Electrical drift, molecular transport

Our initial estimates of molecular transport based on electrical drift should be extended by including convection [e.g., electroosmosis (31)] and diffusion (52). The same general strategy is reasonable A dynamic pore population will be computed, in which electrical interactions are the dominant source of pore creation and expansion. In the case of a pl nar membrane with no osmotic or hydrostatic pressure gradient, the final stages of pore population expansion and collapse should also be governed by purely electrical interactions. By following the pore population over its development, the contribution of each transport mechanism can be estimated. For cell membranes, a nonzero pressure difference will usually exist. In this case, pores of... 

Figure 6.—Continued. C, Predicted electrical drift contribution to molecular transport across one of the two cubic cell membranes. (Weaver, J. C. Barnett, A. Wang, M. W. B/iss, J. G., unpublished). A hypothetical series of molecules, a// with unit charge (zs = l) was used to test the relative importance of different size pores in the pore population. More realistic predictions would use estimates of the size (radius rs), shape (a form factor), and the Bom energy repulsion (zs>eff — zm, where m is a number in the range 1 < m < 2).

Numerous studies of charge transport in amorphous molecular materials have shown that hole drift mobilities of amorphous molecular materials vary widely from 10 6 to 10 2 cm2 NT1 s 1 at an electric field of 1.0 X 105 V cm-1 at room temperature, greatly depending upon their molecular structures. Table 7.6 lists hole drift mobilities of some amorphous molecular materials that function as holetransporting materials in OLEDs. 

A nanochannel is a conduit between two reservoirs of fluid with a characteristic internal diameter of roughly one to several tens of nanometers. A nanometer is a billionth of a meter (10 m). The fluid is assumed to be an electrolyte, that is, water with some dissolved salts that dissociate into positive and negative ions. Electrokinetic flow refers to fluid flow generated in such a chaimel when an externally applied electric field is the primary motive force. Ion transport refers to the average drift of the ions along the channel due to the electric field superimposed on the random molecular motion and collisions with the surrounding water molecules and channel walls. 

In conducting polymers, the extra carriers added upon doping are able to drift under an applied electrical field. In semiconducting polymers, no carriers are available except those thermally excited across the gap. However, negative (positive) carriers can be injected into the material by metallic contacts when the barrier between the metal work function and the LUMO (HOMO) molecular levels is overcome. Then, the injected carriers can move inside the semiconductor if a bias field is applied. Injection of carriers and their transport is a fundamental issue for all electronic devices and transistors in particular. In the following, main transport properties of organic semiconductors (both small molecules and polymers-based) used as active materials in transistors will be reviewed. 

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