The penetration and diffusion of metal atoms in self-assembled monolayers (SAMs) and pentacene solids were studied by first-principles calculations. It was shown in the case of SAM that metal atoms easily penetrate and diffuse in solids, reflecting the weak interaction between metal atoms and molecules. Even when diffusing metal atoms produce a cluster, such a cluster can easily diffuse in solids. In the case of pentacene, on the other hand, since the interaction between metal atoms and molecules is attractive and strong, metal atoms actively enter into solids and are tightly bonded to molecules. We found that Au atoms prefer to produce a cluster at the molecule edge and such a cluster is difficult to move, while Al atoms are scatteringly distributed and last the diffusion owing to the repulsive interaction between Al atoms. We showed that the difference in metal-atom behavior comes from the difference in electronic structure between SAM and pentacene, which are, respectively, σand π-orbital molecular systems.

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Metal-atom contamination often induces the degradation of organic molecular devices. In this work, we studied clustering feature of Au and Al impurity metal atoms in pentacene solids by the first-principles calculations. We found that Au atoms prefer to produce clusters in a molecule-edge space due to the strong bonding among Au atoms, and such clusters can increase their sizes by producing molecule vacancies. On the other hand, Al atom prefers to locate separately around the center of pentacene molecules due to the strong bonding between Al atom and surrounding molecules, which produces the scattering distribution of Al atoms in pentacene solids.

Extended defects such as grain boundaries (GBs) promote serious damage for carrier transport in organic molecular solids. In this work, we studied electronic structures of specific examples of stacking-fault (SF) and GB defects in pentacene films by the first-principles calculation. We found that the SF decreases the carrier transfer for hole carriers, while it works as a scattering potential for electron carriers. In the case of the present GB, we showed that the band offset appears at the GB, reflecting the difference in molecular density and configuration across the GB. Moreover, we showed that the present GB works as a sink of impurity atoms and traps carriers.

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