Over the past decade, humans have witnessed the rapid development of Quantum Dot Light Emitting Diodes (QLED).
With high color saturation, high efficiency, low cost and other advantages, QLED is often considered an ideal candidate for next-generation displays, and is widely used in flexible printed displays and solid-state lighting. However, its electroluminescence (EL) mechanism has not been reasonably explained.
Previously, the scientific research community has a rough definition of the working mechanism behind the upconversion EL of QLED, believing that it may be affected by Factors such as Auger's auxiliary energy upconversion, Coulomb gravity and thermoelectron emission under the action of electric fields, but it is still unknown what mechanism actually dominates.
Recently, a research team from the Department of Electronic and Electrical Engineering of Southern University of Science and Technology found through experiments that thermal energy-assisted thermoelectron emission is the key mechanism affecting the electroluminescence of QLED upconversion. Based on this conclusion, the team further analyzed the injection process of QLED's carriers (particles that carry charge and can move and carry current) to provide a deeper understanding of QLED and its subsequent device designs[1].
Figure | Related papers (Source: Nature Communications)
The paper was published in the journal Nature Communications under the title "Thermal assisted up-conversion electroluminescence in quantum dot light emitting diodes", with Shuming Chen, associate professor of the Department of Electronics and Electrical Engineering at Southern University of Science and Technology, as the corresponding author. The first author is Su Qiang, a doctoral student in his research group.
Thermal energy plays a key role in the upconversion electroluminescence process
"Initially, we didn't find energy upconversion or subband gap onset in QLED." Chen Shuming said.
According to him, his team has always been committed to the exploration of device structure, device physics and preparation processes. At first, the QLED devices developed mainly used the polymer PVK as the hole transport layer material, but because the stability of the PVK material was slightly insufficient, the team began to use another polymer material with better performance, TFB. This is also currently the most common cavity transport layer material in the industry.
Figure | Chen Shuming (Source: Chen Shuming)
However, after using the TFB transport layer material, Chen Shuming's team found that the expected efficiency difference is not obvious, and the biggest change is the increase in the device current and the reduction of the ignition voltage, especially the ignition voltage has been greatly lower than the voltage corresponding to the bandgap of the luminous material.
Previously, researchers generally believed that this was the role of Auger-assisted interface, but from the perspective of the quantum efficiency of the device, this statement is obviously not valid.
Therefore, Chen Shuming's team decided to start from the substrip gap to observe the mechanism behind the phenomenon and the specific behavior of the carrier in this process.
In previous work, Chen Shuming's team has found that in the process of variable temperature photoluminescence of quantum dots, thermal energy will have an impact on the luminous performance of quantum dots, which in turn will lead to the efficiency roll-off phenomenon of QLED under high-current operation, that is, its efficiency will show a rapid decline trend.
Therefore, Chen Shuming's team customized a device that can accurately control the temperature of the device, hoping to conduct some similar temperature control studies on the EL process of QLED to understand the behavior of the carrier during the injection process.
"During this process, we were pleasantly surprised to find that the ignition voltage changed significantly when controlling the temperature of the device's EL operating process." Chen Shuming said.
As shown in the figure, when the temperature of the environment in which the three different colors of QLED changes, the ignition voltage shows a clear trend.
Figure | EL characteristics of QLED over temperature changes (Source: Nature Communications)
This means that thermal energy must play a crucial role in the process of carrier injection.
The operating mechanism is universal and can be used to improve QLED device performance in the future
However, to determine the accuracy of a physical mechanism, you also need to determine whether it can be applied to other things. Therefore, when Chen Shuming's team found that thermal energy-assisted thermal electron emission is a key mechanism affecting the electroluminescence of QLED upconversion, they began to try to verify its universality.
After applying thermal control to other QLED of different structures, the team finally concluded that in the environment of continuously increasing thermal energy, the phenomenon of upconversion electroluminescence is not a flash in the pan, but can be achieved in different structures of QLED.
It is worth noting that the other structureSQ LEDs selected by the team do not have upconversion electroluminescent properties, but after increasing thermal energy, these Q LEDs have changed. This means that the role of thermal energy in the entire elconversion process is unquestionable.
Overall, the results solve a controversial problem that has plagued scientists for many years, and finally reveal the specific mechanism behind the conversion of electroluminescence hidden on QLED, thus laying the foundation for researchers to further clarify the working principle of the device.
In addition, although this work is more focused on mechanism research, it is also quite instructive for application development.
According to Chen Shuming, there are at least two application directions that can be tried: one is to use the subband gap to brighten up and develop QLED devices with energy conversion efficiencies close to or greater than 100%; the other is to improve the performance and brightness of outdoor display or lighting devices through the role of thermal energy. Its future potential should not be underestimated.
At the same time, Chen Shuming said that his team will do more in-depth exploration in subsequent application development, and hopes to make more scientifically meaningful results.
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reference:
1.Qiang Su & Shuming Chen，Nature Communications13，Article number: 369（2022）