high photoluminescence quantum yield (PLQY), wide wavelength tunability, and high color purity, [4][5][6] they have been attractive for light-emitting diode (LED) applications. Since the first demonstration of perovskite LEDs in 2014, [7] the device external quantum efficiency (EQE) has risen rapidly from 0.1% [7] to ≈20%, [2,4,8] and the efficiency enhancements are mainly attributed to passivation and compositional engineering, [2,8] improved charge balance by optimization of device structure, [9] and efficient light extraction. [4] More recently, these materials are considered as optical gain medium for lasers. In 2014, the first amplified spontaneous emission (ASE) was observed from CH 3 NH 3 PbI 3 thin films with a threshold of 12 µJ cm −2 and a gain of 250 cm −1 , which is ascribed to the large absorption coefficient, low bulk defect density, and slow Auger recombination rate. [10] These ASE threshold and gain values are comparable to the state of art gain media such as colloidal quantum dots [11] and organic thin films. [12] Since then, optically pumped lasers have been demonstrated based on various microcavity structures such as Fabry-Pérot cavities, [13,14] distributed feedback (DFB) gratings, [3,15] and whispering gallery cavities. [16] The flexibility of fabricating hybrid perovskite lasers using solution-processed methods enables large-scale production and is attractive for the realization of on-chip integration of photonic circuits. [17] Quasi-2D perovskites, which are also known as Ruddlesden-Popper (RP) perovskites, are mixed phases of 2D and 3D nanocrystals. In the mixture, 2D domains exhibit quantumwell-like electronic properties with strong exciton binding energy due to the reduced dimensionality. [18] Typically, the 2D perovskite (A') 2 A n−1 B n X 3n+1 domains consist of multilayers of BX 6 octahedra separated by intercalating ammonium cations A', which is too large to fit into the crystal structure and hinder the growth of 3D ABX 3 crystals (A = methylammonium (MA + ), formamidinium (FA + ), or Cs + , B = Pb 2+ , and X = I − , Br − , Cl − ). As a result, the number of layers determine the bandgap of 2D quantum-well-like domains. [19] Different from 3D perovskites, thin films of qausi-2D perovskites typically contain a mixture of domains with different layers. Within such inhomogenous Quasi-2D Ruddlesden-Popper halide perovskites with a large exciton binding energy, self-assembled quantum wells, and high quantum yield draw attention for optoelectronic device applications. Thin films of these quasi-2D perovskites consist of a mixture of domains having different dimensionality, allowing energy funneling from lower-dimensional nanosheets (high-bandgap domains) to 3D nanocrystals (low-bandgap domains). High-quality quasi-2D perovskite (PEA) 2 (FA) 3 Pb 4 Br 13 films are fabricated by solution engineering. Grazing-incidence wide-angle X-ray scattering measurements are conducted to study the crystal orientation, and transient absorption spectroscopy measurements are conducted to study the charge-carr...

Organometal halide perovskite light emitting diodes (LEDs) have attracted a lot of attention in recent years, owing to the rapid progress in device efficiency. However, their short operational lifetime severely impedes the practical uses of these devices. The operating stability of perovskite LEDs are due to degradation due to ambient environment and degradation during operation. The former can be suppressed by encapsulation while the latter one is the intrinsic degradation due to the electrochemical stability of the perovskite materials. In addition, perovskites also suffer from ion migration which is a major degradation mechanism in perovskite LEDs. In this review, we specifically focus on the operational stability of perovskite LEDs. The review is divided into two parts: the first part contains a summary of various degradation mechanisms and some insight on the degradation behavior and the second part is the strategies how to improve the operational stability, especially the strategies to suppress ion migration. Based on the current advances in the literature, we finally present our perspectives to improve the device stability.

Perovskite light-emitting diodes have been gaining attention in recent years due to their high efficiencies. Despite of the recent progress made in device efficiency, the operation mechanisms of these devices are still not well understood, especially the effects of ion migration. In this work, the role of ion migration is investigated by measuring the transient electroluminescence and current responses, with both the current and efficiency showing a slow response in a time scale of tens of milliseconds. The results of the charge injection dynamics show that the slow response of the current is attributed to the migration and accumulation of halide ions at the anode interface, facilitating hole injection and leading to a strong charge imbalance. Further, the results of the charge recombination dynamics show that the slow response of the efficiency is attributed to enhanced charge injection facilitated by ion migration, which leads to an increased carrier density favoring bimolecular radiative recombination. Through a combined analysis of both charge injection and recombination dynamics, we finally present a comprehensive picture of the role of ion migration in device operation.

Light‐emitting diodes (LEDs) with directional and polarized light emission have many photonic applications, and beam shaping of these devices is fundamentally challenging because they are Lambertian light sources. In this work, using organic and perovskite LEDs (PeLEDs) for demonstrations, by selectively diffracting the transverse electric (TE) waveguide mode while suppressing other optical modes in a nanostructured LED, the authors first demonstrate highly directional light emission from a full‐area organic LED with a small divergence angle less than 3° and a TE to transverse magnetic (TM) polarization extinction ratio of 13. The highly selective diffraction of only the TE waveguide mode is possible due to the planarization of the device stack by thermal evaporation and solution processing. Using this strategy, directional and polarized emission from a perovskite LED having a current efficiency 2.6 times compared to the reference planar device is further demonstrated. This large enhancement in efficiency in the PeLED is attributed to a larger contribution from the TE waveguide mode resulting from the high refractive index in perovskite materials.

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