Hole blocking material

J. Lu, Y. Tao, M. D iorio, Y. Li, J. Ding, and M. Day, Pure deep blue light-emitting diodes from alternating fluorene/carbazole copolymers by using suitable hole-blocking materials, Macromolecules, 37 2442-2449, 2004. 

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

I. 3,5-triazine ethers as hole-blocking materials in electroluminescent devices, Chem. Mater., 10 3620-3625 (1998). 

Figure 187 (a) The external quantum efficiency of organic LEDs using 6.2mol% [Ir(ppy)3] TCTA as an emitter and three different hole-blocking materials, vs. device current density, (b) The EPH device structure and molecular structures of materials used, (c) The energy level scheme of the EL device part (b). After Ref. 612. Copyright 2001 American Institute of Physics, with permission. 

Figure 11.5 Structures of some hole-blocking materials.

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... 

A hyperbranched polymer 42 comprising oxadiazole subunits has been synthesized, but defect formation in such a structure appears to limit its use as a hole-blocking material [74],... 

Hole-blocking materials for use in OLEDs should fulfill several requirements. 

Bathocuproine (BCP) has been used as a hole-blocking material. However, it tends to form exciplexes with a number of emitting materials with hole-transporting properties, e. g., TPD and m-MTDATA, and emits exciplex emission in the longer wavelength region [51]. Its morphological and thermal stability have not been made clear. 

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. 

For a compound to act as an ideal hole-blocking material, it must have a HOMO level deeper than that of the dopant and host material, and also have a triplet energy high enough to efficiently prevent triplet excitons from migrating out of the luminescent layer (Figure 28). The two most common hole-blocking materials are... 

A number of other organic materials have also been used as hole-blocking materials, for example, fluorinated... 

The effect of a hole-blocking material on electron injection and exciton distribution in the red phosphorescent OLEDs was investigated in detail by Meerheim et In the study, Ir(piq)3 was replaced with the efficient... 

Since this polymer has a high thermal stability and an amorphous morphology, it should be a promising candidate as a hole blocking material for organic light-emitting diodes [57]. 

There are examples in the literature where the balance between holes and electrons is reached by inserting a hole blocking material between the ITO and the active polymers. For example, a simple polymethylmethacrylate (PMMA) monolayer prepared by means of the Langmuir Blodgett technique was reported to give a four-fold efficiency increase if introduced between the FLO and the active layer [113]. Greenham, Niiesch and Yang [114-116] followed similar approaches to control the performance of a MEH-PPV based device and a polyparaphenylene-based device respectively. 

Basic structures of electron transport/hole blocking materials and oligomeric and polymeric materials for charge transport and luminescence... 

Fig. 7.4 Chemical structures of typical electron conducting and hole blocking materials...

---