Spectrally Stable Deep-Blue Light-Emitting Diodes Based on Layer-Transferred Single-Crystalline Ruddlesden-Popper Halide Perovskites.

A novel approach to producing high-color-purity blue-light-emitting diodes based on single-crystalline Ruddlesden-Popper perovskites (RPPs) is reported. The utilization of a pure bromide composition eliminates any possibility of halide segregation, which can otherwise lead to undesired shifts in the emission wavelength or irreversible degradation of the spectral line width. Phase-pure PEA2MAPb2Br7 single crystals with a lateral size exceeding 1 cm2 can be synthesized using the inverse temperature crystallization method. To prepare RPP layers with a thickness of less than 50 nm, we employ a thinning process of the initially thick bulk crystals, followed by a dry-transfer process to place them onto a hole transport layer and an indium-tin-oxide-coated glass substrate. By utilizing polydimethylsiloxane as a handling layer, deformations of the bulk RPP crystal and exfoliated RPP layer, as well as the formation of defects such as pinholes, can be effectively suppressed. Subsequent depositions of an electron transport layer and a metal contact complete the fabrication of electroluminescence (EL) devices. The EL devices utilizing the single-crystalline RPP demonstrate excellent spectral stability across a broad range of the applied bias voltage spanning from 4.5 to 10 V, exhibiting a significantly narrow line width of 14 nm at an emission wavelength of 440 nm that can potentially cover 99.3% of the Rec. 2020 color gamut. The sharp EL emission spectrum can be effectively preserved, avoiding any broadening of the line width, by suppressing Joule heating throughout the device operation, in addition to the intrinsic stability of single-crystalline RPPs.

Ultrafast and Bright Quantum Emitters from the Cavity-Coupled Single Perovskite Nanocrystals.

Perovskite nanocrystals (NCs) have attracted increasing interest in the realization of single-photon emitters owing to their ease of chemical synthesis, wide spectral tunability, fast recombination rate constant, scalability, and high quantum yield. However, the integration of a single perovskite NC into a photonic structure has yet to be accomplished. In this work, the integration of a highly stable individual zwitterionic ligand-based CsPbBr3 perovskite NC with a circular Bragg grating (CBG) is successfully demonstrated. The far-field radiation pattern of the NC inside the CBG exhibits high directionality toward a low azimuthal angle, which is consistent with the simulation results. A 5.4-fold enhancement in brightness is observed due to an increase in collection efficiency. Moreover, a 1.95-fold increase in the recombination rate constant is achieved. This study offers ultrafast (<100 ps) single-photon emission and an improved brightness of perovskite NCs, which are critical factors for practical quantum optical applications.

Highly Efficient Vacuum-Evaporated CsPbBr3 Perovskite Light-Emitting Diodes with an Electrical Conductivity Enhanced Polymer-Assisted Passivation Layer.

Highly efficient vacuum-deposited CsPbBr3 perovskite light-emitting diodes (PeLEDs) are demonstrated by introducing a separate polyethylene oxide (PEO) passivation layer. A CsPbBr3 film deposited on the PEO layer via thermal co-evaporation of CsBr and PbBr2 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 MgCl2. 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.