Direct imaging of carrier motion in organic transistors by optical secondharmonic generation

Interest in the dynamic behaviour of carriers in organic materials is motivated by possible applications that include organic thin-film transistors, organic electro-luminescent devices and organic photoconductors. It can also provide an insight into the modelling of carrier transport and trapping in organic semiconductors and insulators. Here, we use an advanced second-harmonic generation technique to probe and visualize real carrier motion in organic materials. This is a time-resolved microscopic optical second-harmonic generation technique that allows direct and selective probing of dynamic carrier motion in organic materials. Experiments making use of this technique and using pentacene field-effect transistors have revealed dynamic changes of second-harmonic-generation 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.