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.

Ge and Sn in (GeTe)nSb2Te3 (GST) and (SnTe)nSb2Te3 compounds, respectively, are partially substituted by Cd. These layered compounds (n = 1) consist of NaCl‐type slabs that are separated by van der Waals gaps. Resonant X‐ray diffraction yields different Cd‐atom distributions in Cd0.2Ge0.8Sb2Te4 and Cd0.2Sn0.8Sb2Te4, which indicate how van der Waals gaps in metastable cubic GeTe‐rich or SnTe‐rich compounds (n ≥ 7) are surrounded. The latter exhibit promising thermoelectric properties with figures of merit ZT ≈ 0.8 at 450 °C. Alloying with Sb2Te3 increases the ZT value of SnTe by a factor of up to two because of an increased Seebeck coefficient and a reduced thermal conductivity; the hole concentration is significantly reduced. A further improvement in ZT by ≈20% is possible by Cd‐doping, most likely due to a more covalent bonding character, yielding a ZT of ≈1.1 at 450 °C for Cd1.2Sn10.8Sb2Te15. In order to understand the electronic transport properties of these p‐type semiconductors, the single parabolic band model is applied. By contrast, Cd doping in GST materials reduces the Hall mobility and therefore decreases the power factor compared to undoped GST materials.

Pseudobinary phases (SnSe) BiSe 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, SnBiSe ( x = 4) in the NaCl structure type and SnBiSe ( x = 0.5) in the layered defect GeSbTe 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 SnBiSe, L4,5-type SnBiSe, L4,7-type SnBiSe, and L7,7-type SnBiSe). 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 SnBiSe to 0.2 for layered SnBiSe; this is similar to that of the related compounds SnBiTe or PbBiTe. 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-BiSe 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) BiSe is a multipurpose model system with vast possibilities for substitutional and structural modification aiming at the optimization of thermoelectric or other properties.

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.