Defect passivation has been demonstrated to be effective in improving the radiative recombination of charge carriers in perovskites, and consequently, the device performance of the resultant perovskite light‐emitting diodes (LEDs). State‐of‐the‐art useful passivation agents in perovskite LEDs are mostly organic chelating molecules that, however, simultaneously sacrifice the charge‐transport properties and thermal stability of the resultant perovskite emissive layers, thereby deteriorating performance, and especially the operational stability of the devices. We demonstrate that lithium halides can efficiently passivate the defects generated by halide vacancies and reduce trap state density, thereby suppressing ion migration in perovskite films. Efficient green perovskite LEDs based on all‐inorganic CsPbBr3 perovskite with a peak external quantum efficiency of 16.2 %, as well as a high maximum brightness of 50 270 cd m−2, are achieved. Moreover, the device shows decent stability even under a brightness of 104 cd m−2. We highlight the universal applicability of defect passivation using lithium halides, which enabled us to improve the efficiency of blue and red perovskite LEDs.

The formation mechanism of perovskite methylammonium lead triiodide (CH3NH3PbI3) was studied with in situ X-ray photoelectron spectroscopy (XPS) on successive depositions of thermally evaporated methylammonium iodide (CH3NH3I) on a lead iodide (PbI2) film. This deposition method mimics the “two-step” synthesis method commonly used in device fabrication. We find that several competing processes occur during the formation of perovskite CH3NH3PbI3. Our most important finding is that during vapour deposition of CH3NH3I onto PbI2, at least two carbon species are present in the resulting material, while only one nitrogen species is present. This suggests that CH3NH3I can dissociate during the transition to a perovskite phase, and some of the resulting molecules can be incorporated into the perovskite. The effect of partial CH3NH3 substitution with CH3 was evaluated, and electronic structure calculations show that CH3 defects would impact the photovoltaic performance in perovskite solar cells. The possibility that not all A sites in the APbI3 perovskite are occupied by CH3NH3 is therefore an important consideration when evaluating the performance of organometallic trihalide solar cells synthesized using typical approaches.

Two-dimensional (2D) lead halide perovskites with long-chain ammonium halides display high photoluminescence quantum yields (PLQYs), because of their size and dielectric confinement, which hold promise for a high-efficiency and low-cost light-emitting diode (LED). However, the presence of an insulating organic long-chain spacer cation (L) dramatically deteriorates the charge transport properties along the out-of-plane nanoplatelet direction or adjacent nanocrystals, which would limit the device performance of the LED. To overcome this issue, we successfully incorporate small alkaline ions such as sodium (Na+) to replace the long organic molecule. Grazing incidence X-ray diffraction measurements verify 2D layer formation with a preferred crystallite orientation. In addition, the incorporated sodium salt also generates amorphous sodium lead bromide (NaPbBr3) in perovskite as spacers to form a nanocrystal-like halide perovskite film. The PLQY is dramatically improved in the sodium-incorporated film because of its enhanced photoluminescence lifetime. Upon incorporation of a low concentration of an organic additive, this two-dimensional–three-dimensional (2D–3D) perovskite can achieve a compact and uniform film. Therefore, a 2D–3D perovskite achieves a high external quantum efficiency of 15.9% with good operational stability. We develop a type of 2D–3D halide perovskite with various inorganic ions as spacers for promising high-performance optoelectronic devices.

BaFe 2 As 2 exhibits properties characteristic of the parent compounds of the newly discovered iron (Fe)-based high-T C superconductors. By combining the real space imaging of scanning tun-neling microscopy/spectroscopy (STM/S) with momentum space quantitative Low Energy Electron Diffraction (LEED) we have identified the surface plane of cleaved BaFe 2 As 2 crystals as the As terminated Fe-As layer-the plane where superconductivity occurs. LEED and STM/S data on the BaFe 2 As 2 (001) surface indicate an ordered arsenic (As)-terminated metallic surface without reconstruction or lattice distortion. It is surprising that the STM images the different Fe-As orbitals associated with the orthorhombic structure, not the As atoms in the surface plane.

The electronic structure in alkaline earth AeO (Ae = Be, Mg, Ca, Sr, Ba) and post-transition metal oxides MeO (Me = Zn, Cd, Hg) is probed with oxygen K -edge X-ray absorption and emission spectroscopy. The experimental data is compared with density functional theory electronic structure calculations. We use our experimental spectra of the oxygen K -edge to estimate the bandgaps of these materials, and compare our results to the range of values available in the literature. From the calculated partial DOS we conclude that the position of main O K -edge X-ray emission feature in BeO, SrO and BaO is defined by the position of the np-states of the cation while in the other oxides studied here the main O K -edge X-ray emission feature is defined by the position of the (n-1)d (for CaO) or nd-states of the cation.