Hole-Transport Material

For many years, the polymer was thought to have little influence on transport. Recent evidence, however, has shown that the polymer can have a very considerable effect on the mobilities (Sasakawa et al., 1989 Yuh and Pai, 1990, 1990a, 1990b, 1992 Kanemitsu and Eiinami, 1990 Aratani et al., 1990 Kanemitsu, 1992 Hirao et al., 1993). In part, these effects are related to the dipole moment of the polymer repeat unit, as described above. The mobilities of 

Polysilanes (West, 1986 Miller and Michl, 1989) are a class of materials that have high hole mobilities (Kepler et al., 1987 Stolka and Abkowitz, 1987 Stolka et al., 1987 Abkowitz and Stolka, 1990 Abkowitz et al., 1990 Majima et al., 1995 Nagayama et al 1996 Nakayama et al., 1996). These have been used as transport layers with generation layers of H2PC or TiOPc dispersed in poly(vinyl butyral) (Yokoyama and Yokoyama, 1989). A poly(phenylmethyl-silane) transport layer with a TiOPc generation layer performed satisfactorily in a laser printer. For a variety of reasons, including perhaps synthetic complexity, physical and mechanical properties, photoinstability, etc., these materials have not been widely investigated as transport layers. 

Hole injection from an azo pigment into transport layers containing various styiyl donor molecules has been correlated with oxidation potentials (Kakuta et al., 1981). The dependence of the sensitivity on the oxidation potential was somewhat weaker than predicted. In photoreceptors with an azo generation layer and transport layers containing enamine doped polymers, the sensitivity was the highest with donor molecules with low oxidation potentials (Rice et al., 

Others have noted that even within a given class of compounds, correlations between the photoinjection efficiencies and oxidation potentials are complex and depend on the charge-generation layer as well (Murayama et al., 1988). 

Within the framework of current transport theories, the task of the chemist is to prepare donor molecules in which the molecule has the appropriate oxidation potential, orbital delocalization, and solubility. Further, the effects of dipolar disorder and intermolecular dimer sites must be minimized. Finally, the physical and chemical interactions of the transport and generation materials (whether in a single or dual-layer configuration) must promote efficient charge generation and injection. The systematic integration of all of these characteristics is indeed formidable. 

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Photoconductive polymers are widely used in the imaging industry as either photosensitive receptors or carrier (electron or hole) transporting materials in copy machines and laser printers. This is still the only area in which the photoelectronic properties of polymers are exploited on a large-scale industrial basis. It is also one electronic appHcation where polymers are superior to inorganic semiconductors. 

TPD or TAD hole transport material) N,N -diphenyl-N,N -bf.v-(3-mcthylphenyl)-(1,1 )-biphenyl-4,4 -dianiiuc... 

The low efficiencies could be due to lack of intimate contact (interface) between the sensitizer (which is hydrophilic) and the spirobifluorene (which is hydrophobic). Moreover, the surface charge also plays a significant role in the regeneration of the dye by the electrolyte.98 In an effort to reduce the charge of the sensitizer and improve the interfacial properties between the surface-bound sensitizer and the spirobifluorene hole-carrier, amphiphilic heteroleptic ruthenium(II) complexes ((48)-(53)) have been used as sensitizers. These complexes show excellent stability and good interfacial properties with hole-transport materials, resulting in improved efficiencies for the solar cells. 

A better PLED performance was observed by Jenekhe and coworkers [173] for ITO/PEDOT/polymer/Al devices with quinoxaline-phenylene vinylene copolymers 586 and 587 as emitting layers. The el and maximum brightness were measured as 0.012 and 0.01%, and 120 and 35 cd/m2, respectively. The turn-on voltages of these devices were reasonably low, 6.0 and 4.0 V, respectively. The performance of PLEDs with polymer 586 was further improved by blending with 5wt% of a hole transport material, 1, l-Mstdi-d-tolylami-ii ophenyI )cycIohexane (TAPC) that enhanced the d lto 0.06% and the maximum brightness to 450 cd/m2. 

S. Liu, X. Jiang, H. Ma, M.S. Liu, and A.K.-Y. Jen, Triarylamine-containing poly(perfluorocy-clobutane) as hole-transporting materials for polymer light-emitting diodes, Macromolecules, 33 3514-3517, 2000. 

Once the electrons and holes have been injected, they migrate into ETL and HTL to form excited states referred to as polarons by physicists or radical ions by chemists. These polarons or radical ions move, by means of a so-called charge-hopping mechanism, through the electron and hole transport materials (ETMs and HTMs), which typically possess good charge mobility properties, and eventually into the EML. 

SCHEME 3.12 Chemical structures of some hole transport materials based on carbazole units. 

SCHEME 3.18 Composite hole transport material (c-HTM) CuPc (1) and a-NPD (13). 

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, ... 

A. Kimoto, J. Cho, M. Higuchi, and K. Yamamoto, Novel carbazole dendrimers having a metal coordination site as a unique hole-transport material, Macromolecular Symposia 209 (Organo-metallic and Coordination Clusters and Polymers) pp. 51-65 (2004). 

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