Effects of an interface monolayer with downward dipole orientation on pentacene organic field-effect transistors

To further understand the electric field effect of interface monolayer on OFET behavior we studied the current-voltage properties of pentacene OFETs with downward dipole orientation of an inserted monolayer. The results showed compared with the reference sample, the OFETs with downward orientation of a dipole monolayer have a positive threshold voltage shift, whose value as well as sign is as expected from our previously proposed model. However, simultaneously they possess much lower effective mobility, which is outwardly out of our expectation. To understand the reason, we further study the carrier injection and transport property with terms of contact resistance and channel conductance. It is found that the inner electric field generated by the downward interfacial dipole monolayer, as expected, enhances the total electric field in pentacene and thus decreases the contact resistance. However, the downward interfacial monolayer also greatly minimizes the channel conductance, which results in the depression of effective mobility as observed. This study indicates the local electric field is quite important for organic devices.