Relationship between the surface structure of the gate insulator and the performance of organic thin-film transistors

Abstract To investigate the impact on the device performance of different surface structures of the self-assembled monolayer (SAM) used to form the gate insulator of organic field effect transistors (OFETs), sum frequency generation spectroscopy and contact angle measurements were performed on SAM insulators with different surface structures. Furthermore, the surface structure of the SAM was varied with changing its dipping or spin coating formation method. Moreover, the electrical properties of OFETs were examined when only the surface structure of the gate insulator had changed. The results showed that compared with the dipping method, the disorder of alkyl chains (gauche defects) increased on the octadecylphosphonic acid (ODPA) SAM surface, deposited through the spin coating method. Contact angle measurements revealed that the surface free energy of ODPA-SAM increased in tandem with an increase in gauche defects on the SAM surface. Contrary to expectations, the device performance of OFETs with the SAM insulator with gauche defects improved, which is most probably due to the smaller surface free energy difference between ODPA-SAM and poly(3-hexylthiophene) (P3HT), which improved the adhesion at the ODPA-SAM/P3HT interface.