Organic light-emitting diodes (OLED) have the advantages of flexible preparation, self-illumination, low driving voltage, wide color gamut and low power consumption. Coupled with continuous technological breakthroughs in the past year, OLED have become one of the most popular research topics in the development of flat panel display, new lighting, wearable and intelligent electronic products. However, there are still some limiting factors to be solved, such as aggregation-caused quenching effect (ACQ) and low output coupling efficiency. Recent reports have devised good solutions to these problems.
ACQ effect is an important problem that restricts the development of traditional luminescent materials. The luminescent materials with ACQ properties fluoresce strongly in the solution state, on the contrary, they are weak or non-fluoresce in the aggregation state. Until 2001 [1], the discovery of aggregation-induced emission (AIE) phenomenon provided a good idea for solving ACQ. In recent years, AIE systems have been designed and applied in organic electroluminescent devices.
In addition to the influence of ACQ, the carrier transport balance in the device is another important factor to determine the overall performance of the device. Considering these two aspects, Zhang [2] designed five luminescent compounds by using tetraphenylethene (TPE) with AIE properties, carbazole (Cz) with good hole mobility and benzimidazole (PBi) with good electron transport capacity (see Fig. 1). Compare with other compounds, the TPE-2 with both AIE properties and balanced hole and electron moieties, and therefore its non-doped OLEDs showed best performances with maximum luminance (Lmax), current efficiency (CEmax), power efficiency (PEmax), and external maximum efficiency (EQEmax) of 24308 cd m−2, and 15.10 cd A−1, 14.82 lm W−1 and 5.34% respectively. Therefore, we provided a strategy of designing the dipolar AIE emitter, which shows applied in the efficient non-doped OLED device.
Fig.1 Chemical structures of Cz-PBi, TPE and its derivatives TPE-n (n=1-3)
In the design of OLEDs, low carrier mobility requires the use of employ thin-film active layers sandwiched between thin film electrodes. Thus, the output coupling rate is greatly reduced. The main reason may be the small size of the active emitting medium, which leads to the low emission intensity, and the high optical loss caused by the long and long-term interaction between the electroluminescence and the metal electrode.
Huang [3] reported a large-area array of end-emit- ting nano OLEDs, where they intend to reduce the waveguide confinement effects and the optical absorption by the electrodes. More importantly, the nanoscale metallic electrodes supply strong localized surface plasmons (LSPs) that enhance largely the scattering of light and the output coupling of the device. The end-emitting design also facilitates directional output coupling performance. It opens up a new way for the development of OLED and is beneficial to the research and application of miniaturized devices integrating optoelectronics.
Fig.2 Schematic illustration of the design of the periodic array of the end-emitting nano OLEDs with directional output.
These studies provide new thinking directions for the wider application of OLED devices and provide reference for the majority of researchers.
References
- Hong Y.; et al, Aggregation-induced emission. Chemical Society Reviews, 2011; 40(11): 5361-5388.
- Zhang J.; et al. Efficient bipolar AIE emitters for high-performance nondoped OLEDs[J]. Journal of Materials Chemistry C, 2020, 8(34): 11771-11777.
- Huang C.; et al, End-emitting nano organic light emitting diodes (OLEDs) with directional output[J]. Nanophotonics, 2020, 9(9): 2905-2913.
