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Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

Lead-halide perovskites are promising materials for opto-electronic applications. Recent reports indicated that their mechanical and electronic properties are strongly affected by the lattice vibrations. Herein we report far-infrared spectroscopy measurements of CH3NH3Pb(I/Br/Cl)3 thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine the frequencies of the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, electron-phonon coupling constants, and upper limits for the phonon-scattering limited charge carrier mobilities. Our findings place an upper limit in the range of 200 cm 2 V −1 s −1 for the room temperature charge carrier mobility in MAPbI3 single crystals, and are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

The notion that halide perovskite crystals (ABX , where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.

Using a representative model system, we describe here electronic and structural properties of aromatic self-assembled monolayers (SAMs) that contain an embedded, dipolar group. As polar unit we use pyrimidine, varying its orientation in the molecular backbone and, consequently, the direction of the embedded dipole moment. The electronic and structural properties of these embedded-dipole SAMs are thoroughly analyzed using a number of complementary characterization techniques combined with quantum-mechanical modeling.We show that such mid-chain substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the energy levels in the regions above and below the dipoles relative to one another. These SAMs also allow for tuning the substrate work function in a controlled manner independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface.

¶ These authors contributed equallyThe outstanding performance of hybrid organic-inorganic perovskites (HOIPs) in photovoltaic devices is made possible by, among other things, outstanding semiconducting properties: long real charge-carrier diffusion lengths, L, of up to 5 and possibly even 10 μm, as well as a lifetime, τ of ~1 μs or more in single crystal and polycrystalline films. [1][2][3][4][5][6][7][8][9] Top electronic transport materials will have a high "μτ" product, the product of the charge carrier mobility, μ, and lifetime. This is directly related to the diffusion length (L=√(Dτ), where D is the carrier diffusion coefficient given as D=(μq/k B T), where q is the electron charge, k B is the Boltzmann constant, and T is the absolute temperature. Long lifetimes, which imply slow recombination and low trapping probabilities, do not automatically imply high mobilities, which are limited by scattering.Charge carrier mobilities in HOIPs are often described as "high", but this statement warrants some scrutiny. HOIP mobilities are often compared to those of charge carriers in organic semiconductors (see Table I) and are then indeed much higher. 10 But in our opinion, carrier mobilities in HOIPs need to be placed in the context of typical inorganic semiconductors, for several reasons: First, HOIPs exhibit a band structure resembling that of a good inorganic semiconductor, with the conduction and valence band dominated by the inorganic cations and anions, respectively. [11][12][13][14] This leads to small computed charge-carrier effective masses 11,13,14 : a reduced effective mass on the order of 0.1 electron mass, close to that of Si (0.08) or GaAs (0.03) 15 , a value confirmed by magneto-absorption measurements. 16,17 Second, material disorder, which often lowers mobilities dramatically, is low: HOIPs exhibit sharp x-ray diffraction peaks, 18 small Urbach tail energy (~15 meV), 19,20 and low trap-state density (~10 10 cm -3 in single crystals). [5][6][7] Despite these material properties, it is clearly seen in Table 1 that mobilities in HOIPs are in fact rather modest 10 -at least one order of magnitude lower in electron mobility (and at least several times lower in hole mobility) than those of Si, GaAs, and some other inorganic PV materials.What could underlie these rather modest mobility values? The mobility is proportional to the carrier scattering time and inversely proportional to the effective mass. 15 If effective mass values of HOIPs are indeed on par with those of other inorganic semiconductors, then the mobility must be limited by scattering. Then again, the observation of long carrier lifetimes and an inverse power dependence of mobility on temperature (see below) suggests negligible impurity scattering at RT, 15 a fact which is consistent with the observation of slow carrier cooling at room temperature. 21 Therefore, we posit instead that an important hint for the origin of this phenomenon lies in the mechanical and vibrational properties of HOIPs. In particular, the bulk and Young's moduli of HOIP...