Important progress has been made in the field of hot exciton-deep red OLED materials

With its lightweight, flexibility, and self-illumination, organic light-emitting diodes (OLEDs) are widely recognized as the mainstream third-generation display technology. Organic electroluminescent materials are one of the most critical components of OLED. Recently, “heat exciton” materials that can rapidly invert the intersystem transition (RISC) process through high-level channels have attracted intensive attention in the OLED community. Interestingly, a theoretical 100% internal quantum efficiency (IQE) and low roll-off rate can be obtained through the thermal exciton path. However, red thermoexciton materials still inevitably suffer from aggregation-induced quenching (ACQ) in the aggregated and clustered states, resulting in relatively low photoluminescence quantum yields (PLQYs), and there is a lack of clear molecular design strategies to improve PLQYs to date. On the other hand, aggregation-induced luminescence (AIE) is a compelling photophysical phenomenon. However, due to the lack of effective triple exciton utilization strategies, the efficiency of most AIE-based OLEDs is still low.

Recently, Professor Ge Ziyi and Associate Professor Li Wei of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, and Professor Su Shijian of South China University of Technology proposed a novel molecular design strategy to successfully fuse the heat exciton mechanism and AIE properties into a single molecule. In the newly developed molecules, T-IPD and DT-IPD (Figure 1), by adjusting the conjugate length of the receptor unit, the energy of the singleplex excited (S1) state is significantly increased to the second triplet excitation (T2) state, thereby greatly enhancing the inverse interphyletic flyover process (hRISC) of the high-energy state (Figure 2). By introducing TPA and DP-TPA donor groups, T-IPD and DT-IPD can form rigid and twisted three-dimensional geometries with appropriate dihedral angles, which effectively inhibit intermolecular π-π accumulation and intramolecular motion, making them show strong luminescence in solid or aggregated states. At the same time, their AIE properties can be further enhanced by the formation of J-aggregate structures in the aggregate state. Due to the heat exciton mechanism and AIE properties, the researchers achieved an external quantum efficiency of up to 12.2%, which is the highest performance among dark red OLEDs based on the heat exciton mechanism and AIE properties (Figure 3).

Fig. 1 Chemical structure of molecules T-IPD and DT-IPD developed by newly developed materials, as well as schematic diagram of the distribution of front orbitals and natural transition orbitals

Figure 2 Schematic diagram of energy level regulation

Figure 3 Efficient triplet exciton utilization device

To further elucidate the thermal exciton relaxation process in electroluminescent devices by the hRISC process and the triple-triple annihilation (TTA) moiety, the researchers performed transient electroluminescence (TREL) measurements of T-IPD and DP-IPD-based non-doped devices using an electrical pulse width of 100 microseconds. Strikingly, TREL attenuation presents two distinct components: fast EL attenuation and delayed EL decay. After the voltage pulse stops, fast EL attenuation results from single exciton emission within a few nanoseconds, while delayed EL attenuation is the result of the participation of long-lived triplet excitons in the emission process. However, the experimental results show that in HLCT systems, the hRISC process occurs rapidly within a few nanoseconds, resulting in rapid EL decay rather than delayed EL decay.

In addition, the researchers observed that delayed EL decay (Idelayed) fits well with the TTA model, which is due to the low T1 levels of T-IPD and DP-IPD, following the bimolecular upconversion emission formula:

The proportion of delay attenuation components of T-IPD and DP-IPD-based non-doped OLEDs was only 4.0% and 5.6%, indicating that TTA upconversion was limited, mainly due to low T1 exciton density. The low proportion of this delay attenuation component is not enough to explain its significant high efficiency, further verifying the heat exciton mechanism of T-IPD and DP-IPD.

The results were published in Advanced Materials (DOI: 10.1002/adma.202303304) under the title “Hot Exciton Mechanism and AIE Effect Boost the Performance of Deep-Red Emitters in Non-doped OLEDs.” Juniper Juniper, 2020 co-trainee of Ningbo Institute of Materials, Luo Ming, a 2023 master’s graduate, and Dr. Li Deli of South China University of Technology are co-first authors, Ge Ziyi, associate researcher and associate researcher Li Wei of Ningbo Institute of Materials, and Professor Su Shijian of South China University of Technology are co-corresponding authors. This research has been supported by the National Outstanding Youth Fund of China (21925506), the National Natural Science Foundation of China (U21A20331, 51773212, 81903743, 52003088), and Ningbo Key Science and Technology Projects (2022Z124, 2022Z119). (Source: Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences)

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