Barrier lowering and reorientation of dipoles grafted at indium-tin-oxide/polymer interfaces

Dipolar molecules have been grafted on the surface of indium-tin-oxide plates. Using these modified substrates as anodes for organic diodes made from poly-vinylcarbazole enhances the hole current by 1-2 orders of magnitude as compared to diodes with nonmodified anodes. The organization and surface concentration of the grafted monolayer, determined by infrared spectroscopy, show that the grafted layer is not densely packed. The influence of the dipolar molecules on the energy barrier height for the injection of hole current into the organic layer is also measured by internal photoemission, evidencing that the barrier is lowered by the molecular grafting. The free space around the grafted molecules allows for their reorientation under electric field. The energy barrier height has been measured as a function of time during this reorientation.