Hole amorphous materials

Bach et al. have successfully introduced the concept of a solid p-type semiconductor (heterojunction), with the amorphous organic hole-transport material 2,2, 7,7 -tetrakis (, V, V-di-/ -methoxyphcnyl-aminc)9,9 -spirobifluorenc [96]. This hole-conducting material allows the regeneration of the sensitizers after electron injection due to its hole-transport properties. Nevertheless, the incident photon-to-current conversion efficiencies using complex 22 as a charge-transfer sensitizer... 

The structure of M41S-type materials is built up of pores with amorphous walls that are formed around micelles of templating material (surfactants). One of the extreme structures of M41S-type materials (MCM-41) is a hexagonal ordering of the pores, an other extreme is a worm-hole disordered type of arrangement of the pores. A lamellar layered structure is another form in which these type of materials often (partially) appear, but this phase collapses to amorphous material upon removal of the surfactant (eg by calcination). A cubic ordering of the pores is also encountered. This form has been named MCM-48 and will not be discussed in the current paper. 

The theory predicts a strong dependence of photogeneration efficiencies on the field and it approaches unity at high field. The temperature sensitivity decreases with the increase in field. The theory has found satisfactory explanations in the photogeneration process in many organic disordered systems, such as PVK (Scheme la) [25], and triphenylamine doped in polycarbonate [26], Figure 4 shows an example of the field dependence of c() calculated from Eq. (22) (the solid lines) to fit the quantum efficiency data at room temperature for hole and electron generation in an amorphous material. The material consists of a sexithiophene covalently linked with a methine dye molecule (compound 1) (Scheme 2). 

Among arylamine semiconductors, indolo[3,2-ib]carbazole (5) is an appealing system for studies because it has a relatively large and planar molecular structure to enable facile establishment of higher structural order for charge transport [57, 58]. Earlier, we reported the use of 5,ll-bis(l-naphthyl)indolo[3,2-jb]carbazole, a derivative of 5, as a hole transport material in OLEDs [59]. Because of its sterically encumbered naphthyl substituents, this compound forms only an amorphous film on vacuum deposition, and mobility is low in OTFTs, as expected. 

Optical absorption and recombination processes involve two or more particles and so may include correlation effects. Electron-hole pairs form excitons in a crystal, with the result that the absorption and emission spectra are not described by the one-particle density of states distributions. Although excitons can exist in an amorphous material (see Chapter 3), they are not detected in the optical spectra and the absorption is described by the convolution of the one-particle densities of valence and conduction band states. The correlation effects in... 

A spin-dependent recombination rate is another consequence of the electron-hole correlation. The conservation of spin selection rule is preserved in amorphous materials. The final state of the recombination process has zero spin and both radiative and phonon-assisted non-radiative transitions occur without change in spin, so that recombination can only proceed from an initial state of zero spin. A weakly interacting electron-hole pair forms four possible spin states, one singlet and one triplet. Of the four states, only the singlet and one... 

The initial sensitizer anion presence makes recombination of mobile holes possible in the dark regions. Which are the compensator sites Here, there exist different explanations. One possibility is that some of the electro-optic dye molecules present in photorefractive composites to provide refractive index change may become charged positively. An alternative theory in the case of amorphous materials is that the amorphous disorder leads to defect sites forming local potential minima at which positive charge may be immobilized (Figure 5). 

More recently the promising range of applications for photorefractive materials has motivated the rapid development of amorphous, organic materials with a strong photorefractive response [5]. Here the chemical composition of the materials may be varied with relative ease and the opportunity to compare materials from different sources should exist. The various processes necessary for photorefraction may be obtained by a single material, or many different molecular species may be mixed in a composite to provide the range of properties needed. These amorphous materials do not have a well-defined mobility for the photogenerated holes that... 

Discovery of amorphous silicon and its dopability has already had a tremendous impact on industrial applications of amorphous materials. Amorphous Si is now used fairly extensively as a photovoltaic material. In photovoltaic applications, solar photons excite the electrons across the gap and the resulting electron-hole pairs, are driven towards the respective electrodes in order to prevent their recombination. Electron is driven through an external resistance to generate the electrical power. The efficiency of conversion of solar energy to electrical power is characterized by an efficiency factor, r, which is given by. 

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. 

In several higher mobility amorphous hole transporting materials such as PTAA... 

Methods based on quenching from the melt, such as piston and anvil, double piston, torsion catapult and roller casting (Luborsky, 1980) permit larger quantities of amorphous materials to be obtained. The last of these, also known as melt spinning, is probably the method most used to obtain amorphous ribbons of thickness 10-60 pm, width up to 17 cm and virtually infinite length. In this method, the molten alloy is propelled through a small hole or a slot onto a massive, cold metallic disc rotating... 

Hole-transporting materials (HTM) have relatively low ionization potentials (1P). 2 The IP is defined as the energy required to remove an electron from the highest occupied molecular orbital (HOMO) of a substance. It can be measured, for instance, by photoelectron spectroscopy or obtained from electrochemical oxidation potentials in solution. It is also preferable that the HTM have sufficiently high hole drift mobilities. Various classes of materials have been used in the HTL, for example, starburst amorphous molecules, spirocompounds, triarylamines, and tetraarylbenzidines are representative classes of well-known HTMs. The structure of the most commonly used HTM, 4,4 -bis[N-(l-naphthyl)-N-phenylamino]biphenyl (NPB), is shown in Figure 14.2. 

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