The potential of organic electronic devices, over the past few years, has
been proven tremendous. Flexible and transparent electronic devices
will soon enter the market, but there are still some challenging aspects
requiring further improvements. A generic approach to enhance the
charge transport properties of the organic semiconductors is by introducing
molecular additives. In particular for thin lm transistors, doping
the semiconductor layer leads to higher charge carrier mobility, as well
as improving several parameters of the device. The work in this thesis
focuses on identifying suitable molecular additives that can dope organic
semiconductors for thin lm transistors.
Chapter 5 presents the application of tris(penta
uorophenyl)borane
B(C6F5)3 as an e ective p-type dopant. B(C6F5)3 was applied in several
small molecule:polymer blends and the improvement in the charge transport
properties, as measured from organic thin lm transistors (OTFTs),
was remarkable. In particular the blend diF-TESADT:PTAA and the
C8-BTBT:C16-IDT-BT showed hole mobility of 8 and 11 cm2/Vs, respectively.
OTFT parameters, such as the threshold voltage and the contact
resistance were signi cantly reduced with the addition of molecular additives.
The impact of the dopant addition in the blends was studied by
AFM and XRD, which revealed that long-range crystallinity was induced
in certain systems.
Chapter 6 presents the DMBI-BDZC as a new n-type dopant. For
transistors integration into circuits balanced charge transport is required
and thus extensive optimisation is still required for n-type materials.
DMBI-BDZC was used to dope a small molecule (NDI3HU-DTYM2)
and a polymeric (N2200) semiconductor. In both systems free electron generation was observed at an optimum dopant concentration, as measured
by electron paramagnetic resonance (EPR). OTFTs with enhanced
charge carrier mobility and reduced contact resistance was achieved with
the dopant addition. By further analysis of the device characteristics,
reduction in both activation energy and density of traps states was observed
in the doped devices. The morphology studies of the organic
semiconducting thin lms revealed that the dopant addition is not disrupting
the structure. Furthermore, the reduction in activation energy
at low voltages is in accordance with the trap density calculations as
performed from the OTFTs.
In chapter 7 a single organic layer light emitting transistor (OLET) is
demonstrated. The key behind this device structure was the identi cation
of the polymer PDITTT. This polymer combines high charge transport
properties (1 cm2/Vs) as well as having green light emission. The
properties of the polymer allowed the fabrication of a single layer organic
semiconducting OLET. Blending the polymer with the small molecule
C8-BTBT resulted in enhanced charge transport OTFTs. OTFTs and
OLETs were fabricated from both the PDITTT and C8-BTBT:PDITTT.