Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI2Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all‐inorganic perovskites can represent a promising next step for photovoltaic materials.

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

Hybrid metal-halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)-phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the "encapsulated" MA lead-halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cmVs) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 μm for intermediate PEA content (50%).

The crystal structures of 6H-type BaMn 0.15 Ti 0.85 O 3 , BaMn 1/4 Ti 3/4 O 2.95 , and BaMn 1/2 Ti 1/2 O 2.84 and 12Rtype BaMn 2/3 Ti 1/3 O 3 have been established by a combination of X-ray, neutron, and electron diffraction, and high-resolution electron microscopy. The 6H-type structure (space group P6 3 /mmc) can be described by a stacking sequence (hcc) 2 along the c-axis with any anion deficiency located exclusively in the h-BaO 3 layers. Ti atoms display a strong preference for the corner-shared octahedral site, whereas both Mn and Ti are distributed over the octahedral sites in the face sharing dimers. The 12R-type structure (space group R3 j m) can be described by a stacking sequence (hhcc) 3 . Ti atoms again display a strong preference for the isolated corner-sharing octahedral site, whereas Mn atoms occupy the central site of the facesharing trimers. The electrical properties have been characterized by impedance spectroscopy and reveal the fully oxidized compounds to be electrical insulators with relative permittivity values of ∼45-55 at 300 K. The oxygen-deficient compounds are semiconductors, which is attributed to the presence of mixed Mn 3+ and Mn 4+ ions on the B-site sublattice. Antiferromagnetic (AFM) interactions occur within the face-sharing units of the respective structures. The AFM interactions inside the dimers (6H-type) and trimers (12R-type) become stronger with increasing Mn content and result in an increase in the magnitude of the Curie-Weiss constant.