Electroluminescence Efficiency Of Light-emitting Diodes Research Articles

Light–Matter Interaction Enhancement in Anisotropic 2D Black Phosphorus via Polarization-Tailoring Nano-Optics

Black phosphorus (bP), a two-dimensional (2D) layered material, has shown great potential for infrared (IR) optoelectronics owing to the narrow and direct bandgap it exhibits when in multilayer form. However, its thinness and optical anisotropy lead to weak light absorption, which limits the performance of bP-based photodetectors. In this work, we explore plasmonic nanoantennas, optical cavities, and their hybrids that can be integrated with multilayer bP to enhance its light absorption. This is achieved by near-field light intensity enhancement and polarization conversion. In addition, we demonstrate that these nanostructures can boost the spontaneous emission from bP. Light absorption enhancements of up to 185 and 16 times are obtained for linearly polarized and unpolarized IR light, respectively, in comparison with a commonly used device architecture (bP on SiO2/Si). Moreover, IR light emission enhancements of up to 18 times are achieved. The optical nanostructures presented here can be exploited for enhancing the detectivity of photodetectors and electroluminescence efficiency of light-emitting diodes based on bP and other 2D materials.

Charge carrier recombination and ion migration in metal-halide perovskite nanoparticle films for efficient light-emitting diodes

Metal-halide perovskite (MHP) nanoparticles (NPs) have attracted considerable attention as a promising light emitter for efficient light-emitting diodes (LEDs) due to their high color-purity and photoluminescence quantum efficiency (PLQE). In contrast to MHP polycrystalline (PC) bulk films, charge carrier recombination as well as ion migration dynamics in MHP NP films have not been studied yet despite the importance to achieve high electroluminescence efficiency in LEDs. Here, we use steady-state and transient photoluminescence measurements, and photo-responsivity analysis to study the charge carrier recombination and ion migration dynamics in MHP NP films and then, compare these with those of MHP PC films. Results show that MHP NP films do not undergo the severe ion migration and have dominant radiative recombination of excitons by efficiently confining electron-hole pairs. As a result, MHP NP films have high photo-stability, photo-responsibility, and PLQE (> 60%). Our findings provide insight into charge carrier and ion dynamics in MHP NP films, and confirm the potential of NP films for use in efficient LEDs.

Point defects controlling non-radiative recombination in GaN blue light emitting diodes: Insights from radiation damage experiments

The role of Shockley-Read-Hall non-radiative recombination centers on electroluminescence (EL) efficiency in blue multi-quantum-well (MQW) 436 nm GaN/InGaN light emitting diodes (LEDs) was examined by controlled introduction of point defects through 6 MeV electron irradiation. The decrease in the EL efficiency in LEDs subjected to irradiation with fluences above 5 × 1015 cm−2 was closely correlated to the increase in concentration of Ec-0.7 eV electron traps in the active MQW region. This increase in trap density was accompanied by an increase in the both diode series resistance and ideality factor (from 1.4 before irradiation to 2.1 after irradiation), as well as the forward leakage current at low forward voltages that compromise the injection efficiency. Hole traps present in the blue LEDs do not have a significant effect on EL changes with radiation because of their low concentration.

Effect of In incorporation into the quantum well active region on the efficiency of AlGaN‐based ultraviolet light‐emitting diodes

AbstractImproving the internal quantum efficiency is still a major challenge for realizing efficient III‐nitride‐based light‐emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on the influence of a very low In amount of less than 1% on the luminescence properties of single Ga(In)N/AlGaN quantum well (QW) structures and the performance of AlGaN‐based LEDs with an emission wavelength of about 350 nm. The samples were grown by metalorganic vapor phase epitaxy on low defect density AlGaN buffer layers on (0001) sapphire substrates. The temperature dependence of the QW emission energy reveals a clear “S‐shape” behaviour indicating a significant carrier localization for In containing QWs. The electroluminescence efficiency of LEDs with a GaInN QW active region could be improved by a factor of 4 compared to LEDs with GaN QW, resulting in a continuous wave (cw) output power of 8.2 mW at 40 mA. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved efficiency of light-emitting diodes based on polyfluorene blends upon insertion of a poly(p-phenylene vinylene) electron- confinement layer

We report the improvement of the electroluminescence efficiency of light-emitting diodes (LEDs) based on polyfluorene blends, upon insertion of a thin film of poly(p-phenylene vinylene), PPV, between a hole-injection layer of poly(3,4-ethylene dioxythiophene), doped with polystyrene sulfonic acid, and the polyfluorenes emissive layer. For LEDs using a blend of poly(9,9′-dioctylfluorene), with 5 wt % of the green emitter poly(9,9′-dioctylfluorene-altbenzothiadiazole), and calcium cathodes, the efficiency increases from 2.1 to 4.1 cd/A upon insertion of such a PPV layer. We propose that such an improvement is mainly due to the electron-blocking effect of the PPV layer, leading to improved charge carriers balance within the emissive layer.