Submitted to
DOI: 10.1002/adma.((200801725))
High Performance Polymer-Small Molecule Blend Organic Transistors**
R. Hamilton1*, J. Smith2, S. Ogier3, M. Heeney4, J. E. Anthony5, I. McCulloch1, J. Veres6, D.
D. C. Bradley2, T. D. Anthopoulos2
[*]
Departments of Chemistry1 and Physics2, Imperial College London,
South Kensington, SW7 2AZ, (United Kingdom)
E-mail: R.Hamilton@Imperial.ac.uk; Thomas.Anthopoulis@Imperial.ac.uk
UKPETeC3, NETPark, Sedgefield,
County Durham TS21 3FD, (United Kingdom)
Department of Materials4 , Queen Mary, University of London,
Mile End Road, E1 4NS, (United Kingdom)
Department of Chemistry5, University of Kentucky,
Lexington, KY 40506-0055, (U.S.A.)
Eastman Kodak6, 1999 Lake Avenue,
Rochester, NY 146500, (U.S.A.)
We are grateful to the Engineering and Physical Sciences Research Council (EPSRC) and
Research Councils UK (RCUK) for financial support. TDA is an EPSRC Advanced Fellow
and an RCUK Fellow/Lecturer.
Keywords: TIPS, F-ADT, Organic Semiconductors, Organic Transistors, OFETs
Abstract.
Solution processable organic semiconductors have shown significant improvement over
recent years, and are now poised for mainstream commercialisation. Although the electrical
performance of the best devices are now in excess of the first generation application
requirements, increasing complexity will demand improved semiconductor charge carrier
mobilities. Functionalised oligoacenes have demonstrated both solution processability and
high charge carrier mobility, however small molecules may demonstrate limitations in
fabrication compatibility with printing techniques. Here we show that a blended formulation
of semiconducting small molecule and a polymer matrix can provide high electrical
performance within thin film field effect transistors (OTFTs), demonstrating charge carrier
mobilities of greater than 2 cm2V-1s-1, good device to device uniformity and the potential to
fabricate devices from routine printing techniques.
1
Submitted to
Main Text
Solution-deposited, robust alternatives to amorphous silicon have been pursued commercially
for over a decade.[1-3] Introduction of an economically viable technology that enables large
area flexible displays[4,
5]
as well as ubiquitous cheap electronics such as radio frequency
identification tags[6, 7] is expected to be highly disruptive to the silicon dominated market.[8, 9]
Solution-deposited small molecules[10-14] and polymers[15-17] are viable approaches that
promise to meet the challenge. To date, small molecules have provided the highest headline
field effect mobilities,[18, 19] but device-to-device variation due to morphology issues makes
large area deposition via printing difficult,[20,
21]
while high solubility makes finding
orthogonal solvents for further solution processing a considerable constraint. Polymers
however, demonstrate excellent device uniformity[22] and solution rheology, which makes
them ideal for large area printing,[5] but have not yet demonstrated the high mobility to make
them truly useful in commercial devices.[23]
Combining the high mobility of crystalline small molecules with the device uniformity of
polymers is very attractive and has been approached in a number of ways. These include
increasing the crystallinity of a polymer such as p3HT[16] by introducing rigid units into the
polymer back-bone and creating regiosymmetric monomers. Polymer-small molecule blends
have previously been investigated[24, 25] in an attempt to produce ambipolar devices but rather
than enhancing performance, the blending in these investigations appeared to diminish the
peak electron and hole mobility of each component. Using a blend of small molecules and
polymer to enhance the performance and improve deposition has been described in the patent
literature.[26,
27]
By blending a soluble, highly crystalline p-type small molecule organic
semiconductor (OSC) with an inert or field-effect active polymer significant enhancement in
performance is claimed.
2
Submitted to
Here, two acene semiconductors first described by John Anthony, TIPS-pentacene[28] and
diF-TESADT[18, 29] (Figure 1), are blended with both inert and field-effect active polymers
with the expectation that peak device performance will be maintained while device uniformity
is improved. OTFTs fabricated from soluble blends of polymer and small molecule are cast
using spin-coating, which we will show causes preferential vertical phase separation of the
two components. The small molecule is forced to the exposed interface, allowing large
crystals to form within the channel region of the device.
To evaluate the effect of the blend morphology devices were fabricated in a dual gate
structure as well as a more conventional top gate design. The processing conditions are
documented in the methods section, but Figure 1 (c) shows a schematic of the standard OTFT
whilst Figure 2 (b) shows the dual gate device. Constructing transistors that have a bottom
and top gate within the same device will elucidate differences in device performance between
the bottom and top OSC interfaces, but will not differentiate between dielectric effects,
channel injection and morphology changes. The leakage current between the two gates was
always found to be less than the lowest measured off current thus allowing channels to be
probed independently. Devices were operated in dual gate mode by biasing one gate to a
constant voltage whilst sweeping the other to obtain transfer characteristics. This produced a
shift in the threshold voltage dependent on the fixed gate voltage, which is consistent with
dual gate operation.
Figure 2 (a) shows the transfer characteristics for top and bottom channels within a single
device, fabricated using a 1:1 by weight blend of TIPS-pentacene and the insulating polymer
poly(α-methyl styrene) spin-coated onto the bottom gate dielectric. The choice of an inert
polymer matrix for the dual gate device maximised any difference in mobility between top
and bottom gate which might be due to vertical phase separation. Uniform semiconductor
film formation was observed, whilst the saturation mobility of holes within the bottom
3
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channel was 0.10 ± 0.05 cm2V-1s-1 increasing to 0.5 ± 0.1 cm2V-1s-1 in the top channel.
Moving from bottom to top gate also showed an improvement in on/off current ratios, reduced
hysteresis and threshold voltage shifts from greater than +10 V to between -5 and -10 V.
Processing issues, described in the methods section, forced the use of different dielectric
materials for the bottom and top channel, which could account for the difference in
performance seen. However, devices were analysed by a DSIMS technique (results below)
which indicate a higher concentration of TIPS-pentacene in the top-channel. The maximum
mobility achieved using TIPS-pentacene : poly(α-methyl styrene) in a top-gate-only device
was found to be 0.69 cm2V-1s-1, which is only slightly higher than measured in the dual gate
transistors showing that the altered structure does not adversely affect the top channel
operation.
In order to make an improvement in the mobility a change of polymer matrix is needed. It
is suggested that phase separation (not necessarily vertical as described before) of TIPSpentacene and poly(α-methyl styrene) causes a reduction in the effective channel width and
thus a lowering of the measured mobility for devices based on insulating polymers. Replacing
poly(α-methyl styrene) with the amorphous p-type polymer poly(triarlyamine) (PTAA)
(FlexInk) creates conduction pathways between separate crystalline pentacene-rich regions
and improves the performance of the OTFT.
Figure 3 shows typical transfer and output characteristics of top gate devices (channel
length (L) of 60 μm and width (W) of 1000 μm) made from (a) TIPS-pentacene and (b) diFTESADT, both blended with PTAA. The highest mobility devices using TIPS-pentacene had
a slight deviation from ideal square law behaviour at gate voltages greater than -50 V, due to
the drain being biased to -40V. Charge injection appeared to be efficient, since there is a good
linear output at low drain voltages and despite the higher currents within diF-TESADT based
transistors no injection problems were observed. There is also a clear improvement in hole
4
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mobility over the devices made using TIPS-pentacene : poly(α-methyl styrene) blends. The
highest mobility obtained was in a diF-TESADT : PTAA device which had a saturation
mobility of 2.41 ± 0.05 cm2V-1s-1 and a linear mobility of 1.88 ± 0.04 cm2V-1s-1.
In addition to high mobilities the important characterisation parameters for the devices
remained constant and reproducible over the whole sample. A typical, non optimised sample
(having lower quality evaporated source-drain electrodes but more transistors per sample) was
tested and averages taken over 18 devices. The mean saturation mobility was found to be 0.66
± 0.13 cm2V-1s-1 and the best device was 0.91 cm2V-1s-1. Similarly the mean threshold voltage
was -7.2 ± 2.2 V and the on/off current ratio was 10(4.51 ± 0.32). Generally it was found that over
80% of the devices on a sample would show transistor behaviour. Table 1 clarifies the
mobilities and on/off current ratios obtained for the various material and device designs
employed.
Devices fabricated in air using TIPS-pentacene blends had much lower mobilities (~0.1
cm2V-1s-1) compared to those made in nitrogen.
These results are consistent with the
degradation of pentacene OTFT performance due to H2O and O2 acting as a charge trapping
dopants introduced during annealing of the semiconductor layer[30].
However, after
fabrication in nitrogen, exposure to air resulted in only a slow reduction in device
performance since the permeability of CYTOP to H2O and O2 is low. Figure 4 shows how
the mobility as well as on and off currents varied over a period of four weeks for a diFTESADT based and a TIPS-pentacene based transistor. The TIPS-pentacene blend showed a
slight drop in mobility after several hours while diF-TESADT blend transistors showed
excellent stability and maintained a saturation mobility above 1.2 cm2V-1s-1 for the entire test.
There was a gradual decrease in the on/off ratio due to increasing off current as bulk
conduction in the channel became more significant.
Within the dual gate OTFTs the lower performance of the bottom gate can be attributed to
two effects. Firstly, there will be a larger number of charge trapping sites on the BCB5
Submitted to
semiconductor interface due to the oxygen plasma treatment creating polar groups on the
surface. Secondly, we believe that there is vertical separation of components within the
semiconductor layer. During film formation phase separation of the polymer and the small
molecular material occurs, however, due to the high surface energy of the substrate, and in
particular the high polar component of this energy, there is preferential crystallisation of the
TIPS-pentacene (or diF-TESADT) at the semiconductor-atmosphere interface. This therefore
increases the fraction of molecular solid within the conducting channel of the transistor in the
top gate configuration.
Vertical profiling of the device structure by Dynamic Secondary Ion Mass Spectrometry
(DSIMS) was used to confirm the distribution of the blend components. Cs ion bombardment
was used to slowly sputter material from the film, and the resulting ejected ion species
measured by mass spectrometer. Figure 5 shows the first signal to rise is the Si from the silyl
group on the TIPS-pentacene molecule. The nitrogen signal (from the PTAA) can be seen to
rise to a maximum approximately 20 - 30nm beneath the top surface. When scanned over the
channel (a) the Si signal is seen to rise at a probed depth of ca. 50 nm, which could indicate an
increase in concentration of TIPS at the glass substrate surface. This would explain why a
field effect is still seen in bottom gate devices. When scanned over the electrodes, however,
(b) the fluorine peak indicates the position of the self-assembled monolayer of
pentafluorobenzene thiol (PFBT) and here TIPS seems to be excluded from the surface. The
broadening in the F peak (which should in theory be a monomolecular layer <1nm thick) is
due to a combination of a variable rate of etch for different materials in the layer and a slight
spread in the energy density of the Cs ion beam. The gold signal rises immediately afterwards
as the profiling reached the source and drain electrodes, and finally the Si signal rises once
more indicating profiling into the glass substrate. A depth profile of an area between the
source and drain electrodes showed the same distribution of the TIPS and PTAA, indicating
that the phase separation occurs over the entire area of the spin coated sample.
6
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The TIPS-pentacene : PTAA system was chosen to illustrate the suspected vertical phase
separation, as each component contains an element that identifies it as being located at a
certain depth. Using this analysis, it is apparent why a top-gate device may perform better than
a bottom-gate. Although there is sufficient TIPS-pentacene in the bottom channel to allow a
reasonable field-effect mobility, the higher concentration in the top channel provides an
explanation of the better performance. The vertical phase separation in this system is assumed
to apply to all small molecule polymer blends in this study.
The films were also studied using polarised light microscopy to observe the morphology of
the crystallites and atomic force microscopy (AFM) to map the surface of the semiconductor
which forms the conducting channel within the OTFT. Figure 6 shows both diF-TESADT
and TIPS-pentacene based blend films. It is clear that some crystallisation of the molecular
material into spherulitic-type structures has occurred. Crystallisation occurs over the whole
sample when annealed for longer than 2 min at 100 °C, less than this quenches the sample
leaving some amorphous regions which are not suitable for high performance devices. The
AFM shows a clear difference between the amorphous part of the film, with a r.m.s. surface
roughness of 6.4 nm, and the crystallites, with a surface roughness of 18 nm. This increase in
surface roughness and the change in appearance of the film suggest that some of the TIPSpentacene is forming on the top surface during crystallisation – the situation that we require
for good top gate OTFT performance. The optical micrographs also show the effect of
contact induced crystallisation on PFBT coated gold contacts[11]. Much larger and more
spherulitic crystals are produced on the gold surfaces in the case of diF-TESADT blends
where it is suggested that fluorine interactions promote the crystallisation. In the case of
TIPS-pentacene blends the effect is less pronounced as the same F-F interactions do not exist,
which also agrees with the better charge injection observed in the diF-TESADT devices.
Further annealing of the diF-TESADT based transistors at 100 °C after fabrication resulted
in a lowering of the off current, for example after 1 hour the on/off current ratio typically
7
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increased from approximately 103.5 to 104.2. This is also consistent with an improvement in
vertical phase separation on heating which would reduce bulk conduction through the
semiconductor film and thus lower the current in the off state of the device.
Simple unipolar inverter circuits were constructed using TIPS-pentacene and diFTESADT blend transistors showing their possibility for use in real devices.
We have
demonstrated that an inverter gain of greater than 10 and good noise margins can be achieved.
Again with a view to commercial viability we have shown that devices can be fabricated on
poly(ethylene terephthalate) (PET) films with very little loss of performance. Using a TIPSpentacene : PTAA blend for the semiconductor, saturation mobilities of up to 1.13 ± 0.05
cm2V-1s-1 were measured and threshold voltages and on/off current ratios remained the same
as for the devices made on glass. The key feature here was the maximisation of the substrate
surface energy by oxygen plasma in order to enhance uniform film formation and vertical
phase separation of the blend components.
In conclusion, we have shown a significant improvement in peak device performance and
reduced device variability by combining the film-forming properties of polymers and high
mobility due to high degree of crystallinity of small molecules in a blend device as compared
with other small molecule devices.[11] The use of an insulating polymer matrix demonstrated
the improved film forming properties of the blend system, while use of the field-effect active
polymer matrix maintained this and increased the mobility.
The dual gate device structure suggested that vertical phase separation of the small
molecule and the polymer occurs, indicated by improved top channel performance over
bottom channel within the same device. Although this could be explained by the different
dielectrics and their respective interface properties, DSIMS measurements confirmed that
there was a higher fraction of small molecule at the top surface.
8
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The comparison of diF-TESADT and TIPS-pentacene in identical blend systems show that
diF-TESADT is more crystalline, air-stable and higher performing. Optical micrographs show
larger crystallite formation on the electrodes which extend further into the channel, appearing
to correlate with the higher performance seen, reaching a mobility of 2.4 cm2V-1s-1.
Experimental:
The standard top gate transistors were fabricated on glass substrates (EAGLE 2000 from
Corning) with evaporated gold (Au) source and drain electrodes. A 900 nm layer of the
fluorocarbon polymer CYTOP (Asahi Glass) was used as the dielectric and gate electrodes
were evaporated aluminium (Al). The substrates were cleaned by sonication in detergent
solution (DECON 90) and rinsed with deionised water. Source and drain electrodes were
approximately 40 nm thick with a 60 μm channel length. Their surface was modified by
immersion in a 5x10-3 mol l-1 solution of pentafluorobenzene thiol (PFBT) in isopropanol for
5 min and then rinsed with pure isopropanol. The semiconducting layer was spin coated from
a solution of either TIPS-pentacene or diF-TESADT plus the polymer matrix (poly-a-methylstyrene and polytriaryl-amine) 1:1 by weight at 4 wt% concentration of solids in tetralin. Spin
coating was carried out at 500 rpm for 10 sec then 2000 rpm for 20 sec and was followed by
drying at 100 °C for 15 min in nitrogen to obtain layers with a thickness of ~70 nm. CYTOP
was then spin coated at 2000 rpm for 60 sec and dried at 100 °C for 20 min before the gate
electrode was evaporated.
Double
gate
devices
were
fabricated
using
ITO
coated
glass
substrates.
Divinyltetramethyldisiloxane bis-benzocyclobutane (BCB) solution was spin coated at 1500
rpm for 60 sec before being UV cured and then heat cured at 300 °C to form the bottom gate
dielectric. The devices were then completed as for the standard structure with Au source and
drain, CYTOP top gate dielectric and Al top gate contact. It was not possible to use identical
9
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layers for both top and bottom gate dielectrics in the double gate device, due to processing
issues. Although CYTOP uses a fluorinated solvent and will not dissolve the small molecule
when spun on top of the OFET blend layer, it also has a very low surface energy and, when
using it as a bottom gate dielectric, coating layers on top of CYTOP is very difficult.
Conversely, the solvent used with BCB will also dissolve the active layer, thus destroying the
device. Before PFBT treatment the samples were oxygen plasma treated for 2 min at 80W
R.F. power in order to increase the surface energy of the BCB layer from around 35 mJm -2 to
73 mJm-2. This increase is primarily due to the introduction of a polar component to the
surface energy not present on pristine BCB.
The devices using PET films as the substrate were produced in a similar fashion to the
standard devices. Plastic films were cleaned by sonication in acetone and isopropanol, and
oxygen plasma treatment was used to increase the substrate surface energy before PFBT
treatment.
Atomic force microscopy was carried out in close contact mode using a Pacific
Nanotechnology Nano-R2 machine and a Nikon Eclipse E600 POL was used to image the
films between crossed polarisers.
In order to examine the phase separation of the TIPS-pentacene and polymer matrix,
DSIMS (Dynamic Secondary Ion Mass Spectroscopy) was performed on the spin-coated
TIPS-pentacene : PTAA blend film using an Ion-Tof 'ToFSIMS IV' instrument (analysis
performed by Intertek MSG, Wilton, UK). During DSIMS, ion bombardment is used to slowly
sputter material from the film, and the ejected ion species measured by a mass spectrometer.
The primary ion species used were Cs ions and elements were selected so as to uniquely
identify each component: Si in TIPS-pentacene; N in PTAA; F in the PFBT monolayer; Au in
the source-drain electrodes and Si in the bottom layer was assumed to be from the glass
substrate. The results were normalised to their respective maxima, as there is a large variation
10
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in sensitivity for different elements using this technique. Therefore DSIMS cannot be used to
indicate relative concentrations between elements, but it can show the concentration variation
for a particular element throughout the depth of a film. A thin layer of gold was sputtered onto
the surface of the semiconductor film to prevent charging of the sample. The depth of the
trench made by the ion beam was measured by profilometer (Dektak 8) and used to convert
the time signal from the DSIMS into a measure of depth.
Electrical characterisation of the OTFTs was carried out in either a nitrogen atmosphere (<
0.1 ppm oxygen) or in air using a semiconductor parameter analyser.
Mobilities were
calculated in both the linear, μlin, and saturation, μsat, regimes from the transfer characteristics,
using a standard thin film field-effect transistor model:
 sat 
L 1 2ID
W Ci VG2
(1)
 lin 
L 1 I D
W CiVD VG
(2)
In these expressions, Ci is the geometric capacitance of the dielectric layer which for the
case of CYTOP was measured to be 2.10 ± 0.09 nFcm-2 and for BCB was 1.12 nFcm-2. This
measurement was carried out using a parallel plate capacitor on a Solartron 1260 impedance
analyzer. From Ci a value of 2.14 was estimated for the dielectric constant of CYTOP which
is only slightly higher than the value of 2.1 quoted by Asahi Glass Co.
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Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Architecture
Channel
μsat
μlin
ION / IOFF
TIPS-pentacene :
poly(α-methyl styrene)
DUAL GATE
Bottom
Top
0.10 ± 0.05
0.5 ± 0.1
–
–
104.1
104.8
TIPS-pentacene :
poly(α-methyl styrene)
TOP GATE
Top
0.7
0.6
105.3
TIPS-pentacene :
PTAA
TOP GATE
Top
1.1
0.7
105.2
diF-TESADT :
PTAA
TOP GATE
Top
2.41
1.88
104.2
Table 1: Summary of the mobilities and on/off current ratios obtained for the various material
and device designs employed.
(a)
(b)
Si
Si
S
F
F
S
Si
Si
(c)
Figure 1: Chemical structures of (a) diF-TESADT and (b) TIPS-pentacene and (c) a
schematic diagram of the standard device structure (PFBT = pentafluorobenzene thiol).
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Submitted to
(a)
-4
10
5
Bottom channel  = 0.06 cm /Vs
2
Top channel  = 0.44 cm /Vs
2
-5
10
4
-6
3
-3
2
-9
10
1/2
-8
10
ID(sat)
ID / A
/ 10 A
VD = -60 V
-7
10
1/2
10
1
-10
10
VT(bottom) = +12.6 V
VT(top)
-11
10
40
20
0
-20
= -9.0 V
-40
0
-60
VG / V
(b)
Figure 2: Dual gate device using a TIPS-pentacene : poly(α-methyl styrene) blend, (a)
transfer curves for the separate channels and (b) a schematic of the device structure (PFBT =
pentafluorobenzene thiol).
14
Submitted to
-4
VD = -4 V
VG = 0 to -60 V
VD = -40 V
-5
10
(VG= -10 V)
-30
6
-6
-8
ID
1/2
10
-20
ID / A
4
-3
-7
10
/ 10 A
1/2
10
ID / A
-40
8
10
(a)
-10
-9
10
VT = -8.9 V
2
-10
10
0
-11
10
10
0
0
-10 -20 -30 -40 -50 -60
0
-10
-20
-30
-40
-50
-60
VD / V
VG / V
(b)
-140
VD = -2 V
-4
10
14
VD = -40 V
VG = 0 to -60V
-120
-5
12
10
6
-8
10
4
VT = -5.0 V
-10
10
-60
-40
-20
-9
10
10
ID / A
-3
/ 10 A
-7
10
-80
1/2
8
ID(sat)
ID / A
10
1/2
-100
10
-6
0
2
0
0
-10 -20 -30 -40 -50 -60
0
-10
-20
-30
-40
-50
-60
VD / V
VG / V
Figure 3: Transfer and output curves of typical top-gate (a) TIPS-pentacene and (b) diFTESADT blend transistors with saturation mobilities of ~1 cm2V-1s-1 and >2 cm2V-1s-1
respectively.
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Submitted to
-1
1.2
2
(sat) / cm V s
-1
1.4
1.0
0.8
0.6
-5
I/A
10
ON current
-7
10
-9
10
OFF current
0 2 4
10
100
1k
10k
100k
Exposure time in air / min
Figure 4: Air stability of TIPS-pentacene and diF-TESADT (Open and Filled symbols
respectively) measured over a period of four weeks.
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(a)
Silicon
(TIPS)
1.0
Nitrogen
(PTAA)
(b)
Silicon
(TIPS + Substrate)
Silicon
(TIPS)
1.0
Nitrogen
(PTAA)
Gold
Fluorine
(PFBT) (Electrodes)
Silicon
(Substrate)
Normalised Count
0.8
Normalised Count
0.8
0.6
0.6
0.4
0.4
0.2
0.2
Gold
0.0
0.0
0
20
40
60
0
Depth probed into film / nm
20
40
60
80
Depth probed into film / nm
Figure 5: DSIMS depth profile of TIPS-pentacene: PTAA film on silicon substrate with
PFBT treated gold source and drain electrodes. The graphs show the phase separation, (a) in
the channel and (b) over the electrodes of the TIPS-pentacene material (as indicated by the Si
signal) closer to the air interface when compared with the PTAA (seen in the N signal).
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(a)
(b)
Figure 6: Polarised microscopy and AFM images of (a) TIPS-pentacene : PTAA, (b) diFTESADT : PTAA and (c) partially crystallised films with A being the amorphous and B being
the crystalline region.
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The table of contents entry
Here we show a double gate device used to demonstrate that a blended formulation of
semiconducting small molecule and a polymer matrix can provide high electrical performance
within thin film field effect transistors (OTFTs) with charge carrier mobilities of greater than
2 cm2V-1s-1, good device to device uniformity and the potential to fabricate devices from
routine printing techniques.
TOC Keyword: Organic Transistor
J. Smith, S. Ogier, M. Heeney, J. E. Anthony, I. McCulloch, J. Veres, D. D. C. Bradley, T. D.
Anthopoulos, R. Hamilton*
Title: High Performance Polymer-Small Molecule Blend Organic Transistors
ToC figure ((55 mm broad, 40 mm high))
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Supporting Information should be included here (for submission only; for publication, please
provide Supporting Information as a separate PDF file).
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