LED

Researchers at North Carolina State University and Brookhaven National Laboratory have reported a technique for engineering layered hybrid perovskites (LHPs) down to the atomic level, which enables precise control on how the materials convert electrical charge into light. 

Image credit: Matter

The technique opens the door to engineering materials tailored for use in next-generation printed LEDs, lasers and photovoltaic devices.

Soochow University researchers have proposed the in situ treatment of Cl-rich benzene phosphorus oxydichloride (BPOD)as a way to achieve high-quality pure-blue perovskites, by simultaneously enlarging the perovskite bandgap, passivating the halide vacancy defects, and immobilizing the halide ions through the hydrolysis products of chloride ions and phenylphosphonic acid. 

The background for this work is that despite the substantial progress in sky-blue (480−495 nm) perovskite light-emitting diodes (PeLEDs), pure-blue PeLEDs (<480 nm) merely show moderate performances. Bromide-chloride mixed perovskites may have potential to enable a straightforward and effective way to obtain pure-blue emission, but the tricky issue of halide migration in mixed halide perovskites makes it challenging to achieve efficient PeLEDs with stable electroluminescence (EL) spectra. 

A team of researchers, led by Professor David Di from the International College of Zhejiang University and the School of Optoelectronic Science and Engineering, recently achieved a continuous transition from n-type to p-type perovskite semiconductors through molecular doping, while maintaining extremely high luminescence performance. 

Image credit: Zhejiang University

Based on controllable doping, the team developed a perovskite LED with a simple structure and reported a record for the highest brightness of solution-based LEDs, reaching 1.16 million nits.

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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