We
report the synthesis of Bi-based lead-free halide perovskite
nanocrystals (NCs) via a ligand-assisted reprecipitation (LARP) method.
Detailed chemical analysis of the synthesized Cs–Bi–Br
NCs, which are commonly called stoichiometric Cs3Bi2Br9, revealed that the actual composition of the
NCs was extremely Cs deficient. Photoluminescence (PL) spectra from
the Cs-deficient Cs–Bi–Br NCs and BiBr3 NCs
were nearly identical except for a higher emission intensity with
Cs, which suggested that the chemical origin of the PL of the Cs–Bi–Br
NCs was BiBr3, and the inclusion of a few atomic percentages
of Cs improved the PL intensity. Further improvements in the emissive
property of the Cs–Bi–Br NCs were achieved by Cl surface
passivation, which was mediated by transition metal chloride additives,
namely, FeCl3, MnCl2, and NiCl3,
in the precursor solutions. Dispersive Raman spectroscopy studies
suggested that the role of the transition metal in the salt additives
was to facilitate the donation of Cl ions to the growing NCs during
the synthesis. Additionally, a combined incorporation of methylammonium
chloride and FeCl3 significantly enhances the PL quantum
yield compared to pristine Cs–Bi–Br NCs by a 7.5 times
increase from 2 to 15%.
Mixed–halide perovskites have
emerged as a promising
candidate
for optoelectronics, due to their tunable optical properties. However,
photoinduced phase segregation remains an obstacle for stable performance
in solar cells. Here, we have conducted both bulk and nanoscale measurements
to elucidate the mechanism of halide ion migration that leads to phase
segregation. By utilizing Kelvin probe force microscopy (KPFM) under
illumination, we have observed the time-evolution of ion migration
to and from grain boundaries that agreed with bulk photoluminescence
spectra of perovskites with a wide range of band gaps (1.67–1.88
eV), Cs0.15FA0.65MA0.20Pb(I
x
Br1–x
)3 where x = 0.47–0.80. By visualizing the
changes of band bending at grain boundaries, we deduce that halide
segregation is dominantly caused by iodide ions, given faster ion
migration in perovskite materials with a higher iodine content. Further,
we verify that the changing rate of band bending at grain boundaries
is consistent with the emerging rate of I-rich phase at grain boundaries,
suggesting the influence of iodide ion migration toward grain boundaries
on I-rich phase transition. This work will help provide insight for
interpreting the mechanism of light-halide ion interactions.
Highly efficient vacuum-deposited CsPbBr 3 perovskite light-emitting diodes (PeLEDs) are demonstrated by introducing a separate polyethylene oxide (PEO) passivation layer. A CsPbBr 3 film deposited on the PEO layer via thermal co-evaporation of CsBr and PbBr 2 exhibits an almost 50-fold increase in photoluminescence quantum yield intensity compared to a reference sample without PEO. This enhancement is attributed to the passivation of interfacial defects of the perovskite, as evidenced by temperature-dependent photoluminescence measurements. However, direct application of PEO to an LED device is challenging because of the electrically insulating nature of PEO. This issue is solved by doping PEO layers with MgCl 2 . This strategy results in an enhanced luminance and external quantum efficiency (EQE) of up to 6887 cd m −2 and 7.6%, respectively. To the best of our knowledge, this is the highest EQE reported to date among vacuum-deposited PeLEDs.
Epitaxially grown quantum dots (QDs), especially embedded in photonic structures, play an essential role in various quantum photonic systems as on-demand single-photon sources. However, these QDs often suffer from adjacent unwanted emitters, which contribute to the background noise of the QD emission and fundamentally limit the single-photon purity. In this paper, a nanoscale focus pinspot (NFP) technique using focused-ion-beam-induced luminescence quenching enables us to improve single-photon purity from site-controlled QD as a proofof-concept experiment. The optical quality of the QD emission is not degraded while the signal-to-noise ratio of the QD is improved. Moreover, the QD after the NFP technique reveals the single-photon nature at further elevated temperatures owing to the reduced background noise. As the NFP technique is nondestructive, it retains the apparent physical structures and photonic functions, thereby indicating its promising potential for applying diverse high-purity quantum emitters, particularly integrated in photonic devices and circuits.
III‐Nitride semiconductor‐based quantum dots (QDs) play an essential role in solid‐state quantum light sources because of their potential for room‐temperature operation. However, undesired background emission from the surroundings deteriorates single‐photon purity. Moreover, spectral diffusion causes inhomogeneous broadening and limits the applications of QDs in quantum photonic technologies. To overcome these obstacles, it is demonstrated that directly pumping carriers to the excited state of the QD reduces the number of carriers generated in the vicinities. The polarization‐controlled quasi‐resonant excitation is applied to InGaN QDs embedded in GaN nanowire. To analyze the different excitation mechanisms, polarization‐resolved absorptions are investigated under the above‐barrier bandgap, below‐barrier bandgap, and quasi‐resonant excitation conditions. By employing polarization‐controlled quasi‐resonant excitation, the linewidth is reduced from 353 to 272 µeV, and the second‐order correlation value is improved from 0.470 to 0.231. Therefore, a greater single‐photon purity can be obtained at higher temperatures due to decreased linewidth and background emission.
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