Inversion of dominant polarity in ambipolar polydiketopyrrolopyrrole with thermally removable groups

Article abstract of DOI:10.1002/adfm.201200940

A narrow bandgap polymeric semiconductor, BOC-PTDPP, comprising alkyl substituted diketopyrrolopyrrole (DPP) and tert-butoxycarbonyl (t-BOC)-protected DPP, is synthesized and used in organic field-effect transistors (OFETs). The polymer films are prepared by solution deposition and thermal annealing of precursors featuring thermally labile t-BOC groups. The effects of the thermal cleavage on the molecular packing structure in the polymer thin films are investigated using thermogravimetric analysis (TGA), UV-vis spectroscopy, atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) analysis. Upon utilization of solution-shearing process, integrating the ambipolar BOC-PTDPP into transistors shows p-channel dominant characteristics, resulting in hole and electron mobilities as high as 1.32 × 10^-2 cm² V^-1 s^-1 and 2.63 × 10^-3 cm² V^-1 s^-1, which are about one order of magnitude higher than those of the drop-cast films. Very intriguingly, the dominant polarity of charge carriers changes from positive to negative after the thermal cleavage of t-BOC groups at 200 °C. The solution-sheared films upon subsequent thermal treatment show superior electron mobility (μ_e = 4.60 × 10^-2 cm² V ^-1 s^-1), while the hole mobility decreases by one order of magnitude (μ_h = 4.30 × 10^-3 cm² V^-1 s^-1). The inverter constructed with the combination of two identical ambipolar OFETs exhibits a gain of ～10. Reported here for the first time is a viable approach to selectively tune dominant polarity of charge carriers in solution-processed ambipolar OFETs, which highlights the electronically tunable ambipolarity of thermocleavable polymer by simple thermal treatment. A narrow bandgap polymeric semiconductor, BOC-PTDPP, comprising alkyl substituted diketopyrrolopyrrole (DPP) and tert-butoxycarbonyl (t-BOC)-protected DPP, is synthesized and used in organic field-effect transistors (OFETs). The dominant polarity of charge carriers changes from positive to negative after the thermal cleavage of t-BOC groups at 200 °C. Copyright

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Inversion of Dominant Polarity in Ambipolar
Polydiketopyrrolopyrrole with Thermally Removable Groups
Junghoon Lee, A-Reum Han, Jayeon Hong, Jung Hwa Seo, Joon Hak Oh,*
and Changduk Yang*
1
. Introduction
A narrow bandgap polymeric semiconductor, BOC-PTDPP, comprising alkyl
substituted diketopyrrolopyrrole (DPP) and tert -butoxycarbonyl(t-BOC)-
protected DPP, is synthesized and used in organic field-effect transistors
Printed organic field-effect transistors
OFETs) are key building blocks in the
(
context of large-area, flexible and ultralow-
cost electronics, such as radio-frequency
identification (RFID) tags, smart cards,
(
OFETs). The polymer films are prepared by solution deposition and thermal
annealing of precursors featuring thermally labile t-BOC groups. The effects
of the thermal cleavage on the molecular packing structure in the polymer
thin films are investigated using thermogravimetric analysis (TGA), UV-vis
spectroscopy, atomic force microscopy (AFM), Fourier transform infrared
[
1–7]
and organic active matrix displays.
In
comparison to small molecular or oligo-
meric materials, polymer semiconductors
have been suggested as the best candi-
dates for large-scale device fabrication due
to the superior solution processability and
mechanical robustness. It thus comes as
no surprise that to achieve ultimate success
of OFETs and practical organic circuits,
a great deal of effort has been devoted to
both the synthesis of novel polymer-based
semiconductors and the development of
new fabrication techniques.
For instance, on the scientific side of
development in materials design, hole
mobilities in the range of 0.1–1 cm V s
have not only been achieved with p-type
(
FT-IR) spectroscopy, and X-ray diffraction (XRD) analysis. Upon utiliza-
tion of solution-shearing process, integrating the ambipolar BOC-PTDPP
into transistors shows p-channel dominant characteristics, resulting in
−
2
2
−1 −1
hole and electron mobilities as high as 1.32 × 10 cm V
s
and 2.63 ×
−
3
2
−1 −1
1
0
cm V s , which are about one order of magnitude higher than those
of the drop-cast films. Very intriguingly, the dominant polarity of charge
carriers changes from positive to negative after the thermal cleavage of t-
BOC groups at 200 °C. The solution-sheared films upon subsequent thermal
[
8–20]
−2
2
−1 −1
treatment show superior electron mobility (μ = 4.60 × 10 cm V s ),
e
while the hole mobility decreases by one order of magnitude (μ = 4.30 ×
h
2 −1 −1
−3
2
−1 −1
1
0 cm V s ). The inverter constructed with the combination of two iden-
tical ambipolar OFETs exhibits a gain of ∼10. Reported here for the first time
is a viable approach to selectively tune dominant polarity of charge carriers in
solution-processed ambipolar OFETs, which highlights the electronically tun-
able ambipolarity of thermocleavable polymer by simple thermal treatment.
[21–28]
polymers based on thiophene
but
also these high mobilities have recently
been matched by an n-type naphthalene-
bis(dicarboximide) (NDI)-based polymer
2
−1 −1
(
μ = 0.85 cm V s )-a major step toward
e
the realization of polymer-based com-
[
29]
plementary logic circuits.
Also, a par-
J. Lee, Prof. C. Yang
allel and equally heady progress in OFETs has been driven by
advances in fabrication or processing techniques-ranging from
patterning and printing techniques (e.g., photolithography,
inkjet printing, soft lithography) to deposition techniques (e.g.,
friction-transfer and rubbing alignment, photoalignment)-that
have been successfully employed in the fabrication of the key
Interdisciplinary School of Green Energy
KIER-UNIST Advanced Center for Energy
Low Dimensional Carbon Materials Center
Ulsan National Institute of Science
and Technology (UNIST), Ulsan 689-798, South Korea
E-mail: yang@unist.ac.kr
[30]
A.-R. Han, J. Hong, Prof. J. H. Oh
components of organic devices.
School of Nano-Bioscience and Chemical Engineering
KIER-UNIST Advanced Center for Energy
Low Dimensional Carbon Materials Center
Ulsan National Institute of Science and Technology (UNIST)
Ulsan 689-798, South Korea
On the other hand, in order to exploit complementary metal
oxide semiconductor (CMOS)-type logic circuits as well as to
realize efficient light-emitting organic field-effect transistors
(LE-OFETs) as the expanded issue associated with the OFETs,
there is strong interest in developing ambipolar OFETs that can
E-mail: joonhoh@unist.ac.kr
[
31–34]
Prof. J. H. Seo
Department of Materials Physics
Dong-A University
provide both p- and n-channel performance.
As a result of
the aforementioned advances in the alignment techniques and
unipolar material synthesis for hole- and electron-transporting
semiconductors, ambipolar OFETs have been successfully
realized by employing a piled bilayer of a p- and an n-type
Busan 604-714, South Korea
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material,[35] a blend system containing the
(a)
[6]
two types of materials, and a dual nature
S
S
S
S
ambipolar semiconductor.[
32]
R
R
R
R
O
N
O
R
O
N
N
N
O
Among those strategies, polymeric single-
component ambipolar OFETs are a real
prospect for the cost-effective production of
OFETs because they can be deposited in the
simplest single processing step. Recently,
O
N
N
O
N
O
O
N
R
R
R
S
S
S
S
S
S
S
S
BOC
N
O R2
O
HN
O
O
3
,6-(2-thiophenyl)-substituted
diketopyr-
N
O
HN
O
O
N
N
NH
rolopyrrole (thiophenyl DPP)-based poly-
mers have emerged as extremely attractive
R2
O
NH
BOC
S
S
S
S
materials for both OFETs and solar cell
devices.[
1,5,36–44]
The DPP core with electron-
S
S
S
S
R
Thermal Tretment
R
R
R
deficient nature exhibits a planar conjugated
bicyclic structure, which leads to strong π–π
interactions. Besides, the two thiophene
units adjacent to the DPP can alleviate their
steric repulsions with DPP to maintain high
coplanarity of the polymer backbone, which
is essential for achieving efficient charge
transport properties and low bandgaps. Thus,
N
O
O
N
O
N
N
O
O
N
N
O
N
O
O
N
R
R
R
R
S
S
S
S
S
S
S
S
O
BOC
R
N
O
O
t-BOC:
O
HN
O
O
N
HN
O
O
O
N
N
NH
R2
O
NH
BOC
Hydrogen-Bonding
S
S
S
S
many polymers based on DPP have been
reported for OFET applications[
45,46]
and the
ambipolar OFETs with DPP-containing poly-
mers have recently been accomplished by
p-type dominant
n-type dominant
Bürgi,[ Janssen, and Li and coworkers,
44]
[42]
[46]
respectively. Independently, we found
a
nearly equivalent ambipolar polymeric semi-
conductor composed of benzothiadiazole
(
(b) Shearing direction
BT) and DPP.[
1]
Enhanced Molecular
Orientation
We are now interested in feasibility of
the formation of a hydrogen-bonded net-
work within the DPP-based polymers, which
would enhance intermolecular charge-carrier
hopping. However, to make the highly con-
jugated organic materials solution process-
able, introduction of solubilizing groups in
the NH group of the DPP unit is necessary,
which leads to transient disruption of inter-
molecular hydrogen-bonding interactions as
well as reduction of the density of chromo-
phores in the polymer.
Shearing substrate
Semiconductor solution
OT S- treated SiO / Si ( Stationary)
2
Mild heating ( < bp of solvent)
Figure 1. (a) Thermocleavable polymer based on diketopyrrolopyrrole (DPP) and (b) schematic
Therefore, a design strategy maintaining
solubility without sacrificing the hydrogen
bonding would be valuable for high perform-
illustration of solution-shearing technique.
ance OFETs. In addressing this issue, inspired by the work on
thermally-removable solubilizing groups in DPP-based mate-
rials, we design new version of an original thiophenyl DPP-
based polymer (PTDPP) bearing tert-butoxycarbonyl (t-BOC)
protective groups (Figure 1). The t-BOC groups are known to
undergo thermolysis at ∼180 °C, affording the parent chromo-
phore in high purity and quantitative yield.[47] Therefore, they
are advantageous to the formation of hydrogen-bonded network
with a high chromophore density in a post-processing step.
One promising approach to the thermal cleavage of solubilizing
groups based on pyrrole and thiophene-based materials was
reported by Fréchet group.[
planar substrates by a facile solution-sheared deposition, where
a small volume of an organic semiconductor solution is placed
between two preheated silicon wafers that move relative to each
[
50,51]
other at a controlled rate.
Thereby, OFETs with several
small molecules prepared through solution-shearing process
produce mobilities that are comparable and more often supe-
rior to those of drop-cast devices. Furthermore, the solution-
shearing offers another notable advantage for highly crystalline
thin film preparations from a small volume of dilute organic
solution without special additives or post-processing. There-
fore, it can not only be a simple and rapid tool for screening the
performance of solution-processable organic semiconductors,
but also be adapted to high-throughput manufacturing proc-
esses, such as roll-to-roll printing methods. A recent in-depth
48,49]
Small-molecule semiconductors can form highly-crystalline
elongated and aligned grains along the shearing direction on
2
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study by Giri et al. revealed that the lattice strain achieved by
solution-shearing is highly beneficial to an increase in the
charge transfer integral of π-planes and allows an efficient
charge transport.[ In this study, we utilize the intermolecular
hydrogen-bonding interaction within thermocleavable mate-
rial PTDPP as an additional driving force to further enhance
molecular packing structures in the thin films obtained with
the solution-shearing technique. Figure 1 illustrates the
core concept of our works. To the best of our knowledge, not
only is the solution-shearing method firstly applied to study
polymer-based OFETs but also there exist no examples showing
the inversion of dominant polarity in ambipolar PTDPP OFETs
subjecting to thermal treatment.
DPP[53] according to the literature method using di-tert-butyl
[54]
dicarbonate,
which was then converted to dibrominated
DPP 3 (yield 75%) as a monomer for metal (Ni or Pd)-catalyzed
cross-coupling polymerizations.
52]
In the first attempt, it was found that polymerization of
dibrominated DPP 3 with only t-BOC groups on nitrogen atoms
results in the polymer with extremely limited solubility. To enable
the solution processability of the DPP-based polymer, we used a
long branched alkyl side chain, 2-decyl-1-tetradecyl to substitute
DPP at the nitrogen atoms, which was transformed into the cor-
responding diboronic ester co-monomer 4via lithiation (LDA)
and subsequent reaction with 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (79%). A soluble DPP-containing polymer
with thermally removable solubilizing groups (BOC-PTDPP)
was prepared via Suzuki coupling polymerization between
bis-borolylated and bis-brominated monomers. After Soxhlet
extraction using methanol and acetone sequentially, whereby
impurities and undesired low molecular weight oligomers were
removed, a dark purple solid was obtained in a high yield (75%).
According to size-exclusion chromatography (PS standards),
the resulting polymer has a number-average molecular weight
2
. Results and Discussion
2
.1. Synthesis and Chemical/Thermal Characterizations
The synthetic routes to PTDPP are shown in Scheme 1 and
detailed in the Experimental section. t-BOC-protected DPP pig-
ment precursor was conveniently prepared from the thiophenyl
(M ) = 13,520 g/mol and a polydispersity index (PDI) = 3.56.
n
O
NH
S
(i)
S
(ii)
C10H21
C12H25
HN
O
O
O
O
O
N
N
S
S
S
S
N
N
O
O
O
C12H25
O
2
1
C10H21
(
(iii)
iv)
C10H21
O
O
O
C12H25
S
O
N
N
S
O
O
O
O
Br
Br
B
B
S
S
N
O
N
O
O
C12H25
O
3
4
C10H21
(v)
C10H21
O
O
S
C12H25
O
O
N
N
S
n
S
S
C12H25
N
N
O
O
O
O
C10H21
BOC-PTDPP
Scheme 1. Synthesis of Polymer BOC-PTDPP. Reagents and conditions: (i) di-tert-butyl dicarbonate, dimethylaminopyridine (DAMP), THF, under Ar,
4 h, 85%, (ii) 2-decyltetradecylbromide, K CO , DMF, 120 °C, under Ar, 24 h 55%, (iii) NBS, Chloroform, dark, under Ar, 48 h, 75%, (iv) LDA, 2-Isopro-
2
2
3
poxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF, −40 °C, under Ar, 24 h, 79%, (v) Pd (dba) /P-(o-Tol) , K PO , toluene/H O, 95°C for 48 h, 75%.
2
3
3
3
4
2
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BOC-PTDPP is readily soluble in common organic solvents
(a)
(THF, chloroform, toluene, etc) at room temperature because of
its long branched side chain substitution.
To precisely clarify a ‘latent pigment technology’ of the t-BOC-
protected polymer films upon subsequent thermal treatment,
inspections by thermogravimetric analysis (TGA) and UV-vis
spectral changes were carried out to reveal the thermal behavior
of the t-BOC-protected DPP pigment precursor which could
serve as a model process for the spectroscopic characterization
of the polymer BOC-PTDPP. TGA of the precursor 1 confirms
the occurrence of the thermolysis reaction at about 180 °C. The
mass loss is approximately 45%, which corresponds well to the
loss of the t-BOC groups, indicating that the parent pigment is
obtained in high purity and quantitative yield (Figure 2a). We
prepared a uniform thin orange colored film by spin casting of
the pigment solution in chloroform. Thermal treatment of this
film at 200 °C for 5 min resulted in the in situ regeneration
of polycrystalline red colored film as a consequence of a pro-
nounced red shift (Figure 2b). The change of photophysical
properties is attributed to the NH–O hydrogen-bonding effect
of DPP.
The thermal cleavage of the t-BOC groups in the polymer
BOC-PTDPP also starts to occur at around 180 °C (Figure 3),
whereas the UV-vis spectra of the thin films before and after
thermal treatment were almost identical in shape and absorp-
tion, exhibiting a broad band centered at 800 nm. This obser-
vation is most likely due to the existence of the long branched
alkyl side chain on the counterpart comonomer in which the
hydrogen-bonding network along the polymer backbone can
be somewhat disrupted. However, note that after the post-
processing step, the final film became completely insoluble,
implying the strong intermolecular π–π interactions in planar
closely packed DPP systems.
100
8
6
0
0
1
00
200
300
o
Temperature ( C)
(b)
0.6
0.4
0.2
0.0
Precursor film (R.T)
o
Precursor film (200 C)
4
00
600
800
To certify that the intermolecular interactions had success-
fully gone to hydrogen-bonding network, the drop-cast and
solution-sheared BOC-PTDPP fi lms before and after the thermal
treatment at 200 °C for 30 min were characterized by Fourier
transform infrared (FT-IR) spectroscopy (Figure 4). The FT-IR
spectra obtained after the thermal treatment reveal that the loss
of the carbamate protecting groups induces the disappearance
Wavelength (nm)
Figure 2. (a) TGA of t-BOC-protected DPP precursor with heating rate of
10 °C/ min in N . (b) UV-vis absorption spectra of t-BOC-protected DPP,
spin-coated precursor film and precursor film after thermal cleavage at
2
2
00 °C.
−1
of the C=O stretching vibration bands at 1749 cm1 as well as
carbon working electrode (GC) coated with polymer films, a
platinum-wire auxiliary electrode as a counter electrode, and a
Ag wire pseudo-reference electrode at a scan rate of 100 mV/s,
and Fc/Fc as the external standard. The cyclic voltammo-
grams of BOC-PTDPP fi lms before and after thermal annealing
at 200 °C for 5 min are shown in Figure 5 and the CV data
, HOMO and LUMO energy levels, Eg ) are
summarized in Table 1. Not only do both the t-BOC-protected
and deprotected polymers exhibit reversible p-doping/dedoping
the appearance of the N-H band at near 3436 cm− (Figure 4a).
It is found that the υ
(amide) has shifted to lower energies
C=O
−
1
+
(
by ∼23 cm ) after thermal treatment, which is indicative of the
hydrogen bonding (C=O—H-N) along the polymer backbone
Figure 4b). Furthermore, the solution-sheared polymer films
(
onset
onset
ec
with thermal treatment show further shift of υC=O (amide),
compared to the drop-cast films, presumably due to more pro-
nounced intermolecular interactions in the solid state.
(Eox
/Ered
(
oxidation/rereduction) processes at positive potentials but
also the n-doping/dedoping (reduction/reoxidation) processes
at negative potential range are clearly reversible. According to
2
.2. Electrochemical Properties
onset
onset
the empirical equation E
= −[Ered
–Eferrocene
+
HOMO/LUMO
[
55]
Cyclic voltammetry (CV) was performed to estimate the highest
occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) energy levels of BOC-PTDPP
before and after thermal treatment. The experiments were car-
ried out using tetra-n-butylammonium hexafluorophosphate
4.8] eV, the HOMO and LUMO energy levels are estimated
as −5.31 and −3.79 eV for BOC-PTDPP, whereas after thermal
treatment, the polymer possesses slightly downshifted HOMO
(−5.41 eV) and LUMO (−3.83 eV) with a nearly identical
ec
bandgap (Eg ). The molecular energy levels, in particular the
(
n-Bu NPF ) (0.1 M) as the supporting electrolyte, a glassy
LUMO levels, well correspond to the empirical energy window
4
6
4
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(a)
(a)
1
00
Before thermal cleavage
After thermal cleavage
8
0
3
500
3000
2500
2000
1500
1000
1
00
200
300
-
1
ο
Wavenumber (cm )
Temperature ( C)
(
b)
(
b)1.0
Before thermal cleavage
Polymer film (R.T)
o
Polymer film (200 C)
0
0
0
0
0
.8
.6
.4
.2
.0
After thermal cleavage
3
500
3000
2500
2000
1500
1000
4
00
600
800
1000
1200
-
1
Wavenumber (cm )
Wavelength (nm)
Figure 4. FT-IR spectra of (a) the drop-cast thin film and (b) the solution-
sheared film of BOC-PTDPP before (upper) and after (bottom) thermal
cleavage at 200 °C. The dotted line is a guide line for C=O stretching
bands.
Figure 3. (a) TGA of BOC-PTDPP precursor with heating rate of
0 °C/ min in N . (b) UV-vis-NIR absorption spectra of BOC-PTDPP in
thin film before and after thermal cleavage at 200 °C.
1
2
(
−3.1 ∼ −3.8 eV) for ambipolar behaviors for OFETs with regard
reduced d(001)-spacing indicates that π-planar distance might
be shortened, which is beneficial to charge transport. Interest-
ingly, an additional broad peak is observed at about 2θ = 21.30°,
corresponding to a d-spacing of ∼4.20 Å (Figure 6b). This can
be assigned to either the π–π stacking peak or the formation
of another polymorph in the annealed film. The presence of
the smaller d-spacing value suggests the strong intermolecular
interaction forces between fused ring moieties (DPP) and
the thiophene segments comprising the deprotected-PTDPP
backbone.
[
56–58]
to gold contacts.
2
.3. X-ray diffraction (XRD) analysis
To further elucidate the molecular packing structure in the
polymer thin films depending on the thermal cleavage, the
X-ray diffraction (XRD) analysis was carried out on the BOC-
PTDPP thin films. As shown in Figure 6a, the as-cast thin film
exhibits a sharp primary diffraction peak at 2θ = 3.96°, which
arises from the ordered interlayer stacking of the polymer and
corresponds to a d(001)-spacing of 22.29 Å. The secondary
small peak (002) is also observed at 2θ = 7.80°, indicating
that the as-cast thin film has a relatively long-range ordering.
Upon thermal cleavage by annealing at 200 °C for 30 min, the
deprotected-polymer exhibits a primary diffraction peak at 2θ =
2.4. DFT Electronic Structure Calculation
To provide an insight into the different molecular architecture
of the polymers, molecular simulation was carried out for BOC-
PTDPP and deprotected-PTDPP with a chain length of n = 1
using density functional theory (DFT) at the B3LYP/6-31G∗
4
.26°, corresponding to a d(001)-spacing of 20.72 Å. The slightly
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fe rro c e n e
(
001)
(
a )
0
.1 m A
(
002)
(001)
(
b )
0
.1 m A
5
10
15
20
25
30
2
θ (degrees)
Figure 6. XRD data obtained from drop-cast BOC-PTDPP thin films
before (top) and after (bottom) thermal cleavage at 200 °C.
-
1 .5
-1 .0
-0 .5
0 .0
0 .5
1 .0
1 .5
+
thin films prepared through this method would stimulate the
preferential intermolecular hydrogen-bonding network after
thermal treatment, on account of aligned nature of molec-
ular packing structures in the organic thin films. To test the
hypothesis, the BOC-PTDPP fi lm morphologies were first
studied by tapping mode atomic force microscopy (AFM), as
shown in Figure 8. The as-cast thin film before thermal treat-
ment shows a uniform surface with very fine grains. Notably,
P o te n tia l (V v s A g /A g )
Figure 5. Cyclic voltammograms of BOC-PTDPP thin film on the glassy
carbon (GC) electrode in 0.1 M n-Bu NPF acetonitrile solution at room
temperature before (a) and after (b) thermal cleavage at 200 °C.
4
6
with Gaussian 03 package (Figure 7). Dihedral angles between
thiophene and DPP are susceptible to the substituents on the
N-atoms of the DPP units. Thereby, the dihedral angle in BOC-
o
the polymer thin film thermally cleaved at 200 C is still com-
o
posed of clustered non-fibrillar structures with small grains,
but a very few pinhole-like voids appear on the grains due to
the vaporization of t-BOC groups on the surface during the
thermolysis. On the other hand, when the solution-shearing
technique is applied, the BOC-PTDPP thin film reveals a com-
pletely-different morphology in which an array of unidirectional
valleys is observed. This suggests that the solution-shearing can
impact the molecular packing and orientation of the polymer
chains, in addition to the small molecule semiconductors. The
PTDPP is 18 , whereas in the case of the deprotected-PTDPP,
the completely planar conformation is observed. This implies
that the deprotected-PTDPP can be easily stacked through π–π
interactions of the layers of molecules. The calculation results
are in considerable coincidence with those of XRD analysis.
The HOMO and LUMO orbitals of the deprotected-PTDPP are
further well-delocalized due to the absence of the substituents
on the DPP units. In particular, it is noteworthy that the LUMO
orbitals of the deprotected-PTDPP are far more delocalized
compared with BOC-PTDPP, indicating that n-channel behavior
may be facilitated after the thermolysis. Thus, it is clear that t-
BOC groups on DPP can be used as a tuning means to control
the torsional angle and, therefore, to control the electronic and
optical properties of the polymer.
(a)
(b)
Therefore, we turned our attention to alignment techniques
such as solution-shearing process because the BOC-PTDPP
HOMO
LUM O
HOMO
T a b le 1 . Electrochemical properties.
E oxonset
Eredonset
(V)
Egec
(eV)
LUM O
BOC-PTDPP
HOMO
(eV)
LUMO
(eV)
a)
b)
c)
(
V)
o
72
Without
0.85
−0.67
−5.31
−5.41
−3.79
−3.83
1.52
annealing
Thermal
cleavage
0.95
−0.63
1.58
To p v i e w
Top view
Figure 7. DFT-optimized geometries and charge-density isosurfaces for
the HOMO and LUMO levels and the top views of PTDPP polymer with
(a) and without (b) t-BOC groups.
a)
onset
- Eferroceneonset + 4.8); b)LUMO (eV) = −(Eredonset
HOMO (eV) = −(Eox
-
onset
Eferrocene
+ 4.8); Eg (eV) = Eoxonset - Ered
c) ec
onset
.
6
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Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201200940

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atmosphere. Further details on the surface treatment and OFET
fabrication are included in Experimental section. The OFET
properties of BOC-PTDPP thin films are summarized in Table 2.
Figure 9 exhibits the typical ambipolar characteristics of OFETs
based on the solution-sheared BOC-PTDPP thin films with
gold electrodes. The V-shape transfer characteristics are clearly
observed in hole-enhancement (VDS = −100 V) and electron-
enhancement (VDS = 100 V) mode operations. This is consistent
with the prediction from the empirical LUMO window (−3.1 ∼
−
3.8 eV) for ambipolar organic semiconductors with gold con-
[
56–58]
tacts.
The BOC-PTDPP exhibits the maximum hole and
−2
−3
2 −1 −1
electron mobilities of 1.32 × 10 and 2.63 × 10 cm V s ,
respectively. The hole mobility in the BOC-PTDPP is about one
order of magnitude higher than the electron mobility, which
may result from the larger injection barrier for electrons with
regard to the gold contacts. In general, more densely packed
polymer π-planar backbones after thermal cleavage facilitate
the charge transport of both hole and electron. Interestingly,
o
however, after thermal cleavage of the t-BOC groups at 200 C,
the magnitude of hole and electron mobilities is switched: hole
−
3
2 −1 −1
mobility of 4.30 × 10 cm V s and electron mobility of 4.60 ×
−2
2 −1 −1
1
0
cm V s . In other words, hole mobility decreases by
about one order of magnitude, while electron mobility increases
after thermal cleavage of the t-BOC groups. The inversion of
dominant polarity in ambipolar PTDPP OFETs may arise from
several factors. First, as can be seen from DFT calculation, the
LUMO orbitals of the deprotected PTDPP become far more
delocalized compared to the BOC-PTDPP, which is profitable
to n-channel conduction. On the other hand, the enhancement
in the delocalization of the HOMO orbitals is not as high as
the LUMO orbitals. Second, the downshift of the HOMO-
LUMO energy levels for BOC-PTDPP after thermal treatment
may decrease the injection barrier for electrons, while relatively
increasing the injection barrier for holes. Third, we cannot rule
out that the removal of nonconjugated t-BOC groups that can
act as electron traps facilitates electron transport.
Figure 8. Tapping mode AFM images of (a),(b) drop-cast and (c),(d)
solution-sheared films of BOC-PTDPP before (a),(c) and after (b),(d)
thermal cleavage at 200 C.
o
modified polymer chain networks, which are the result of the
harmonized effects of the solution-shearing technique and
the hydrogen-bonding in the deprotected-PTDPP would form
highly efficient pathways for charge carrier transport along the
elongated and aligned grains in the polymer film.
2
.5. OFET Performance
OFET devices based on BOC-PTDPP thin films were prepared
in top-contact bottom-gate geometry by either drop-casting or
solution-shearing in chlorobenzene solution onto n-octadecylt-
The solution-sheared BOC-PTDPP polymer thin films
exhibits about one order of magnitude higher mobilities, com-
pared with drop-cast films (Figure S1 in Supporting Informa-
tion), which is a typical trend for small-molecule organic semi-
rimethoxysilane (OTS)-treated SiO /Si substrates. The films
2
were annealed on a hot plate at 200 °C for 30 min in a nitrogen
[49–51]
conductors.
In addition to the aligned nature and strained
T a b le 2 . Electrical performance of OFET devices based on BOC-PTDPP thin film.
Conditiona)
p-type
n-type
μh,max
−1
μh,avg
−1 −1 b)
Ion/Ioff
VT
[V]
μe,max
−1
μe,avg
−1 −1 b)
Ion/Ioff
VT
[V]
cm V s−1
2
]
[cm2V
s
]
[cm V s−1
2
]
[cm2V
s
]
[
1.32 × 10−
2
3
3
3
1.28 × 10−2
2.45 × 10−3
2.57 × 10−3
1.37 × 10−3
2.09 × 105
2.64 × 104
3.22 × 104
1.18 × 106
−25.00
−50.39
−19.69
−33.43
2.63 × 10−3
4.60 × 10−2
7.78 × 10−4
1.63 × 10−2
2.43 × 10−3
2.56 × 10−2
7.27 × 10−4
1.45 × 10−2
1.64 × 101
6.01 × 105
4.78 × 102
3.41 × 104
Solution-
shearing
Without
60.69
57.56
71.46
44.90
annealing
4.30 × 10−
2.59 × 10−
1.61 × 10−
Thermal
cleavage
Drop-casting
Without
annealing
Thermal
cleavage
a)The p-type and n-type characteristics of ambipolar BOC-PTDPP OFETs were measured with VDS= −100 V and +100 V, respectively; b)The average mobility of the OFET
devices (L = 50 μm and W = 1000 μm)
Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201200940
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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7

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-
-
-
-
5
6
7
8
3
0
2.0
1.5
1.0
(b) 10
10
0.2
0.1
(
a)1
V
= 40V
V
= 0V
-
5
GS
0V
GS
0V
0
2
20
2
0
V
-6
40V
60V
80V
10
-20V
-40V
-60V
20
-7
10
1
00V
-80V
-8
-100V
10
10
-9
10
10
10
0.5
-
10
10
-
11
10
10
0
0.0
0
0.0
0.6
4
0 20 0 -20-40-60-80-100
0
-20 -40 -60 -80 -100
VDS (V)
0
20 40 60 80 100
VGS (V)
0
20 40 60 80 100
VDS (V)
VGS (V)
-5
(c)
(d)10
V
= 0V
-5
VGS = 0V
GS
10
-20V
-6
20V
40V
60V
80V
20
0.4
0.2
0.0
-40V
-60V
1
1
1
1
0
0
0
0
20
-6
10
-80V
-7
0.4
-7
-100V
100V
10
-
8
9
-8
10
10
10
-
0.2
-9
10
-
10
10
-
10
10
-
11
0
10
0
0.0
4
0 20 0 -20-40-60-80-100
0
-20 -40 -60 -80 -100
VDS (V)
0
20 40 60 80 100
VGS (V)
0
20 40 60 80 100
VDS (V)
VGS (V)
Figure 9. Transfer and output characteristics of OFET devices based on solution-sheared BOC-PTDPP thin films. The BOC-PTDPP ambipolar transistor
operating in (a) hole-enhancement and (b) electron-enhancement mode before thermal cleavage. Results of the thin films obtained from (c) hole-
o
enhancement and (d) electron-enhancement mode after thermal cleavage at 200 C (|VDS| = 100 V).
lattice of solution-shearing, deprotected-PTDPP polymer thin
films can form the hydrogen-bonded network along the pol-
ymer backbone, allowing an efficient charge transport, which
is also confirmed by the results of FT-IR analysis. The same
trend in the inversion of dominant polarity by thermal cleavage
is also observed for BOC-PTDPP OFETs with aluminum elec-
trodes (Table S1 and Figure S2).
be conveniently fabricated into films. By utilizing t-BOC ther-
molysis at ∼200 °C, the BOC-PTDPP fi lms are directly converted
to the deprotected-PTDPP fi lms that can potentially possess a
hydrogen-bonded network, supported by FT-IR spectral changes
and XRD analysis. Solution-shearing technique in this study is
used to facilitate the strong interlayer interactions through the
proper alignment of the BOC-PTDPP fi lms. The ambipolar per-
formance with dominant p-type conduction is observed from
We applied two identical ambipolar transistors to CMOS-
like inverters with a common gate as the input voltage (VIN).
Although the characteristics were measured in air with the lack
of optimization, the inverter operated well with the maximum
gain of ∼10 (Figure 10), which is comparable to that of state-
−2
2 −1 −1
solution-sheared BOC-PTDPP (μ = 1.32 × 10 cm V s ; μ =
h
e
−3
2 −1 −1
2.63 × 10 cm V s ). Very interestingly, after thermal cleavage at
200 °C, the dominant polarity of the ambipolar OFETs is switched
(μ = 4.30 × 10 cm V s ; μ = 4.60 × 10 cm V s ). This can
−3
2
−1 −1
−2
2 −1 −1
h
e
of-the-art CMOS-like inverters based on two-
component organic semiconductors.[
59,60]
The
100
12
10
8
successful implementation of an inverter with
BOC-PTDPP highlights the promise of the
DPP family of polymers in CMOS-like logic
applications since the devices based on ther-
mally deprotectable materials should be pos-
sible to give rise to better operational stability
in the ambient atmosphere for a long period.
VDD = 100V
80
60
40
20
0
6
4
2
3
. Conclusions
0
In summary, we have synthesized a thermo-
cleavable narrow bandgap polymer based
on diketopyrrolopyrrole (DPP) bearing tert-
butoxycarbonyl (t-BOC) and alkyl groups
0
20
40
60
80
100
VIN
(
BOC-PTDPP). The BOC-PTDPP precursor is
Figure 10. Inverter characteristics of ambipolar BOC-PTDPP in air. The gain of inverter is ∼10
soluble in common organic solvents and can at a constant supply bias, VDD=100 V.
8
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Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201200940

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arise from the far more delocalized LUMO orbitals, the lower
injection barriers for electrons, and the removal of t-BOC groups
that can act as electron traps. The inverter constructed with the
combination of two identical BOC-PTDPP OFETs exhibits a gain
of ∼10. The thermocleavable DPP-containing ambipolar polymer
in harmony with solution-shearing method would become poten-
tially useful for high-throughput, roll-to-roll manufacturing of
low-cost OFET circuits and arrays for a wide range of practical
electronic applications.
Calcd for C H B N O S : C, 72.52; H,10.36, N, 2.29. Found: C, 72.40;
H, 10.59, N, 2.57.
7
4
126
2
2
6 2
Synthesis of Poly[3,6-dithien-2-yl-2,5-di(t-butoxycarbonyl)-pyrrolo[3,4-c]-
pyrrole-1,4-dione-5′,5″-diyl-alt-3,6-dithien-2-yl-2,5-di(2-decyltetradecanyl)-pyr-
rolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl] (BOC-PTDPP) : A mixture of 3 (104
mg, 0.16 mmol), 4 (200 mg, 0.16 mmol), tris(dibenzylidenacetone)-
dipalladium (0) (10 mg, 0.011 mmol) were taken together in a Schlenk
flask. Tri(o-tolyl)phosphine (5 mg, 0.016 mmol) and K PO4 (220 mg)
3
in toluene (5 ml) with demineralized water (1 ml) were added to this
solution and the reaction mixture was heated at 95 °C under vigorous
stirring for 48 h. The crude product was poured into a mixture of
methanol (300 ml) and water (100 ml). The resulting solid was filtered
off and subjected to sequential Soxhlet extraction with methanol (1 d),
acetone (1 d) and hexane (1 d) to remove low molecular weight fraction
of the materials. The residue was extracted with chloroform to give a
dark purple product after precipitating again from methanol and drying
in vacuo. Isolated yield of polymer BOC-PTDPP = 150 mg (75%). GPC
4
. Experimental Section
Materials and Instruments: All starting materials were purchased either
from Aldrich or Acros and used without further purification. All solvents
are ACS grade unless otherwise noted. Anhydrous THF was obtained
by distillation from sodium/benzophenone prior to use. Anhydrous
toluene was used as received. 3,6-Dithien-2-yl-2,5-dihydropyrrolo[3,4-
analysis M = 13,520 kg/mol, M = 48,360 kg/mol, and PDI = 3.58
n
w
1
(
2
against PS standard,). H NMR(CDCl , 600 MHz): δ ppm 8.96-8.91 (br,
H), 8.37-8.33 (br, 2H), 7.63-7.07 (br, 4H), 4.03-3.93 (br, 4H), 2.94-1.20
3
c]pyrrole-1,4-dione (DTDPP),[
52]
2-decyl-tetradecylbromide,
[61]
and
(t, 94H), 0.85-0.84 (br, 12H). Anal. Calcd for C H N O S : C, 70.45; H,
88 130
4
8 4
8.73 N, 3.73 Found: C, 70.61; H, 9.00, N, 3.51.
1
,4-dioxo-3,6-di-thiophene-2-yl-pyrrolo[3,4-c]pyrrole-2,5-dicarboxylic
[
54]
Fabrication and Characterization of OFETs: A highly n-doped (100)
acid di-tert-butylester,
pyrrolo[3,4-c]pyrrole-2,5-dicarboxylic acid di-tert-butyl ester (3)
prepared according to established literature procedures. H NMR
and C NMR spectra were recorded on a VNMRS 600 (Varian, USA)
and 3,6-bis-(5-bromo-thiophen-2-yl)-1,4-dioxo-
[54]
silicon wafer (<0.004 Ωcm) with a thermally grown 300-nm-thick SiO
2
were
2
−
1
layer (C = 10 nFcm ) was utilized as the substrate for OFETs. The SiO
i
2
1
3
surface was treated with n-octadecyltrimethoxysilane (OTS) in solution
phase.[ After cleaning the SiO /Si wafer with piranha solution (7:3
62]
spectrophotometer using CDCl as solvent and tetramethylsilane (TMS)
2
3
mixture of H SO4 and H O2 by volume ratio) and UV-ozone plasma
as the internal standard and MALDI MS spectra were obtained from
Ultraflex III (Bruker, Germany). Thermogravimetric analysis (TGA) was
carried out using Q200 (TA Instrument, USA) with heating from 50 to
2
2
treatment, 3 mM solution of OTS in trichloroethylene was spin-coated
on the cleaned wafer at 3000 rpm for 30 sec. Then, the wafer was
exposed to ammonia vapor in a desiccator for about 12 h. The wafer
was washed sequentially with toluene, acetone and isopropyl alcohol,
and then dried under a nitrogen stream. The contact angle of DI water
on the OTS-treated wafer was typically over 104°. In the drop-casting,
800 °C at a heating rate of 10 °C/min. FT-IR spectra were recorded on
670-IR/620-IR Imaging (Varian USA). UV-vis-NIR spectra were taken on
a Cary 5000 (Varian USA) spectrophotometer. Atomic force microscope
AFM) images were obtained using Agilent 5500 (Agilent, USA). X-Ray
(
−1
the BOC-PTDPP was dissolved in chlorobenzene (2 mgmL ), and the
diffraction (XRD) was performed by D/MAZX 2500V/PC (Rigaku, Japan).
solution (∼80 μL) was drop-cast on the OTS-modified SiO /Si substrate.
Number-average (M ) and weight average (M ) molecular weights, and
2
n
w
In the solution-shearing method, the BOC-PTDPP solution (∼50 μL) was
polydispersity index (PDI) of the polymer products were determined
by gel permeation chromatography (GPC) with Agilent 1200 HPLC
Chemstation using a series of mono disperse polystyrene as standards
in THF (HPLC grade) at 308 K. Cyclic voltammetry (CV) measurements
were performed on Solartron SI 1287 with a three-electrode cell in
a 0.1 M tetra-n-butylammonium hexafluorophosphate (n-Bu NPF )
placed onto an OTS-treated SiO /Si substrate on a hot plate at 80 °C.
2
Then, another OTS-treated substrate was put down on the solution-
deposited substrate as a shearing tool and moved at a shearing rate of
−1
0
.12 mms by a digital syringe pump. To completely remove residual
solvent molecules, the solution-processed polymer thin films (∼50 nm)
were placed in a vacuum oven at 100 °C for 12 h, and the deprotected-
PTDPP films were obtained by annealing at 200 °C for 30 min on a hot
plate under nitrogen atmosphere. Gold contacts (∼40 nm) were then
thermally evaporated onto the polymer thin film to form source and drain
electrodes with a channel length (L) of 50 μm and a channel width (W) of
4
6
solution in acetonitrile at a scan rate of 100 mV/s at room temperature
under argon. A silver wire, a platinum wire and a glass carbon disk
were used as the reference electrode, counter electrode and working
+
electrode respectively. The Ag/Ag reference electrode was calibrated
using a ferrocene/ferrocenium redox couple as an external standard,
whose oxidation potential is set at −4.8 eV with respect to zero vacuum
level. The HOMO energy levels were obtained from the equation HOMO
1
000 μm using a shadow mask. The transfer and output characteristics
of OFETs were recorded in a N -filled glove box by using a Keithley
2
onset
onset
4200 semiconductor parametric analyzer. The field-effect mobility was
calculated in the saturation regime using the following equation:
(
eV) = −(Eox
–Eferrocene
+ 4.8). The LUMO levels of polymers were
onset onset
obtained from the equation LUMO (eV) = −(Ered
–E
+ 4.8).
ferrocene
Synthesis of 3,6-Di(2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)thien-
2
I
DS = 1/
2
(W/L)µ C
i
(V − V )
G
T
5
-yl)-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (4) : A lithium
diisopropylamide (LDA) solution (1.28 ml, 2.5 mmol) was added slowly
over 5 min) to a solution of 3,6-dithien-2-yl-2,5-di(2-decyltetradecyl)-
where IDS is the drain current, W and L are the semiconductor channel
(
width and length, respectively, μ is the mobility, C is the capacitance per
i
pyrrolo[3,4-c]pyrrole-1,4-dione (1 g, 1.0 mmol) and 2-isopropyl-4,4,5,5-
tetramethyl-1,3,2-dioxoborane (0.42 g, 0.46 ml, 2.5 mmol) in THF (30 ml)
under argon at −40 °C. The resulting mixture was stirred for 1 h at 0 °C
and then quenched with water (200 ml). The compound was extracted
in CHCl , washed, and dried with MgSO . The solvent was evaporated
unit area of the gate dielectric, and V and V are the gate voltage and
G
T
threshold voltage, respectively.
DFT Calculation: DFT calculations were performed using the
Gaussian 03 package with the nonlocal hybrid Becke three-parameter
Lee–Yang–Parr (B3LYP) function and the 6-31G∗ basis set to elucidate
the HOMO and LUMO levels after optimizing the geometry of BOC-
PTDPP using the same method.
3
4
under a reduced pressure. The crude product was washed with methanol
200 ml) three times. Isolated yield = 1.0 g (79%) as a thick viscous
(
1
dark purplish oil. H NMR (CDCl , 600 MHz): δ ppm 8.91 (d, J = 3.17,
2
1
3
H), 7.71 (d, J = 3.17, 2H), 4.05 (m, 4H), 1.91 (m, 2H), 1.36 (s, 24H),
.27-1.20 (m, 80H), 0.87-0.86 (m, 12H). C NMR (CDCl3, 150 MHz): δ
13
Supporting Information
ppm 161.71, 140.48, 137.62, 136.11, 135.63, 108.70, 84.55, 46.24, 37.77,
3
2
1.91, 31.27, 30.01, 29.80, 29.68, 29.66, 29.64, 29.62, 29.57, 29.35, 29.33,
6.32, 24.75, 22.67, 14.10. MALDI-TOF MS (m/z) 1225.68(M+). Anal.
Supporting Information is available from the Wiley Online Library or
from the author.
Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201200940
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
9

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