Pentacene crystals

FIGURE 2.1.22 Temperature dependence of the hole mobility in ultrapure pentacene crystals extracted from the SCLC measurements. The open symbols correspond to the valnes of p calculated under an assumption of uniform current flow across the crystal thickness solid symbols take into account the anisotropy of conductivity in pentacene. Below room temperature, the mobility increases with decreasing T as p T T where y 2.4. (From Jurchescu, O. D. et ah, Appl. Phys. Lett., 84, 3061, 2004.)... 

In particular, it was shown that 6,13-pentacenequinone (the product of pentacene oxidation) forms electronic defects in pentacene, and careful purification of the commercially available material by vacuum sublimation is required to achieve a significant reduction of this impurity [85]. Following an extensive purification procedure, it has been estimated from SCLC measurements that the bulk density of traps in ultrapure pentacene crystals can be as low as 2 10" cm. However, the density of traps can be considerably larger close to the crystal surface, which is normally exposed to the environment during the OFET assembly. 

As seen in Figure 4.3.5, similar full first layer coverage and layer-by-layer formation of dendritic pentacene crystals are observed for oxidized and H-terminated Si surfaces. However, at film surface coverage (0) = 0.14 ML (calculated from AFM... 

The regime of intermolecular interaction energies shown in Figure 2.10 most probably spans the value of 7 relevant for pentacene crystals [19]. The low-field mobility can be directly deduced fi om the calculated polaron velocity. These results set an upper limit on the mobility since the simulations are performed for a defect fi-ee system. Possibly, the mobility could change sKghtly if we would go from a ID to 2D system. The fact that the polaron is rather delocalized even in the case of /= 50 meV indicates that the pure nonadiabatic (or hopping) treatment, such as that given by the Marcus theory [16] or the... 

In Figure 3.13, the results of pentacene [45] are shown as an example. For penta-cene (Pn), the values of mj in the F—Xp and F— Ypn directions at 300 K are determined to be 3.02mo and 1.86mo, respectively, by applying Eq. (3.14) to the measured dispersion rdations. This result demonstrates the presence of the anisotropy of the hole mobility in pentacene crystals also at room temperature. Furthermore, by comparing with the other experimental E k) relations in pentacene films [46-48], it was confirmed that the band structure of pentacene films is very sensitive to the minor difference in the film structure, espedally molecular tilt angle, because of bumpy special distribution of MO of the molecule. Tliis was also... 

Sadaharu, J., Kentaro, K., Mitsuru, T., 2014. Growth process of pentacene crystals obtained by physical vapor transport technique. Japanese Journal of Applied Physics 53, 115506. 

Tsiper EV, Soos ZG (2003) Electronic polarization in pentacene crystals and thin films. Phys Rev B 68 085301... 

Lang et al. have analyzed the tail of the density of states using optical (photocurrent) and electrical (LET) methods [221]. They considered various existing models that describe the tail of the density of states and they concluded that their observation is consistent with the presence of some residual disorder which is not due to impurities (they used ultrapure crystals). The band tails are similar to those observed in amorphous inorganic solid, in agreement with the model of [172] which predicts that, from the electronic point of view, the pentacene crystal at room temperature can be seen as a disordered material. 

The p-terphenyl pentacene crystals used in the experiment described below were grown from the melt by the Bridgman technique using extensively zone-refined p-terphenyl. The pentacene obtained from Aldrich was vacuum sublimed once before... 

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