Using Sum-Frequency Generation (SFG) to probe electric-fields within organic field-effect transistors

Abstract

Organic Field-Effect Transistors (OFETs) have attracted much research interest due to their potential for unique applications, such as flexible electronics. The operation of OFETs depends on the charge accumulation at the interface between an organic semiconductor and a dielectric material, induced by the voltage applied at the gate electrode. Direct measurements of the electric-field distribution in an operating device are useful for proposing and validating theoretical models for OFET operation. Here we propose using the second-order nonlinear optical process of Sum-Frequency Generation vibrational spectroscopy (SFG spectroscopy) to probe the presence of an electric-field in the dielectric layer of OFETs, in a non-invasive, non-destructive and remote fashion. The OFETs were fabricated with a dielectric layer consisting of poly(methyl-methacrylate) — PMMA, and an active layer based on poly(3-hexyl thiophene) — P3HT, and SFG spectra were acquired from the channel region of operating OFETs. It was observed the appearance of vibrational bands due to carbonyl groups (∼ 1720 cm−1) of the PMMA layer, whose χ(2) were induced by the electric-field within the dielectric, similarly to a reversible poling of polymers. This phenomenon opens up the possibility of mapping the spatial charge distribution in the conducting channel using SFG microscopy in operating devices.