Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, especially given the significantly reduced toxicity of Bi when compared with Pb. The compounds A 3 Bi 2 I 9 (A = K, Rb, Cs) examined here crystallize in two different structure types: the layered defectperovskite K 3 Bi 2 I 9 type, and the Cs 3 Cr 2 Cl 9 type. The latter structure type features isolated Bi 2 I 9 3− anions. Here, the crystal structures of the ternary iodides are redetermined and a corrected structural model for Rb 3 Bi 2 I 9 , as established by single crystal X-ray diffraction and solid state 87 Rb NMR spectroscopy and supported by density functional theory (DFT) calculations is presented. A variety of facile preparation techniques for single crystals, bulk materials, as well as solution-processed thin films are described. The optical properties and electronic structures are investigated experimentally by optical absorption and ultraviolet photoemission spectroscopy and computationally by DFT calculations. Absolute band positions of the valence and conduction bands of these semiconductors, with excellent agreement of experimental and calculated values, are reported, constituting a useful input for the rational interface design of efficient electronic and optoelectronic devices. The different structural connectivity in the two different structure types, somewhat surprisingly, appears to not impact band positions or band gaps in a significant manner. Computed dielectric properties, including the finding of anomalously large Born effective charge tensors on Bi 3+ , suggest proximal structural instabilities arising from the Bi 3+ 6s 2 lone pair. These anomalous Born effective charges are promising for defect screening and effective charge carrier transport. The structural, electronic, and optical properties of the complex bismuth iodides are to some extent similar to the related lead iodide perovskites. The deeper valence band positions in the complex bismuth iodides point to the need for different choices of hole transport materials for Bi-iodide based solar cell architectures.

Stable s 2 lone pair electrons on heavy main-group elements in their lower oxidation states drive a range of important phenomena, such as the emergence of polar ground states in some ferroic materials. Here we study the perovskite halide CsSnBr 3 as an embodiment of the broader materials class. We show that lone pair stereochemical activity due to the Sn s 2 lone pair causes a crystallographically hidden, locally distorted state to appear upon warming, a phenomenon previously referred to as emphanisis.The synchrotron X-ray pair distribution function acquired between 300 K and 420 K reveals emerging asymmetry in the nearest-neighbor Sn-Br correlations, consistent with dynamic Sn 2+ off-centering, despite there being no evidence of any deviation from the average cubic structure. Computation based on density functional theory supports the finding of a lattice instability associated with dynamic off-centering of Sn 2+ in its coordination environment. Photoluminescence measurements reveal an unusual blueshift with increasing temperature, closely linked to the structural evolution. At low temperatures, the structures reflect the influence of octahedral rotation. A continuous transition from an orthorhombic structure (P nma, no. 62) to a tetragonal structure (P 4/mbm, no. 127) is found around 250 K, with a final, first-order transformation at 286 K to the cubic structure (P m3m, no. 221).2

The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI 3 ) and formamidinium lead iodide (FAPbI 3 ), has been an enigmatic subject of great interest. Beyond aiding in the ease of processing of thin films for photovoltaic devices, there have been suggestions that many of the remarkable properties of the halide perovskites can be attributed to the dipolar nature and the dynamic behavior of these cations. Here, we establish the dynamics of the molecular cations in FAPbI 3 between 4 K and 340 K and the nature of their interaction with the surrounding inorganic cage using a combination of solid state nuclear magnetic resonance and dielectric spectroscopies, neutron scattering, calorimetry, and ab initio calculations. Detailed comparisons of the reported temperature dependence of the dynamics of MAPbI 3 are then carried out which reveal the molecular ions in the two different compounds to exhibit very similar rotation rates (≈8 ps) at room temperature, despite differences in other temperature regimes.For FA, rotation about the N···N axis, which reorients the molecular dipole, is the dominant motion in all phases, with an activation barrier of ≈21 meV in the ambient phase, compared to ≈110 meV for the analogous dipole reorientation of MA. Geometrical frustration of the molecule-cage interaction in FAPbI 3 produces a disordered γ-phase and subsequent glassy freezing at yet lower temperatures. Hydrogen bonds suggested by atom-atom distances from neutron total scattering experiments imply a substantial role for the molecules in directing structure and dictating properties. The temperature dependence of reorientation of the dipolar molecular cations systematically described here can clarify various hypotheses including large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.2

Abstract:The structure of the hybrid perovskite HC(NH2)2PbI3 (formamidinium lead iodide) reflects competing interactions associated with molecular motion, hydrogen bonding tendencies, thermally activated soft octahedral rotations, and the propensity for the Pb 2+ lone pair to express its stereochemistry. High-resolution synchrotron X-ray powder diffraction reveals a continuous transition from the cubic α-phase (Pm 3 m, #221) to a tetragonal β-phase (P4/mbm, #127) at around 285 K, followed by a first-order transition to a tetragonal γ-phase (retaining P4/mbm, #127) at 140 K. An unusual reentrant pseudosymmetry in the β-to-γ phase transition is seen that is also reflected in the photoluminescence. Around room temperature, the coefficient of volumetric thermal expansion is among the largest for any extended crystalline solid.Photovoltaic absorbers based on the hybrid perovskite HC(NH2)2PbI3 (formamidinium lead iodide) and its alloys exhibit impressive performance, [1] but the description of the crystal structure of this material is incomplete. In addition to this technological motivation, [2] dense hybrid materials with 3-D inorganic connectivity and isolated organic molecular ions combine features of traditional inorganic solids and open framework materials, and their composition-structure relations are of fundamental interest.The initial report of the preparation and characterization of HC(NH2)2PbI3 proposed perovskite structures of trigonal symmetry for the α-and β-phases on the basis of laboratory single crystal X-ray diffraction, [3] while a subsequent report assigned the structure of the cubic perovskite aristotype for the α-phase from neutron powder diffraction. [4] The structure of the γ-phase has not been reported. Figure 1. X-ray scattering intensity from HC(NH2)2PbI3 around selected lowangle Bragg peaks between 90 K and 490 K, normalized to maximum peak intensity. (a) The 211t tetragonal Bragg peak emerges upon cooling through a continuous phase transition around 285 K from the cubic α-phase to the tetragonal β-phase. (b) The 200c cubic Bragg peak splits continuously on cooling due to the emergent tetragonality. A first order transition to the pseudocubic γ-phase with tetragonal symmetry occurs at 140 K. [5] The 002t and 220t tetragonal peaks "fuse" across the β-γ transition while the 211t tetragonal peak remains.The disordered molecular cation, challenges associated with twinning in single crystals, and issues of pseudosymmetry led us to employ high-resolution synchrotron X-ray powder diffraction to follow the structure evolution of HC(NH2)2PbI3 between 90 K and 490 K. The temperature-dependent scattering intensity around instructive low-angle Bragg peaks is given in Figure 1. The continuous α-β phase transition around 285 K is evident in the emergence of the 211 tetragonal peak and the splitting of the 200 cubic peak into the 002 and 220 tetragonal peaks on cooling. The first-order β-γ transition can be seen in the abrupt change in intensities and peak positions at 140 K (transition temperature fro...

Rapid recent improvement in photovoltaic efficiency in hybrid lead halide perovskite materials has provided the impetus for understanding other, related main-group halide systems. Here, we show that the closely related but less toxic bismuth iodide BiI3 can show promising optoelectronic properties. Layered binary BiI3 is used here as the active layer in planar solar cell architectures (efficiency approximately 0.3%). Experimental and computational studies of absolute band positions of BiI3 are also presented, to help in the rational design of device architectures that would allow efficient charge transfer at the interfaces.

The temperature-dependent structure evolution of the hybrid halide perovskite compounds, formamidinium tin iodide (FASnI, FA = CH[NH]) and formamidinium lead bromide (FAPbBr), has been monitored using high-resolution synchrotron X-ray powder diffraction between 300 and 100 K. The data are consistent with a transition from cubic Pm3m (No. 221) to tetragonal P4/mbm (No. 127) for both materials upon cooling; this occurs for FAPbBr between 275 and 250 K, and for FASnI between 250 and 225 K. Upon further cooling, between 150 and 125 K, both materials undergo a transition to an orthorhombic Pnma (No. 62) structure. The transitions are confirmed by calorimetry and dielectric measurements. In the tetragonal regime, the coefficients of volumetric thermal expansion of FASnI and FAPbBr are among the highest recorded for any extended inorganic crystalline solid, reaching 219 ppm K for FASnI at 225 K. Atomic displacement parameters of all atoms for both materials suggest dynamic motion is occurring in the inorganic sublattice due to the flexibility of the inorganic network and dynamic lone pair stereochemical activity on the B-site. Unusual pseudocubic behavior is displayed in the tetragonal phase of the FAPbBr, similar to that previously observed in FAPbI.

We reveal here a key aspect of the inorganic framework of hybrid halide perovskites that potentially impacts the electronic, thermal, and dielectric properties.

ABSTRACT:While recent improvements in the reported peak power conversion efficiency (PCE) of hybrid organic-inorganic perovskite solar cells have been truly astonishing, there are many fundamental questions about the electronic behavior of these materials. Here we have studied a set of electronic devices employing methylammonium lead iodide ((MA)PbI 3 ) as the active material and conducted a series of temperature-dependent measurements. Field-effect transistor, capacitor and photovoltaic cell measurements all reveal behavior consistent with substantial and strongly temperature-dependent polarization susceptibility in (MA)PbI 3 at temporal and spatial scales that significantly impact functional behavior. The relative PCE of (MA)PbI 3 photovoltaic cells is observed to reduce drastically with decreasing temperature, suggesting that such polarization effects could be a prerequisite for high-performance device operation. The pace at which new materials and designs for solar cells emerge is very slow, [1][2][3][4][5][6] and is arguably comparable to the discovery of high T c superconductors. [7][8] The finding that hybrid organic metal halide solar cells based on CH 3 NH 3 PbI 3 can lead to high power conversion efficiency (PCE) using simple coating methods, in a material comprising earth-abundant elements has therefore garnered significant interest. 6, 9-10 Not only has the peak reported PCE exceeded 20% in a short time, [11][12][13] but the processing techniques widely employed suggest that commercial products could be fabricated using low-cost, large-area techniques, potentially compatible with flexible substrates. 14The ABX 3 perovskite crystal structure is characterized by three-dimensionally corner connected network of BX 6/2 octahedra that is filled by the A ions. Perovskite compounds where the A is an organic cation, B is usually a main group element, and X is a group 7 anion (halide) We found that the electrical characteristics of (MA)PbI 3 had a complex temperature dependence. At room temperature these devices exhibit low source-drain currents and no fieldinduced current modulation (see Figure 1(a)). However when the temperature is reduced below 220K, a field-effect is observed and the drain current continues to increase as the temperature is reduced. Where the drain current is observed to be substantially in excess of gate current the gradual-channel approximation 44 has been applied to extract a value of field-effect mobility. The gradual channel approximation for extraction of carrier mobility in field-effect devices assumes a single electronic charge carrier, uniform carrier accumulation in the channel, and timeindependent behavior and consequently does not provide a simple estimation of the carrier motion for measurements with non-ideal effects. This is consistent with our observation of absolute values of electron mobility substantially lower than those previously reported using other techniques. 16 Nonetheless it can be applied as a proxy for relative transistor performance, assuming a high transcondu...