Bipolar transport electron mobilities

The multi-ion pair theories discussed in this section suffer from several deficiencies or simplifications. First of all, the electrons are considered to be classical species, and the quantum mechanical nature of all interaction processes is not considered. Even so, the variation of the transport properties with electric field strength (see Section 3.1) and the dependence of electron/ion recombination on the electron mobilities (see Section 3.5) is not taken into account. Bipolar diffusion should be considered in the dense plasma column around the track of an a-particle (Loeb, 1955). 

In photoconductive polymers the intersite hopping distance is not variable it is determined by the structure and morphology of the polymer. In molecularly doped polymers the average hopping distance can be varied at will, simply by changing the concentration of the transport-active species in the host polymer. In the PVK/TNF bipolar system there is evidence that holes move via uncomplexed carbazole groups, while electrons move via both complexed and uncomplexed TNF molecules [14]. The hole mobility decreases and the electron mobility increases as the TNF concentration increases [14] (Fig. 8.10). 

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

Mobility data on bipolar charge-transport materials are still rare. Some bipolar molecules with balanced mobilities have been developed [267], but the mobilities are low (10 6—10 8 cm2/Vs). Up to now, no low molecular material is known that exhibits both high electron and hole conductivity in the amorphous state, but it is believed that it will be only a matter of time. One alternative approach, however, is to use blends of hole and electron transporting materials [268]. 

The bipolar single-trap model assumes that both electrons and holes share identical trap centers. Since sequential trappings of the electrons and holes by the identical centers mean the neutralization of the electric charge, the effective space-charge field will depend on the relative power (i.e., the mobilities) of electron and hole transports. The expressions for the writing and erasing diffraction efficiency are [100] ... 

---