Carboxylic acid anchoring groups for the construction of self-assembled monolayers on organic device electrodes

We have investigated the surface adsorption of carboxylic acid derivatives on both an aluminium cathode and an indium-tin oxide anode, commonly used in light emitting devices. For this purpose, a series of benzoic acids with systematically varying dipole moments were used to form self-assembled monolayers on these electrodes. The work function shifts induced by the adsorbed molecules were measured by the Kelvin probe method, while the surface densities of the adsorbed molecules were inferred from Langmuir isotherm measurements. It appeared that the work function shifts correlate linearly with the molecular dipole moment measured in the gas phase. Furthermore, the dipole moment which is formed upon adsorption depends crucially on the surface acido-basicity of the electrode. The difference of the adsorption dipoles on indium-tin oxide and aluminium is as high as 3.5 D indicating a very different binding mechanism in the two cases.