Interest in dynamic behaviour of carriers in organic materials is motivated by possible applications that include organic thin film transistors, organic electroluminescent (EL) devices, and organic photo-conductors. It can also provide insight into modelling of carrier transport and trapping in organic semiconductors and insulators Here, we employ advanced SHG technique to probe and visualize real carrier motion in organic materials. This is a time-resolved microscopic optical SHG (TRM-SHG) technique that allows direct and selective probing of dynamic carrier motion in organic materials. TRM-SHG experiments using pentacene field effect transistors (FET) revealed dynamic changes of SHG intensity profiles arising from pentacene. Carrier velocity in organic solids is thus determined from the visualized carrier motion. We anticipate that this direct visualization technique will find wide application in the illustration of space charge field formation in organic and inorganic materials, including biomaterials and polymers.

The pentacene field effect transistor (FET) is analyzed as a Maxwell-Wagner effect element. As a result of the Maxwell-Wagner effect, carriers injected from source electrode are accumulated at the interface between pentacene and SiO2-gate insulator. They are then conveyed along the FET channel by the electric field formed between source and drain electrodes. The drain current Ids shows characteristic behavior depending on the force of the electric field. The transit time and charging time of injected carriers are key parameters to specify FET characteristics ruled by the Maxwell-Wagner effect. Results also show that our pentacene FET characteristics are well explained based on the present theoretical analysis.

Electric field distribution in channel of pentacene field effect transistor (FET) was successfully probed by microscopic optical second-harmonic generation (SHG) observation. Microspot SHG signals were acquired at various points in the channel with scanning a spot position along source-drain direction. For the FET at off state, enhanced SHG signal was observed, indicating the Laplace field formation reflecting the device geometry. This clearly supports the insulating nature of pentacene layer at off state. After turning on the FET, SHG profile changed drastically, indicating change in the field distribution by the space charge formation in the channel due to the carrier injection.

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The surface potential across Alq 3 [tris(8-quinolinolato)aluminum] evaporated films on a metal (Au, Al) electrode was measured by the Kelvin probe method and the results were discussed taking into account the orientational ordering of Alq 3 molecules, the presence of surface charges on Alq 3 films and the displacement of excess charge (electrons and holes) from the electrode into Alq 3 films. The very large surface potential established in as-deposited Alq 3 films was mainly due to the alignment of dipoles of Alq 3 molecules, whereas the small surface potential that remained in the films after photoirradiation was due to excess charge. In order to clarify the drastic decrease of surface potential by photoirradiation, the surface potential decay was examined.