Molecular-scale properties of MoO₃ -doped pentacene

The mechanisms of molecular doping in organic electronic materials are explored through investigation of pentacene p -doped with molybdenum trioxide (MoO₃). Doping is confirmed with ultraviolet photoelectron spectroscopy. Isolated dopants are imaged at the molecular scale using scanning tunneling microscopy (STM) and effects due to localized holes are observed. The results demonstrate that donated charges are localized by the counterpotential of ionized dopants in MoO₃ -doped pentacene, generalizing similar effects previously observed for pentacene doped with tetrafluoro- tetracyanoquinodimethane. Such localized hole effects are only observed for low molecular weight MoO₃ species. It is shown that for larger MoO ₃ polymers and clusters, the ionized dopant potential is sufficiently large as to mask the effect of the localized hole in STM images. Current-voltage measurements recorded using scanning tunneling spectroscopy reveal that electron conductivity decreases in MoO₃ -doped films, as expected for electron capture and p -doping.