beyond the reach of the quasi-harmonic appro…
We do a search for topological insulators which are associated with ferromagnetic ordering and show anomalous quantum Hall effect, among transition metal oxide superlattices taking the parent compounds as LaAlO3 and SrTiO3. Among the various superlattices which are studied here, (LaAlO3)10/(LaOsO3)2 exhibits a ferromagnetic ground state with a topologically non-trivial energy gap when a spin-orbit interaction is turned on. The study of transverse conductivity shows that the system has quantized Hall conductivity inside the topological energy gap without applying any external magnetic field. Th…
The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI3 thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 microsecond and 10 microseconds correspond to the lifetimes of electrons and holes in FACsPbI3, respectively. Strikingly, the diffusion mapping indicates that the diff…
