The development of quantum-dot and  organic hybrid tandem light-emitting diodes (LEDs) has led to progress  made in a single LED being able to emit any color. They have the  potential to replace traditional RGB LEDs and dramatically improve the  resolution and aperture ratio of displays. Research conducted by  scientists at Southern University of Science and Technology (SUSTech)  has continued to progress the field of quantum-dot LEDs.

On June 4, Associate Professor Shuming Chen (Electrical and Electronic Engineering) led his team to make significant progress through the publication of a paper in the high-impact academic journal, Nature Communications (IF = 11.878), titled “Quantum-dot  and organic hybrid tandem light-emitting diodes with  multi-functionality of full-color-tunability and white-light-emission.”

CdSe-based quantum-dot light-emitting diodes (QLEDs) have been  extensively explored for applications in display and lighting due to  their unique merits of high color saturation, tunable emission color,  high brightness, and simple solution processability. Recent advances  have enabled QLEDs to show high external quantum efficiency (EQE) and  long operational lifetime. Although the efficiency of QLEDs could meet  the requirements of display applications, the realization of the QLED  full-color display remains challenging.

Figure 1. The device structure of a multi-function stacked LED.

One reason is that the blue (B) QLEDs are unstable. By substituting  the B-QLEDs with relative stable B organic (O), LEDs can enjoy both the  high saturation of QLEDs as well as the high stability of OLEDs.  However, integrating QLEDs and OLEDs could be challenging since they  come from different families. Another reason is that the inkjet  printing, which is used to deposit and pattern the light-emitting layers  (EMLs) is far from mature for mass-producing QLED displays. The RGB  side-by-side color pixels can also be realized by combining the white  devices with patterned color filters (CF). However, the introduction of  CFs significantly reduces the brightness of the displays.

A practical approach to eliminate both the absorptive CFs and EMLs  patterning is to develop a full color-tunable device. Unfortunately, all  the reported full color-tunable devices require four independently  addressable electrodes, which complicates the fabrication and the drive  circuits.

Figure 2. (a) The principle diagram of  the yellow QLED device’s luminous color changes with voltage. (b) to (d)  are the CIE graphs, AC voltage signal graphs, and electroluminescence  spectrum graphs of the AC drive device luminous color changes from red  to green, blue to red, blue to orange, and blue to green, respectively.

The research team aimed to develop a two-terminal, full color-tunable  LED that combines B-OLED and R/G-QLED. They could demonstrate a  multi-functional hybrid tandem LED by stacking a yellow (Y) QLED with a  B-OLED using indium–zinc oxide (IZO) intermediate connecting electrode  (ICE). Due to the introduction of ICE, the Y-QLED and B-OLED can be  connected in series or parallel. Under parallel connection and  alternate-current (AC) driving, a full-color-tunable LED with the  emission colors covering a 63% NTSC color triangle is achieved. Under  series connection and direct current (DC) driving, an efficient  white-light-emission LED with a peak EQE of 26.02% is obtained. By using  a novel AC driving method, the white LED can emit stable colors with  color coordinates fixed at (0.34, 0.36) over a wide range of brightness  (1000~50000 cd/m²). Also, the color coordinates can be tuned  to trace the blackbody locus over a wide range of correlated color  temperatures (1500 ~10000 K). We believe this hybrid tandem LED could  find potential applications in both full-color-display and  solid-state-lighting.

Figure 3. (a) to (d) are the  photoelectric performance of the stacked white light device under DC  driving. (e) to (g) are the optical properties of the stacked white  light device during AC driving.

Previous papers have been published in Advanced Functional Materials (IF =15.621), ACS Nano in 2018 and 2019 (IF = 13.903), and Advanced Science (IF = 15.804).

Doctoral candidate Heng Zhang is the first author of the paper, and Shuming Chen  is the only correspondent author, with SUSTech as the only  communication unit of the paper. This work was supported by the National  Natural Science Foundation of China, the Guangdong Natural Science  Funds for Distinguished Young Scholars, and the Guangdong Special Funds  for Science and Technology Development.

Link to paper: https://www.nature.com/articles/s41467-020-16659-x