Organic light-emitting diode (OLED) is also known as organic laser display and organic light-emitting semiconductor. OLED is a kind of current type organic light-emitting device, and the luminous intensity is proportional to the injected current. Under the action of an electric field, the electrons generated by the anode and the hole generated by the cathode will move and inject into the hole transport layer and electron transport layer respectively. When they meet at the luminescent layer, they create an energy exciton, which excites the luminescent molecules and eventually produces visible light. The practical organic light-emitting diode (OLED) device was firstly reported in 1987, as shown in figure 1. With the advantages of high brightness, low power consumption, fast response, high definition, good flexibility and high luminous efficiency, OLED can meet the new demands of display technology.
Figure 1 The diagram of OLED device.
OLED has the advantages of low power consumption, fast response speed, wide viewing angle, high resolution display and wide temperature tolerance, which suits for many fields.
- Business field: Bright colors often attract people's attention and are suitable for commercial advertising. OLED screen is flexible, light, and high anti-aging, which is both beautiful and practical and can be used as a business advertising screen.
- Electronics field: OLED is the most widely used in smartphone, laptop, tablet, and digital cameras. The color of the OLED display is bright, and can be adjusted by setting different display modes.
- Transportation field: The display screen size of ships, aircraft instruments, GPS, video phones and car display is small, so the wide viewing angle performance of OLED can clearly see the screen content even though not looking directly.
According to the exciton properties and luminescence mechanisms, organic light-emitting diode (OLED) materials can be divided into three categories: fluorescence, phosphorescence and TADF materials.
- Fluorescence materials: Fluorescent is the photon emission produced by the attenuation of singly excited state radiation to the ground state. The internal quantum efficiency (IQE) of fluorescent-based devices is capped at 25%. Three primary colors (red, green and blue, RGB) have been obtained with high brightness and high efficiency.
Figure 2 Structures of fluorescence materials.
- Phosphorescence materials: Phosphorescence materials have a large stokes shift between absorption and emission spectra, so they are not prone to spontaneous quenching. Phosphorescent OLED devices can achieve nearly 100% internal quantum efficiency. Such materials are typically organic metal complexes containing heavy metal atoms, such as platinum, iridium, ruthenium and osmium. These complexes have strong phosphorescent emission, and their color depends largely on the energy of the ligand, so the luminous color of the complexes can be adjusted by changing the structure of the ring metal ligand.
Figure 3 Molecular structures of typical phosphorescent materials.
- Thermally activated delayed fluorescence (TADF) materials: TADF materials have the advantages of easy synthesis, easy modification and low cost. Therefore, the TADF material is expected to be used in OLED based solid-state lighting sources and displays. The TADF molecule usually has a distorted D-A molecular structure.
Figure 4 An example of TADF molecule.
- Tang C W, Vanslyke S A. Organic Electroluminescent Diodes[J]. Applied Physics Letters, 1987, 51(12):913-915.
- Hatakeyama T, Shiren K, Nakajima K, Nomura S, Nakatsuka S, Kinoshita K, et al. Ultrapure Blue Thermally Activated Delayed Fluorescence Molecules: Efficient HOMO–LUMO Separation by the Multiple Resonance Effect [J]. Advanced Materials. 2016;28(14):2777-81.