Self-assembled monolayers as interfaces for organic opto-electronic devices

Charge injection into an organic semiconductor can be improved by using a s elf-assembled monolayer of functionalized molecules grafted on the electrod e. This new interface can be designed in order to reduce the Schottky barri er between the conductive electrode and the organic semiconductor. The pola rizability of the molecules involved can also be chosen in order to increas e the adhesion of the molecular semiconductor onto the electrode. We presen t Kelvin Probe experiments and saturated photovoltage measurements performe d on a number of such derivatized electrodes. They permit a quantitative de scription of the potential shifts due to the self-assembled monolayers whic h are related to the electrical dipoles of the individual molecules constit uting them. When conjugated sites contributing to the band states of the or ganic semiconductor are placed too close to the electrode in the negative p art of the image-force potential, two new effects unfavorable to charge inj ection can appear. We demonstrate that it is convenient to separate the att achment group of the molecule from the conjugated core by a spacer of non-c onjugated sites in order to reduce these undesirable effects.