Nanostructural Effect of ZnO on Light Extraction Efficiency of Near-Ultraviolet Light-Emitting Diodes

Abstract

The effect of ZnO nanostructures on the light output power of 375 nm near-ultraviolet light-emitting diodes (NUV-LEDs) was investigated by comparing one-dimensional (1D) nanorods (NR-ZnO) with two-dimensional (2D) nanosheets (NS-ZnO). ZnO nanostructures were grown on a planar indium tin oxide (ITO) by solution based method at low temperature of 90°C without degradation of the forward voltage. At an injection current of 100 mA, the light output efficiency of NUV-LED with NR-ZnO was enhanced by around 30% compared to the conventional NUV-LEDs without ZnO nanostructures. This improvement is due to the formation of a surface texturing, resulting in a larger escape cone and a multiple scattering for the photons in the NUV-LED, whereas the light output efficiency of NUV-LED with NS-ZnO was lower than that of the conventional NUV-LEDs due to the internal reflection and light absorption in the defective sites of NS-ZnO.

ZnO nanowires/YAG:Ce functional heterostructure coatings with tunable optical properties

“…Among the existing light extraction management strategies based on this increase, one can cite surface roughening [4,9], use of photonic crystals [2], nanorods or nanopillars [7] and many others. In particular, cheap and tunable [10,11] ZnO nanostructures such as nanorods, nanopillars, nanowires and nanosheets have attracted much attention to this end over last years since they can contribute to LEDs improvement in more than one way: 1) they are characterized by a refractive index (nZnObulk2.0-2.1) intermediary between GaN-based LED (n2.4) and air, expanding the escape cone value C [12][13][14], 2) their high surface-to-volume ratio allows a better thermal dissipation, entailing a longer LED lifetime [14], 3) depending on their morphology and inherent structural defects, they can be used either as light waveguides [15] or to increase light scattering through surface nanotexturing [16].…”

“…If they have already been used to improve emission efficiency or quality of GaN-based LEDs [12,14,16], ZnO nanostructures have scarcely been combined with phosphors to yield an enhanced white light, whether in terms of light extraction or in terms of photometric parameters [3,21]. However, they could successfully be used for this latter issue thanks to their tuneable emission profile [11].…”

High transparency and conductivity of heavily In-doped ZnO thin films deposited by dip-coating method

“…Many recent works have been devoted to the one-dimensional growth of ZnO nanostructures such as nanorods and nanotubes, considered as the most promising candidates for future nanodevices, such as light-emitting diodes [1,2], dye sensitized solar cells (DSSC) [3,4] and gas sensors [5]. These nanostructures can also be used as photocatalysts [6] and in biomedical applications [7].…”

Zinc oxide light-emitting diodes: a review

“…For instance, at 50 mA forward current, their LED was 2.6 times brighter than a conventional planar GaN LED. There have been other reports [86][87][88][89] of ZnO being used in this index-matching role with GaN LEDs, especially because it can be doped n-type easily and can thus serve as the n-type contact to the LED.…”