Threshold Voltage Shift in Organic Field Effect Transistors by Dipole-Monolayers on the Gate Insulator

TL;DR

This study demonstrates how self-assembled monolayers on gate insulators can controllably shift threshold voltages in organic FETs, affecting device characteristics and charge carrier densities.

Contribution

It introduces a method to tune threshold voltages in organic FETs using organosilanes, revealing effects on device behavior and trap state formation.

Findings

Threshold voltage shifts from -2 to 50 V due to SAMs.

Charge carrier density at zero bias reaches 4×10^12/cm^2.

SAMs create trap states, broadening the subthreshold region.

Abstract

We demonstrate controllable shift of the threshold voltage and the turn-on voltage in pentacene thin film transistors and rubrene single crystal field effect transistors (FET) by the use of nine organosilanes with different functional groups. Prior to depositing the organic semiconductors, the organosilanes were applied to the SiO2 gate insulator from solution and form a self assembled monolayer (SAM). The observed shift of the transfer characteristics range from -2 to 50 V and can be related to the surface potential of the layer next to the transistor channel. Concomitantly the mobile charge carrier concentration at zero gate bias reaches up to 4*10^12/cm^2. In the single crystal FETs the measured transfer characteristics are also shifted, while essentially maintaining the high quality of the subthreshold swing. The shift of the transfer characteristics is governed by the built-in electric field of the SAM and can be explained using a simple energy level diagram. In the thin film devices, the subthreshold region is broadened, indicating that the SAM creates additional trap states, whose density is estimated to be of order 1*10^12/cm^2.