As an electro-optic converter, QLED converts electrons into photons. According to the law of conservation of energy, the energy of injected electrons must be greater than or equal to the energy of emitted photons, that is, it must satisfy: (where is the driving voltage). Therefore, the turn-on voltage of QLED should satisfy. However, the turn-on voltage of most QLEDs is low. For example, for a red QLED emitting 2.0 eV photons, its turn-on voltage is often as low as 1.6 V. This phenomenon is called upconversion EL or sub-bandgap turn on, and the physical mechanism behind it has been controversial. The working mechanisms widely discussed by researchers in the past more than ten years include: defect-assisted upconversion EL, Coulomb attraction, and electric field-assisted hot electron injection. Based on the research reported in existing literature, the Chen Shuming research group, with a brand-new experimental argument and detailed theoretical analysis, concluded that the main mechanism of upconversion EL in QLED is thermally assisted hot electron injection.


(1) The Key Role of Thermal Energy
Figure 1.1 is the temperature-dependent EL characteristics made for this purpose. From Figures 1.1a-1.1c, it can be clearly seen that during the temperature change of red, green, and blue QLEDs in the ambient environment, the turn-on voltage all shows a significant decreasing trend. Taking a red QLED as an example, when the temperature increases from -140 °C to 160 °C, the corresponding turn-on voltage (defined as the voltage at 0.1 cd·m⁻² brightness) decreases from ~2.0 V to ~1.25 V. For an electron, the energy required to emit photons close to 2.0 eV with an energy of 1.25 eV from the electric field is amazing, and its EL upconversion efficiency reaches an astonishing 156% (see Figure 1.1e). At the same time, Figure 1.1g also shows that under the sub-bandgap voltage (1.6 V), the brightness of the red QLED increases significantly with the increase of temperature. This obvious change proves that thermal energy plays a key role in the upconversion EL process.

Figure 1.1 EL Characteristics of QLED with Temperature Change

(2) The Role of Thermal Energy in the Kinetics of Carrier Injection
A typical QLED uses P-type polymer semiconductors and N-type ZnO inorganic nanoparticles as hole and electron transport layers, respectively. Due to the differences in energy level matching of materials and carrier mobility, the injection of holes is more difficult than that of electrons. Therefore, the key to understanding upconversion EL lies in analyzing the injection kinetics of holes. Taking red QLED as an example, Figure 1.2 shows the energy level structure of the device and the change of energy level from voltage to the entire process. The detailed analysis of the voltage change of QD has two nodes to help us understand this problem:

(a) When the voltage reaches the flat-band voltage of QD (); as shown in Figure 1.2c, the additional external electric field at this time pulls up the energy band on the electron side, allowing electrons to enter QD smoothly, but holes still face the built-in potential and interface barrier at the TFB (hole transport layer)/QD heterojunction interface. At room temperature (RT), the energy level of holes is low and cannot reach and accumulate at the interface, so it is difficult to form effective hot electron injection, and QLED cannot be turned on; but at high temperature (HT), the kinetic energy of holes increases. According to the thermal excitation effect and the Maxwell-Boltzmann distribution, the energy and proportion of high-energy particles in holes will both increase. These high-energy particles in holes have enough energy to accumulate at the heterojunction interface. At the same time, they are injected into QD by hot electron injection, which improves the environmental thermal energy and reduces the turn-on voltage of QLED, thereby increasing the upconversion efficiency.

(b) When the voltage reaches the turn-on voltage (); as shown in Figure 1.2d, the increased external voltage further reduces the built-in potential of TFB, thereby greatly reducing the injection barrier of holes. At this time, at room temperature, an effective hole accumulation layer can be formed at the heterojunction interface, and some high-energy particles among them are successfully injected into QD by hot electron injection. It is precisely because of the successful injection of this part of high-energy particles that the red QLED realizes the energy upconversion from 1.6 eV to 2.0 eV under RT conditions. This work reveals the key role of environmental thermal energy in the process of hole sub-bandgap injection, and clarifies the necessary conditions for upconversion EL: QD reaches the flat-band voltage and the heterojunction interface forms effective carrier accumulation. The data calculated according to this mechanism is in good agreement with the experimental results.

Figure 1.2 Carrier Injection Kinetics Process of Red QLED

(3) The Universality of the Mechanism Elucidated in This Work
The correctness of the physical mechanism is largely judged by whether it is universal. To verify this, this work transfers the control of thermal energy to QLEDs with two other structures. The results show that QLEDs with different energy level structures can achieve upconversion EL when the operating environment thermal energy is increased (as shown in Figure 1.3). Especially considering that the two verification structures selected in this work do not have upconversion EL characteristics at RT, it further confirms the decisive role of thermal energy in the entire upconversion EL (sub-bandgap turn-on).

Figure 1.3 Universality of Thermally Assisted Upconversion EL (Verification in Other QLED Structures)

This work reveals and answers the long-standing upconversion mechanism problem that has puzzled researchers for many years, and analyzes the carrier injection kinetics process in QLED in detail. This work not only provides a reference for researchers to better understand the working mechanism of devices, but also provides new ideas for realizing upconversion EL devices with an upconversion efficiency exceeding 100%.

In addition, the Chen Shuming research group continues to innovate in QLED structure development and has developed a full-color suspended QLED with individually addressable red, green, and blue. This work was recently published in the cooperative publishing journal Nature under the title "Flexible and tandem quantum-dot light-emitting diodes with individually addressable red/green/blue emission".

Suspended QLED has the potential of ultra-high brightness and long lifetime. However, for suspended quantum dot light-emitting diodes fabricated by solution methods, there have always been challenges in the operational reliability of multi-layer continuous solution processes and the independent control of each light-emitting unit. To solve the above problems, the authors use high-performance red, green, and blue three-color flexible transparent QLEDs as building units, use transparent flexible substrates as intermediate separators, and use ultraviolet curing glue as a binder, and successfully prepare a full-color suspended transparent QLED with excellent performance and individually addressable. When using this scheme to vertically stack red and green emission layers, not only can external quantum efficiencies of 12.0%, 8.5%, and 4.5% be achieved when emitting red, green, and blue alone, but each unit can also work in series and parallel modes as needed, with external quantum efficiencies of 24.8% and 8.2%. In addition, while ensuring the performance of the device, this structure avoids the damage problem caused by multi-step solution processes very well.

This work provides a new experimental strategy for realizing new suspended QLEDs and developing individually addressable full-color display and lighting devices.

 Figure 2.2 Structure, Preparation and Performance Characteristics of Full-Color Suspended QLED with Individually Addressable Red, Green and Blue

The first authors of the above research are all 2019 master's students in the Chen Shuming research group, and the corresponding author is Chen Shuming. Southern University of Science and Technology is the first unit of the paper. The research was supported by projects such as the National Natural Science Foundation General Project, Guangdong Provincial Higher Education Research Project, and Shenzhen Basic Research Project.

Paper 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