Hole organic materials

There is one added layer which deserves special mention, namely a thin copper phthalocyaninc layer, which has been placed [103] between an 1TO anode and the hole transport layer. It is not an injection layer in the sense just discussed, because its HOMO is not well aligned with the 1TO Fermi energy and it slightly raises the operating voltage of the structure. It does, however, dramatically improve the stability of the device and appears to act as an adhesion layer for the organic materials above it. The inechanism(s) for these improvements is not yet well understood. 

One has to consider that in Eqs. (9.15)—(9.17) the mobility /t occurs as a parameter. As it will be pointed out below, // shows a characteristic dependence on the applied electric field typical for the type of organic material and for its intrinsic charge transport mechanisms. For the hole mobility, //, Blom et al. obtained a similar log///,( ) const. [E dependency [88, 891 from their device modeling for dialkoxy PPV as it is often observed for organic semiconductors (see below). 

Just as metals can be ductile or brittle, so can organic materials. The Brittle Fracture Index is a measure of the brittleness of a material. It is a measure of the ability of a compact of material to relieve stress by plastic deformation. The Brittle Fracture Index (BFI) is determined [29,31] by comparing the tensile strength of a compact, stress concentrator) in it, o-T0, using the tensile test we have described. A hole in the center of the compact generally weakens a tablet. If a material is very brittle, theoretical considerations show that the tensile strength of a tablet with a hole in it will be about one-third that of a solid tablet. If, however, the material can relieve stress by plastic deformation, then the strength of the compact with a hole in it will approach that of a compact with no hole. The Brittle Fracture... 

O. Stephan, F. Tran-Van, and C. Chevrot, New organic materials for light emitting devices based on dihexylfluorene-co-ethylenedioxythiophene copolymers exhibiting improved hole-injecting properties, Synth. Met., 131 31 40, 2002. 

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

The HIL acts as an interface connection layer between the anode and the HTL so as to improve the film forming property of the subsequent organic layer and to facilitate efficient hole injection. Hole injection materials (HIMs) should have good adhesion to the anode and should serve to smooth the anode surface. The most common HIMs are... 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

J. Blochwitz, M. Pfeiffer, T. Fritz, and K. Leo, Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material, Appl. Phys. Lett., 73 729-731 (1998). 

C. Adachi, K. Nagai, and N. Tamoto, Molecular design of hole transport materials for obtaining high durability in organic electroluminescent diodes, Appl. Phys. Lett., 66 2679-2681 (1995). 

D.F. O Brien, P.E. Burrows, S.R. Forrest, B.E. Koene, D.E. Loy, and M.E. Thompson, Hole transporting materials with high glass transition temperatures for use in organic light-emitting devices, Adv. Mater., 10 1108-1112 (1998). 

R.D. Hreha, C.P. George, A. Haldi, B. Domercq, M. Malagoli, S. Barlow, J. Bredas, B. Kippelen, and S.R. Marder, 2,7-fe(diarylamino)-9,9-dimethylfluorenes as hole-transport materials for organic light-emitting diodes, Adv. Func. Mater., 13 967-973 (2003). 

J.P. Chen, H. Tanabe, X. Li, T. Thoms, Y. Okamura, and K. Ueno, Novel organic hole transport material with very high Te for light-emitting diodes, Synth. Met., 132 173-176 (2003). 

S. Tokito, K. Noda, K. Shimada, S. Inoue, M. Kimura, Y. Sawaki, and Y. Taga, Influence of hole transporting material on device performance in organic light-emitting diode, Thin Solid... 

M.S. Bayerl, T. Braig, O. Nuyken, D.C. Muller, M. Gross, and K. Meerholz, Crosslinkable hole-transport materials for preparation of multilayer organic light emitting devices by spin-coating, ... 

E. Bacher, S. Jungermann, M. Rojahn, Y. Wiederhirn, and O. Nuyken, Photopatterning of crosslinkable hole-conducting materials for application in organic light-emitting devices, Macro-mol. Rapid Commn., 25 1191-1196 (2004). 

Charge transport The holes and electrons must move through the device under the influence of the applied electrical field. The mobility of holes in typical hole transport organic materials is approximately 10 cm2/(V s) [65], For electrons the mobility is usually one or more orders of magnitude lower [66],... 

To facilitate good charge transport in an OLED, the organic materials must satisfy three key requirements they must have a high mobility for either electrons or holes, a good injection efficiency from the contact electrode, and suitable band offsets with other organic layers within the device. These processes are discussed in detail by, for example, Kalinowski [73] and Greenham and Friend [74],... 

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