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With the aim of understanding the relationships between organic small molecule field-effect transistors (FETs) and organic conjugated polymer FETs, we investigate the thickness dependence of surface morphology and charge carrier mobility in pentacene and regioregular poly (3-hexylthiophene) (RR-P3HT) field-effect transistors. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility is largely related to the morphology of the organic active layer. We observe that the change trends of the surface morphologies (average size and average roughness) of pentacene and RR-P3HT thin films are mutually opposite, as the thickness of the organic layer increases. Further, we demonstrate that the change trends of the field-effect mobilities of pentacene and RR-P3HT FETs are also opposite to each other, as the thickness of the organic layer increases within its limit.

This paper investigates the morphology and crystallization properties of the two crystalline phases of pentacene grown by thermal evaporation on p+-Si substrates at room temperature by the methods of atomic force microscopy and x-ray diffraction. This kind of substrate induces a thin film phase and a triclinic phase which are formed directly onto p+-Si substrates and constitute a layer consisting of faceted grains with a step height between terraces of 15.8 A (1 A=0.1 nm) and 14.9 A, respectively. Above the critical thickness of the thin film phase, lamellar structures are found with an increasing fraction with the increase of the film thickness. When the film thickness is fixed, the fraction of lamellar structures increases with the increase of annealing temperature. These lamellar structures are identified as the second phase with a interplanar distance of 14.9 A corresponding to the pentacene triclinic phase. Furthermore, the thin film phase consisting of several micrometre sized uniformly oriented grains at an annealing temperature of less than 80°C and a deposition rate of 0.6 A/s is observed.