Introducing a Surface-Enhanced-Raman-Scattering Enhancer for Experimental Estimation of the Debye Screening Length in Organic Field-Effect Transistors

The Debye screening length LD is one of the key parameters for the field-effect channel geometry. However, to the best of our knowledge, there are little reports on the experimental estimation of LD. In this study, we have reported our recent observation of in situ surface-enhanced Raman scattering (SERS) mapping during the investigation of the operating organic field-effect transistors. Placing SERS enhancers in different positions of the device can obtain charge and potential information at different locations, which can help to analyze the spatial distribution of charge and quantitatively estimate the Debye screening length. The enhancing factor of Raman intensity that is testified somehow increases exponentially with the external electrical field in both experiments and theoretical calculations. The experimental estimated Debye screening length is from 6.5 to 3.6 nm when the gate voltage changed from −30 to −50 V. This value is larger than the traditional consideration that the Debye screening length is less than 1 nm, which is due to the presence of the light field and the corresponding photogenerated carriers distributed in the whole active layer. Besides, the charge trapping effect of SERS enhancers has been testified to introduce weak error during the accumulation mode with large current density. These conditions have well mimicked the working mode of phototransistors, which is of great significance for optimization of phototransistor performance.