Light Extraction Efficiency Enhancement of Colloidal Quantum Dot Light‐Emitting Diodes Using Large‐Scale Nanopillar Arrays

Abstract

A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced light extraction effi ciency by applying a layer of largescale, low-cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, effi cient method of non-wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current effi ciency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum effi ciency (EQE) of 9.34% reaches the highest EQE value for green-emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light-extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the fi rst account for QLEDs offering detailed insight into the light extraction effi ciency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high-performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.

Colloidal nanocrystals for quality lighting and displays: milestones and recent developments

“…The researchers used these patterns for incorporation of green-emitting and red-emitting QDs on a blue LED. The resulting white LED exhibited a CRI of 74 at a CCT of 6100 K. To improve the light-extraction efficiency of QD-LEDs that are based on electrical charge injection, Yang et al studied the effect of ZnO nanopillars [57]. In this work, ZnO nanopillars were grown on the LED surface from which the light outcouples.…”

“…Therefore, increasing the amount of outcoupled light is very essential to realize high-efficiency light sources. This problem has been addressed for different types of LEDs including organic LEDs [30][31][32] and epitaxially grown LEDs [52][53][54][55] in addition to LEDs using QDs for color conversion [32,56] and charge injection [57][58][59]. These works in general make use of scattering structures that alter the exiting angle of the light beam leaving the device and indirectly overcomes the angular limitation imposed on the planar structures by Snell's law.…”

Enhanced Light Extraction From Green Quantum Dot Light-Emitting Diodes by Attaching Microstructure Arrayed Films

“…Similar to the planar structure of OLEDs [13], [14], only a small fraction of the total photons generated inside the QD film are generally useable because of the total internal reflection (TIR) and wave-guiding effects of the high-index layers of QDs, ZnO films, organic conducting films, and transparent conductive films (Indium Tin Oxide, ITO). In fact, the extracted efficiency of QLEDs is only ~ 20% of the internally generated photons in the QD layer when any kind of out-coupled structure is not implemented in the planar structure of QLEDs [15]. Therefore, to further improve the EQE and catch up the performance of OLEDs, one has to find solutions that increase the light extraction efficiency as well as the IQE or find the strategies to reduce the guided mode in QLEDs.…”

“…However, there have been only a few approaches reported in previous journals, to date, that enhance the extraction efficiency of QLEDs [15], [22], [23]. [15]. Based on contrasting results between a few studies on QLEDs and many studies on OLEDs [16]- [21], simple microlens and microprism outcoupling approaches were applied in this study, which are representatively used in OLEDs to make bright and efficient green QLEDs.…”

“…To date, a lot of out-coupling schemes have been implemented in the hope of improving light extraction from OLEDs [16]- [21]. However, there have been only a few approaches reported in previous journals, to date, that enhance the extraction efficiency of QLEDs [15], [22], [23]. [15].…”