No abstract

We determined the shifts in the energy levels of approximately 15 nm thick poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] films deposited on various substrates including self-assembled monolayer (SAM) modified Au surfaces using photoelectron spectroscopy. As the unmodified substrates included Au, indium tin oxide, Si (with native oxide), and Al (with native oxide), a systematic shift in the detected energy levels of the organic semiconductor was observed to follow the work function values of the substrates. Furthermore, we used polar SAMs to alter the work function of the Au substrates. This suggests the opportunity to control the energy level positions of the organic semiconductor with respect to the electrode Fermi level. Photoelectron spectroscopy results showed that, by introducing SAMs on the Au surface, we successfully increased and decreased the effective work function of Au surface. We found that in this case, the change in the effective work function of the metal surface was not reflected as a shift in the energy levels of the organic semiconductor, as opposed to the results achieved with different substrate materials. Our study showed that when a substrate is modified by SAMs (or similarly by any adsorbed molecules), a new effective work function value is achieved; however, it does not necessarily imply that the new modified surface will behave similar to a different metal where the work function is equal to the effective work function of the modified surface. Various models and their possible contribution to this result are discussed.

We have investigated the reaction of trimethylaluminum (TMA) with -CH 3 , -OH, and -COOH terminated self-assembled monolayers (SAMs) adsorbed on Au, using time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. TMA is a well-known atomic layer deposition precursor that is employed commercially to deposit compound semiconductors, alumina, and nitrides. We demonstrate that TMA can be employed to deposit both alumina and aluminum on SAMs at room temperature. TMA reacts with -OH and -COOH terminated SAMs to form a surface-bound dimethyl aluminum complex but does not react with -CH 3 terminal groups. If deposition is performed in a nitrogen-purged glovebox, an alumina film is grown on -CH 3 , -OH and -COOH terminated SAMs. The alumina film can be removed from -CH 3 terminated SAMs by rinsing with organic solvents. However if the base pressure of the deposition chamber is below 10 -8 Torr, a metallic Al overlayer is selectively deposited on -OH and -COOH terminated SAMs, and no reaction is observed on -CH 3 terminated SAMs. Using these reactions, we demonstrate that alumina and aluminum can be selectively deposited on patterned SAMs. The possible reaction pathways involved in the film growth on these different surfaces are discussed.

No abstract

The chemistry and the morphology of metal-deposited organic semiconductor interfaces play a significant role in determining the performance and reliability of organic semiconductor devices. We investigated the aluminum metallization of poly(2-methoxy-5,2′-ethyl-hexyloxy-phenylene vinylene) (MEH-PPV), polystyrene, and ozone-treated polystyrene surfaces by chemical (x-ray and ultraviolet photoelectron spectroscopy) and microscopic [atomic force microscopy, scanning electron microscopy (SEM), focused ion beam (FIB)] analyses. Photoelectron spectroscopy showed the degree of chemical interaction between Al and each polymer; for MEH-PPV, the chemical interactions were mainly through the C–O present in the side chain of the polymer structure. The chemical interaction of aluminum with polystyrene was less significant, but it showed a dramatic increase after ozone treatment of the polystyrene surface (due to the formation of exposed oxygen sites). Results showed a strong relationship between the surface reactivity and the condensation/sticking of the aluminum atoms on the surface. SEM analysis showed that, during the initial stages of the metallization, a significant clustering of aluminum takes place. FIB analysis showed that such clustering yields a notably porous structure. The chemical and the morphological properties of the vapor-deposited Al on organic semiconductor surfaces makes such electrical contacts more complicated. The possible effects of surface chemistry and interface morphology on the electrical properties and reliability of organic semiconductor devices are discussed in light of the experimental findings.