HKUST Engineering Researchers Develop High-Performance Quantum Red QR-LEDs to Boost Colour Vividness and Brightness in Displays

A research team led by the School of Engineering of The Hong Kong University of Science and Technology (HKUST) has made significant advances in quantum rod light-emitting diodes (QR-LEDs), setting record-high efficiency level for red QR-LEDs. This innovation is poised to revolutionize next-generation display and lighting technologies, offering smartphone and television users a vibrant and enhanced visual experience.

LEDs have been widely used in electronic products for decades. Recent developments in quantum materials have given rise to quantum dot LEDs (QD-LEDs) and QR-LEDs. QD-LEDs offer superior color purity (color vividness) and higher brightness compared to current mainstream LEDs. However, outcoupling efficiency has now become the primary obstacle, as it sets a fundamental ceiling for external quantum efficiency (EQE), thereby hindering any further performance improvements.

Quantum rods, for which QR-LEDs are based on, are a type of elongated anisotropic nanocrystals with unique optical properties that can be engineered to optimize the light emission direction and ultimately improve outcoupling efficiency. However, QR-LEDs encounter two significant technical challenges: first, the ratio of emitted to absorbed photons (photoluminescence quantum yield) is relatively low after the material absorbs photons; second, there is a substantial leakage current due to poor thin-film quality.

To address these challenges, a research team led by Prof. Abhishek K. SRIVASTAVA, Associate Professor of the Department of Electronic and Computer Engineering (ECE), focused on boosting optical performance of QR-LEDs via refined synthesis engineering. They achieved an excellent photoluminescence quantum yield of up to 92% for both green and red quantum rods, along with uniform size distribution and shape confinement, all of which are essential for optimizing QR-LED performance.

In previous studies, the carrier leakage caused by irregular quantum rod films and its impact on diminishing the light coupling efficiency of QR-LEDs has often been overlooked. To tackle this issue, the team built an equivalent circuit model that illustrates the detrimental effects of leakage current in traditional QR-LED structures. This model provided valuable insights into the device’s operation, enabling the formulation of targeted solutions to suppress current leakage. By strategically transforming the QR-LED device structure, the team achieved a dual breakthrough: simultaneously enhancing balanced carrier injections and suppressing leakage current.

The results were remarkable – the optimized red QR-LEDs achieved a peak EQE of 31% and a peak brightness of 110,000 cd m⁻², setting a new record in previous red QR-LED research. Moreover, to validate the universality of their strategies, the team applied the same approach to green “dot-in-rod” quantum rods. The green devices also yielded impressive results, reaching a peak EQE of 20.2% and an ultra-high luminance of 250,000 cd m⁻². These outcomes not only demonstrate the effectiveness of the innovations but also highlight their potential for application across different color variants of QR-LEDs.

Prof. Srivastava, the corresponding author of the study, remarked, “Previous QD-LED research primarily focused on optimizing quantum dot structures for high efficiency, but this approach does not apply to elongated shaped quantum rods, such as QR-LED. By  utilizing equivalent circuit models and quantum rod micromorphology, we revealed that QR-LED have widespread pinholes due to their shape, which leads to critical leakage currents – an issue not encountered in tightly packed QD-LED. By modifying the device structure, we addressed the quality issues in emissive layer and verified fundamental advantages of quantum rod over quantum dots. This work is set to guide research on similar anisotropic nanocrystal and advance their commercial applications.

The team’s research work, titled “Inverted Device Engineering for Efficient and Bright Quantum Rod LEDs”, has been published in the prestigious journal Advanced Materials. Members of the team are from HKUST’s State Key Laboratory of Displays and Opto-Electronics, Center for Display Research, Department of Electronic and Computer Engineering and Department of Physics, and collaborators from the University of Edinburgh, UK. The first author is ECE PhD student LIAO Zebing, supervised by Prof. Srivastava.