The breakthrough in gallium nitride based high brightness blue leds in the 1990s ushered in a new era of LED lighting and display (for which three Japanese scientists won the Nobel Prize in Physics in 2014). QLED based on semiconductor quantum dots has a broad application prospect in the field of display and lighting due to its advantages such as better monochromaticity, color saturation and lower production cost. After the rapid development in recent years, its luminance, external quantum efficiency (EQE), life and other main performance indicators have been greatly improved. However, in previous work, there is a contradiction between low efficiency at high brightness and low brightness at high efficiency. How to keep high efficiency, long life and high stability while keeping high brightness is an urgent problem to be solved in THE FIELD of QLED, which is also a key technical bottleneck restricting its application in the field of display and lighting.
The main reason for the above dilemma of "having your cake and eat it" is that the quantum dot valence band level in QLED luminescent layer is usually deep, which does not match with the hole transport layer, resulting in low efficiency of hole injection and imbalance with electron injection. To solve this problem, the research team started from the design of luminescent layer quantum dots, and based on the technology of "low temperature nucleation, high temperature long shell", synthesized CdSe/ZnSe new core-shell quantum dots with high fluorescence quantum yield and strong stability through selenium anions (Figure 1). As a luminescent layer, such high-quality core-shell quantum dots can improve the match between the energy level and the transport layer, effectively reduce the hole injection barrier, improve the efficiency of carrier injection, overcome a series of problems caused by insufficient hole injection and excessive electron injection in QLED, and thus greatly improve the overall performance of the device.

Based on this new structure system, the maximum brightness and external quantum efficiency of red, green and blue QLED devices obtained by the research team reached 356,000 CD /m2, 614,000 CD /m2, 62,600 CD /m2 and 21.6%, 22.9% and 8.05%, respectively. The brightness, efficiency and blue brightness of red and green QLED devices are the highest records in the world at present (FIG. 2). This work breaks through the previous key problems of low efficiency and high efficiency at high brightness under QLED, and realizes the red, green and blue primary color QLED devices with high brightness and high efficiency for the first time.

To more accurately describe the luminescence characteristics of the new QLED, the team introduced a new concept called "Effective brightness (EFL)", which is defined as the product of peak EQE and its corresponding luminescence intensity. Figure 3 is the comparison of THE EFL of this three-color QLED and the reported work in the literature. It can be seen that the EFL of green devices has doubled, and the red and blue colors have been improved by nearly order of magnitude. Moreover, from the brightness and efficiency requirements of lighting, the three-color QLED obtained by this work has exceeded the corresponding threshold.

The stability (lifetime) of the device is another key factor restricting its application. The new QLED devices developed by this work also perform well in terms of life span. The life span of red and green QLED devices reaches more than 1.6× 106 h, and the life span of blue devices reaches more than 7000 h. The life span of green and blue devices is also the world's longest record at present.

These research results and the device model established not only demonstrate the possibility of the application of QLED in the field of high-brightness and high-efficiency display and lighting in principle, but also provide a new idea for the material system design and device structure optimization of QLED in the future.

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