Increased light extraction efficiency from top-emitting organic light-emitting diodes employing a mask-free plasma-etched stochastic polymer surface

Abstract

Top-emitting organic light-emitting diodes (TEOLEDs) are drawing interest as future devices for both high-quality display and lighting. However, the current TEOLEDs have external quantum efficiencies of approximately 20%, which still need improvement. To attain high device efficiencies in TEOLEDs, waveguide and surface plasmon polariton modes should be minimised. Many efforts have been made using nano- or micrometre-scale periodic gratings to extract the confined photons. However, significant angular distortion or colour shifts occur as the Bragg condition calls for. Here, an effective method is demonstrated for enhancing the light extraction efficiency from TEOLEDs employing a mask-free plasma-etched polymer surface with stochastically distributed nano-hemispheres. TEOLEDs with the stochastic polymer surface allowed us to achieve a device efficiency enhancement up to 1.55-fold relative to conventional devices without introducing spectral changes and angular emission distortion, unlike periodic grating-embedded devices. Because of the independence of the improvement from the particular emission wavelengths, the proposed TEOLEDs are attractive and practical for use in full colour and white lighting as well as display applications.