Space-charge-limited current mechanisms

Charge transport in thin polypropylene films has been observed to be due to tunneling mechanism al low temperatures and Schottky etnisskn at high tcmpcratuies. Space charge-limited currents have been observed to flow in polystyrene in the steady state [322]. 

For PMMA and PMMA/BaTlOi composites with volume fractions of BaTiO, lower than 03, R< 1. In this case, the charge transport may be connected with the diffusion mechanism. Besides, if the composite contains more than 03 volume fraction BaTK)] ceramic, then ft > 1. In this case the charge transport can be described by space charge-limited current (SCLC). 

Important electrical informations about OLEDs, such as charge transport, charge injection, carrier mobility, etc., can be obtained from bias-dependent impedance spectroscopy, which in turn provides insight into the operating mechanisms of the OLED [14,15,73,75 78]. Campbell et al. reported electrical measurements of a PLED with a 50-nm-thick emissive layer [75], Marai et al. studied electrical measurement of capacitance-voltage and impedance frequency of ITO/l,4-Mv-(9-anthrylvinyl)-benzene/Al OLED under different bias voltage conditions [76], They found that the current is space-charge limited with traps and the conductivity exhibits power-law frequency dependence. 

Space-Charge Limited Versus Injection-Limited Current Mechanisms... 

The possibility of space charge limited transport was verified by measuring the dependence of the conducting current on film thickness. That this mechanism does make a contribution could, in accordance with the SCLC theory (3JI), follow from the second branch of the Inj-lnV characteristics where n > 2 (Fig. 3). However, in no case do the results obtained confirm the j a/ d n dependence required by the SCLC theory, where d is film thickness, and n - a parameter depending on trap distri-... 

Hickmott has discussed the possible mechanisms in considerable detail. The tentative theory which he has advanced is, briefly, that the production of conductivity in the forming process is due to the development of impurity-band conduction, in a concentrated band of impurity states which are ionized in the forming process. The current is then space-charge limited. The impurity-band nature of the conduction is supposed to account for the small temperature dependence. To account for the negative resistance it is supposed that as the field is increased the impurity-band states are filled by electrons tunneling from valence band or lower impurity states. The process involved in the development of the impurity band conduction is somewhat uncertain. 

Electrokinetic phenomena of the second kind have received mostly academic interest, but there may be important and unexpected applications. Already, second-kind electroosmosis has been implicated as a mechanism for superUmiting current in electrodialysis. Space-charge formation at nanochannel/microchannel junctions has also been exploited to trap biomolecules, although in that case the flow is undesirable since it interferes with trapping. The speed of second-kind electrophoresis may be useful in separation or mixing, but the need to apply a large DC voltage may limit its applicability. 

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