Cornucopia of Structures in the Pseudobinary System (SnSe) xBi2Se3: A Crystal-Chemical Copycat.

Pseudobinary phases (SnSe) xBi2Se3 exhibit a very diverse structural chemistry characterized by different building blocks, all of which are cutouts of the NaCl type. For SnSe contents between x = 5 and x = 0.5, several new phases were discovered. Next to, for example, Sn4Bi2Se7 ( x = 4) in the NaCl structure type and SnBi4Se7 ( x = 0.5) in the layered defect GeSb2Te4 structure type, there are at least four compounds (0.8 ≤ x ≤ 3) with lillianite-like structures built up from distorted NaCl-type slabs (L4,4-type Sn2.22Bi2.52Se6, L4,5-type Sn9.52Bi10.96Se26, L4,7-type Sn11.49Bi12.39Se30, and L7,7-type Sn3.6Bi3.6Se9). For two of them (L4,7 and L7,7), the cation distributions were determined by resonant X-ray scattering, which also confirmed the presence of significant amounts of cation vacancies. Thermoelectric figures of merit ZT range from 0.04 for Sn4Bi2Se7 to 0.2 for layered SnBi4Se7; this is similar to that of the related compounds SnBi2Te4 or PbBi2Te4. Compounds of the lillianite series exhibit rather low thermal conductivities (∼0.75 W/mK for maximal ZT). More than other "sulfosalts", compounds in the pseudobinary system SnSe-Bi2Se3 adapt to changes in the cation-anion ratio by copying structure types of compounds containing lighter or heavier homologues of Sn, Bi, or Se and can incorporate significant amounts of vacancies. Thus, (SnSe) xBi2Se3 is a multipurpose model system with vast possibilities for substitutional and structural modification aiming at the optimization of thermoelectric or other properties.

Temperature-dependent Cu and Ag ion mobility and associated changes of transport properties in pavonite-type Cu1.4Ag0.4Bi5.4Se9.

Cu1.4Ag0.4Bi5.4Se9, the first quaternary compound in the system Cu/Ag/Bi/Se, was obtained from the elements by melt synthesis after pre-reaction and annealing steps. It exhibits a 4P pavonite-type crystal structure in the spacegroup C2/m, which consists of NaCl-type building blocks extending in two dimensions that are separated by rods of edge-sharing BiSe7 polyhedra and "clusters" of several partially occupied Cu atom sites. Temperature-dependent single crystal X-ray diffraction up to 350 °C helped to explain thermoelectric properties. Cu atoms occupy more and different disordered sites at ≥200 °C. Ag atoms share Bi atom sites and prefer different sites as a function of temperature. Cu1.4Ag0.4Bi5.4Se9 is a metallic n-type thermoelectric material with Seebeck coefficients up to -150 µV K-1. Lattice thermal conductivity significantly decreases from 0.55 W mK-1 to 0.42 W mK-1 with increasing structural disorder. Cu1.4Ag0.4Bi5.4Se9 may thus be described with the phonon-liquid electron-crystal (PLEC) concept and reaches a figure of merit of zTmax = 0.23 at 450 °C. B-factor analysis based on a single parabolic band model shows that the chemical potential of electrons approaches the one corresponding to optimal charge carrier concentration, especially at higher temperatures. With respect to most properties and the easily reproducible synthesis, the compound can well compete with other sulfosalt-like thermoelectric materials and is considerably less toxic than most of them.

Structure and thermoelectric properties of the silver lead bismuth selenides Ag5Pb9Bi19Se40 and AgPb3Bi7Se14.

In the system Ag/Pb/Bi/Se, two new thermoelectric phases derived from lillianite (Pb3Bi2S6) have been characterized. The crystal structures correspond to the 8,8L- and 5,5L-types, respectively, both in the space group Cmcm. The room-temperature unit-cell parameters of 8,8L-Ag5Pb9Bi19Se40 are a = 4.2151(8) Å, b = 13.951(3) Å and c = 35.284(7) Å and those of 5,5L-AgPb3Bi7Se14 are a = 4.2337(8) Å, b = 13.864(2) Å and c = 24.653(3) Å. The temperature-dependent evolution of the lattice parameters of Ag5Pb9Bi19Se40 becomes steeper at temperatures above 300 °C and hints at the mobility of Ag+ ions. Samples containing both phases exhibit thermoelectric figures of merit up to ZT = 0.23 at 250 °C. HRTEM investigations on such samples showed well-ordered areas of the lillianite-like phases separated by large slabs that exhibit a high concentration of defects and may compensate for lattice misfits between the lillianite-type phases.

Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide.

The development of medium-bandgap solar cell absorber materials is of interest for the design of devices such as tandem solar cells and building-integrated photovoltaics. The recently developed perovskite solar cells can be suitable candidates for these applications. At present, wide bandgap alkylammonium lead bromide perovskite absorbers require a high-temperature sintered mesoporous TiO2 photoanode in order to function efficiently, which makes them unsuitable for some of the above applications. Here, we present for the first time highly efficient wide bandgap planar heterojunction solar cells based on the structurally related formamidinium lead bromide. We show that this material exhibits much longer diffusion lengths of the photoexcited species than its methylammonium counterpart. This results in planar heterojunction solar cells exhibiting power conversion efficiencies approaching 7%. Hence, formamidinium lead bromide is a strong candidate as a wide bandgap absorber in perovskite solar cells.