Metal-halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for wide variety of compounds, the physical origins of these changes, namely the variations in valence band and conduction band positions, are not well characterized. Knowledge of these band positions is of importance for optimizing the energy level alignment with charge extraction layers in optoelectronic devices. Here, we determine ionization energy and electron affinity values of all primary tin and lead based perovskites using photoelectron spectroscopy data, supported by first-principles calculations. Through analysis of the chemical bonding, we characterize the key energy levels and elucidate their trends via a tight-binding analysis. We demonstrate that energy level variations in perovskites are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions. Secondary changes in the perovskite energy levels result from the cation-anion interaction strength, which depends on the volume and structural distortions of the perovskite lattices. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.
We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred …
Despite their puzzling vibrational characteristics that include strong signatures of anharmonicity and thermal disorder already around room temperature, halide perovskites exhibit favorable optoelectronic properties for applications in photovoltaics and beyond. Whether these vibrational properties are advantageous or detrimental to their optoelectronic properties remains, however, an important open question. Here, this issue is addressed by investigation of the {finite-temperature optoelectronic properties} in the prototypical cubic CsPbBr$_3$, using first-principles molecular dynamics based o…
Metal halide perovskite semiconductors have outstanding optoelectronic properties. Although these perovskites are defect-tolerant electronically, defects hamper their long-term stability and cause degradation. Density functional theory (DFT) calculations are an important tool to unravel the microscopic structures of defects, but results suffer from the different approximations used in the DFT functionals. In the case of metal halide perovskites, qualitatively different results have been reported with different functionals, either predicting vacancy or interstitial point defects to be most domi…
Naturals and synthetic perovskites are widely used functional materials thanks to their particular physical properties, such as superconductivity, ferroelectricity and photo-activity. Many of these properties are related to static or dynamic motion of octahedral units; Yet a full understanding of the relationships between perovskite crystal structure, chemical bonding and physical properties is currently missing. Several studies indicate the existence of dynamic disorder generated by anharmonic motion of octahedral units e.g. in halide perovskite structures. In this work, we derive all possibl…
A method for improving the efficiency of solar cells is combining a low-bandgap semiconductor with a singlet fission material (which converts one high energy singlet into two low energy triplets following photoexcitation). Here we present a study of the interface between singlet fission molecules and low-bandgap halide pervoskites. We briefly show a range of experiments screening for triplet transfer into a halide perovskite. However, in all cases triplet transfer was not observed. This motivated us to understand the halide perovskite/singlet fission interface better by carrying out first-prin…
Favorable optoelectronic properties and ease of fabrication make NiO a promising hole transport layer for perovskite solar cells. To achieve maximum efficiency, the electronic levels of NiO need to be optimally aligned with those of the perovskite absorber. Applying surface modifiers by adsorbing species on the NiO surface, is one of the most widespread strategies to tune its energy levels. Alkali halides are simple inorganic surface modifiers that have been extensively used in organic optoelectronics, however, rarely studied in perovskite solar cells. Using density functional theory (DFT) cal…
Organic-inorganic halide perovskite solar cells have attracted much attention due to their low-cost fabrication, flexibility, and high-power conversion efficiency. More recent efforts show that the reduction from three- to two-dimensions (2D) of organic-inorganic halide perovskites promises an exciting opportunity to tune their electronic properties. Here, we explore the effect of reduced dimensionality and heterostructure engineering on the intrinsic material properties, such as energy stability, bandgap and transport properties of 2D hybrid organic-inorganic halide perovskites using first-pr…
The centrosymmetric halide perovskites undergo a continuous phase transition from a normal insulator to a topological insulator at the critical value of strain. Contrarily, in noncentrosymmetric halide perovskites, this phase transition is discontinuous. The noncentrosymmetry does not stabilize the gapless state, causing a discontinuity in the bandgap. We have employed the density functional theory and Slater-Koster formalism-based tight-binding Hamiltonian studies to understand the evolution of band topology under the compressive strain in the halide perovskites. Our study shows that both cub…
Halide perovskites make efficient solar cells due to their exceptional optoelectronic properties, but suffer from several stability issues. The characterization of the degradation processes is challenging because of the limitations in the spatio-temporal resolution in experiments and the absence of efficient computational methods to study the reactive processes. Here, we present the first effort in developing reactive force fields for large scale molecular dynamics simulations of the phase instability and the defect-induced degradation reactions in inorganic CsPbI$_{3}$. We find that the phase…
Photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1-xBrx)3 compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. Evolution of light absorption and the corresponding structural modifications in the films were recorded by UV-vis spectroscopy and X-ray diffraction (XRD), respectively. MAPbBr3 film exhibited no degradation. In MAPbI3 and mixed halide MAPb(I1-xBrx)3 films, decomposition of the perovskite material with crystallization of inorganic PbI2, and the corresponding degradation of light absorpti…
Metal halide perovskites represent a flourishing area of research, driven by both their potential application in photo-voltaics and optoelectronics, and for the fundamental science underpinning their unique optoelectronic properties. The advent of colloidal methods for the synthesis of halide perovskite nanocrystals has brought to the attention inter-esting aspects of this new type of materials, above all their defect-tolerance. This review aims to provide an updated survey of this fast-moving field, with a main focus on their colloidal synthesis. We examine the chemistry and the ca-pability o…
Hybrid halide perovskite semiconductors exhibit complex, dynamical disorder while also harboring properties ideal for optoelectronic applications that include photovoltaics. However, these materials are structurally and compositionally distinct from "traditional" compound semiconductors composed of tetrahedrally-coordinated elements with an average valence electron count of silicon. As discussed here, the additional dynamic degrees of freedom of hybrid halide perovskites underlie many of their potentially transformative physical properties. Neutron scattering and spectroscopy studies of the at…
Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport
卤化铅钙钛矿是一种很有前途的光电子材料。最近的报道表明，它们的力学和电子性能受到晶格振动的强烈影响。本文报道了CH $_{3}$ NH $_{3}$ Pb（I/Br/Cl）$_{3}$薄膜和单晶在室温下的远红外光谱测量和光谱的详细定量分析。我们发现所有三种卤化物钙钛矿的晶格振动的强加宽和非谐性，这表明在室温下的铅卤化物笼的动态无序。我们确定的横向和纵向光学声子的频率，并使用它们来计算，通过适当的模型，静态介电常数，极化子质量，电子-声子耦合常数，和上限的声子散射有限的载流子迁移率。在所用模型的限制内，我们可以将MAPbI $_{3}$单晶中室温载流子迁移率的上限设为200 $\,$ cm $^{2}$ V $^{-1}$ s $^{-1}$。我们的研究结果对于理解金属卤化物钙钛矿中的电荷传输过程和机械性能具有重要意义。
Reliable chemical state analysis of Sn semiconductors by XPS is hindered by the marginal observed shift in the Sn 3d region. For hybrid Sn-based perovskites especially, errors associated with charge referencing can easily exceed chemistry-related shifts. Studies based on the modified Auger parameter $α'$ provide a suitable alternative and have been used previously to resolve different chemical states in Sn alloys and oxides. However, the meaningful interpretation of Auger parameter variations on Sn-based perovskite semiconductors requires fundamental studies. In this work, we perform a compreh…
Halide perovskites are perceived to be the promising class of materials for optoelectronics, spinorbitronics and topological electronics due to presence of strong spin-orbit coupling and polarized field. Here, we develop a Hamiltonian using quasi-degenerate perturbation theory to describe polarized field driven band structures and unique topological phases such as Dirac semimetallic rings for the universal class of Halide perovskites including both inorganic and hybrid members. The spin textures obtained through this theoretical model captures minute effects of polarization and hence can be us…
