Rubrene Hall mobility

FIGURE 2.1.21 Left the Hall mobility versus T for a rubrene OFET in the double-log scale (compare with Figure 2.1.20). (From Podzorov, V. et al., Phys. Rev. Lett., 95, 226601, 2005.) Right the calculated T-dependences of the hole mobility for different crystallographic directions in tetracene. (From Hannewald, K. and Bobbert, P. A.,AIP Conf. Proc. 772, 1101, 2005.)... 

Fig. 2 (Top) Temperature dependence of the mobUity in pentacene (reprinted from [46]) showing a band-like mobdity up to room temperature. (Bottom) Temperature dependence of the Hall mobility of holes in rubrene (full circles), essentially due to the free holes in the solid (reprinted from [50]). The total mobdity (empty circles) is simdar to the Had mobdity beyond 230 K. At lower temperature a fraction of carriers are trapped and the initial increase of total mobility with temperature is due to detrapping...

From the Hall data, the density of mobile carriers in the OFET s channel can be directly determined for the first time, without the assumptions regarding the gate-channel capacitance Q. Hall effect studies in other organic semiconductors (e.g., pentacene and tetracene) are highly desirable because most of the OFETs based on those materials still operate in the trap-dominated regime and the intrinsic mobilities at the surface of these semiconductors, To(7), are unknown. The Hall measurements, however, are complicated by a very high sheet resistance of the conduction channel typical for OFETs, which often exceeds 10 MO/square. For this reason, the first demonstration of the Hall effect appeared only recently, due to the high carrier mobility in rubrene [82,83]. 

The high quality of rubrene crystals has allowed detailed measurements of the transport characteristics, including the recent observation of the Hall effect [26]. Charge transport in rubrene single crystals, while trap-limited at low temperature, appears to occur via delocalized states over the 150-300 K temperature range with an (anisotropic) hole mobility of up to 20 cm /V s at room temperature [27,28]. 

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