Charge amorphous molecular materials

Usually, in amorphous molecular materials, charge transport is described by a disorder formalism that assumes a Gaussian distribution of energetic states of the molecules between which the charges jump [246]. The mobility is then given by... 

Recently, a novel series of amorphous molecular materials based on carbazole and methine dyes has been synthesized [89], These molecular materials exhibit a very interesting charge-transfer complex formation and large PR responses. 

Charge-transporting and Charge-blocking Amorphous Molecular Materials for Organic Light-emitting Diodes... 

Charge carrier drift mobilities of a number of amorphous molecular materials have been determined by a time-of-fhght method, and their electric-field and temperature dependencies have been analyzed in terms of the disorder formalism [56, 57] ... 

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. 

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]. 

When charge transport fails to reach a steady state during the time available, the most likely reason is that the transit time is dominated by the time required to escape from the slowest site(s) that a carrier encounters as it crosses the sample. Furthermore, the distribution of release times is such that the carrier continues to encounter slower and slower sites as it crosses a sample. Transport under such conditions is called dispersive, and has been the subject of much study since a seminal paper by Scher and Montroll [73a-e]. The term dispersive alludes to the wide dispersion in release times and/or the fact that carriers that are injected simultaneously spread out, disperse, to an anomalous extent as they cross the sample. The literature has several examples of studies of this subject in amorphous molecular solids [66b, 73f-h]. Some materials undergo a transition from essentially dispersive transport at low temperatures to essentially nondispersive transport at higher temperatures, and this dispersive-to-nondispersive transition has been the subject of significant attention [73i-p]. 

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