Colloidal quantum dot light-emitting diodes (QLEDs) have enjoyed fast development over the past 30 years. One of the greatest challenges in QLEDs is their low light outcoupling efficiency due to their planar structure, which limits the external quantum efficiency (EQE) of QLEDs for commercial applications. To increase the EQE, different kinds of optical optimizations, such as micro-structure and microcavity, have been applied to retrieve the photons that are trapped in waveguide modes or lost in surface plasmon modes.  

A planar microcavity is a wavelength-scale resonator with a planar Fabry-Perot architecture that can be easily integrated into QLEDs.  

In a study published in ACS APPLIED NANO MATERIALS, a research group led by Prof. LIU Xingyuan from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences, cooperating with Prof. ZOU Bingsuo's group from Guangxi University, realized a series of high efficiency red, green and blue (RGB) microcavity QLEDs (MQLEDs) with narrow line widths.  

By incorporating microcavity structures with distributed Bragg reflector (DBR) and Al as reflectors, the researchers first realized bottom-emitting RGB MQLEDs with substantially improved electroluminescence (EL) performance. As a result, very narrow spectra are obtained with 50% or more narrowing of full width at half maximum (FWHM) for RGB devices. 

They then fabricated and tested MQLEDs with different length of cavity to investigate different factors, such as thickness of the transport layer, reflectivity and position of the emitting layer, which affect EL properties of microcavity. They observed that when deviating from the EL peak of devices without microcavity structure, current efficiency decreases and FWHM of EL spectrum increases quickly. The highest performance was obtained at the condition that the optical confinement must match the electrical transport process in one microcavity device. 

As a consequence, the FWHM can be further narrowed by means of increasing the quality (Q) value when the reflectance of reflectors is improved. For the EL peak wavelength of 632 nm, its FWHM can be further compressed into 3.5 nm with a high Q value of 180. Further improvement of its Q value can lead to an extremely narrow FWHM and a low-loss optical resonator, indicating that a high Q microcavity is a promising and practical structure for achieving electrically pumped quantum dot (QD) lasers. 

This study demonstrates a general applicability of microcavity structure to obtain high-quality device and narrow EL spectra, and microcavity structure can be applied in low-threshold laser, cavity quantum electrodynamics, biological detection, and high-performance filters.