The hole transport properties of 5'-[4-[bis(4-ethylphenyl)amino]phenyl]-N,N,N',N'-tetrakis(4-ethylphenyl)-[1,1':3',1''-terphenyl]-4,4''-diamine (pEFTP), an aromatic polyamine, have been studied extensively before. We report on the hole injection processes from metal electrodes, or redox solutions, into polymers molecularly doped with pEFTP. By use of the redox couple Fe2+/Fe3+ at large fields, the injection of charges seems to be limiting the current density. At lower fields, competition between recombination and escape processes could be observed in the presence of the reduced form of the redox couple. The influence of the work function of the metals on the values of the dark currents suggests injection controlled currents characterized by almost exclusive hole transport.
The field, temperature and concentration dependence of the dark injection of holes and electrons from metal electrodes into molecular dispersions of respectively 5′-[4-[bis(4-ethylphenyl)amino]phenyl]-N,N,N',N'-tetrakis(4-ethylphenyl)-[1,1′:3′,1′'- terphenyl]-4,4′'-diamine and 1,3-bisdicyanomethylene-2-methyl-allyl-indane in polymers has been investigated. All results point to a two step process: in a first step charge carriers are injected to the bottom of the potential well created by an image potential between the charge transport material and the electrode. The competition between escape of the charge carriers from this well and recombination with empty levels in the electrode governs the final rate of charge carrier injection. While in a qualitative way this approach resembles an extension to doped polymers of the Willig-Gerischer model, developed for organic single crystals, a quantitative description of this system requires us to consider also the diagonal and non-diagonal disorder of the hole transport material.
Upon investigation of the photoconductive properties of polycarbonate doped with 1,3-bis(dicyanomethylene)indans, a large, temperature independent quantum yield for charge generation was observed. The field and temperature dependence of the charge carrier mobility could be analyzed in the framework of the disorder model, developed by Bässler et al. The prefactor mobility was found to be of the same order of magnitude as that observed for hole transport materials with a comparable dipole moment. In the framework of the used disorder model, this relationship would suggest a dependence of the wave function decay constant on the molecular dipole moment. This would result in a material dependence of the prefactor mobility, which has also been found for other charge transport molecules.
The electroluminescent properties of 5′-[4-[bis(4-ethylphenyl)amino]phenyl] -N,N,N′,N′-tetrakis(4-ethylphenyl) -[1,1′:3′,1″-terphenyl]-4,4″-diamine (pEFTP) were investigated in a vapor deposited layer and dispersed in a polymer matrix. Blue–violet electroluminescence was observed, after applying voltages beyond 12 V for the single layer devices and 22 V for the double layer devices. The electroluminescence (EL) spectrum shows two maxima, of which one corresponds to that of the photoluminescence spectrum. Either direct radiative recombination of the hole and the electron residing at two neighboring molecules or phosphorescence is causing this red shifted electroluminescence maximum. Transient electroluminescence measurements allow us to estimate the mobilities of the charge carriers in the different transport layers. The occurrence of an EL overshoot after switching off a voltage pulse, confirms the importance of detrapping and interfacial phenomena in the radiative recombination in pEFTP.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Electron transport layers , comprised of polymers doped with electron acceptor compounds have proved difficult to develop as teh inclusion of the acceptors generally results in poor solubility in coating solvents, limited miscibility and even toxic properties. The title compounds (see Figure) are soluble, miscible with a wide range of polymers, and show essentially to absorption in the visible region of the spectrum. magnified image
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