In situ study of pentacene interaction with archetypal hybrid contacts: Fluorinated versus alkane thiols on gold

One approach developed to improve the performance of bottom contact source/drain electrodes is to treat the contacts with thiols before deposition of the semiconductor. There is evidence indicating that improvement is due to both morphological effects and improved work function matching. Especially suggestive evidence shows that thiols that increase the effective work function of the contacts (e.g., fluorinated thiols) yield better device performance than work function decreasing thiols (e.g., alkane thiols). Here, we compare two technologically relevant thiol treatments, an alkane thiol (1-hexadecanethiol), and a fluorinated thiol (pentafluorobenzenethiol), in pentacene organic field effect transistors. Using in situ semiconductor deposition, x-ray photo-emission, and x-ray absorption spectroscopy, we are able to directly observe the interaction between the semiconductor and the thiol-treated gold layers. Our spectroscopic analysis suggests that there is not a site-specific chemical reaction between the pentacene and the thiol molecules. A homogeneous standing-up pentacene orientation was observed in both treated substrates, consistent with the morphological improvement expected from thiol treatment in both samples. Our study shows that both the highest occupied molecular orbital-Fermi level offset and C 1s binding energy are shifted in the two thiol systems, which can be explained by varied dipole direction within the two thiols, causing a change in surface potential. The additional improvement of the electrical performance in the pentafluorobenzenethiol case is originated by a reduced hole injection barrier that is also associated with an increase in the density of states in the lowest unoccupied molecular orbital.