Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs

Abstract

While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron-hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.

Fig. 1. Evolution-inspired design of disordered metasurface for light extraction.

a Photo images of Photinus, Phausis reticulata and Pyrocoelia rufa. b The SEM image of lantern cuticles of Pyrocoelia rufa. c Enlarged view of microstructures on the lantern cuticle of the firefly. The inset shows the microstructures with higher magnification. The disorder in both position and size of stripes can be observed. d High magnification SEM image of microstructure on the lantern cuticle. The curvature on the top surface for each stripe is clearly demonstrated. e The design flow of the metasurface. Meta-I is an ordinary metasurface composed of periodic stripes with squared cross-sections. Meta-I evolves to Meta-II and Meta-III by utilising two bio-inspired features, the curved top surfaces and disorder, respectively. Meta-IV endows extend features to another dimension, transforming stripes to nanoparticles

Fig. 2. FDTD simulations for the evolution-inspired light extraction.

a The field distribution E_y of a substrate without any light extracting structures. When the incident light is beyond critical angle, no photons can escape. b–d The field distribution E_y of a substrate with (b) Meta-I, (c) Meta-II and (d) Meta-III. The amplitude of the electric field coupled outside is gradually increased, demonstrating the improvement of light extraction by utilising the bio-inspired features. e, A far-field intensity for three different metasurfaces. For simulations in (a–d), the incident angle is chosen as θ_i = 32^o while the critical angle for TIR is θ_c = 30^o. The wavelength of the light is at 450 nm. f–h The transition spectra of the substrate at different incident angles beyond θ_c; (f) Meta-I, (g) Meta-II and (h) Meta-III. For disordered Meta-III, 3 samples with different sets of random variables are investigated. The white dashed line corresponds to θ_c. A prominent improvement is observed for different angles and wavelength from Meta-I to Meta-II. The region with efficient light extraction (reddish colour) is enlarged by the introduction of disorder (Meta-III)

Fig. 3. Characterisations of the fabricated disordered metasurface Meat-IV.

a Low magnification SEM image of the disordered Ag metasurface. b Two-dimensional Fourier power spectrum of the position of the Ag nanostructures, which is calculated from the SEM image of disordered Ag metasurface. c The tilted view of the disordered metasurfaces with high magnification. Inset is image of a single Ag nanostructure. d A schematic illustration of the light extraction measurement setup. e Size distribution histogram of the Ag nanoparticles in the optimised disordered metasurface and corresponding Log-Normal fitting (black solid line). f Transmission spectrum from the hemispherical glass prisms with Meta-IV for θ_i = 50^o. The SiO₂ substrate covered with disordered Ag metasurface and the prism were bonded together by using index-matching fluid (n = 1.46). Inset: optical image of the spot of transmitted light extracted from the glass prism with disordered Ag metasurface

Fig. 4. Performance of commercialised LED with disordered metasurface Meat-IV.

a Schematic illustration (not to scale) of a GaN-based LEDs with disordered metasurface deposited on the top. b Photoluminescence spectra of LEDs with/without Meat-IV, with an enhancement of 170%. c Electroluminescence spectra of LEDs with/without Meat-IV, with an enhancement of 140% d Polar plot for the far-field light intensity of LEDs with/without Meat-IV. The data is fitted with Lambert’s cosine law for each case. e The voltage and output power as a function of current for LEDs with/without Meat-IV. The metasurface has negligible impact on light generation of the LED beneath (from I-U) but prominent impact on light extraction (from I-P). f IQE and EQE of LEDs with/without Meat-IV. After packaging, the EQE is increased from 31.6% to 51.5% by aid of Meat-1V