Electron blocking material

STABILIZERS AND HOLE- AND ELECTRON-BLOCKING MATERIALS 3.7.1 Stabilizers... 

Low molecular weight and polymeric heterocyclics as electron transport/hole-blocking materials in organic light-emitting diodes... 

The criteria for good electron transport materials are that they should transport electrons, block holes, and have a small barrier to electron injection from the metal cathode. The most commonly used ETL in vacuum-deposited OLEDs is tris-(8-hydroxyquinoline) aluminum (Alq3), as shown in Figure 7.7. Alq3 for example, has a LUMO energy level of 3 eV [65] and an electron mobility of 5x 10 5 cm2/(V s) [66]. 

Since a large number of low-molar-mass emitters are also efficient electron-transport materials, there is a clear requirement for efficient, non-emissive, holetransporting, electron-blocking LMMMs. Fortunately, a large range of holetransporting, low-molar-mass organic materials had already been developed as... 

Figure 4.46. Molecular structures of commonly used OLED/PLED materials. Shown are (a) Alq3 (tris(quinoxalinato)Al (III)) used as an electron-transport material (b) DIQA (diisoamylquinacridone) used as an emissive dopant (c) BCP (2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline) used as an exciton/ hole blocking agent (d) NPB (l,4-bis(l-napthylphenyl amino)biphenyl) (e) PFO (9,9-dioctylfluorene) used as an emissive polymer in PLEDs (f) PEDOT-PSS (poly-3,4-ethylenedioxythiophene-polystyrene sulfonate) used as a hole transport material in PLEDs.

The results revealed that holes and electrons were more easily confined to this complex by constructing a proper device configuration. To prevent the hole from entering the AIQ layer, a 20 nm layer of hole blocking material, BCP, was inserted between complex 38 and AIQ in the device with configuration ITO/TPD (20 nm)/complex 38 (50 nm)/BCP (20 nm)/AlQ... 

New classes of hole-blocking amorphous molecular materials have recently been developed, which include the families of triarylbenzenes and triarylboranes (Table 7.4). These compounds readily form amorphous glasses with well-defined Tgs and possess weakly electron-accepting properties. These hole-blocking materials enabled fabrication of high-performance blue- and blue-violet-emitting OLEDs using a-NPD, p-TTA and TPD as emitters. The performance of some devices is summarized in Table 7.5. 

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