Identification of polymorphs of pentacene

Abstract

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 [J. Phys. Chem. B. 106 (2002) 8288]. We have synthesized four different polymorphs of pentacene, identified by their layer periodicity, d(0 0 1): 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 (ED). 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/V s at room temperature.