Barrier Modification of the Metal Contacts by an Ultrathin Oxide Layer in Organic Field-Effect Transistors*

Reducing the Schottky barrier height and addressing the issue of Fermi level pinning in metal-organic semiconductor contacts are crucial for the performance of organic field-effect transistors (FETs). Typically, the application of self-assembled monolayers (SAM) on metal contacts tunes the metal work function and reduces the contact resistance in organic FETs; however, it does not necessarily lead to de-pinning the Fermi level. We demonstrate the reduction of the Schottky barrier height in bottom-contact top-gate organic field-effect transistors by adding 1 nm of atomic layer deposited Al₂O₃ layer on Au contacts. A donor-acceptor–type conjugated polymer (DPP-DTT) was used as the active layer along with a polymer gate dielectric. Temperature-dependent current-voltage measurements from non-treated, SAM treated, and Al₂O₃ treated Au source-drain contact FETs with varying channel lengths were carried out. The carrier mobility shows a typical hopping transport with a higher activation energy between 200 K – 300 K for the Al₂O₃ treated FET. The drain current versus drain voltage at zero gate voltage can be described by the thermionic emission model at temperatures above 150 K, allowing the estimation of the Schottky barrier height (φ_B). The Al₂O₃ treated FET, which demonstrates the highest carrier mobility, shows a 40% lower φ_B compared with the non-treated device.