Charge-Carrier Mobilities in Organic Molecular Crystals

Charge-Carrier Mobilities in Organic Molecular Crystals 263... 

The mobihties /t of the charge carriers in the materials from which OLEDs are fabricated are low in comparison to the mobilities in organic molecular crystals (see Chap. 8). Furthermore, they are very different in the different materials there are some materials in which predominantly electrons are transported (fig ft),), and some in which mainiy holes participate in the transport (/t), /tg). Hole-transport materials, e.g. the naphthyl-phenyldiamine-biphenyl derivative NPB, have a relatively low ionisahon energy and therefore form radical cations preferentially and reversibly. Electron transport materials, e.g. Alqs, have a relatively high electron affinity and thus form radical anions preferentially and reversibly. 

The charge carrier mobilities p differ fundamentally in ultrapure aromatic molecular crystals from those in less-perfect organic crystals or disordered organic sohds or polymers. Fig. 8.1 shows the temperature dependence of /c in an ultrapure perylene crystal between 300 K and 30 K, the mobility increases with decreasing temperature. This holds generally for high-purity single crystals. In this particular case, p increases from about 1 cm /Vs at room temperature to around... 

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