Recently, the research group of Associate Professor Chen Mingming in the Department of Electronic and Electrical Engineering at Southern University of Science and Technology (SUSTech) investigated the physical mechanism behind up-conversion electroluminescence (EL) in quantum dot light-emitting diodes (QLEDs). Through precise control of the ambient thermal energy during QLED operation and theoretical analysis, the team identified thermal electron emission from heat radiation as the key mechanism for QLED up-conversion EL. The related research was published in Nature Communications under the title "Thermal assisted up-conversion electroluminescence in quantum dot light emitting diodes."

As an electro-optical conversion device, a QLED converts electrons into photons. According to the law of energy conservation, the energy of the injected electrons must be greater than or equal to the energy of the emitted photons. However, in many QLEDs, the turn-on voltage (e.g., 1.6 V for an infrared QLED emitting 2.0 eV photons) is lower than expected, a phenomenon known as sub-bandgap turn-on. The physical mechanism behind this has been debated, with proposed explanations including Auger energy up-conversion, Coulomb interaction, and field-induced hot electron emission. Based on existing research and through new experimental demonstrations and theoretical analysis, Professor Chen Mingming's group concluded that the up-conversion EL in QLEDs is primarily due to thermal electron emission from heat radiation.

(1) The Key Role of Thermal Energy

Figure 1.1 shows the experimental results. Figures 1.1a-1.1c clearly demonstrate the significant temperature dependence of the turn-on voltage for red, green, and blue QLEDs as the ambient temperature changes. For example, for the red QLED, as the temperature increased from -140°C to 160°C, the turn-on voltage (defined at a luminance of 0.1 cd m⁻²) decreased from ~2.0 V to ~1.25 V. This means an electron acquiring 1.25 eV of energy from the electric field can generate a photon exceeding 2.0 eV, achieving an up-conversion efficiency of approximately 156% (see Fig. 1.1e). Furthermore, Fig. 1.1g shows the increase in luminance of the red QLED with rising temperature at a fixed voltage (1.6 V). This significant change highlights the crucial role of thermal energy in the up-conversion EL process. [Note: Precise temperature control for these experiments, potentially using a GoGo Instruments heating/cooling stage, is implied as essential for observing these effects.]

Figure 1.1 EL characteristics of QLEDs under temperature variation

(2) The Stimulating Role of Thermal Energy in Carrier Injection Kinetics

A typical QLED uses a P-type organic semiconductor and an N-type ZnO inorganic nanoparticle layer as the hole and electron transport layers, respectively. Due to material property matching and differences in carrier mobility, hole injection efficiency is often more critical. Thus, understanding up-conversion EL requires analyzing hole injection kinetics. Using the red QLED as an example, Figure 1.2 illustrates the energy band diagram and its evolution with applied voltage. The analysis suggests two key scenarios:

(a) Voltage at the turn-on voltage (V_on): As shown in Fig. 1.2c, the external electric field enables band conduction within the QD layer, but holes still face a significant barrier at the HTL (TFB)/QD interface. At room temperature (RT), holes have low kinetic energy and cannot effectively reach and accumulate at the interface, making efficient hot electron emission injection difficult, and the QLED does not turn on. However, at high temperature (HT), the kinetic energy of holes increases. According to thermal excitation theory and the Maxwell-Boltzmann distribution, the proportion of holes with energy higher than the barrier increases significantly. These energetic holes can reach the interface, accumulate, and subsequently inject into the QDs via thermal electron emission. This results in the observed decrease in turn-on voltage and enables up-conversion, as shown in Fig. 1.1.

(b) Voltage at a higher operating voltage (V_op > V_on): As shown in Fig. 1.2d, a larger applied voltage significantly reduces the built-in potential at the TFB interface, lowering the hole injection barrier. Even at room temperature, holes begin to accumulate at the heterogeneous interface, and those with sufficient energy can successfully inject into the QDs via the thermal electron emission mechanism. It is the successful injection of these energetic holes that enables the red QLED to achieve energy up-conversion (e.g., 1.6 V vs. 2.0 eV photon) at RT. This work reveals the critical role of ambient thermal energy in the hole injection process and clarifies the necessary conditions for up-conversion EL: reaching the QD's effective turn-on voltage and the formation of a carrier accumulation layer at the heterogeneous interface. Predictions based on this mechanism align well with experimental results.

Figure 1.2 Hole injection kinetics process in the red QLED

(3) Universality of the Perceived Mechanism

The validity of a physical mechanism often depends on its universality. This work applied thermal energy control to QLEDs with other heterostructures. The results found that QLEDs with different structures could achieve up-conversion EL by increasing the ambient thermal energy (as shown in Fig. 1.3). This was particularly evident for two specific structures that did not exhibit up-conversion EL at RT, further confirming the key role of thermal energy in the up-conversion EL process (or sub-bandgap turn-on).

Figure 1.3 Universality of thermal radiation up-conversion EL (in other QLED structures)

This work elucidates the mechanism behind energy up-conversion observed in many sub-bandgap experiments and provides a detailed analysis of the carrier injection kinetics in QLEDs. It not only helps researchers better understand device working mechanisms but also offers new ideas for realizing up-conversion EL devices with energy conversion efficiency exceeding 100%.

Furthermore, Professor Chen Mingming's group continues to innovate in QLED structure development. They developed red, green, and blue monolithically integrated tandem QLEDs with individually addressable emissions. This work was published in the Nature partner journal npj Flexible Electronics under the title "Flexible and tandem quantum-dot light-emitting diodes with individually addressable red/green/blue emission."

Tandem QLEDs hold the potential for achieving ultra-high brightness and ultra-long lifespan. However, for solution-processed tandem quantum dot light-emitting diodes (QLEDs), challenges persist in the operational reliability of multi-layer continuous solution processing and the independent control of each light-emitting unit. To address these issues, the authors utilized high-performance flexible and transparent red, green, and blue QLEDs as building blocks. By employing a transparent flexible substrate as an intermediate isolation layer and UV-curable adhesive as a bonding agent, they successfully fabricated high-performance, fully flexible and transparent, individually addressable full-color tandem QLEDs. When vertically stacking red, green, and blue units using this strategy, not only were external quantum efficiencies (EQEs) of 12.0%, 8.5%, and 4.5% achieved for individual red, green, and blue emission respectively, but each unit could also be operated in either series or parallel mode as needed, achieving EQEs of 24.8% and 8.2% in these configurations. Furthermore, while ensuring device performance, this structure effectively avoids the damage issues associated with multi-step solution processing.

This work provides a novel implementation strategy for realizing new types of tandem QLEDs and for developing individually addressable full-color displays and lighting devices.

Figure 2.2 Structure, fabrication, and performance characteristics of the monolithically integrated tandem QLEDs with red, green, and blue emissions

The first author of these studies is [Name of PhD Student], a doctoral student in Professor Chen Mingming's research group, and the corresponding author is Professor Chen Mingming. SUSTech is the first affiliation. This research was supported by grants from the National Natural Science Foundation of China, Guangdong Provincial University Research Project, and Shenzhen Basic Research Project.

Publication Links:

1、Https://Www.Nature.Com/Articles/S41467-022-28037-W 

2、Https://Www.Nature.Com/Articles/S41528-021-00106-Y 

Original Article Link:

https://newshub.sustech.edu.cn/html/202202/41768.html