Controlling charge injection by self-assembled monolayers in bottom-gate and top-gate organic field-effect transistors

We investigate the modulation of the charge injection in organic field-effect transistors with self-assembled monolayers (SAMs) using both a bottom-gate and a top-gate geometry. The current modulation by using SAMs is more pronounced in the top-gate geometry due to the better defined upper surface of the bottom source and drain electrodes. By modifying Ag electrodes with a perfluorinated monolayer an injection barrier as high as 1.6 eV into poly(9,9-dioctylfluorene) can be surmounted, enabling the measurement of the saturated field-effect mobility of 6 × 10^?5 cm² V^?1 s^?1.     

Intermolecular segregation of siloxane in P3HT: surface quantification and molecular surface-structure

Surfaces of spin-coated and solution-cast poly(3-hexylthiophene) (P3HT) films are analysed by X-ray Photoelectron Spectroscopy and Low Energy Ion Scattering. Here, we use the P3HT-siloxane system with only 2% siloxane monomers in the bulk as a model system to study segregation and surface orientation of molecules in polymers. The surfaces are enriched in siloxane due to the intermolecular segregation of the siloxanes present in P3HT. The siloxane coverage fraction was found to depend on the preparation parameters such as spinspeed and solution-concentration, and ranges from 25 to 100%.The molecular orientation of segregated siloxanes on P3HT was found to resemble that on pure PDMS. Furthermore, siloxane molecules prefer specific sites on P3HT, such that sulphur atoms are screened from being at the outermost surface to lower the surface free energy. The results presented here demonstrate clearly the unique ability of LEIS to quantify the composition of the outermost atomic layer, and to obtain detailed information on the surface structure.