Nonlinear Optical Imaging of Carrier Transport at the Semiconductor-Insulator Interface in Organic Field-Effect Transistors

Organic field-effect transistors (FETs) are witnessing a revolution in their performance, in part, due to a reduction in contact resistance, and through controlled morphology of the organic semiconducting film. Another major consideration is the role of the dielectric layer in dictating transport properties. The transient electric-field-induced second-harmonic generation (EFISHG) technique allows visualization of carrier dynamics in the channel region of a FET by capturing the movement of the induced dielectric polarization. A microscopic imaging system using a broadband femtosecond laser and a pulse compensation arrangement is constructed for imaging transient EFISHG from organic FETs by synchronized laser and voltage pulses. By varying the dielectric layer in organic FETs comprising pentacene and a donor-acceptor conjugated polymer as semiconducting layers, we show that EFISHG images provide an accurate estimate of the carrier mobility along with providing insights into channel formation and the electric field distribution within the channel region. We further correlate the interface charging effects observed in EFISHG to the interface trap density obtained by capacitance-voltage and conductance-voltage from metal-insulator-semiconductor diodes.