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

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...

Hybrid main group halide perovskites hold great technological promise in optoelectronic applications and present rich and complex evolution of structure and dynamics. Here we present low-temperature dielectric measurements and calorimetry of APbI3 [A = CH3NH3 +, HC­(NH2)2 +] that suggest glassy behavior on cooling. In both compounds, the dielectric loss displays frequency-dependent peaks below 100 K characteristic of a glassy slowing of relaxation dynamics, with HC­(NH2)2PbI3 exhibiting greater glass fragility. Consistent with quenched disorder, the low-temperature heat capacity of both perovskites deviates substantially from the ∼T 3 acoustic phonon contribution predicted by the Debye model. We suggest that static disorder of the A-site molecular cation, potentially coupled to local distortions of the Pb–I sublattice, is responsible for these phenomena. The distinct low-temperature dynamics observed in these two perovskites suggest qualitative differences in the interaction between the molecular cation and the surrounding inorganic framework, with potential implications for defect screening and device performance at ambient temperatures.