Improvement of the light extraction efficiency of light-emitting diodes based on ZnO nanotubes

• The light extraction behavior and mechanisms of ZnO nanotubes LEDs have been investigated.
• The influences of ZnO nanotubes configurations have been studied.
• The LEE over the whole visible spectrum has also been demonstrated.
• An optimization of ZnO nanotubes geometries which leaded to more than four times enhancement of LEE has been presented.

Although ZnO nanotubes are emerging as a very promising approach for getting high-brightness light-emitting diodes (LEDs), the light extraction behavior of ZnO nanotubes LEDs has not been reported in detail previously. In this paper, numerical simulations based on finite-difference time-domain (FDTD) method were carried out to investigate the mechanisms through which ZnO nanotube arrays could effectively improve the light extraction efficiency (LEE). The effects of several parameters, including the inner radius, wall thickness, height and period of ZnO nanotubes on improving the LEE were studied. Furthermore, the LEE over the whole visible spectrum has also been investigated, two strong peaks and broadband visible spectrum has been demonstrated. An optimization of ZnO nanotubes geometries with respect to the optical properties has been presented. With the optimized structure, more than four times enhancement of LEE has been achieved. This enhancement is related to the wave-guiding and optical properties of ZnO nanotubes. The optimized structure presented here can be expected as a promising candidate for high efficiency LEDs.