LED

Researchers from Seoul National University, University of Pennsylvania, Weizmann Institute of Science and Korea Basic Science Institute (KBSI) recently reported an advancement in the development of ultra-high efficiency perovskite nanocrystal light-emitting diodes (LEDs). Their work involved reinforcing the perovskite lattice and mitigating the material's natural low-frequency dynamics. 

The team identified a critical challenge in the reduction of luminescence efficiency due to the ionic nature of perovskite. The weak ionic bonds in perovskite materials can cause large-amplitude displacement of atoms within the crystal lattice, resulting in dynamic disorder that interferes with the radiative recombination process, leading to exciton dissociation and decreased luminescence efficiency. Addressing this issue, however, has been underexplored until now. The team proposed a novel mechanism to enhance the luminescence efficiency of perovskite emitters by incorporating conjugated molecular multipods (CMMs). These CMMs bind to the perovskite lattice, strengthening it and reducing dynamic disorder, which in turn improves the luminescence efficiency.

Researchers from North China Electric Power University, Beijing University of Chemical Technology and Sichuan Normal University have created bright blue perovskite LEDs (PeLEDs), among the LED colors needed to enable commercial applications.

With high efficiency and stability, PeLEDs could be a promising new option for full-color displays and solid-state lighting technology. However, while red and green PeLEDs have nearly reached their theoretical external quantum efficiencies, blue PeLEDs do not yet reach the efficiency, stability, or luminosity required for commercial applications. The novel method presented in this work seeks to address this challenge. Using an in situ spin-coating method, the authors created Dion-Jacobson phase quasi-2D perovskite nanocrystals. A mixture of mixed inorganic cesium bromide and two organic bidentate molecules in the perovskite precursor solution regulates growth and crosslinking in the nanocrystals. The resulting perovskites demonstrated effective emission in PeLEDs, brightly glowing from sky blue to deep blue.

Colloidal perovskite nanoplatelets (NPLs) have shown promise in tackling blue light-emitting diode challenges based on their tunable band gap and high photoluminescence efficiencies. However, high quality and large area dense NPL films have been proven quite hard to prepare due to their chemical and physical fragility during the liquid phase deposition.

Recently, researchers from University of Groningen reported a perovskite-polymer composite film deposition strategy with fine morphology engineering obtained using the blade coating method. The effects of the polymer type, solution concentration, compounding ratio and film thickness on the film quality were systematically investigated and the team found that a relatively high-concentration suspension with an optimized NPL to polymer ratio of 1 : 2 is crucial for the suppression of phase separation and arriving at a uniform film.

Metal halide perovskite light-emitting diodes (PeLEDs) that emit deep-blue color with high efficiency have not yet been fully achieved and become more difficult in the thin film of confined perovskite colloidal quantum dots (PeQDs) due to particle interaction. Recently, researchers from Seoul National University and University of Toronto demonstrated that electronic coupling and energy transfer in PeQDs induce redshift in the emission by PeQD film, and consequently hinder deep-blue emission.

Scheme illustrating QD-QD interaction related to emission spectrum shifts in the A) QD-only film and B) QD-mCP solid solution. Image credit: Advanced Materials

To achieve deep-blue emission by avoiding electronic coupling and energy transfer, a QD-in-organic solid solution was introduced, to physically separate the QDs in the film. This physical separation of QDs reduces the interaction between them yielding a blueshift of ≈7 nm in the emission spectrum. 

Solar cells and light-emitting diodes strive to maintain the excited state kinetics of molecules. A major loss mechanism, especially in the highest efficiency systems, is called exciton-exciton annihilation, leading to lowering of solar efficiency and of light output in LEDs. Controlling the amount of exciton-exciton annihilation is therefore an important goal that affects efficiency.

National Renewable Energy Laboratory (NREL) researchers, working with researchers from University of Colorado Boulder, sought to control exciton/exciton annihilation by coupling excitons with cavity polaritons, which are essentially photons caught between two mirrors, to combat energy dissipation and potentially increase efficiency in optoelectronic devices. As detailed in their recent article, the scientists used transient absorption spectroscopy to demonstrate control of the loss mechanism by varying the separation between the two mirrors forming the cavity enclosing the 2D perovskite (PEA)₂PbI₄ (PEPI) layer. This perovskite material is a candidate for future LED applications.

Researchers from the U.S and Ghana recently examined the fracture behavior of Perovskite Light Emitting Devices (PLEDs), emphasizing the importance of interfacial toughness in device performance. This can influence future materials and interface engineering strategies in optoelectronic devices.

Understanding the interfacial fracture toughness of PLEDs can guide the design of more robust devices by improving the adhesion between layers and reducing defect propagation. This can lead to enhanced performance and durability of PLEDs.

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Researchers from National Renewable Energy Laboratory (NREL), University of Utah, Université de Lorraine CNRS and University of Colorado Boulder have improved upon their previous work, that included incorporating a perovskite layer that allowed the creation of a new type of polarized light-emitting diode (LED) that emits spin-controlled photons at room temperature without the use of magnetic fields or ferromagnetic contacts. In their latest work, they have gone a step further by integrating a III-V semiconductor optoelectronic structure with a chiral halide perovskite semiconductor. 

The team transformed an existing commercialized LED into one that also controls the spin of electrons. The results could provide a pathway toward transforming modern optoelectronics, a field that relies on the control of light and encompasses LEDs, solar cells, and telecommunications lasers, among other devices.

Researchers from Zhejiang University, LinkZill Technology, Jilin University, and Linköping University have found that the electroluminescence rise time of perovskite LEDs (PeLEDs) can be reduced to microseconds using an individual-particle passivation strategy. This addresses a known issue with PeLEDs, that tend to have electroluminescence rise times over milliseconds due to ion migration in crystal structure, which is problematic for the development of high-refresh-rate displays.

The team demonstrated a second-generation digital display screen that uses perovskite light-emitting diodes instead of standard LED technology. In their study, the group made improvements to the device and demonstrated its sensing capability.

Researchers at China's Shanghai University, Jilin University, University of Science and Technology of China, Chinese Academy of Sciences, Korea's Pohang University of Science and Technology (POSTECH) and UK's University of Cambridge have reported efficient and color-stable perovskite LEDs (PeLEDs) across the entire pure-red region, with a peak external quantum efficiency reaching 28.7% at 638 nm, enabled by incorporating a double-end anchored ligand molecule into pure-iodine perovskites.

Light-emitting diodes (LEDs) based on metal halide perovskites (PeLEDs) with high color quality and facile solution processing are promising candidates for full-color and high-definition displays. However, the team explained that despite the great success achieved in green PeLEDs with lead bromide perovskites, it is still challenging to realize pure-red (620–650 nm) LEDs using iodine-based counterparts, as they are constrained by the low intrinsic bandgap.