Effect of the silanization and annealing on the morphology of thin poly(3-hexylthiophene) (P3HT) layer on silicon oxide

The role of the silanization of silicon oxide (SiOx) with different alkyl-silane molecules, in determining the morphology and structure of the overlying poly(3-hexylthiophene) (P3HT) layer has been studied by atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. Particular attention has been paid to the first thin layers close to the interface P3HT/SiOx. For each case the effect of the annealing temperature has been studied. For all the considered silanizations XRD investigations on the P3HT layers, 5-20 nm thick, reveal an edge-on configuration of the thiophene rings. However, a strong morphology dependence has been observed on the length and polarity of the silanizer. By using silanizer with substituents of short length (dimethyldichlorosilane (DMDS) and hexamethyldisilazane (HDMS)) and of significant polarity (3-aminopropyl-trietoxysilane (APS)), P3HT layer organizes into filaments where the macromolecular backbones, perpendicular to the surface, form Pi-Pi interaction. By moving to a longer alkyl substituents (octadecyltrichlorosilane (OTS) and propyltrichlorosilane (PTS)), a different globular morphology occurs where the same Pi-Pi edge-on stacking gives rise to shorter and thinner needle-like fundamental structures. The morphological features have been demonstrated to be highly related to the electron mobility of field effect transistors, having as acting layer P3HT film deposited over the two most significant silanized surfaces.