Direct imaging of carrier motion in organic transistors by optical second-harmonic generation

Abstract

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.

Direct observation of the charge carrier concentration in organic field-effect transistors by electron spin resonance

“…9͒ or pentacene. 10 On the other hand, direct determination of the injected carrier concentration is also the key issue to characterize the device properties of the organic FETs. According to the standard FET theory, the change in the carrier concentration in the FET channel region is induced by the application of the drain-source voltage ͑V ds ͒ together with the gate-source voltage ͑V gs ͒, which dominates the output characteristics of the devices.…”

Electrochemical mechanism of ion current rectification of polyelectrolyte gel diodes

“…This is in contrast with the cases of p-n diodes, where voltages mostly drop at their p-n junctions for both forward and reverse voltages; these diodes rectify electric currents because the densities of hole and electrons in p-type and n-type semiconductors are already asymmetric. Organic diodes that have the junction of (solid) p-type and n-type semiconducting organic materials [29][30][31] are operated by injecting carriers (electrons or holes) from the electrodes [31][32][33][34] . At a first glance, one may think these organic diodes are analogous to PGDs because electric carriers in both of the two diodes are brought from the exterior by electron transfer.…”

“…Our theory predicts the following experimentally accessible predictions: first, voltages are mostly distributed to the interfaces between the two gels for reverse voltages and to the surfaces of the electrodes for forward voltages. This prediction may be accessible by using experimental techniques that measure local electric fields, for example, electric field-induced second harmonic generation 32 , and/or techniques that capture the signature of redistribution of counterions, for example, cyclic voltammetry, fluorescence microscopes with charged probes 10 and electrochemical impedance spectroscopy 28 . Second, the asymmetry of electric currents increases (forward currents increase and reverse currents decrease) with decreasing values of k f l 0 ðn 3=2 f Þ, see Fig.…”

Probing of Electric Field Distribution and Carrier Behavior in Double-Layer Organic Light-Emitting Diodes by a Novel Microscopic Electric-Field-Induced Optical Second-Harmonic Generation Measurement system

“…The EFISHG measurement detects nonlinear polarization induced in organic layer in the presence of static electric field by laser irradiation, and it is given by �⃗(2 ) = 0 (3) ⋮ �⃗(0)�⃗( )�⃗( ) [8]. Here, 0 is the permittivity of a vacuum, (3) is the 3rd-order nonlinear susceptibility tensor of material and depends on , �⃗ ( ) is the electric field of pulsed laser.…”

“…Using this developed system, we have studied the electric field distributions in active layers in OFETs, OSCs, OLEDs, etc. [8][9][10][11][12]. However, to probe two-dimensional electric field distribution in devices, it is very useful to develop a system that allows electric field along the film-thickness direction to be directly probed.…”