Perovskite light-emitting diodes (LEDs) have attracted broad attention due to their rapidly increasing external quantum efficiencies (EQEs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . However, most high EQEs of perovskite LEDs are reported at low current densities (< 1 mA cm -2 ) and low brightness. Decrease in efficiency and rapid degradation at high brightness inhibit their practical applications. Here,
The structural coloration of arthropods often arises from helicoidal structures made primarily of chitin. Although it is possible to achieve analogous helicoidal architectures by exploiting the self‐assembly of chitin nanocrystals (ChNCs), to date no evidence of structural coloration has been reported from such structures. Previous studies are identified to have been constrained by both the experimental inability to access sub‐micrometer helicoidal pitches and the intrinsically low birefringence of crystalline chitin. To expand the range of accessible pitches, here, ChNCs are isolated from two phylogenetically distinct sources of α‐chitin, namely fungi and shrimp, while to increase the birefringence, an in situ alkaline treatment is performed, increasing the intensity of the reflected color by nearly two orders of magnitude. By combining this treatment with precise control over ChNC suspension formulation, structurally colored chitin‐based films are demonstrated with reflection tunable from blue to near infrared.
Hybrid‐perovskite‐based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF4−, introduced via methylammonium tetrafluoroborate (MABF4) in a surface treatment for MAPbI3 perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non‐radiative charge‐carrier recombination with a reduction in first‐order recombination rate from 6.5 × 106 to 2.5 × 105 s−1 in BF4−‐treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). 19F, 11B, and 14N solid‐state NMR is used to elucidate the atomic‐level mechanism of the BF4− additive‐induced improvements, revealing that the BF4− acts as a scavenger of excess MAI by forming MAI–MABF4 cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non‐radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF4−.
The interaction of high‐energy electrons and X‐ray photons with beam‐sensitive semiconductors such as halide perovskites is essential for the characterization and understanding of these optoelectronic materials. Using nanoprobe diffraction techniques, which can investigate physical properties on the nanoscale, studies of the interaction of electron and X‐ray radiation with state‐of‐the‐art (FA0.79MA0.16Cs0.05)Pb(I0.83Br0.17)3 hybrid halide perovskite films (FA, formamidinium; MA, methylammonium) are performed, tracking the changes in the local crystal structure as a function of fluence using scanning electron diffraction and synchrotron nano X‐ray diffraction techniques. Perovskite grains are identified, from which additional reflections, corresponding to PbBr2, appear as a crystalline degradation phase after fluences of 200 e− Å−2. These changes are concomitant with the formation of small PbI2 crystallites at the adjacent high‐angle grain boundaries, with the formation of pinholes, and with a phase transition from tetragonal to cubic. A similar degradation pathway is caused by photon irradiation in nano‐X‐ray diffraction, suggesting common underlying mechanisms. This approach explores the radiation limits of these materials and provides a description of the degradation pathways on the nanoscale. Addressing high‐angle grain boundaries will be critical for the further improvement of halide polycrystalline film stability, especially for applications vulnerable to high‐energy radiation such as space photovoltaics.
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