Electronic transport, field effect and doping in pentacene nanorods and monolayer thin film prepared by combination of nano-fabrication and self-assembly

Abstract

The transport in organic semiconductors is investigated on the nanometer scale. Field effect transistor with device-active layers in the monolayer (ML) range are characterized in-situ right after the controlled deposition of pentacene. By using a bottom contact geometry, the thickness of the channel is varied in several steps and the thin film transistors (TFTs) are characterized after each additional deposition. The mobility is found to saturate at a channel thickness of 3-4 ML. This allows for the operation ofML TFT, where the effect of dopant molecules deposited on top of the channel is studied. Fluorinated tetracyanoquinodimethane (F4 TCNQ) is found to increase the charge carrier concentration by a charge transfer process with pentacene without degradation of the mobility. For a separation of contact effects from bulk effects, TFTs with channel length between 20 µm and 200 µm have been processed in parallel. The contact resistance Rc and the contact free mobility µcf have been extracted and analyzed by the transmission line method (TLM). In temperature dependent studies, an approximately linear log(µ) vs. 1/T relation describes the experimental results. The fit is slightly better for the undoped TFT compared to the doped TFT. This is consistent with the accepted theoretical models based on a disordered Gaussian density of states (DOS). In fact, the experimental determination of the DOS in doped TFTs shows an additional broad peak at 0.14 eV above the HOMO edge caused by the dopant molecules. A second pronounced effect of doping is the reduction of the Rc by a factor 20 between the gold electrode and pentacene. The gate field dependent decrease of Rc in undoped TFTs is related to a lowering of the Schottky barrier at the interface. In doped TFTs, the opposite effect is observed. This demonstrates the influence of the dopant molecules on the interface barrier. The injection process determining Rc is found to change from a thermionic emission mechanism to a tunneling regime. For the investigation of the injection properties at the interface, a new manufacturing method to produce metallic nanojunction is presented. In combination with a self-assembly process leading to pentacene rod-like islands connecting the two electrodes right through their growth, the transport in the 10-nm scale is investigated. At this scale length, the current-voltage (I-V) characteristics of pentacene is changing from rectifying Schottky-like behavior to fully linear behavior upon F4TCNQ doping. Variable temperature investigations show that the conduction in the doped pentacene nanojunction is thermally activated, with an activation energy very close to the energy position of the dopant induced peak in the DOS and to the thermal activation energy of Rc.