Chemistry Reference
In-Depth Information
films. Single crystals of rubrene, with a parylene polymer film as the gate insulator,
exhibit p-type conductivity with
1cm
2
V
−
1
s
−
1
(Podzorov
et al.
, 2003).
High-performance OFETs have also been fabricated on the surface of freestand-
ing organic single crystals with the lamination technique. This conceptually simple
method consists of laminating a monolithic elastomeric transistor stamp against the
surface of a crystal and has enabled the fabrication of rubrene transistors with charge
carrier mobilities as high as
µ
h
∼
0
.
1
−
15 cm
2
V
−
1
s
−
1
(Sundar
et al.
, 2004). The fabrication
process uses a flexible elastomer as a substrate (e.g., PDMS) on which the transistor
stamp is constructed. The gate and source-drain electrodes are evaporated through
shadow masks and are separated by an additively transferred thin film (2-4
µ
m) of
PDMS. The flexibility of both the dielectric and the substrate enables assembly of
devices through simple lamination of the organic crystal and the elastomeric stamp
surface. Slight pressure applied to one edge of the crystal initiates contact with the
stamp. The ease of this assembly process together with its inherently non-invasive
nature enable the systematic analysis of the semiconducting properties of pristine
organic crystals.
The simple drop casting crystallization method, discussed in Section 3.1,
has also shown its validity in the determination of hole mobilities at least for
the TTF-derivatives DTTTF (Mas-Torrent
et al.
, 2004), EDT-TTF-(CONHMe)
2
(Colin
et al.
, 2004) and DBTTF (Mas-Torrent
et al.
, 2005). In all cases,
µ
h
∼
∼
1cm
2
V
−
1
s
−
1
. In this case the formation of high-quality organic/insulating
interfaces is a matter of serendipity (see the discussion on the formation of nano-
volcanoes at the surfaces of drop-cast single crystals in Section 3.1).
Table 6.1 lists the experimental values of the mobilities obtained for some
selected MOMs. Such values vary depending on the preparation of the materials as
thin films or single crystals as well as on the preparation of the interfaces and metallic
contacts. OFETs obtained from rubrene single crystals exhibit mobility values
between 0.1 and 15 cm
2
V
−
1
s
−
1
, depending on the quality of the crystal/dielectric
interface. Once the preparation of the interface has been mastered, the maximum
RT
µ
e
,
h
values should keep increasing with time with the aim of soon obtaining
their intrinsic values. For pentacene, mobilities as high as 3 cm
2
V
−
1
s
−
1
have
been claimed (Dimitrakopoulos & Malenfant, 2002; de Boer
et al.
, 2004). In the
case of thin films, orientation is also a major issue, in particular for anisotropic
molecules. When planar molecules stand perpendicularly to the insulator surface,
the mobilities are higher because of the more effective
π
−
π
interactions, in clear
contrast to when the molecules lie flat on the surface.
Charge mobilities determined by the time of flight (TOF) method show that
values of 1-10 cm
2
V
−
1
s
−
1
are not rare for MOMs (Karl
et al.
, 1999). In TOF
τ
experiments the transit time
for a sheet of photoinjected carriers to move across
a sample of thickness
L
is monitored. The sheet of carriers is usually generated
