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.

The crystal structure of BaMn 0.4 Fe 0.6 O 2.73 , a recently identified oxygen-deficient 10H-type hexagonal perovskite in the system BaMn 1-x Fe x O 3-y, has been established by a combination of electron microdiffraction and neutron diffraction. The structure (space group P6 3 /mmc, a ) 5.74435(5) Å, and c ) 24.0331(3) Å) can be described by a stacking sequence (hch′ch) 2 along the c-axis with the anion deficiency located exclusively in the h′-BaO 3 -type layers. The anion distribution in the h′-BaO 3 layers differs significantly from that observed for 10H BaFeO 2.8 and results in a 70:30 random distribution of corner-sharing tetrahedral Fe 2 O 7 dimers and face-sharing octahedral (Mn,Fe) 2 O 9 dimers as opposed to the exclusive Fe 2 O 7 dimers in 10H BaFeO 2.8 . The difference is attributed to the preference of Mn for octahedral coordination. The compound is a "leaky" insulator at room temperature with a permittivity of ∼20. The conduction mechanism has low activation energy, ∼0.3 eV, and is consistent with polaronic hopping associated with the Fe and/or Mn ions.

The complete structural characterization of the hexagonal cobaltite BaMn0.4Co0.6O2.83 has been performed by combining high-resolution transmission electron microscopy and electron and neutron diffraction. The structure is closely related to the 12H (cc‘chhh)2 with oxygen deficient cubic c‘-BaO2 layers. This framework forms a laminar structure in which tetramers of face-sharing octahedra occupied by manganese and cobalt are connected by corners to two tetrahedral layers mostly occupied by cobalt. A magnetic study shows only the presence of short-range magnetic interactions indicating that there are no interactions between the tetramers through the tetrahedral layers, thus impeding the three-dimensional magnetic ordering of the system. A spin-glass-like state was found to be compatible with the observed phenomenology.

A combination of X-ray, neutron and electron diffraction, and high-resolution electron microscopy have been used to establish the 5H structural type of a new hexagonal-type perovskite BaMn0.2Co0.8O2.80. The structure can be described as a (cc′chh) 5H hexagonal polytype with ordered oxygen vacancies where the cubic c′ layer corresponds to a composition of [BaO2] as opposed to [BaO3]. The resulting layer structure consists of [MnCo2O12] blocks of three sharing faces octahedra linked by corners to two unconnected [CoO4] tetrahedra. Electron Energy Switch order Loss Spectroscopy shows Mn to be present only as Mn(+IV) and therefore Co is present as mixed +III/+IV. Mn(+IV) and Co(+III) ions are distributed over the face sharing octahedral sites whereas Co(+IV) ions are located on the tetrahedral sites. The magnetic behavior is more complex than is observed for BaCoO2.80 (a ferromagnet with T c = 47 K) and can be described by a Stoner–Wohlfarth model of random-anisotropic, noninteracting monodomain ferromagnetic clusters. The ferromagnetic clusters occur below ∼35 K and are assigned to groups of Co ions in octahedral and/or tetrahedral sites; however, incorporation of Mn ions in the octahedral sites disrupts the transition into long-range three-dimensional ferromagnetic order. Impedance Spectroscopy data reveals semiconducting grain conductivity at room temperature (∼1 × 10−2 S cm−1); however, subambient data reveal an unusual temperature dependence with a smooth changeover from a thermally activated process (∼0.07 eV) in the range 40–300 K to a low-temperature state below 40 K with a near-zero activation energy. The data cannot be described by conventional Arrhenius or variable-range hopping conduction models and the conduction mechanism(s) remain unresolved. Several possible suggestions for the conductivity behavior are made, including Anderson localization, anisotropic conduction associated with the 5H crystal structure or some complex correlated mechanism between the magnetic and electronic transport properties. The electrical microstructure of BaMn0.2Co0.8O2.8 ceramics consist of semiconducting grains and constrictive grain boundaries and therefore exhibit internal barrier layer capacitor (IBLC) behavior, with a high and temperature-stable apparent permittivity of ∼10 000 (at 10 kHz) above 100 K.

The crystal and magnetic structures and electrical properties of two new Mn-rich 6H′-type hexagonal perovskites in the BaMn1−x Fe x O3−δ system have been investigated. Structural characterization performed by X-ray, electron, and neutron diffraction and high resolution electron microscopy indicates that both BaMn0.85Fe0.15O2.87 and BaMn0.6Fe0.4O2.72 crystallize in the 6H′ hexagonal polytype (P6̅m2 space group). The structure is formed by tetramers and dimers of face-sharing octahedra that are linked by corners. The anion deficiency is located at random through the hexagonal layers and increases with the Fe-content. In both phases, the central position of the tetramers is fully occupied by Mn, the remaining Mn and Fe cations being randomly distributed over different polyhedra. The Mössbauer spectroscopy data show Fe to be present only as FeIII in octahedral and tetrahedral coordination. The magnetic structure is formed by ferromagnetic sheets with the magnetic moments aligned along the x-axis and stacked antiferromagnetically perpendicular to the c-axis. The electrical properties have been characterized by impedance spectroscopy and reveal both compounds behave as leaky insulators at room temperature with bulk permittivity values <20.