Versatile α,ω-disubstituted tetrathienoacene semiconductors for high performance organic thin-film transistors

Article abstract of DOI:10.1002/adfm.201101053

Facile one-pot [1 + 1 + 2] and [2 + 1 + 1] syntheses of thieno[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene (tetrathienoacene; TTA) semiconductors are described which enable the efficient realization of a new TTA-based series for organic thin-film transistors (OTFTs). For the perfluorophenyl end-functionalized derivative DFP-TTA, the molecular structure is determined by single-crystal X-ray diffraction. This material exhibits n-channel transport with a mobility as high as 0.30 cm ²V^-1s^-1 and a high on-off ratio of 1.8 × 10⁷. Thus, DFP-TTA has one of the highest electron mobilities of any fused thiophene semiconductor yet discovered. For the phenyl-substituted analogue, DP-TTA, p-channel transport is observed with a mobility as high as 0.21 cm²V^-1s^-1. For the 2-benzothiazolyl (BS-) containing derivative, DBS-TTA, p-channel transport is still exhibited with a hole mobility close to 2 × 10^-3 cm²V ^-1s^-1. Within this family, carrier mobility magnitudes are strongly dependent on the semiconductor growth conditions and the gate dielectric surface treatment. Copyright

Full text of DOI:10.1002/adfm.201101053

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Versatile α,ω-Disubstituted Tetrathienoacene
Semiconductors for High Performance Organic Thin-Film
Transistors
Jangdae Youn, Peng-Yi Huang, Yu-Wen Huang, Ming-Chou Chen,* Yu-Jou Lin,
Hui Huang, Rocio Ponce Ortiz, Charlotte Stern, Ming-Che Chung, Chieh-Yuan Feng,
Liang-Hsiang Chen, Antonio Facchetti,* and Tobin J. Marks*
charge transport properties, reflecting
extensive molecular conjugation and
strong intermolecular S•••S interactions
which promote close molecular packing.
tors are described which enable the efficient realization of a new TTA-based
Several fused thiophene derivatives have
Facile one-pot [1 + 1 + 2] and [2 + 1 + 1] syntheses of thieno[3,2-b]-
thieno[2′,3′:4,5]thieno[2,3-d]thiophene (tetrathienoacene; TTA) semiconduc-
series for organic thin-film transistors (OTFTs). For the perfluorophenyl end-
already been demonstrated to have good
functionalized derivative DFP-TTA, the molecular structure is determined by
single-crystal X-ray diffraction. This material exhibits n-channel transport with
a mobility as high as 0.30 cm2V−1s−1 and a high on-off ratio of 1.8 × 107. Thus,
DFP-TTA has one of the highest electron mobilities of any fused thiophene
semiconductor yet discovered. For the phenyl-substituted analogue, DP-TTA,
p-channel transport is observed with a mobility as high as 0.21 cm2V−1s−1.
For the 2-benzothiazolyl (BS-) containing derivative, DBS-TTA, p-channel
transport is still exhibited with a hole mobility close to 2 × 10−3 cm2V−1s−1.
Within this family, carrier mobility magnitudes are strongly dependent on the
semiconductor growth conditions and the gate dielectric surface treatment.
p-type charge transport performance
(Figure 1). For example, those with three
fused thiophene units, compounds 1[22]
and 2,[23] and those with four thiophene-
fused units, 3,[19] 4,[24] 5,[25] and 6[26] exhibit
hole mobilities up to 0.42, 0.89, 0.14,
0.06, 0.70, and 2.75 cm2 V−1 s−1, respec-
tively. Furthermore, five-ring fused pen-
tacene analogs 7[27] and 8[28] achieve hole
mobilities of 0.51, and 1.7 cm2 V−1 s−1,
respectively. Relative to p-type fused thi-
ophenes, the potential of n-type fused
thiophene-based semiconductors has not
been fully explored until recently, and
then only for a limited range of materials.[29–31] One approach
to realizing electron transport in organic semiconductors is
to functionalize p-type semiconductors with strong electron-
withdrawing substituents, such as perfluoroaryl,[32] carbonyl[33]
groups. Previously we investigated perfluorophenyl substi-
tuted dithieno[2,3-b:3′,2′-d]thiophene (DFP-DTT; 9) and per-
fluorobenzoyl (C6F5CO) substituted DTTs (FBB-DTT; 10 and
DFB-DTT; 11). All three of these new DTTs exhibit decent elec-
tron mobilities, as high as 0.07, 0.03, and 0.003 cm2 V−1 s−1,
respectively, for vapor-deposited films.
1. Introduction
Organic semiconductors have attracted growing attention over
the past decade due to their potential application in organic
thin-film transistors (OTFTs) for low cost/printable electronics,
such as flexible displays, RF-ID components, and e-papers.[1–12]
Compared with well-known organic semiconductors, such
as pentacenes[13–15] and anthradithiophene derivatives,[16–18]
fused-thiophenes[19–21] offer the attraction of relatively higher
ambient stability originating from large band gaps, and good
Dr. J. Youn, Dr. H. Huang, Dr. R. P. Ortiz, Dr. C. Stern,
Prof. A. Facchetti, Prof. T. J. Marks
Department of Chemistry and the Materials Research Center
Northwestern University
Dr. R. P. Ortiz
Department of Physical Chemistry
University of Malaga
29071, Malaga, Spain
2145 Sheridan Rd., Evanston, IL 60208-3113, USA
E-mail: a-facchetti@northwestern.edu; t-marks@northwestern.edu
L.-H. Chen
Process Technology Division
Display Technology Center
Industrial Technology Research Institute
Chung Hsing Rd., Chutung, Hsinchu, Taiwan 31040, R.O.C.
Dr. P.-Y. Huang, Y.-W. Huang, Prof. M.-C. Chen, Y.-J. Lin,
M.-C. Chung, C.-Y. Feng
Department of Chemistry
National Central University
Jhong-Li 32054, Taiwan
E-mail: mcchen@ncu.edu.tw
DOI: 10.1002/adfm.201101053
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DP-DTT (1)
PV-TTA (4)
D-DTT (2)
DP-TTA (3)
P-BTDT (5)
C13-BTBT (6)
s
DTBTD (7)
DBTDT (8)
DFP-DTT(9)
FBB-DTT(10)
DFB-DTT(11)
Figure 1. Several examples of fused thiophene organic semiconductors.
Following
a
molecular design strategy analogous to
efficient route to obtain tetrathiophene cores at a low cost. Fur-
thermore, we show that TTA can also be generated in a reverse
[2 + 1 + 1] order, unprecedented in TTA chemistry. With the
help of electron-withdrawing groups, this TTA core is shown to
achieve good n-type charge transport in OTFT devices. The per-
fluorophenyl end-functionalized derivative DFP-TTA exhibits
n-channel transport with a mobility as high as 0.30 cm2 V−1 s−1,
rendering it one of the highest performing n-type semiconduc-
tors among fused thiophenes reported to date. Two other TTA
derivatives are synthesized to better understand molecular
structure-device performance relationships. Phenyl-substituted
derivative DP-TTA and the 2-benzothiazolyl (BS-) containing
derivative DBS-TTA exhibit p-channel transport with mobilities
as high as 0.21 cm2 V−1 s−1 and 0.002 cm2 V−1 s−1, respectively.
Materials properties such as crystal structure, HOMO-LUMO
localization/energetics, and film microstructure are discussed
and compared/contrasted with the DTT analogs (Figure S1).
In addition, film growth conditions including substrate tem-
perature, and dielectric surface treatment are investigated and
shown to strongly influence TFT device response in a readily
understandable way.
DTTs, the fused tetrathiophene-based tetrathienoacene
(TTA) system, end-functionalized with electron-withdrawing
groups, is investigated in this contribution to better under-
stand correlations between molecular structure and n-type
TFT performance (Figure 2). To the best of our knowledge,
the first example of a TTA-based small molecule semicon-
ductor with a p-channel mobility of 0.14 cm2 V−1 s−1 was
reported by Y. Liu et al. in 2009.[19] The second TTA, end-
capped with styrenyl groups, was reported by the same team
in 2010 with a mobility of 0.06 cm2 V−1 s−1.[24] Recently a
dicyanomethylene substituted fused tetrathienoquinoid was
reported with a good mobility of 0.9 cm2 V−1 s−1 after solu-
tion processing.[34] Since then, no other TTA-based small
molecules have been reported, due to synthetic difficulties
in accessing the tetrathiophene core, which was obtained
in only 15 ∼ 26% overall yield from reaction of 3-bromothi-
ophene and 3-bromothieno[3,2-b]thiophene.[19]
To address this issue, we report here facile “one-pot”
[1 + 1 + 2] and [2 + 1 + 1] syntheses of TTA. Compared to the
previous synthetic routes, this one-pot synthesis offers a more
DP-TTA
DBS-TTA
DFP-TTA
Figure 2. Chemical structures of the tetrathienoacene (TTA) derivatives synthesized in this study.
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generated in situ from 3-bromothienothiophene,
synthesized as shown in Scheme S1.[37,38] Without
product isolation, the crude mixture is subse-
quently dilithiated with n-BuLi and ring closure is
achieved with CuCl2 to afford TTA in >27% yield.
An alternative/reversed one-pot [2 + 1 + 1] syn-
thesis of TTA was also explored and offers a com-
parable yield (∼22%).[39] For comparison, TTA was
prepared following the known fused thiophene
synthetic route in which 3-bromothiophene is
first ring-fused,[40] and then brominated to give
2,3,5,6-tetrabromothieno[3,2-b]thiophene, then double
One pot [1 + 1 + 2]
4.
1. n-BuLi
5. n-BuLi
2. S8
3. TsCl
6. CuCl2
TTA
TTA
One pot [2 + 1 + 1]
4.
1. n-BuLi
5. n-BuLi
2. S8
6. CuCl2
3. TsCl
Scheme 1. One-pot [1 + 1 + 2] and [2 + 1 + 1] synthetic routes to the TTA core.
ring-fused, following
a known TTA synthetic
route,[41] to afford tetrathienoacene in an total yield
of 16% (Scheme 2). Undoubtedly, the latter route is much more
time-consuming and labor-intensive compared to the one-pot
synthetic route developed here.
2. Results and Discussion
In this section we first describe the synthesis of the new semi-
conductors, followed by molecular characterization. Next, we
examine details of the solid state packing, and the microstruc-
ture of the vapor-deposited films based on X-ray diffraction data.
Finally, we discuss thin-film transistor characterization and cor-
relate the results with semiconductor film morphology, such as
crystalline domain size, surface coverage, and nucleation den-
sity at the semiconductor-dielectric interfacial region.
Since perfluorophenyl functionalization is likely to result in
good candidates for n-channel semiconductors, the synthesis of
C6F5-functionalized TTA (DFP-TTA) was explored and achieved
in a yield of 75% via Stille coupling (Scheme 3). For compar-
ison, DP-TTA was also prepared via Stille coupling and the yield
is comparable to the Suzuki coupling approach (Scheme 3).
More conjugated benzothiazolyl substituents, in addition to
contributing the electron-withdrawing capacity of the carbonyl
group (C = O), tend to form planar molecular structures,[42]
thus possibly resulting in lower-lying LUMOs, and hence pos-
sible n-channel transport. Therefore, 2-benzothiazolyl (BS-)
end-capped DBS-TTA was prepared and achieved in a yield of
∼51% by refluxing 2-aminobenzenethiol with TTA-(COCl)2. The
2.1. Synthesis
Since the key building block in this investigation is the
tetrathiophene-fused core, a one-pot [1 + 1 + 2] synthesis of
TTA was first explored. Our new approach originates from inex-
pensive, commercially available materials and dispenses with
the requirement of expensive bis(phenyl-sulfonyl)sulfide, which
is conventionally used in the fused-thiophene syntheses,[35–37]
and the results for TTA derivatives are much shorter synthetic
times with comparable yields. For this one-pot synthesis,
3-bromothiophene is first lithiated with n-BuLi, followed
first by S8 and then by TsCl addition, as shown in Scheme 1.
Next, the mixture is treated with 3-lithiumthienothiophene,
[41]
later was generated in situ by refluxing TTA-(COOH)2 with
SOCl2 in situ as shown in Scheme 4.
2.2. Semiconductor Thermal and Optical Properties
The three TTA-based materials synthesized in this study are
thermally stable and lack any detectable phase transitions at low
HSCH2CO2Et
10% NaOH
LDA / THF
N-Formylpiperidine
K2CO3
82%
71%
Br2
N-BuLi / THF
AcOH
N-Formylpiperidine
79%
85%
95%
HSCH2CO2Et
K2CO3
10% NaOH
77%
97%
Cu
Quinoline
58%
Scheme 2. Synthesis of tetrathienoacene from 3-bromothiophene.
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1. n-BuLi, Bu3SnCl / THF
2. Ar-Br, Pd(PPh3)4
TTA
R = F; DFP-TTA
R = H; DP-TTA
Scheme 3. Synthetic route to DFP-TTA and DP-TTA.
temperatures, indicating they are suitable for TFT fabrication.
As might be expected, the highest molecular weight compound,
DBS-TTA, exhibits the highest melting point and TGA weight
loss temperature. For DFP-TTA and DP-TTA, differential scan-
ning calorimetry (DSC) does not reveal obvious endothermic
features at temperatures below 280 °C. The thermogravimetric
analysis data indicate weight loss (5%) only on heating above
330 °C, as summarized in Table 1. Note that the present TTA
compounds have significantly higher melting points and higher
weight loss temperatures in comparison to their DTT analogs,
DFP-DTT, DP-DTT, and DBS-DTT (Figure S1). Interestingly, with
perfluorophenyl groups end-capping the TTA core, DFP-TTA has
a lower melting point and a lower weight loss temperature than
its nonfluorinated analogue, DP-TTA, and exhibits higher vola-
tility, doubtless due to the perfluoroaryl substituents.[30,31]
delocalization extends from the TTA (or DTT) core to the two
benzothiazolyl substituents. Interestingly, derivatives with
C6F5- substituents exhibit slightly blue-shifted absorptions
versus those with C6H5 substituents in both the TTA and DTT
series, as seen in DFP-TTA (λmax ∼ 384 nm) vs. DP-DTT (λmax
389 nm) and DFP-DTT (λmax ∼ 367 nm) vs. DP-DTT (λmax
∼
∼
373 nm). The HOMO-LUMO energy gaps calculated from the
onset of the optical absorption (2.6–3.1 eV) increase in the
order: DBS-TTA < DP-TTA < DFP-TTA as well as DBS-DTT <
DP-DTT < DFP-DTT, as shown in Table 1.
These results seem contradictory at first glance considering
well-known electron-withdrawing group (EWG) effects that
typically red-shift absorption maxima in optical spectra.[43–50]
However, the present DPV and DFT studies (See more below)
reveal that the electron-withdrawing C6F5 group lowers both the
HOMO and LUMO, also consistent with known EWG effects.
Moreover, the HOMO levels of DFP-TTA and DFP-DTT are low-
ered versus DP-TTA and DP-DTT with respect to their LUMOs.
Consequently, the energy gaps for C6F5- substituted deriva-
tives DFP-TTA and DFP-DTT become slightly larger, thereby
explaining the slightly blue-shifted absorption maxima.
The optical absorption spectra of the TTA compounds in
o-C6H4Cl2 are significantly red-shifted versus their DTT analogs
as shown in Figure 3, verifying the smaller band gaps of TTA com-
pounds than their DTT analogs. As expected, the benzothiazolyl-
substituted derivative, DBS-TTA, exhibits the smallest energy
gap among the compounds in this study, arguing that the
SOCl2
TTA-(COCl)2
DBS-TTA
Scheme 4. Synthetic route to DBS-TTA.
Table 1. Thermal, optical, and electrochemical comparison of the properties of TTA and DTT compounds.
Compound
DSC
Tm(°C)
TGA
(°C, 5%)
UV–visb)
λmax (nm)
Reduction Potential Oxidation Potential
ΔEgap(eV)
(Optical)b)
(V)c)
(V)c)
(DPV)c)
DFP-TTA
DP-TTA
280
280a)
442
258
290
262
330
371
443
270
394
388
384
389
1.46
1.15
1.35
1.64
1.21
1.47
2.93
2.87
2.60
3.06
3.02
2.69
3.12
3.09
2.83
3.26
3.16
2.93
−1.66
−1.94
−1.48
−1.62
−1.95
−1.46
DBS-TTA
DFP-DTT
DP-DTT
DBS-DTT
426,451
367
373
417,441
a)see ref. 19; b)in o-C6H4Cl2; c)by DPV in o-C6H4C12 at 25 °C (using ferrocene/ferrocenium as internal standard, set at +0.60 V).
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Figure 4. The electrochemically-derived HOMO and LUMO energy levels
of TTA and DTT molecules.
Figure 3. Optical spectra of TTA compounds (solid lines) and DDT com-
pounds (dashed lines) in o-C6H4Cl2 solution.
The photooxidative stability of the present TTA derivatives was
investigated by monitoring the absorbance decay at λmax in aer-
ated CHCl3 solutions exposed to white light (fluorescent lamp)
at room temperature. Under these conditions over the course of
3 days, no decomposition is observed for any of these com-
pounds, demonstrating the photo-oxidative stability of these
materials.
the order DBS-TTA < DP-TTA < DFP-TTA (as well as DBS-DTT <
DP-DTT < DFP-DTT; Table 1, Figure 4), which is consistent with
the values obtained from optical spectroscopy.
It is interesting to highlight the different effects of the two
electron-deficient end-groups on the HOMO and LUMO ener-
gies. While the perfluorophenyl substituents lower both frontier
orbital energies due to almost identical contribution of the per-
fluorophenylunitstobothHOMOandLUMOorbitals(FigureS2),
benzothianolyl substitution lowers the LUMO energy to a far
greater extent than the HOMO. Note that despite the fact that
both substituents are formally electron-withdrawing groups,
the nature of their effects differs substantially; for the perfluor-
ophenyl substituents a σ-(-I) isomeric effect is expected versus a
π-mediated mesomeric (-M) electron-withdrawing effect for the
benzothianolyl substitution.[54] These different effects explain
the rather different modulations of the frontier MO energies,
and may result in very different electrical behavior. Further-
more, if we analyze the theoretical charge distributions on both
neutral DFP-TTA and DBS-TTA molecules (see Figure S2 in
Supporting Information) we observe that the atoms bearing the
greatest negative charge in DBS-TTA are the nitrogen atoms,
with negative charges of −0.518 e. each. Furthermore, the sulfur
atoms in the benzothianolyl group bear a much less positive
charge than in the corresponding thiophene rings (Figure S2),
underscoring the electron-deficient character of the lateral sub-
stituent sulfur atoms. These two electron-deficient atoms are
involved to a greater extent in the LUMO (as evidenced in the
frontier orbital topologies in Figure S2) than in the HOMO,
which explains the unequal destabilization of both frontier
orbitals as discussed above. In contrast, the negative charge on
the electron-withdrawing perfluorophenyl substituents is local-
ized on the fluorine atoms, which are equally involved in both
the HOMO and LUMO topologies (Figure S2), in agreement
with the equal stabilization of both orbitals discussed above.
2.3. Electrochemical Characterization
Differential pulse voltammograms (DPVs) of the TTA com-
pounds were recorded in dichlorobenzene at 25 °C, and the
resulting reductive and oxidative potential data are summarized
in Table 1.[51] The electrochemically-derived HOMO levels of the
TTAs are significantly up-shifted versus their DTT analogs as
shown in Figure 4. More π-electron delocalization is observed for
the fused-tetrathiophene TTA system than for the DTTs, and the
HOMO-LUMO energy gaps of the TTAs obtained from the DPV
data are smaller than those of their DTT analogs. For C6F5- sub-
stituted DFP-TTA, the electron affinity increases with the larger
number of fluoroaryl rings, and fluoroaryl substitution in DFP-
TTA strongly lowers both the HOMO and LUMO energies versus
those in DP-TTA. Similar trends are observed for DFP-DTT com-
pared to its nonfluorinated derivative DP-DTT. The DPV of DFP-
TTA exhibits an oxidative peak at +1.46 V and a reductive peak at
−1.66 V (using ferrocene/ferrocenium as the internal standard,
set at +0.60 V). For comparison, the oxidation and reduction
potentials of DP-TTA (Eox = +1.15 V, Ered = −1.94 V) are shifted
to more negative values, which can be attributed to the electron-
withdrawing effects of the perfluorophenyl substituents. Similarly,
the benzothiazolyl substitution in compound DBS-TTA (Eox
=
+1.35 V, Ered = −1.48 V) strongly shifts the HOMO and LUMO
energies to higher values compared to DP-TTA, and induces the
most extensive π-electron delocalization within the TTA series,
and also has the smallest band gap. The HOMO-LUMO energy
gaps obtained from the DPV data are 3.12 eV for DFP-TTA,
3.09 eV for DP-TTA, and 2.83 eV for DBS-TTA (assuming fer-
rocene/ferrocenium oxidation at 4.8 eV).[52,53] Overall, the electro-
chemically-derived HOMO-LUMO energy gaps can be ranked in
2.4. Semiconductor Solid State and Thin-Film Structure
Thin films of the new semiconductors were vapor-deposited
with the goal of investigating the thin-film microstructure
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and morphology for TFT device fabrication. The morphology/
TFT properties of the TTA films were investigated on doped Si
(gate)/SiO2 (gate insulator) substrates with several different gate
dielectric surface treatments. Hexamethyldisilazane (HMDS)-
modified substrates were prepared by exposing the Si/SiO2
substrates to HMDS vapor for 7 days in a nitrogen atmosphere
to yield a trimethylsilyl-coated surface. Octadecyltrichlorosilane
(OTS)-modified substrates were fabricated by immersion of the
Si/SiO2 substrates in 3.0 mM hexane solutions of the silane
reagent in air for 1 hour after 10 hours of solution aging under
55–60% of relative humidity. All substrates were characterized
by advancing aqueous contact angle measurements, which
indicate increasing hydrophobicity in the order: SiO2 (28°) <
HMDS (95°) < OTS (104°). Additionally, the surface rough-
nesses were evaluated by tapping mode atomic force micro-
scopy (AFM), revealing a root-mean square (RMS) roughness
of 0.15 nm for SiO2, 0.20 nm for HMDS, and 0.35 nm for OTS.
All semiconductor films (∼50 nm thick) were vapor-deposited
while maintaining the substrates at the temperatures (TDs) of
25, 50, and 110 °C, and with a film growth rate of 0.1 Å/s. All
films were characterized by θ–2θ X-ray diffraction (XRD) scans
and tapping mode AFM.
Before discussing the XRD results for the TTA films, it is
useful to begin by correlating film X-ray θ–2θ scan data with
a compound of known crystal structure. This initial examina-
tion allows a much more thorough analysis of the molecular
ordering in the solid film. The perfluorinated TTA derivative
DFP-TTA crystallizes in the monoclinic space group P21/c.
The unit cell packing viewed perpendicular to the molecular
stacking direction is illustrated in Figure 5B, and along the
c-axis in Figure 5C.
cell parameters, a = 28.500 Å, b = 3.9449 Å, c = 11.3467 Å,
α = 90.00°, β = 91.014°, γ = 90.00°, and Z = 2 (Figure 5). The
fluorinated phenyl moiety is slightly twisted from the plane
of the fused tetrathiophene core, with a dihedral angle of
10.2o (Figure 5B). The cofacial stacking distance between TTA
cores is 3.59 Å (Figure 5B), and the shortest intermolecular
sulfur•••sulfur contact is 3.58 Å (Figure 5C). This planar mole-
cular structure, short packing distances, and high crystal den-
sity (2.086 gcm−3) suggest ideal conditions to achieve significant
charge transport in solid films. In general, DFP-DTT[30] exhibits
a very similar crystal structure to DFP-TTA. This latter molecule
also has a typical herringbone packing with cell parameters, a =
11.7702 Å, b = 37.048 Å, c = 3.8830 Å, α = 90.00°, β = 90.000°, γ =
90.00°, and Z = 4 (Figure S3) and the perfluorophenyl groups are
slightly twisted by 11.1° from the DTT core plane (Figure S3B).
The stacking distance between planar DTT cores is 3.60 Å
(Figure S3B) with a shortest intermolecular sulfur-to-sulfur dis-
tance of 3.48 Å (Figure S3C). In addition, the slipping angle of
the DFP-DTT stack is 68° similar to 65° in DFP-TTA. It is well
known that the herringbone motif is the result of pulling two
adjacent molecular stacks together with a roll angle in the oppo-
site direction.[55] When the slipping angle is larger, the overlap
of the π-conjugated cores is substantially increased along the
stacking direction. Since their slipping angle is almost identical,
the degree of spacial overlap between adjacent π-conjugated
cores depends on the core sizes of molecules. The effect of the
greater core size in the DFP-TTA versus that in DFP-DTT is
reflected in more efficient charge transport properties for the
corresponding thin film transistors (see more below).
With the crystal structure data in hand, it is straightforward
to simulate the XRD powder pattern and therefore assign the
reflections observed in the thin film XRD measurements. Thus,
d spacings calculated from the XRD data, and the molecular
Similar to other fused thiophenes, the unit cell of DFP-TTA
exhibits a commonly observed herringbone packing motif with
10.2°
A
B
C
3.59Å
c
65°
o
a
a
o
a
3.58Å
3.58Å
c
DFP-TTA
Figure 5. Crystal structure of DFP-TTA (A) molecular length of DFP-TTA = 19.7 Å, (B) slipping angle of adjacent TTA cores = 65°; cofacial distance between
TTA cores = 3.59 Å; torsion angle between core and perfluorophenyl plane = 10.2° (C) the shortest intermolecular sulfur - sulfur distance = 3.58 Å.
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Table 2. Film microstructural parameters for TTA and DTT molecular systems.
Compound
Substrate
temperature (°C)
d-spacing (A)
Film XRD
Molecular length (A)
2θ (°)
DFT calc.
19.8
Single crystal data
DFP-TTA
DP-TTA
25,50
25,50
4.3
4.6
3.8
4.7
2.6
4.2
20.7
19.4
23.3
18.8
16.9
21.0
19.7
NAa)
NA
17.3
DBS-TTA
DFP-DTT
DP-DTT
DBS-DTT
25, 50, 110
25,50
22.8
17.5
17.4
NA
25,70
17.0
25, 70, 90
20.5
NA
a)Not available
lengths computed from the geometry optimization and the
single-crystal structure analysis are summarized in Table 2.
Figure 6 shows a graphical comparison of the experimental
and simulated data, with a 2θ scan of a DFP-TTA film grown
at TD = 50 °C on an OTS-coated substrate, and a powder pat-
tern generated from the single-crystal data. The (h 0 0) reflec-
tion family is particularly pronounced, from (1 0 0) to (6 0 0).
The (1 0 0) reflection in the film XRD is observed at 2θ = 4.25°,
corresponding to a d spacing of 20.7 Å (Table 2), approximately
the length of the unit cell a axis. This indicates that the films
are highly textured and that DPF-TTA molecules in the films
are predominantly aligned with their long molecular axes along
the substrate normal; that is, film growth is favored in the a
direction. It is also apparent that higher deposition tempera-
tures (TD) make this a-directional alignment even more favo-
rable, as indicated by sharpening and increased intensity of the
(h 0 0) reflections in the θ–2θ scans of the corresponding films
of the same thickness (Figure S4, Supporting Information).
In general, both TTA and DTT molecular types appear to be
aligned approximately vertically at the semiconductor-dielectric
interface since none of the d spacings is smaller than molec-
ular lengths (Table 2). The slight estimated size difference
between TTA and DTT may reflect the intrinsic inaccuracies of
the method. This edge-on type molecular arrangement is well
known to promote optimum charge transport in organic TFT
devices. Over the entire TD range examined here, the TTA films
deposited on OTS substrates exhibit reflections having the
same 2θ values (Figure S4). As the deposition temperature (TD)
is increased, the peaks become sharper and their intensities
increase. The consistency in reflection positions indicates the
presence of a single polymorph and growth orientation across
the TD range. The increase in relative intensity of the higher
order peaks on going from room temperature to higher growth
temperatures indicates enhanced long range order. The effect
of surface treatment is not as obvious as the effect of substrate
temperature on the XRD data (Figure S5).
Organic semiconductor thin films (50 nm) were vapor-deposited
onto the Si/SiO2, HMDS-treated, and OTS-treated substrates as
described previously. Then, 50 nm gold source and drain elec-
trodes were vapor-deposited at 2 × 10−6 Torr through a shadow
mask in a high vacuum deposition chamber. Devices were fabri-
cated with typical channel lengths of 100 μm and a channel width
of 2000 μm. Current–voltage (I–V) transfer and output plots were
measured for each device under vacuum and in air. To illustrate
the precision of each measurement, the reported data are an
average of at least five devices tested on different regions of the
semiconductor film. Key device performance parameters such as
field-effect carrier mobility (μ), threshold voltage (VT), and on-to-
off current ratio (Ion/Ioff), were extracted using standard proce-
dures.[56] The results are summarized in Table 3.
DFP-TTA and DFP-DTT exhibit good n-type charge transport.
DFP-TTA exhibits electron mobility μe = 0.30 cm2 V−1 s−1 and
Ion/Ioff = 2.9 × 107 for films grown on an OTS-coated substrate
at 25 °C (Figure 7A, B). DFP-DTT exhibits μe = 0.05 cm2 V−1 s−1
and Ion/Ioff = 107 for films grown on an HMDS-coated substrate
at 25 °C.[25] The relatively low electron injection barriers evident
in the HOMO-LUMO energy diagram (Figure 4) doubtless con-
tribute to this high n-type charge transport. As discussed in the
single-crystal structural analysis, the differences in core sizes
2.5. Organic Thin Film Transistor Fabrication
and Characterization
Thin film transistors were fabricated in bottom gate-top con-
tact configurations. Highly doped p-type (100) silicon wafers
were used as gate electrodes as well as substrates, and 300 nm
thermally grown SiO2 on the Si was used as the gate insulator.
Figure 6. Comparison of the θ–2θ XRD scan of a DFP-TTA film grown at 50 °C
on an OTS-coated substrate (A) with the simulated powder pattern (B).
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Table 3. TFT device performance of TTA materials in this study.
Vacuum
Threshold
Air
Compound
Substrate
Temperature
Substrate Carrier
Ion/IOff
Threshold
Voltage VT
(V)
Ion/IOff
Mobility μ
Mobility μ
a)
(cm2 V−1s−1
)
(cm2 V−1s−1
)
Surface
Sign
Voltage
VT (V)
TD (°C)
Treatment
25
Bare
HMDS
OTS
N
NAc)
NA
NA
NA
NA
NA
b)
(6.7 ± 1.0)
× 10−3
(7.7 × 10−3
(0.05)
)
72 ± 14
56 ± 7
56 ± 5
47 ± 0
57 ± 5
44 ± 2
(4.1 ± 1.9)
× 104
N
0.04 ± 0.01
(3.2 ± 0.2)
× 106
N
(0.30)
0.29 ± 0.01
(1.8 ± 1.5)
× 107
50
Bare
N
(7.6 ± 0.0)
× 10−3
(7.6 × 10 −3
)
(5.3 ± 0.0)
× 107
HMDS
OTS
N
(0.08)
0.05 ± 0.02
(2.8 ± 1.1)
× 107
N
(0.03)
0.03 ± 0.00
(3.4 ± 2.0)
× 105
DFP-TTA
25
50
Bare
HMDS
OTS
P
P
P
P
P
P
(0.04)
(0.07)
(0.11)
(0.06)
(0.10)
(0.21)
0.04
0.09
0.13
0.05
0.11
0.19
0.04 ± 0.00
0.07 ± 0.00
0.11 ± 0.01
0.05 ± 0.01
0.10 ± 0.00
0.21 ± 0.00
−(19 ± 0)
−(17 ± 0)
−(13 ± 4)
−(19 ± 4)
−(10 ± 1)
−(22 ± 0)
(1.5 ± 0.0) 0.04 ± 0.00
× 106
−(17 ± 1)
−(18 ± 2)
−(19 ± 6)
−(16 ± 2)
−(14 ± 1)
−(25 ± 13)
(4.7 ± 2.7)
× 105
(3.3 ± 0.0) 0.08 ± 0.01
× 106
(1.3 ± 0.2)
× 106
(4.3 4.0)
× 107
0.11 ± 0.02
(4.8 ± 0.2)
× 106
Bare
(1.9 ± 1.2) 0.04 ± 0.00
× 105
(1.3 ± 0.5)
× 105
HMDS
OTS
(4.0 ± 3.6) 0.10 ± 0.01
× 106
(2.4 ± 1.3)
× 106
DP-TTA
(1.1 ± 0.0) 0.16 ± 0.05
× 106
(1.1 ± 1.0)
× 107
(2.8 × 10−5
(3.0 × 10−4
(5.0 × 10−4
(2.4 × 10−5
(8.5 × 10−4
(8.8 × 10−4
(5.5 × 10−4
(1.7 × 10−3
(1.9 × 10−3
)
−(25 ± 4)
−(42 ± 3)
−(45 ± 4)
−(31 ± 2)
−(42 6)
(8.3 ± 8.2) (1.2 ± 0.1)
(1.3 × 10−5
(3.4 × 10−4
(4.8 × 10−4
(1.1 × 10−5
(7.3 × 10−4
(8.2 × 10−4
(4.4 × 10−4
(1.4 × 10−3
(2.3 × 10−3
)
)
)
)
)
)
)
)
)
−(15 ± 0)
−(34 ± 2)
−(41 ± 5)
−(10 ± 4)
−(39 ± 4)
−(40 ± 8)
−(22 ± 3)
−(36 ± 1)
−(29 ± 3)
(2.0 ± 1.0)
× 102
25
50
Bare
HMDS
OTS
P
P
P
P
P
P
P
P
P
(2.3 ± 0.4)
× 10−5
× 103
× 105
(2.6 ± 0.4)
× 10−4
)
(3.7 ± 2.4) (2.9 ± 0.5)
× 104 × 104
(3.1 ± 1.6)
× 103
(4.1 ± 0.9)
× 10−4
)
)
)
)
)
)
)
(5.7 ± 1.9) (4.2 ± 0.6)
× 103 × 104
(5.7 ± 2.5)
× 103
Bare
(2.3 ± 0.1)
× 10−5
(1.0 ± 0.8) (1.0 ± 0.1)
× 104 × 105
(2.6 ± 0.2)
× 102
HMDS
OTS
(6.7 ± 2.1)
× 10−4
(1.5 ± 0.9) (6.8 ± 0.5)
× 105 × 104
(2.7 ± 1.6)
× 104
DBS-TTA
(8.6 ± 0.3)
× 10−4
−(44 ± 11) (4.2 ± 3.0) (7.3 ± 0.8)
× 104 × 104
(2.4 ± 0.2)
× 104
110
Bare
(5.0 ± 0.3)
× 10−4
−(32 ± 4)
−(34 ± 4)
−(12 ± 5)
(1.3 ± 0.4) (4.1 ± 0.3)
× 104 × 104
(2.2 ± 1.1)
× 103
HMDS
OTS
(1.6 ± 0.1)
× 10−3
(4.1 ± 1.8) (1.3 ± 0.0)
× 104 × 103
(5.8 ± 2.9)
× 103
(1.8 ± 0.5)
× 10−3
(5.9 ± 2.6) (2.1 ± 0.2)
× 104 × 103
(1.5 ± 0.1)
× 104
a)The average values obtained for at least 5 devices; b)The maximum mobility recorded; c)Not active.
of DFP-DTT (Figure 5B) versus DFP-TTA (Figure S3B) can be
correlated with the lower mobility of DFP-DTT. DFT calcula-
tions predict the reorganization barrier for electron transport in
the case of DFP-DTT to be 0.33 eV, slightly greater than the
corresponding one for tetrathienoacene derivative DFP-TTA,
0.28 eV (Table S2). This factor, among others, likely contributes
to the mobility differences in these two bis(perfluorophenyl)
derivatives.
In marked contrast to the above DFP-TTA and DFP-DTT
results, neither DBS-TTA nor DBS-DTT exhibit the anticipated
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10-4
10-5
10-6
10-7
10-8
10-9
B
D
F
A
1.0x10-2
8.0x10-3
6.0x10-3
4.0x10-3
4.0x10-5
3.0x10-5
2.0x10-5
VSG = 120V
F
F
F
F
F
S
V
SG = 100V
S
S
10-10
10-11
10-12
S
2.0x10-3
0.0
F
F
1.0x10-5
0.0
F
F
VSG = 80V
SG = 60V
V
F
0
20 40 60 80 100 120
-20
0
20 40 60 80 100 120
(Source -Drain Voltage (V))
(Source -Gate Voltage (V))
1.0x10-2
8.0x10-3
6.0x10-3
4.0x10-3
6.0x10-5
5.0x10-5
4.0x10-5
3.0x10-5
2.0x10-5
C
VSG = -120V
VSG = -100V
10-5
10-6
10-7
S
10-8
V
SG = -80V
S
S
10-9
2.0x10-3
0.0
S
VSG = -60V
SG = -40V
10-10
10-11
10-12
1.0x10-5
V
0.0
0
20 40 60 80 100 120
-(Source -Drain Voltage (V))
-20
0
20 40 60 80 100 120
-(Source -Gate Voltage (V))
1.4x10-6
1.2x10-6
1.0x10-6
8.0x10-7
6.0x10-7
4.0x10-7
E
VSG = -120V
VSG = -100V
10-6
10-7
10-8
10-9
10-10
1.6x10-3
1.2x10-3
8.0x10-4
S
N
V
SG = -80V
S
S
S
VSG = -60V
VSG = -40V
S
4.0x10-4
0.0
2.0x10-7
N
S
V
SG = -20V
0.0
0
20 40 60 80 100 120
-20
0
20 40 60 80 100 120
-(Source -Drain Voltage (V))
-(Source -Gate Voltage (V))
Figure 7. Transfer and output plots of OTFT devices fabricated from TTA films grown on an OTS-coated substrate: (A) transfer plot; (B) output plot.
DFP-TTA, substrate temperature = 50 °C, μ = 0.30 cm2 V−1 s−1, VT = 60 V, Ion/Ioff = 2.9 × 107 in vacuum. (C) transfer plot; (D) output plot. DP-TTA,
substrate temperature = 50 °C, μ = 0.21 cm2 V−1 s−1, VT = −22V, Ion/Ioff = 1.1 × 106 in vacuum (E) transfer plot; (F) output plot. DBS-TTA, substrate
temperature = 110 °C. μ = 1.9 × 10−3 cm2 V−1 s−1, VT = −5 V, Ion/Ioff = 4.7 × 104 in vacuum. Channel width = 2000 μm, channel length = 100 μm.
n-type charge transport, despite having the lowest-lying LUMOs
in this series. Instead, they exhibit p-type charge transport. Thus,
DBS-TTA achieves μ = 0.0019 cm2 V−1 s−1 and Ion/Ioff = 4.7 × 104
on OTS-coated substrates for TD = 110 °C (Figure 7E, F),
and DBS-DTT exhibits μ = 0.01 cm2 V−1 s−1 and Ion/Ioff = 105 for
films grown on HMDS-coated substrates at 90 °C.[30] To under-
stand the reversed polarity of the charge transport in these
perfluorophenyl and benzothianolyl derivatives, we optimized
the molecular geometries of the corresponding radical anions
using DFT computation, and their charge distributions are
shown in Figure S6. Upon injection of an electron into DBS-
TTA, 63% of the charge is localized in the external electron-
withdrawing groups while only 37% of the remaining charge
is delocalized over the conjugated tetrathienoacene skeleton. In
contrast, the negative charge on DFP-TTA is more uniformly
distributed, with the external electron-withdrawing substituents
bearing ca. 44% of the charge and the central conjugated fused
unit the remaining 56%. The more evenly delocalized charge
in the perfluorophenyl derivative likely stabilizes the injected
charge to a greater extend, thus facilitating electron transport
even though the DBS-TTA LUMO energy is lower in energy.
These contrasting results can be ascribed to the different nature
(σ-inductive vs. π-mesomeric) of the two electron-withdrawing
substituents, as discussed above. The situation for the DBS-
TTA radical cation is very different, and we find that the posi-
tive charge is evenly distributed, with ca. 47% located in the
external substituent groups and the remaining 53% on the
conjugated tetrathienoacene core (Figure S6), probably favoring
hole transport within the thin film. The theoretical calculations
also show that DBS-TTA has the smallest reorganization energy
within the series (Table S2), with the reorganization energy for
hole transport less than that for electron transport (0.214 eV
vs. 0.282 eV, respectively). Note that the present results for
DBS-TTA and DBS-DTT are in accord with other observations
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that appending electron-withdrawing thiazole units to oligothi-
ophene skeletons seldom enhances electron mobility.[57] In fact,
as shown here for DBS-TTA vs. DP-TTA, lower hole mobilities
are usually obtained versus the corresponding oligothiophenes.
As expected from the HOMO-LUMO energy diagram (Figure 4),
DP-TTA and DP-DTT exhibit good p-type charge transport. Since
the work function of the gold electrode is about −5.0 eV, these
materials have the lowest hole injection barrier in the series:
DFP-TTA (0.66 eV) < DBS-TTA (0.55 eV) < DP-TTA (0.35 eV) and
DFP-DTT (0.84 eV) < DBS-DTT (0.61 eV) < DP-DTT (0.47 eV).
on device performance. The most distinctive effect on surface
morphology is substrate temperature (TD; Figures S7, S8, S9).
As the substrate temperature is increased, semiconductor
molecules diffuse more rapidly, and are captured in islands
facilitating grain growth. Therefore, films deposited at higher
temperatures exhibit larger grain sizes. For example, 50 nm
thick DFP-TTA films have small ball-shaped grains with dia-
meters of 100 nm − 200 nm for room temperature growth and
become web-like 2–3 μm long wire meshes in films grown at
50 °C (Figure S7). DP-TTA films exhibit the largest grain sizes
in the series. While small gains with 100−200 nm diameters
grow at room temperature, the grains grow in pentacene-like
ordered crystalline structures[62–64] with 2–3 μm diameters as
the substrate temperature is increased to 50 °C (Figure S8).
DBS-TTA exhibits relatively small grains compared to DFP-
TTA and DP-TTA, reflecting the highest molecular weight and
melting temperature in the series. These grains evolve from
small facets with 20–40 nm diameters to 200–300 nm wide balls
at 110 °C growth temperature (Figure S9). XRD data shown in
Figure S4 verify that the crystallinity increases along with these
film morphology changes. TFT parameters summarized in
Table3indicatecorrespondingdeviceperformanceenhancement
as well.
While bulk film morphology evolution satisfactorily
explains the effects of substrate temperature on semicon-
ductor film crystallinity, it does not correlate well with
dielectric surface treatment. Thus, 50 nm thick TTA film mor-
phologies do not vary greatly for the three different types of
substrates. Even the XRD data do not exhibit significant dif-
ferences related to surface treatment (Figure S5). If the bulk
film morphologies are similar, the key factors defining device
performance on these three substrates may involve variations
in the interface microstructure. To investigate possible differ-
ences in molecular level interface microstructure, the surface
morphologies of the TTA semiconductor films having sub-
monolayer thicknesses were next investigated on these three
substrates.
AFM images of sub-monolayer TTA films grown on the
different substrates clearly explain the origin of the device
performance differences as a function of dielectric surface
treatment (Figure 8). In general, TTA semiconductors grow in
large, widely spaced domains on bare Si/SiO2 along with large
empty boundary regions. On the other hand, these materials
form small grains with greater nucleation densities on HMDS
and OTS SAMs, resulting in tightly integrated film structures.
The most densely populated small grains grow on OTS-coated
Si/SiO2 and the SAM plays an important role in modulating
charge transport. DFP-TTA forms the most densly populated
film structure on the OTS SAM (Figure 8A). This tightly inte-
grated initial semiconductor film growth structure achieves
the highest n-type device performance on OTS SAM-deposited
gates. Note that DP-TTA films exhibit smaller, but densely pop-
ulated grains on OTS SAMs compared with the HMDS SAMs
and the bare SiO2 surface (Figure 8B). DBS-TTA also exhibits
the same trend with widely dispersed grain patches on bare
SiO2 (Figure 8C). In contrast, DBS-TTA forms webs of micro-
wire structures on HMDS and OTS SAMs. However, the wire
diameters are much smaller on OTS SAMs than on HMDS
SAMs. These tightly integrated initial semiconductor layers
As a result, DP-TTA exhibits μ = 0.21 cm2 V−1 s−1 and Ion/Ioff
=
1.1 × 106 on OTS-coated substrates at 50 °C (Figure 7C, D).
Note that DP-DTT grown at 70 °C exhibits μ = 0.42 cm2 V−1 s−1
and Ion/Ioff = 5 × 106 on OTS-coated substrates.[22]
The effect of substrate temperature and surface treatment are
also well reflected in the device performance. As the film crys-
tallinity is enhanced by increasing the substrate temperature,
the device performance is substantially enhanced. For DP-TTA
devices, the parameters μ = 0.11 cm2V−1 s−1 and Ion/Ioff = 2.8 ×
107 for room temperature growth change to μ = 0.21 cm2 V−1 s−1
and Ion/Ioff = 1.1 × 106 for films grown at 50 °C on OTS-coated
substrates. For DBS-TTA devices, μ = 5.0 × 10−4 cm2 V−1 s−1 and
Ion/Ioff = 3.8 × 104 for room temperature growth changes to μ =
1.9 × 10−3 cm2 V−1 s−1 and Ion/Ioff = 4.7 × 104 for 110 °C growth
on OTS-coated substrates.
Organic self-assembled monolayer (SAM) treatment of gate
dielectric surfaces is known to enhance TFT performance by
minimizing surface charge traps and by increasing the micro-
structural order of semiconductor growth.[58] Thus, growth on
HMDS and OTS SAM-treated Si/SiO2 substrates yields signifi-
cantly enhanced device performance than on bare Si/SiO2 sub-
strates. OTS surface treatment particularly enhances OTFT per-
formance versus the other treatments for the TTA semiconduc-
tors. For example, DFP-TTA devices exhibit μ = 0.30 cm2 V−1 s−1
and Ion/Ioff = 2.9 × 107 on OTS for 25 °C growth, but μ =
0.05 cm2 V−1 s−1 and Ion/Ioff = 3.1 × 106 on HMDS, compared
to μ = 0.0077 cm2 V−1 s−1 and Ion/Ioff = 2.7 × 104 on bare SiO2.
DP-TTA and DBS-TTA OTFTs follow the same pattern. One pos-
sible explanation for the superior device performance on OTS
SAMs versus HMDS SAMs is the high structural quality of the
OTS SAMs. If long alkyl chains in OTS molecules are tightly
packed and vertically aligned, the SAM has order approaching
in-plane crystallinity which enhances the interconnectivity of
initial semiconductor film growth as well as the film
crystallinity.[59–61] To understand the origin of these device per-
formance variations induced by substrate temperature during
semiconductor growth and dielectric surface treatment, the sur-
face morphology of semiconductor films was next examined by
tapping mode AFM.
2.6. Semiconductor Film Morphology and TFT Performance
The surface morphology of organic semiconductor thin films
is often used to evaluate crystalline microstructure based
on grain sizes. The highest carrier mobilities are generally
obtained for films having the appropriate balance of large grain
size and space-filling grain connectivity. AFM images of the
TTA films clearly reflect the influence of surface morphology
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Figure 8. Sub-monolayer (1.5 nm) film morphologies of TTA films on the three different Si/SiO2 substrates indicated. A. DFP-TTA grown at 25 °C,
B. DP-TTA grown at 50 °C, C. DBS-TTA grown at 110 °C (tapping mode, topography, 10 μm × 10 μm).
composed of small but highly crystalline grains must have
enhanced charge transport on OTS SAMs to achieve the best
device performance among the substrates examined here. In
short, interface semiconductor film microstructures, very dif-
ferent from bulk film morphologies, indicate that the superior
device performance for growth on OTS SAMs originates from
well-connected as well as highly crystalline initial semicon-
ductor film growth in the dielectric-semiconductor interfacial
region.
considerable potential as versatile n-type organic semiconduc-
tors. The TTA building block can now be obtained via an effi-
cient one-pot synthetic route and the first TTA-based n-channel
molecule is one result of this study. Enlarged TTA cores are
effective in reducing the bandgap and introducing n-type prop-
erties. Solid-state packing motifs revealed in single crystal
analysis, substrate temperature dependence of film growth,
and semiconductor–gate dielectric interface morphology are
all shown to be critically importanct factors in optimizing TFT
performance. We believe the newly designed one-pot synthetic
routes offer a scalable way to produce practical building blocks
for n-type organic semiconductors, and are now currently
pursuing these molecular design strategies further by incor-
porating other building blocks into perfluorophenyl-function-
alized molecular structures.
3. Conclusions
In this contribution, we established that tetrathiophenes
with proper electron-withdrawing functional groups offer
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Supporting Information is available from the Wiley Online Library or
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Acknowledgements
We thank AFOSR (FA9550-08-01-0331) and the NSF-MRSEC program
through the Northwestern Materials Research Center (DMR-0520513) for
support of this research at Northwestern University. Financial assistance
for this research was also provided by the National Science Council,
Taiwan, Republic of China (Grant Numbers NSC100-2628-M-008-004
and NSC100-2627-E-006-001) and partially provided by Industrial
Technology Research Institute of Taiwan (Grant Numbers A301AR1D12).
R. P. O acknowledges funding from the European Community’s Seventh
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