Electron-Transport Material

It is the interaction of the above requirements that has made the search for acceptor molecules difficult. To achieve the desired reduction potential, it is necessaiy to attach electron-withdrawing substituents, such as nitro and dicyanomethylene, on a parent aromatic ring system such as fluorene (Kuder et al, 1978). Unfortunately, many of these compounds show severe electron trapping and are highly polar. Also, the introduction of electron-withdrawing substituents frequently reduces the solubility such that amorphous layers with high acceptor concentrations cannot be prepared. 

Alkyl-substituted nitrated fluorene-9-ones were synthesized with the expectation of improved solubility and lowered toxicity (Loutfy and Ong, 1984 Loutfy et al., 1984 Ong et al., 1985). These showed improved solubility and somewhat improved mobilities compared to TNF. Studies of dual-layer photoreceptors prepared with (4-H-butoxycaibonyl-9-fluorenylidene)malonitrile transport layers have been described by Loutfy et al. (1984) and Murti et al. (1987). Low-field trapping was observed. The mobilities of this molecule in a polyester (PE) have been described by Borsenberger et al. (1990) and Borsenberger and Bassler (1991a). A transport layer with this molecule pendant lias been prepared and used with a TiOPc generation layer (Sim et all., 1996). A very significant residual potential was observed. For reasons that are not well understood, this is a common observation for dual-layer photoreceptors prepared with acceptor doped transport layers. The residual potential usually increases with cycling such that the useful process lifetime is limited. 

Thus far, electron transport layers have not been used in commercial applications. Positively charged photoreceptors have been either single-layer or have the generation layer uppermost. Because of the influence of dipolar disorder on mobilities, acceptor molecules must have a combination of a low reduction potential, high solubility, and low dipole moment. This combination has thus far proven elusive. 

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Other electron-deficient heterocyclic systems have also been investigated as electron-transporting materials. In particular, devices employing poly(phenyl qui-noxaline) 43 as an ECHB layer have shown improvements in device efficiency when used in conjunction with an emissive PPV layer [75]. 

Scheme 1-13. Examples of oxadiazole-containing electron-transporting materials.

Kim SH, Han SK, Park SH, Park LS (1998) A new dithiosquarylium dye for use as an electron transport material in an organic electroluminescent device having poly(p-phenylene vinylene) as an emitter. Dyes Pigm 38 49-56... 

C Adachi, MA Baldo, SR Forrest, and ME Thompson, High-efficiency organic electrophosphor-escent devices with tris(2-phenylpyridine)iridium doped into electron-transporting materials, Appl. Phys. Lett., 77 904-906, 2000. 

Electron transport materials for organic light-emitting diodes A.P. Kulkarni, C.J. Tonzola, A. Babel, and S.A. Jenekhe Chem. Mater., 16 4556-4573... 

Electron-transporting materials for organic electroluminescent and electrophosphorescent devices G. Hughes and M.R. Bryce J. Mater. Chem., 15 94-107... 

Due to relatively high electron affinity and very good PL efficiency, molecular materials based on oxadiazole, particularly, PBD (21) are among the most popular electron transport materials for OLEDs. The oxadiazole moieties, including PBD, were introduced as pendant... 

Polybenzobisthiazoles 608 and polybenzobisoxazole 609 have been used as efficient electron transport materials in PLEDs [71] (Chart 2.143). Although these polymers show poor fluorescence quantum yields in thin films likely due to excimer formation [700], double-layer devices ITO/PEDOT/polymer/ETL/Al with PPV or MEH-PPV as emissive polymers and... 

M.M. Alam, and S.A. Jenekhe, Polybenzobisazoles as efficient electron-transport materials for improving the performance and stability of polymer light-emitting diodes, Chem. Mater., 14 4775-4780, 2002. 

E.-N. Chang and S.-A. Chen, Cyano-containing phenylene vinylene-based copolymer as blue luminescent and electron transport material in polymer light-emitting diodes, J. Appl. Phys., 85 2057-2061, 1999. 

C.J. Tonzola, M.M. Alam, B.A. Bean, and S.A. Jenekhe, New soluble n-type conjugated polymers for use as electron transport materials in light-emitting diodes, Macromolecules, 37 3554-3563,2004. 

S. Dailey, W.J. Feast, R.J. Peace, I.C. Sage, S. Till, and E.L. Wood, Synthesis and device characterization of side-chain polymer electron transport materials for organic semiconducting applications, J. Mater. Chem., 11 2238-2243, 2001. 

M. Uchida, T. Izumizawa, T. Nakano, S. Yamaguchi, K. Tamao, and K. Furukawa, Structural optimization of 2,5-diarylsiloles as excellent electron-transporting materials for organic electroluminescent devices, Chem. Mater., 13 2680-2683 (2001). 

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