Triisopropylsilylethynyl-functionalized anthradithiophene derivatives for solution processable organic field effect transistors

Abstract

Two different solution processable anthradithiophenes were used as semiconductor material in top gate organic field effect transistors (OFETs) on flexible substrates. We observed good crystallization of the material in thin layers and determined charge carrier mobilities of up to 0.6 cm2/Vs. This value is only half of the mobility of the today best material triisopropylsilylethynyl-pentacene. Furthermore our OFETs show very good air stability over more than 20 months without significant changes of the mobility or on/off-ratio.

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Triisopropylsilylethynyl-functionalized anthradithiophene derivatives for Triisopropylsilylethynyl-functionalized anthradithiophene derivatives for
solution processable organic field effect transistorssolution processable organic field effect transistors
K. Schulze, T. Bilkay, and S. Janietz

Citation: Appl. Phys. Lett. 101101, 043301 (2012); doi: 10.1063/1.4739241
View online: http://dx.doi.org/10.1063/1.4739241
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Published by the American Institute of Physics.

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Triisopropylsilylethynyl-functionalized anthradithiophene derivatives for
solution processable organic field effect transistors
K. Schulze, T. Bilkay, and S. Janietz
Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, Potsdam-Golm 14476, Germany
(Received 30 May 2012; accepted 11 July 2012; published online 24 July 2012)
Two different solution processable anthradithiophenes were used as semiconductor material in
top gate organic field effect transistors (OFETs) on flexible substrates. We observed
good crystallization of the material in thin layers and determined charge carrier mobilities of
up to 0.6cm2/Vs. This value is only half of the mobility of the today best material
triisopropylsilylethynyl-pentacene. Furthermore our OFETs show very good air stability over more
than 20 months without significant changes of the mobility or on/off-ratio. V
Institute of Physics. [http://dx.doi.org/10.1063/1.4739241]
C 2012 American
A lot of research was done in the field of material devel-
opment for organic field effect transistors (OFETs) in the
last years. Today 6,13-bis(triisopropylsilylethynyl)pentacene
(TIPS-PEN) seems to be the most promising solution proc-
essable semiconductor material. TIPS-PEN shows a hole
mobility in OFETs of approximately 1cm2/Vs.1Anthradi-
thiophenes as structurally analogous of pentacene seem to be
a second interesting class of solution processable small mol-
ecules for the use in OFETs. The advantage of anthradithio-
phenes over solution processable pentacene derivatives is to
be seen in the thiophene rings which can be much more eas-
ily functionalized then benzene rings.
The syntheses and properties of anthradithiophene deriv-
atives were published several years ago.2,3These materials
were investigated because of their enhanced p-stacking inter-
actions which support the hopping process of the charge car-
riers. The performance of these materials in solution
processed OFETs has been investigated.3,4Heavily doped
silicon wafer with a thermally grown oxide layer have
been used as gate electrode and dielectric to analyze
triisopropylsilylethynyl-anthradithiophene (TIPS-ADT); fur-
thermore, the gold electrodes were covered with a monolayer
of pentafluorobenzenethiol (PFBT), and TIPS-ADT from a
toluene solution was spin coated on top.4In the work of
Payne et al. the mobility in the TIPS-ADT-films was lower
than 10?4cm2/Vs, and the authors observed only an amor-
phous anthradithiophene layer. Additionally Park et al.
obtained a high mobility of 1cm2/Vs for a similar
material, namely, triisoethylsilylethynyl-anthradithiophene
(TES-ADT). In this case only the triisopropyl-side chains in
the TIPS-ADT were replaced by triisoethyl-side chains. The
authors calculated the crystal order of the materials and
claimed that TIPS-ADT arrange in a 1-D “slippedstack”-
packing and TES-ADT in a 2-D “bricklayer”-packing and
this results in amorphous layers for TIPS-ADT and crystal-
line layers for TES-ADT. According to this result a lot of
work was published using TES-ADT as semiconductor in
OFETs.5–7
Another aspect is introducing fluorine end groups to the
anthradithiophene core to accelerate the crystallization of the
semiconductor layer. It was determined that TIPS-ADT-F
packs in a 2-D stacking arrangement with interplanar
spacings of 3.27A˚, whereas TIPS-ADT packs in a 1-D
arrangement with interplanar spacings of 3.46A˚.2,3These
differences in packing as well as the differences in polarity
of the molecules will influence the charge transport proper-
ties of the materials. We found only one publication of a
fluorinated triisopropylsilylethynyl-anthradithiophene (TIPS-
ADT-F) used as semiconductor in an OFET.3In the work of
Subramanian et al. the preparation of a TIPS-ADT-F-layer
via spin coating from a chlorobenzene solution was not pos-
sible, but the authors prepare an OFET using a solution-
grown TIPS-ADT-F-crystal and parylene-N as dielectric.
The observed mobility in this device was 0.1cm2/Vs.
In our work the performance of TIPS-ADT and TIPS-
ADT-F were determined in a top gate OFET setup. We syn-
thesized both materials using the synthesis route published in
Ref. 2 for TIPS-ADT and Ref. 3 for TIPS-ADT-F using com-
merciallyavailable 5-fluorothiophene-2,3-dialdehyde
starting material. The chemical structures of the two materi-
als are shown in Figs. 1(a) and 1(b). From cyclic voltamme-
try measurements we determined that the fluorination of the
TIPS-functionalized anthradithiophene leads to an increase
of the oxidation potential of approximately 100meV. For the
transistor preparation a PET-foil was used as substrate. The
substrates were cleaned by sonification in acetone and iso-
propanol. Approximately 40nm thick source-drain electro-
des were prepared by thermal evaporation of gold in
vacuum. The channel length and width of the transistors
were 130lm and 26400lm, respectively. A monolayer of
PFBT was prepared on top of the gold electrodes as it was
published in literature.8The PFBT-monolayer increases the
work function of the gold-electrodes and supports the charge
carrier injection. The semiconductor materials were spin
coated on top from a solution of 2wt.% of TIPS-ADT or
TIPS-ADT-F in tetralin. The spin coating process was car-
ried out at 500rpm for 10s followed by 2000rpm for 20s.
The layers were dried on a hotplate at 100?C for 15min.
Cytop was used as dielectric and the preparation was done
by spin coating as well: 1200rpm for 30s followed by a dry-
ing process at 120?C for 30min. The thickness of the dielec-
tric is approximately 1.2lm as it was determined on glass
reference substrates using a Dektak 150 profilometer from
Veeco. With a permittivity of 2.2 the capacitance of the
cytop-layer was calculated to be approximately 1.6nF/cm2.
As a final step the preparation of the gate electrode was done
as
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C 2012 American Institute of Physics 101, 043301-1
APPLIED PHYSICS LETTERS 101, 043301 (2012)
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in vacuum by thermal evaporation of silver with a thickness
of 100nm. All preparation steps except the preparation of
the monolayer were done in glovebox or in high vacuum
without any contact to air. Electrical characterization of the
devices was carried out in air.
In Figs. 1(c) and 1(d) typical transfer characteristics of
OFETs using TIPS-ADT or TIPS-ADT-F as semiconductor
are shown.
For several devices we determined the characteristical
values like charge carrier mobility, threshold voltage, and
on/off-ratio. The charge carrier mobilities were calculated
from the saturation regime of the transfer characteristics.
The charge carrier mobility of TIPS-ADT in OFETs is nearly
one order of magnitude higher compared to TIPS-ADT-F.
TIPS-ADT reach a mobility in the range of 0.3–0.6cm2/Vs,
whereas a mobility of 0.03–0.11cm2/Vs was determined for
TIPS-ADT-F. The on/off-ratio of both devices is comparable
in the range of 104–105. The off-current of the TIPS-ADT-
device shown in Fig. 1(c) is a little higher compared to the
TIPS-ADT-F-device in Fig. 1(d), but this is only an experi-
mental error, deriving from the preparation. The threshold
voltage is high for both materials, approximately ?24V for
TIPS-ADT and in the range of ?12 to ?20V for TIPS-
ADT-F. This means a high gate voltage is necessary for the
development of a conductive channel, and so we believe the
interface between the dielectric and the semiconductor can-
not be ideal. But compared to the results in Ref. 4, a working
transistor with a promising charge carrier mobility of
0.6cm2/Vs for TIPS-ADT could be fabricated. As well we
were able to prepare TIPS-ADT-F films by spin coating with
a mobility of up to 0.1cm2/Vs which is comparable to the
value determined for a solution-grown crystal of TIPS-ADT-
F in literature.3
In contrast to the results in literature we observe crystal-
line semiconductor layers in our devices. In Fig. 2 polarized
microscopy images recorded with an Olympus BX51 of
TIPS-ADT and TIPS-ADT-F films on PFBT treated gold are
shown.
The crystals of TIPS-ADT are visibly larger compared
to crystals of TIPS-ADT-F. Additionally it seems that TIPS-
ADT-F-films show higher inhomogeneity with obviously
higher thickness variations then TIPS-ADT-layer. These two
aspects might be the reasons for the higher charge carrier
mobility in TIPS-ADT-films compared to TIPS-ADT-F-
layers. Therefore we see that a fluorination of the anthradi-
thiophene which should accelerate the crystallization of the
semiconductor layer results in a higher inhomogeneity of the
layer. Maybe, this is influenced by a faster film formation,
and at the end this leads to a lower charge carrier mobility in
our case.
FIG. 1. Chemical structure of (a) TIPS-
ADT and (b) TIPS-ADT-F; transfer char-
acteristic of a typical organic field effect
transistor using (c) TIPS-ADT and (d)
TIPS-ADT-F as semiconductor material.
FIG. 2. Polarized microscopy images of
anthradithiophene films on PFBT treated
gold: (a) TIPS-ADT and (b) TIPS-ADT-F.
Obviously the crystal size in TIPS-ADT-
films is much larger then in TIPS-ADT-F-
layers.
043301-2 Schulze, Bilkay, and Janietz Appl. Phys. Lett. 101, 043301 (2012)
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Page 4

A comparable tendency is seen in the optical absorption of
both materials. In Fig. 3 the optical absorption spectra of TIPS-
ADT, TIPS-ADT-F, and TIPS-PEN in solution and thin film
determined using a Perkin Elmer Lambda 950 UV/VIS spec-
trometer are shown. For all three materials a redshift or dis-
placement (D) of the absorption from solution to thin film is
observed. The redshift is caused by the interaction of the mole-
cules with its neighboring molecules in the film. As the dis-
placement depends on the polarity of the solvents we used for
all three materials the same, in this work chloroform. We deter-
mined the following redshift values: 17nm (D¼537 1/cm) for
TIPS-ADT, 13nm (D¼459 1/cm) for TIPS-ADT-F, and
27nm (D¼627 1/cm) for TIPS-PEN. It was observed by
Ostroverkhova et al. that a low redshift of the optical absorption
corresponds to amorphous layers and a high displacement to
polycrystalline films.9According to this we observed compara-
ble results. The displacement of TIPS-ADT is approximately
78 1/cm higher than for TIPS-ADT-F, and we observed a
higher crystallinity of the TIPS-ADT-films (Fig. 2). In compari-
son to TIPS-PEN both materials exhibit a lower displacement,
and a lower charge carrier mobility in OFETs which supported
the results from Refs. 9 and 10.
In the investigated device architecture the OFETs using
TIPS-ADT as semiconductor have a larger subthreshold re-
gime compared to the OFETs using TIPS-ADT-F. Although
we observed comparable onset voltages Vonfor both devices
(10V for TIPS-ADT-device and 6V for TIPS-ADT-F-
device) the difference in the threshold voltage leads to a sig-
nificant different subthreshold regime. For the devices shown
in Figs. 1(c) and 1(d) we determined a subthreshold regime
of approximately 34.2V for the OFET based on TIPS-ADT
and 23.2V for the TIPS-ADT-F-OFET. We observed the
same difference of the subthreshold regime for all prepared
devices. In comparison of the two materials the lower sub-
threshold regime leads to a better charge carrier channel de-
velopmentinTIPS-ADT-F than in TIPS-ADT.The
development of the channel is connected to the band bending
behavior of the energy level of the semiconductor induced
by the gate voltage over the dielectric. Here the interface
between the semiconductor and the dielectric plays an im-
portant role. We think that a difference in the arrangement of
the anthradithiophene molecules in the films displays the
quality of the channel development in the semiconductor.
This means that the arrangement of the TIPS-ADT-F-mole-
cules in the crystals fits better to the dielectric cytop than the
arrangement of the TIPS-ADT-molecules. As the difference
is induced by the fluorination of the thiophene, this has a
positive impact on the device performance.
In the anthradithiophene OFETs we analyzed the air sta-
bility behavior additionally. Therefore we store our devices in
air in a dark surrounding and repeat the electrical characteriza-
tion after several days to months. In Fig. 4 transfer characteris-
tics at several days over a time period of about 20 months of a
TIPS-ADT-OFET are shown. Here the on-current of the devi-
ces increase slowly from 95.2lA at day 0 to 115.8lA at day
602 whereas the off-current varies between 10?10and
5?10?9A at a gate voltage of 20V. The increase of the on-
current is connected with an increase of the charge carrier mo-
bility. This is additionally to be seen in Fig. 5(a). Here the
charge carrier mobilities of four different TIPS-ADT-OFETs
over a time period of ca. 20 months are summarized. In Fig.
5(b) the threshold voltage of the same devices are shown.
For all four devices the charge carrier mobility increase
by a factor between 14% and nearly 29% from day 0 to day
115 and stay almost constant from that day on. This effect
could not be induced by oxygen doping, because the off-
current does not increase at the same time. Therefore we think
that the arrangement of the anthradithiophene molecules
changes in the first 4 months in a preferred way and leads to
an increase of the charge carrier mobility. In the same way the
threshold voltage decrease from day 7 to day 289 about 3-4V
which means that lower gate voltages are necessary to create
charge carrier channels. This underlines our hypothesis of a
better molecule arrangement after several weeks as well.
Beside this result the OFETs show high air stability over
a time span of nearly 20 months (experiments not finished
yet). A degradation of the OFET performance induced by
oxygen doping was not observed as it is mostly the case for
organic materials in electronic devices. In our devices the
cytop-layer as well as the gate electrode act as a passivation
FIG. 3. Optical absorption of TIPS-ADT and TIPS-ADT-F in solution
(upper graph) and thin film (lower graph). Additionally the optical absorp-
tion spectra of TIPS-pentacene are shown as well.
FIG. 4. Transfer characteristics of an OFET using TIPS-ADT as semicon-
ductor determined over a time period of about 20 months.
043301-3Schulze, Bilkay, and Janietz Appl. Phys. Lett. 101, 043301 (2012)
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Page 5

layer for the semiconductor. This property is a positive
aspect of choosing a top gate setup as OFET layout.
In conclusion we could prepare OFET devices based on
two anthradithiophenes using spin coating processes. In con-
trast to earlier results from literature we were able to achieve
crystalline TIPS-ADT-films with charge carrier mobilities
for TIPS-ADT of up to 0.6cm2/Vs in a flexible OFET setup.
Compared to other results we were also able to prepare crys-
talline TIPS-ADT-F-films by spin coating with a comparable
mobility to a solution-grown single-crystal of 0.1cm2/Vs.
Furthermore we observed a high air stable behavior of
OFETs based on TIPS-ADT over a time period of at least 20
months. An increase of the mobility in these devices could
be evaluated as an improvement in the arrangement of the
semiconductor molecules in thin films.
We thank K. Fink for the help of sample preparation and
the Bundesministerium fu ¨r Bildung und Forschung (Contract
No. 13N10480) for financial support.
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FIG. 5. (a) Mobility and (b) threshold volt-
age of different TIPS-ADT-OFETs over a
period of approximately 20 months. The
charge carrier mobilities increase until day
115 and the threshold voltages decrease of
about 3–4V from day 7 to day 289.
043301-4Schulze, Bilkay, and Janietz Appl. Phys. Lett. 101, 043301 (2012)
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