  Pentacene crystallizes in a layered structure with a herringbone arrangement
within the layers. The electronic properties depend strongly on the stacking of
the molecules within the layers (Haddon et al., 2002). We have synthesized four
different polymorphs of pentacene, identified by their layer periodicity,
d(001): 14.1, 14.4, 15.0 and 15.4 A. Single crystals commonly adopt the 14.1 A
structure, whereas all four polymorphs can be synthesized in thin film form,
depending on growth conditions. We have identified part of the unit cell
parameters of these polymorphs by X-ray and electron diffraction. The 15.0 and
15.4 A polymorphs transform at elevated temperature to the 14.1 and 14.4 A
polymorphs, respectively. Using SCLC measurements, we determined the mobility
of the 14.1 A polymorph to be 0.2 cm^2/Vs at room temperature.
