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Electron Transport and Hole Blocking Materials
Multilayer OLED introduce functional layers, such as hole blocking layer (HBL) and electron transpor layer (ETL). ETL and HBL play the role of transporting electrons and blocking holes, and the ETL and the HBL are crucial to the device performance. Its main functions are:
- Reduce the energy level barrier between semiconductor layers, reduce energy consumption and open voltage
- Transfer hole and electron to ensure carrier balance in the device
- For devices with long exciton lifetime, such as phosphorescent OLED, the ETL also acts as an exciton barrier.
Oxadiazole derivatives
For the first time, oxadiazole derivative PBD is used as ETL in double-layer OLED devices. BND, another oxadiazole derivative, is also widely used as an ETL for OLED devices. Dimeric oxadiazole and star oxadiazole derivatives with high glass transition temperature have been widely studied and used as ETL materials.
Pyridine derivatives
ETL with high mobility and high triplet energy level is one of the requirements for the fabrication of phosphorescent OLEDs. However, to obtain higher triplet energy levels, the conjugation length of the material molecule needs to be limited, and in order to obtain high mobility, the conjugation length of the material molecule needs to be prolonged, which is contradictory. Two pyridine derivatives, TMPYPB and TPPYPB, solve this problem perfectly because of their high electron mobility and triplet energy levels, it is widely used in simplifying device structure and preparing high-efficiency OLED devices.
TPBI
With good electron transport properties and low LUMO energy levels, benzimidazole is a potential ETL material. TPBI, a benzimidazole derivative, is widely used as an ETL in fluorescent and phosphorescent OLED. Unlike ALq3, TPBI can act as an ETL for blue OLED because of its wide optical band gap. TPBI has high ionization energy, so it can be used as ETL or HBL in phosphorescent OLED.
Aluminum octahydroxyquinoline
Aluminum octahydroxyquinoline (Alq3) is an ETL material and green light-emitting material due to its excellent optoelectronic properties, thermal stability, carrier mobility, film stability and other factors. The Alq3 can form a perfect amorphous thin film state, which is very suitable for the preparation of OLED devices, and is an excellent semiconductor material that is enduring.
O-phenanthroline derivatives
Phenanthroline has a large rigid plane, and its derivatives usually have a high Electron mobility, which can be used as electron transport materials in OLED devices. Among them, 1,10-phenanthroline derivatives BCP and Bphen are widely used as excellent ETL/HBL materials. In addition, because of the low HOMO energy level of BCP, it can be used as a HBL to fabricate OLED devices.
References
- Wang C, Jung G Y, Hua Y, et al. An Efficient Pyridine- and Oxadiazole-Containing Hole-Blocking Material for Organic Light-Emitting Diodes: Synthesis, Crystal Structure, and Device Performance [J]. Chemistry of Materials, 2001, 13 (4): 1167-1173.
- Padmaperuma A B, Sapochak L S, Burrows P E. A New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence: 2,7–Bis(diphenylphosphine oxide)–9,9–dimethylfluorene[J]. Chemistry of Materials, 2006, 18(9):2389-2396.
- Jin L. Electroluminescence properties of poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene]/tris(8-hydroxyquinoline)aluminum two-layer devices[J]. Synthetic Metals, 2000.
- BW D’Andrade, Forrest S R, Chwang A B. Operational stability of electrophosphorescent devices containing p and n doped transport layers[J]. Applied Physics Letters, 2003, 83(19):3858-3860.
