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By analyzing electric field migration in the pentacene organic field-effect transistor channel (OFET), visualized using the time-resolved microscopic optical second harmonic generation (TRM-SHG) is analyzed the propagation of injected carriers. We find that the accumulated charge propagation on the pentacene-gate insulator interface of the three-electrode system is clearly different from the drift in electric field of the two-electrode system. The propagation of injected carriers is evaluated on the basis of the Maxwell-Wagner effect model and the transmission line approximation. We show that the interface charge accumulation has a significant contribution to the charge transport in OFET. Proposed model for the transient state is beyond the limits of common used impedance spectroscopy models and represents extension of the small-signal analysis. Found relation between mobility and transit time helps in analysis of OFET transit time sensitive experiments such as the time-of-flight technique (TOF) or TRM-SHG. #

The microscopic electric field induced second-harmonic generation technique is used for direct observation of electric field profiles and evaluation of the potential drop on the injection electrode in the organic field-effect transistors with various channel lengths L. It is found that the potential drop on injection electrode is not a function of L. We show that the analysis of the transmission line model (TLM) cannot distinguish channel length independent contact resistance and potential drop. Tracing back to the conceptional idea of contact resistance proposed by Shockley in 1964, the TLM approach is discussed to explain L-dependent contact resistance.

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The effect of silver nanoparticles self-assembled monolayer (Ag NPs SAM) on charge transport in pentacene organic field-effect transistors (OFET) was investigated by both steady-state and transient-state methods, which are current-voltage measurements in steady-state and time-resolved microscopic (TRM) second harmonic generation (SHG) in transient-state, respectively. The analysis of electronic properties revealed that OFET with SAM exhibited significant charge trapping effect due to the space-charge field formed by immobile charges. Lower transient-state mobility was verified by the direct probing of carrier motion by TRM-SHG technique. It was shown that the trapping effect rises together with increase of SAM layers suggesting the presence of traps in the bulk of NP films. The model based on the electrostatic charge barrier is suggested to explain the phenomenon.