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Light-emitting diodes (LEDs) have been known as the most widely used light source in lighting and displays for more than 60 years. There is still room for progress in the performance of LEDs, especially since the current devices with various types of different light-emitting layer materials have converged to unity in terms of internal quantum efficiency, and there is an urgent need to improve the light extraction efficiency. Metasurfaces (MSs) have received attention from researchers as structures that can be integrated with LEDs to efficiently modulate the phase and amplitude of light through resonance and scattering, which can reduce light loss. This paper reviews the development of metasurfaces in LEDs so far. The different working mechanisms of metasurfaces composed of different materials are first analyzed in depth. Subsequently, three aspects of light extraction, angle change, and polarization modulation are described in detail according to different applications of metasurfaces in LEDs. Finally, the current status of metasurface applications in LEDs is summarized, and the future development prospects are envisioned. [ABSTRACT FROM AUTHOR]

Quasi‐two‐dimensional (quasi‐2D) perovskites are emerging as efficient emitters in blue perovskite light‐emitting diodes (PeLEDs), while the imbalanced crystallization of the halide‐mixed system limits further improvements in device performance. The rapid crystallization caused by Cl doping produces massive defects at the interface, leading to aggravated non‐radiative recombination. Meanwhile, unmanageable perovskite crystallization is prone to facilitate the formation of nonuniform low‐dimensional phases, which results in energy loss during the exciton transfer process. Here, we propose a multifunctional interface engineering for nucleation and phase regulation by incorporating the zwitterionic additive potassium sulfamate into the hole transport layer. By using potassium ions (K+) as heterogeneous nucleation seeds, finely controlled growth of interfacial K+‐guided grains is achieved. The sulfamate ions can simultaneously regulate the phase distribution and passivate defects through coordination interactions with undercoordinated lead atoms. Consequently, such synergistic effect constructs quasi‐2D blue perovskite films with smooth energy landscape and reduced trap states, leading to pure‐blue PeLEDs with a maximum external quantum efficiency (EQE) of 17.32 %, spectrally stable emission at 478 nm and the prolonged operational lifetime. This work provides a unique guide to comprehensively regulate the halide‐mixed blue perovskite crystallization by manipulating the characteristics of grain‐growth substrate. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of positional isomerization on the photoelectric properties of thermally activated delayed fluorescence (TADF) emitters, thereby fully stimulating the potential of the molecular backbone, herein, two red TADF isomers, oTPA-PPP and mTPA-PPP, were developed and synthesized by linking triphenylamine (TPA) donor into different positions of the pyrido [2′,3′:5,6] pyrazino [2,3-f] [1,10] phenanthroline (PPP). Their photoluminescent quantum yields (PLQYs) and external quantum efficiencies (EQEs) were significantly different by manipulating the substitution position of the donor. Compared with mTPA-PPP, oTPA-PPP exhibited higher PLQY benefiting from intramolecular hydrogen bonding and larger fluorescence radiative decay rate, as confirmed by both theoretical and experimental analyses. As a result, oTPA-PPP-based organic light-emitting diode (OLED) realized high EQE max of 19.2 % with red emission at 603 nm, whereas mTPA-PPP achieves an EQE max of 7.9 % at 596 nm. This work provides a simple and feasible molecular design strategy to design and select red TADF emitters with excellent properties. [Display omitted] • Two emitters' photophysical properties can be finely regulated and their molecular skeleton potential is excited. • Multi-nitrogen atom receptors promote RISC and radiative transition processes. • High k r of 5.29 × 107 s−1 for oTPA-PPP suppressed the non-radiative decay and enhanced PLQY to 92.8 %. • OTPA-PPP-based OLED realized high EQE of 19.2 % with red emission peaked at 603 nm. [ABSTRACT FROM AUTHOR]