Unravelling Molecular Disorder at SAM-functionalized Charge Injection Interfaces in Organic Field-Effect Transistors

Charge injection in Organic Field Effect Transistors is investigated by monitoring in real-time the electrical conduction, while in situ growing pentacene films. The source and drain electrodes are made of gold functionalized with alkanethiol Self-Assembled Monolayers (SAMs), whose number of carbon atoms ranges from 5 to 10. Measuring the current flowing into the channel allows us to establish a correlation between the charge injection and the pentacene film growth. In particular, the thickness dependence of both the charge mobility and the charge carrier accumulation can be explained in terms of the molecular order at the interface between the pentacene films and the SAMs. Specifically, the threshold voltage, contact resistance and temperature-dependent constant exhibit parabolic trends for increasing alkanethiol chain length. Conversely, both the band mobility and the charge mobility are characterized by oscillations that follow an odd-even trend with a linear decay for increasing number of the carbon atoms. These results corroborate the theoretical interpretation, previously proposed by Stoliar et al., that the odd-even effect is ascribable to the order degree and orientation of the pentacene molecules interacting with the SAMs, while the parabolic trend depends on the chain length of the alkanethiol SAMs.