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

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.