Temperature Dependence and Anisotropy of the Mobilities

The ultrapure organic molecular crystals naphthalene, anthracene, tetracene, pery-lene and others have mobilities /r both for holes and for electrons whose values at room temperature all lie around Icm /Vs, but are not aU the same (Table 8.2). Their temperature dependencies /r(T) a T (n 0) at T 300 K are also similar but not identical. In particular, there are clear deviations from the ideal value of the exponent n = -3/2 (see Fig. 8.31). The temperature above which the relation 

How strongly a high defect concentration affects the temperature dependence of the mobility can be seen from the comparison of the experimental results for a perylene crystal with a concentration of 0.17% traps of depth Et = 270 meV (Fig. 8.33). Even near room temperature, the mobility ii decreases steeply with decreasing temperature. In this crystal with defects, hopping conductivity occurs even in the temperature region near 300 K. 

Finally, from Fig. 8.32, one can see a characteristic dependence of the mobility on the magnitude of the electric field F. It will be treated in detail in Sect. 8.5.3. 

In ultrapure naphthalene crystals [20] also, the exponents n are different for holes and electrons (Fig. 8.35 and Table 8.2). For holes, the value of n = -2.8 also deviates strongly from the ideal value -3/2. The reason for this is probably a continuous energy distribution of the shallow charge-carrier traps. Down to the lowest measurement temperature of about 4 K, no decrease in the mobility with decreasing temperature was observed. And also in naphthalene crystals at high electric fields, a decrease of /x with increasing field strength is found. 

Qualitatively similar values are found in ultrapure anthracene crystals [21] (Table 8.2). To be sure, the values of the mobilities at low temperature and the ex- 

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