Enhanced light out-coupling efficiency of quantum dot light emitting diodes by nanoimprint lithography.

Extracting light from quantum dot light emitting diodes (QLEDs) by applying optical-functional nanostructures inside and outside the devices is essential for their commercial application in illumination and displays. In this paper, we demonstrate the highly effective extraction of waveguided light from the active region of QLEDs by embedding internal grating patterns fabricated using a nanoimprint lithography technique. The grating couples out waveguide mode power into the substrate without changing the device's electrical properties, resulting in an increase in both the external quantum efficiency and luminous efficiency for a green QLED from 11.13% to 13.45%, and 29 010 cd m-2 to 44 150 cd m-2, respectively. The observed improvement can be ascribed to the elimination of the waveguide mode by the grating nanostructures introduced in the device. Furthermore, the finite-difference time-domain (FDTD) simulation also demonstrated that the power loss due to the waveguide mode was reversed. The results indicate that internal nano-scattering pattern structures are attractive for enhancing the out-coupling efficiency of QLEDs.