Influence of interface structure on performance of organic field effect transistors

Abstract

In organic field effect transistors (OFETs), charge transport in the conducting channel and charge injection from metal electrodes into the conducting channel is highly dependent on the quality and reliability of the insulator-organic semiconductor (OSC) and electrodes-OSC interfaces respectively. In this regard, the choice of the dielectric and the metal is as important as that of OSC. In this paper, simulation is performed using organic transistor modelling software for Rubrene based OFETs with different dielectric materials and metal contacts. Further, influence of interface traps at the insulator-OSC interface and impact of charge injection barrier and density of states (DOS) at the source electrode-OSC interface on OFETs electrical characteristics is studied. In addition, surface Fermi level pinning effect and influence of the semiconductor layer thickness is also discussed. The obtained simulation data shows that the results can be further improved by introducing high dielectric material and high work function hole injecting source/drain contacts. It is found that there is a reduction in the charge carrier mobility in disordered OSC with interface traps as compared to crystalline OSC without interface traps. Variation of both threshold voltage and practical onset voltage is observed due to the Fermi level pinning. Also, increase of threshold voltage is observed when non-linear injection occurs due to injection barrier. It is observed that the reduction of OSC layer thickness results in an increase in the drain current and variation of sub threshold slope. Steeper sub-threshold slope correlates to the lower trap density that results in better switching behaviour of OFETs. This simulation clarifies a number of issues that can help in design, fabrication and characterisation of organic electronic devices.