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Photo-Patternable Stretchable Semi-Interpenetrating
Polymer Semiconductor Network Using Thiol–Ene
chemistry
1
Chemistry for Field-Effect Transistors
thio
-
ene
Hsin-Chiao Tien, Xin Li, Chang-Jing Liu, Yang Li, Mingqian He, and Wen-Ya Lee*
are essential for novel applications,
including wearable electronics, implanted
devices, electronic skins, electronic eyes,
and prosthetics. However, for traditional
silicon-based devices, it is challenging
to prepare stretchable integrated circuits
due to the brittleness of the silicon semiconductor. Therefore, many approaches
have been developed for stretchable
electronics, including rigid-island architectures,[5–7] buckled surfaces,[8,9] and
intrinsically stretchable materials.[4,10–13]
Compared to the other approaches, intrinsically stretchable materials have been
considered promising for stretchable
electronics. They do not need to be like
the rigid-island architecture with the large
space of serpent-shaped electrodes with
the disadvantages of reduced device density and less resolution. Furthermore, the
intrinsically stretchable materials would
not require a rough, wrinkled surface to
release stress. Therefore, the intrinsically
stretchable semiconductor is ideal for fabricating stretchable electronics.[4] Intrinsically stretchable semiconductors can be
accomplished by either the backbone/side-chain engineering
of conjugated polymers[10,14–27] or by blending with elastic
polymers.[2,11–13,17,28] Compared with the modification of the
chemical structure of conjugated polymer backbone or side
chains, which requires complicated synthesis skills, the
blending of a conjugate polymer and an insulating elastomer
provides a facile way to prepare a stretchable semiconducting
layer. Recently, Wang et al. reported a stretchable field-effect
transistor (FET) array based on the blend of the donor–acceptor
copolymer
and
polystyrene-b-poly(ethylene-co-butylene)-bpolystyrene (SEBS). The average mobilities of the stretchable
transistors are ≈0.82 cm2 V−1 s−1.[11,12] The same group applied
solution shearing to align the polymer semiconductor films to
further enhance device performance in stretchable polymer transistors.[29] The solution-sheared stretchable transistor showed
ideal characteristics with an average mobility of 1.50 cm2 V−1 s−1.
Nowadays, the mobility of stretchable transistors has been
further improved. However, the fabrication of a large-scale
stretchable circuit is still challenging due to the limitation of
scalable patterning of stretchable polymer semiconductors. The
polymer commonly suffers severe solvent etching during photolithography, dramatically degrading device performance.
Stretchable polymer semiconductors are an essential component for skininspired electronics. However, the lack of scalable patterning capability of
stretchable polymer semiconductors limits the development of stretchable
electronics. To address this issue, photo-curable stretchable polymer blends
consisting of a high-mobility donor–acceptor conjugated polymer and an
elastic rubber through thiol–ene chemistry are developed. The thiol–ene
reaction can selectively cross-link the rubber with alkene or vinyl groups
without damaging the electronic properties of the conjugated polymer. The
conjugated polymer chains embedded in the elastic polymer matrix induce
a semi-interpenetrating polymer network (SIPN). The thiol–ene-cross-linked
network provides great solvent resistance and enhances stretchability for the
embedded conjugated polymer. The well-defined patterned film with a feature
size of ≈10 µm can be obtained using UV light at 365 nm through conventional photolithography processes. Furthermore, the SIPN-based transistors
show increased mobilities from 0.61 to 1.18 cm2 V−1 s−1 when applying the
strain from 0% to 100%. Moreover, the hole mobility can still maintain at
0.87 cm2 V−1 s−1 after 1000 strain-and-release cycles at the strain of 25%. This
study sheds light on the molecular design of photo-curable polymer semiconductors for the mass production of stretchable circuits.
1. Introduction
Stretchable electronics have attracted great attention recently
due to their great mechanical compliance to tolerate a large
degree of deformation without sacrificing electronic properties.[1–4] Deformable devices with high electronic performance
H.-C. Tien, C.-J. Liu, W.-Y. Lee
Research and Development Center for Smart Textile Technology
and Department of Chemical Engineering and Biotechnology
National Taipei University of Technology
Taipei 106, Taiwan
E-mail: wenyalee@mail.ntut.edu.tw
X. Li, Y. Li
Corning Research Center China
Shanghai 201206, P. R. China
M. He
Corning Incorporated
Corning, NY 14831, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202211108.
DOI: 10.1002/adfm.202211108
Adv. Funct. Mater. 2023, 2211108
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RESEARCH ARTICLE
可拉伸介電層 paper
phenol
Several approaches have been demonstrated to cross-link
polymer chains to develop scalable patterning approaches and
reduce solvent etching damage. Previously, much effort has
been put into the development of dielectric layers in field-effect
transistors.[30–37] Lim et al. have reported a cross-linked polymer
dielectric consisting of hydrophilic poly(4-vinyl phenol) (PVP)
and poly(melamine-co-formaldehyde) with negligible current
hysteresis.[38] Yoon et al. have demonstrated a low-voltage-driven
transistor using silane-cross-linked gate dielectrics. Using
thermal treatments, they employed three trichlorosilanes with
various chain lengths to cross-link hydrophilic PVP as gate
dielectrics. Low-voltage-driven devices with the leakage
ultrathin dielectric material can be achieved by trichlorosilane
cross-linking.[34] Cheng et al. cross-linked an amorphous fluoropolymer dielectric, poly(perfluorobutenylvinylether) (Cytop),
using 1,6-bis(trichlorosilyl)hexane.[39] The cross-linked Cytop
showed great bias and air stability due to its 荣
inert fluorinated
chemical structure. Chua et al. used a siloxane-based material,
divinyltetramethyldisiloxane-bisbenzocyclobutene (BCB), as a
gate dielectric. BCB can form a defect-free surface to facilitate
charge transport.[40] However, the BCB film requires a thermal
treatment at 290 °C. Hallani et al. synthesized an alkoxy-substituted benzocyclobutene (BCB)-based polymer for gate dielectric
materials to reduce process temperatures.[33] The BCB moiety
with alkoxy groups can be cross-linked through the electrocyclic
opening at 120 °C.
Although cross-linked polymer dielectrics are commonly
obtained using thermal curing processes, photo-curable
polymer dielectrics are preferred for the industry. This photocurable feature provides an efficient direction for patterning a
large-scale integrated circuit. Kwon et al. have reported a solventfree photo-patternable dielectric using poly[(mercaptopropyl)
methyl-siloxane] based on thiol–ene cross-linking.[31] The
poly[(mercaptopropyl)methyl-siloxane] pattern with a feature
size of 3 µm can be achieved using UV-nanoimprint lithography.
Wang et al. have developed cinnamate-modified carbohydrates
as gate dielectrics. The cinnamate-modified carbohydrates can
proceed [2+2] cycloaddition after UV exposure and then form a
robust cross-linked dielectric.[32] Kwon et al. have demonstrated
a cross-linker series with fluorophenyl azide (FPA) groups. The
FPA groups can react with alkyl groups of polymer chains.[41]
Low/high-k or amorphous/crystalline polymers can be crosslinked as gate dielectrics. Although much progress has
been made in cross-linked polymer dielectrics, the development of photo-cross-linked polymer semiconductors significantly lags.
The concept of cross-linking semiconductors has been
reported in organic solar cells to solve the issues of longterm stability.[42] Cross-linking chemistry is also applicable
for stretchable polymer semiconductors. The Bao group first
demonstrated an azide-cross-linking dielectric for stretchable
transistor array with inkjet printing and traditional photolithography processes. They used azide to allow the photo
patterning of the insulating polymer, SEBS, as a stretchable
dielectric layer.[12] Sequentially, the stretchable semiconductor is
deposited on the dielectrics using inkjet printing. Combining
with a fluorinated polymer as a sacrificial layer, photo-patterned
stretchable dielectrics, and inkjet-printed polymer semiconductors, a large-scale array of stretchable transistors with a
Adv. Funct. Mater. 2023, 2211108
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向
device density of 347 transistor per cm−2 has been made. The
same group recently applied a light-triggered carbine insertion
chemistry to cross-link stretchable polymer semiconductors by
UV light.[13] They synthesized a diazirine cross-linker, which
can induce carbene groups by UV light. The carbene group
is a highly reactive agent and can react with the CH bonds
in the polymer for the cross-linking reaction. The
carbene-based photo-cross-linking enables the layer-by-layer
deposition of stretchable polymer semiconductors during
photolithography.
Here, we demonstrate a mild approach to pattern stretchable polymer semiconductors without reducing their charge
transport properties using thiol–ene chemistry. The thiol–ene
reaction is a highly efficient way of bonding thiols and enes.
A donor–acceptor conjugated polymer is embedded in a thiol–
ene-cross-linked elastic polymer matrix based on poly(styrenebutadiene-styrene) (SBS) to induce a semi-interpenetrating
polymer network (SIPN). The SIPN using thiol–ene chemistry
was selected for the following reasons: i) improved stretchability of the polymer semiconductor layer due to the elastic
polymer; ii) relative selectivity of cross-linking reaction of thiol–
ene chemistry; iii) enhanced solvent resistance due to the semiinterpenetrating polymer network. Compared to the highly
reactive agent, azide or carbene group, which can directly react
with alkyl groups (CH3), the thiol–ene reaction is a selective
reaction, which only allows cross-linking with double bonds to
form a thioether. Instead of using SEBS commonly reported
in the literature, we selected the elastic polymer, SBS, as our
elastic polymer. SBS contains the vinyl and alkene groups,
which can act as a cross-linking site for thiol–ene click chemistry. Two thiol reactions have been developed: thiol–ene freeradical and catalyzed thiol-Michael reaction.[43–45] Herein, this
work used free-radical thiol–ene addition for the cross-linking
reaction. In the free-radical thiol–ene reaction, the reaction rate
is dependent on the electron density of the alkene. Vinyl double
bonds can react faster than conjugated ones.[43] Therefore, the
thiol group has a higher reactivity with the electron-rich ene,
e.g., a vinyl group, than electron-poor conjugated alkenes.[45]
During the thiol–ene cross-linking reaction, it is expected that
the thiol group tends to cross-link with the vinyl group in SBS
instead of conjugated polymer chains. Hence, the thiol–ene
reaction has less chance of altering the molecular packing
structures of the conjugated polymer. Moreover, the selectively
cross-linking SBS may form a semi-interpenetrating polymer
matrix to encapsulate the conjugated polymer, protecting the
conjugated polymer from solvent etching.
The main difference between carbene insertion reaction and
thiol–ene chemistry is cross-linking target materials. The carbene groups induced from the diazirine cross-linker can insert
into the CH bonds of the non-conjugated alkyl chains of a
conjugated polymer for cross-linking.[13] The advantage of this
carbene reaction is that the carbene groups can react with a
large variety of polymers with alkyl chains. However, the carbene insertion reaction may alter the molecular packing of
polymer chains since the alkyl chains act an important role in
molecular packing. On the other hand, the thiol–ene reaction
prefers cross-linking with non-conjugated double bonds of the
SBS elastomer instead of the conjugated polymer. Therefore,
the thol-ene cross-linking approach should have a lower chance
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of altering the molecular packing of the conjugated polymer.
Furthermore, all the photoinitiator and thiol cross-linkers are
commercially available, compared to the above-mentioned
carbene cross-linkers require additional chemical syntheses.
This commercial availability is another advantage of the
thiol–ene cross-linked system for industrial production.
In our system, the high mobility donor–acceptor polymer,
poly(tetrathienacene-diketopyrrolopyrrole) (P4TDPP), has
been chosen as the polymer semiconductor for charge transport.[17] The fused-ring tetrathienacene moiety has high charge
mobilities of >1 cm2 V−1 s−1 and provides low resonance
energy and great chemical stability.[46,47] It is expected that the
combination of the SIPN and highly stable P4TDPP will facilitate the fabrication of scalable photo-patterned stretchable
transistors for large-scale production in the industry. Note that
our devices are only partially stretchable. The drain, source,
gate electrodes, and dielectric layer are not stretchable. However, through the PDMS transfer processes, we can focus on
evaluating the polymer semiconductors under various strains
without concerning the degradation of drain/source/gate electrodes or dielectrics under deformation. Therefore, this work
emphasizes investigating the electronic properties of polymer
semiconductors under various strains. Despite lacking the
integration of fully stretchable devices, this work can still provide new insight for designing patternable stretchable polymer
semiconductors.
2. Results and Discussion
2.1. Photo-Cross-Linking Reaction Based on
Thiol–Ene Click Chemistry
To incorporate conjugated polymer films with thiol–ene chemistry, we first blended the diketopyrrolopyrrole (DPP)-based
conjugated polymers (P4TDPP) with the elastomer (SBS) to
enhance the stretchability of the polymer film, as shown in
Figure 1a. Then, trimethylolpropane tris(3-mercapto propionate) (TRIS) and trimethyl benzoyl-diphenylphosphine oxide
(TPO) were mixed with the polymer blends to form a photocurable polymer network. SBS has both alkene (CHCH)
and vinyl group (CHCH2) in the polymer structures, which
provide reactive sites for thiol–ene reaction. TPO can produce
free radicals initiated by UV light at 365 nm, and the free
radicals can be transferred to the thiol group of TRIS. The
transferred free radicals form thiyl radical species (Figure 1b).
The free-radical thiol–ene reaction is efficiently employed to
cross-link SBS as a polymer network. Compared to the air-sensitive azide or carbene reaction, the thiol–ene reaction is insensitive to oxygen or moisture. The whole thiol–ene cross-linking
process can be done in the ambient atmosphere condition.
Furthermore, the thiol groups prefer reacting with the electronrich group, such as the alkene or vinyl group. The reaction rate
of conjugated dienes with the thiol groups is much slower.[48]
The different reactivity is a great benefit for photo pattern technology because the cross-linking between the alkene groups
of SBS does not change the chemical structure of the conjugated polymer. Furthermore, the different reactivity will cause
the formation of the P4TDPP polymer chains embedded in the
Adv. Funct. Mater. 2023, 2211108
cross-linked SBS network, the so-called semi-interpenetrating
polymer network (SIPN) (Figure 1c). The SIPN structure may
offer the encapsulation of the conjugated polymer protected
from the etching of organic solvents.
To verify the photo-cross-linking reaction of blend film, we
employed Fourier transform infrared (FTIR) to analyze the
photo-cross-linking reaction of the conjugated polymer blend
films. Figure 2a-1 shows the IR absorption peaks of TRIS, the
pristine polymer, and the polymer blend films. The signals
near 2800–3000 cm−1 are assigned to CH functional groups.
The strong absorption peak at 1730 and 1662 cm−1 are assigned
to the CO functional group of TRIS and P4TDPP, respectively. The absorption peak at 1600 cm−1 belongs to the CC
group of SBS. After exposure to UV light at 365 nm, the peak
significantly decreases. The decreased signal indicates that a
large amount of the alkene groups forms the thioether group
through the thiol–ene reaction. We have also used H-NMR
spectra to investigate the cross-linking reaction. However,
unfortunately, we could not find any meaningful signal to
support the thiol—ene reaction. This may be attributed to the
low percentage of the TRIS/TPO (3 wt.%). Despite the lack
of the NMR spectra to support our hypothesis, the photocured P4TDPP/SBS films showed great solvent resistance for
common solvents, e.g., toluene, p-xylene, butyl acetate, and
acetone. The great solvent resistance indicates the successful
cross-linking of the P4TDPP/SBS film. Owing to the photocross-linking characteristics, we can successfully prepare a
well-defined pattern using UV exposure (Figure 2; Figure S2,
Supporting Information). To investigate the influence of thiol–
ene cross-linking, we tested the P4TDPP/SBS film without
TRIS. Without the thiol cross-linker, the polymer film did not
proceed with the thiol–ene cross-linking. As a result, the crosslinking of the film is required a much higher percentage of
TPO (≈10 wt.%) and increased exposure time (6 min) to obtain
a pattern. However, the device performance of P4TDPP/SBS
exhibited a significant decrease. According to these results, we
conclude that the addition of the thiol cross-linkers (TRIS) is
necessary to realize high-mobility photo-patternable stretchable
polymer semiconductors.
2.2. Photo-Patterning of the Conjugated Polymer Blend Films
Various pattern sizes of the conjugated polymer blend films
were fabricated by conventional photolithography. The desired
patterns of the spin-coated P4TDPP/SBS films were obtained via
a quartz mask with UV light exposure at 365 nm for a minute.
The untreated area of polymer films can be fully removed by
immersing it in toluene for 10 min. Figure 2b shows the welldefined patterns of the P4TDPP/SBS blends with various sizes.
The rectangular area of the conjugated polymer films with
the same length of 1000 µm and various widths of 1000, 200,
and 100 µm can be obtained, respectively. The smallest line
width based on our current accessible UV curing equipment is
≈10 µm. As shown in Figure 2c, the AFM height image reveals
the feature size of the patterned P4TDPP/SBS film with a width
of ≈10 µm. Although the etching profile angle is not ideal at
90o, which requires further optimization, a well-defined pattern
can still be observed. Figure 2d exhibits The AFM cross-section
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Figure 1. a) Chemical structure of conjugated polymer P4TDPP, poly(styrene-butadiene-styrene) (SBS), and trimethylolpropane tris(3-mercapto propionate) (TRIS). b) Thiol–ene reaction of the light-induced cross-linked polymer network. c) Illustration of SIPN (semi-interpenetrating polymer network)
structure.
profile of the P4TDPP/SBS spin-coated film. The film thickness estimated from the cross-section profile is ≈25–30 nm
(Figure 2d). These results show that the P4TDPP/SBS film can
be successfully patterned via the conventional photolithography
processes. Furthermore, the success of the small, well-defined
pattern indicates that this patterning approach can be applicable to OLED applications, which require a micrometer-scale
device array for display panels.
Adv. Funct. Mater. 2023, 2211108
2.3. Electrical Characterizations of FETs Based on Patterned
Stretchable Polymer Semiconductor Films
The electrical performance of the bottom-gate-top-contact FETs
is based on the P4TDPP/SBS films, as shown in Figure 3a. All
the charge mobilities of FETs are estimated from the saturation regime in the transfer curves. The pristine P4TDPP films
and P4TDPP/SBS blend films were prepared by spin coating
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Figure 2. a) Thin film FTIR results of pure P4TDPP, pure SBS, pure TRIS, SBS/P4TDPP/TPO, SBS/TRIS/TPO, SBS/P4TDPP/TRIS/TPO with UV exposure, and SBS/P4TDPP/TRIS/TPO without UV exposure. b) Optical microscope images of patterns (scale bar was 200 µm). c) AFM height image of
the SBS/P4DPP/TRIS/TPO pattern with the size of 10 µm × 10 µm. d) The cross-section profile of Figure 2c.
from a p-xylene solution with a 5 mg mL−1 concentration. The
blend films comprised 60 wt.% SBS and 40 wt.% P4TDPP. All
the polymer films were treated with UV light for photo-curing
without thermal annealing to investigate the charge transport properties of the photo-cross-linked polymer films. The
average mobility (µavg) of the neat P4TDPP film and P4TDPP/
SBS blend film was 0.46 and 0.32 cm2 V−1 s−1, respectively,
as shown in Table 1. The blending with SBS showed slightly
improved charge mobility (Figure S1, Supporting Information). The performance evaluation indicates that the mixing
with the insulating SBS will not obstruct charge transport in
the channel of FETs. Furthermore, the 40% to 80% P4TDPP
exhibited similar mobilities, while the 20% P4TDPP device
showed the lowest performance. However, the charge mobilities of the 60% P4TDPP show a clear degradation under the
strain of 60% (Figure S1b, Supporting Information). On the
other hand, the 40% P4TDPP exhibited better stretchability and
increased mobilities even at the strain of 100%. Considering
the mechanical compliance and photo pattern effect, we chose
the 40% P4TDPP/60 wt.% SBS as the main blending ratio for
stretchable polymer semiconductors.
Adv. Funct. Mater. 2023, 2211108
The dual sweeping transfer curves of the blend films and
neat films at different strains are given in Figure 3a,b, respectively. All the transfer curves in the dual sweeping showed
negligible hysteresis, indicating that the semiconductor/
dielectric interface has a low density of defects even in the
SBS blends. Figure 3c,d exhibits the well-defined output
curves of blend films at 0 and 100% strains, respectively. The
output curves in both conditions showed linear behavior in
the low drain voltage, indicating no significant contact resistance even in the blends. Figure 3e shows the mobility trend
in blended and neat films with various strains. Note that the
average mobilities under various strains were estimated from
the charge transport direction along with the stretching direction. The pristine P4TDPP devices showed significant degradation of ≈73% under high-level strains. Compared with the neat
film, the blend film with a high ratio of SBS exhibited gradually
increased charge mobilities from 0.61 to 1.18 cm2 V−1 s−1 with
increased strains from 0% to 100%. This enhanced mobility
may be attributed to the improved orientation of the molecular packing of the conjugated polymer during the alignment
of polymer chains under stretching. In contrast, the charge
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Figure 3. a,b) Hysteresis curves (VD = −100 V) of the neat film and the blend films under strains. c,d) The output curves of the blend film at strains of
0% and 100%. e) The mobility of the neat film and the blend film at different strain ratios. f) Charge mobilities of the P4TDPP/SBS films before and
after solvent etching.
mobilities did not show significant enhancement in the direction vertical to the stretching direction (Figure S1c, Supporting
Information). This may be attributed to the compression in the
vertical direction. When the polymer film is stretched, the film
tends to compress in a vertical direction to the stretching force.
Therefore, the polymer film did not show significant mobility
enhancement in the vertical direction.
Note that the average mobilities of the neat and semiinterpenetration polymer semiconductor showed slightly
decreased mobility after UV exposure. However, the change of
Adv. Funct. Mater. 2023, 2211108
the mobilities is not significant for the neat polymer, indicating
that the UV exposure process did not significantly affect the
device performance. On the other hand, the semi-interpenetration P4DPP/SBS blends showed a significant decrease after UV
exposure. Intriguingly, we observed that the stretching process
could recover the reduced performance. This might be affected
by the cross-linked SBS matrix and the rearrangement of the
conjugated P4TDPP chains. During UV exposure, although
the conjugated polymer was not significantly damaged, the formation of the insulating SBS matrix may suppress the charge
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Table 1. A summary of FETs (L = 50 µm, W = 1000 µm) is based on the neat and blended film at different strains, respectively.
µavg [cm2 V−1 s−1]
Conditions
P4TDPP
P4TDPP/SBS
µmax [cm2 V−1 s−1]
Ion/Ioff
4
Vth [V]
Without UV
0.58 ± 0.25
0.87
10
0±1
As cast (UV)
0.46 ± 0.21
0.69
104
−6 ± 5
Strain 0%
0.38 ± 0.05
0.46
105
0 ± 10
Strain 50%
0.14 ± 0.01
0.16
104
−4 ± 6
Strain 100%
0.10 ± 0.02
0.12
105
−7 ± 1
4
−19 ± 7
Without UV
0.73 ± 0.08
0.78
10
As cast
0.32 ± 0.03
0.34
104
−12 ± 2
Strain 0%
0.61 ± 0.06
0.72
103
−6 ± 5
3
−11 ± 6
Strain 25%
0.64 ± 0.01
0.69
10
Strain 50%
0.96 ± 0.05
1.06
104
−20 ± 5
Strain 75%
1.11 ± 0.06
1.29
104
−17 ± 7
1.45
4
−19 ± 5
Strain 100%
1.18 ± 0.13
transport in the conjugated polymer. However, the stretching
process facilitates the rearrangement of the conjugated polymer
chains and rebuilds the channel for charge transport, thus
leading to the enhancement of charge mobilities.
Figure 3f compares the performance of the photo-cured
P4TDPP/SBS FETs before and after solvent etching in toluene
for 10 min. The photo-cured polymer blend FETs exhibited
10
great solvent resistance. After immersing in toluene, the FETs
can still maintain charge mobilities ≈0.2–0.3 cm2 V−1 s−1 even
under the strain of 100%. On the contrary, the non-crosslinked polymer-blend film was fully removed in toluene. This
feature indicates that the photo-cross-linked SIPN structure
provides a facile way to obtain a selected pattern of the polymer
semiconductor film. Additionally, we have applied this SIPN
Figure 4. a) Hysteresis curves (VD = −100 V) of the blend film strain to 25% for 100, 500, and 1000 cycles. b) The mobility of the blend film strain to
25% for 100, 500, and 1000 cycles. c,d) The output curves of the blend film strain to 25% for 100 and 1000 cycles.
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Figure 5. AFM tapping phase and height images of a,b) P4TDPP neat film, c,d) P4TDPP/SBS blend film under UV exposure for 1 min, and AFM tapping
phase images of P4TDPP/SBS blend film e) at 0% strain and f) 100% strain.
approach to other conjugated polymers, including p-type
poly(diketopyrrolopyrrole-thienothiophene) (PTTDPP) and
n-type poly(naphthalene diimide-alt-bithiophene) (PNDI2T),
respectively, and evaluate their device performance (Figure S2,
Supporting Information). The mobility of these polymers only
showed a slight decrease after the photo-curing processes. The
mobilities of these photo-cured polymer semiconductors can
still maintain the same order of magnitude after immersing in
toluene for a few minutes. These results show that our approach
can be applied to different conjugated polymers. We also
tested the patterning capability of the PNDI2T/SBS film. The
polymer blend films showed similar behaviors of great solvent
resistance and formed a well-defined polymer semiconductor
pattern. These results suggest that the semi-interpenetrating
structure has the potential for scalable photo-patterning of
polymer semiconductors for stretchable field-effect transistor
applications.
Adv. Funct. Mater. 2023, 2211108
We also investigated the electrical properties of a cyclic strain
of blend films at 25% strain. After the blend film stretching at
25% for 100, 500, and 1000 cycles, the electrical properties show
great performance in the transfer curves, as shown in Figure 4a.
The mobility was measured parallel to the strain direction. The
values of µavg increase from 0.48 cm2 V−1 s−1 at 100 cycles to
0.87 cm2 V−1 s−1 at 1000 cycles (Figure 4b). It is consistent with
the enhanced drain current of the transfer and output curves
of P4TDPP/SBS FETs with the increased stretching cycles
(Figure 4c). This indicates that the P4TDPP/SBS blends have
remarkably long-term robustness.
2.4. Morphological Characterizations
To further understand the influence of the stretching processes on the morphology, we have investigated the surface
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Figure 6. a) UV–vis spectra of the P4TDPP/SBS film at the strain of 100%. b) The dichroic ratio of blend films at different strain ratios. c) SIMS results
of the blended film.
morphology of the polymer films by using atomic force microscopy (AFM). The height and phase images of the neat P4TDPP
and P4TDPP/SBS films are shown in Figure 5a,b. The P4TDPPonly film showed a smoother surface with small aggregates. On
the other hand, the polymer blend film exhibited well-defined
nanofiber morphology with large grains. To compare the
morphology of the polymer blend films under different strains,
we employed Scotch tape to peel off the polymer blend films
and measured their backside morphology. The polymer blend
films did not show any significant cracking on the surface,
even under the strain of 100%. Furthermore, after stretching
at the strain of 100%, the nanofibers exhibited slightly aligned
wrinkles on the surface, as shown in Figure 5c. Next, we studied
their optoelectronic properties using polarized UV–vis spectra.
Figure 6a,b shows the polarized UV–vis spectra of the P4TDPP/
SBS films under different strain ratios in the parallel (0°) and
perpendicular (90°) directions. The polymer films showed
strong anisotropic absorption. We calculate their dichroic
ratio from the polarized UV–vis spectra. We can also see that
the dichroic ratio increases from 1 to 2.1 when the strain ratio
increases from 0 to 100%. The enhanced dichroic ratio indicates that the orientation of polymer chains has been aligned
after stretching. The alignment of the polymer chains facilitates better charge transport, which may explain the increased
charge carrier mobility with stretching.
The surface elemental analysis of blend film was analyzed
by SIMS (Secondary Ion Mass Spectroscopy), as shown in
Figure 6c. The purple line and blue line represent the content
of sulfur and cyanide groups, respectively, which should be
Adv. Funct. Mater. 2023, 2211108
contributed from the conjugated polymer, P4TDPP. The red
line represents the content of carbon, which can be attributed
to both P4TDPP and SBS. The black and orange lines represent the silicon and oxygen content from the Si/SiO2 substrate
signals. We found a vertical phase separation in the polymer
blend film from the SIMS results. The thickness of the
blend film is ≈20–30 nm. The conjugated polymer prefers to
distribute in the top and bottom positions, and SBS is rich in
the middle of the blend film, which is consistent with the other
polymer blends observed in the literature.[11] Because of the
vertical phase separation, the conjugated polymer can still form
a channel for charge transport even though blending with a
high ratio of SBS.
The molecular packing of the thin films was analyzed by
grazing incidence X-ray diffraction (GIXD). Figure 7a,b showed
the 2D-GIXD patterns of the cross-linked P4TDPP/SBS films
under 0% and 100% strain. The P4TDPP/SBS films showed
well-defined diffraction peaks in the out-of-plane direction,
which were attributed to a lamellar structure of the P4TDPP.
The estimated lamellar spacing of the P4TDPP/SBS is 31.66 Å,
slightly higher than the value of the pristine P4TDPP (30.28 Å).
The increased lamellar spacing should originate from the SBS
blending, which probably induces a less dense structure. When
stretching from 0% to 100%, the lamellar spacing increases to
32.8 Å. Similar behavior has also been observed in the literature.
The increased spacing implies that the external mechanical
stress may reduce the interdigitated structure of the P4TDPP
polymer chains. Additionally, the lamellar spacing of the
P4TDPP/SBS blend is slightly higher than the long-branched
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Figure 7. 2D GIXD images of a) P4TDPP/SBS blend film with the strain of 0% and b) 100%, respectively. c) Crystal sizes of the P4TDPP/SBS with
various strains.
chains of the P4TDPP nanowires reported in the literature.[49,50]
This difference may be attributed to the morphology. Compared
to our thin-film device, the nanowire of P4TDPP reported in
the literature may have a denser interdigitated packing or a
tilted backbone.
Although the pristine P4DTPP polymer showed a pi–pi
stacking distance of 3.72 Å (Figure S4, Supporting Information), we did not observe any (010) signals (π–π stacking) from
the P4TDPP/SBS blends. This result should be attributed to
the reduced concentration of the conjugated polymer. The
blend film consisted of 40 wt.% P4TDPP and 60 wt.% SBS.
The concentration of P4TDPP was significantly diluted due to
the blending with a large ratio of SBS. Therefore, there are no
detectable (010) signals from GIXD patterns of the P4TDPP/
SBS blends.
To further investigate the crystallinity, we evaluate the crystal
sizes of the polymer films according to the Scherrer equation,
Adv. Funct. Mater. 2023, 2211108
which calculates the crystal sizes from the full width at half
maximum (FWHM) of the out-of-plane (200) peak. As shown in
Figure 7, the crystal sizes increased from 5.13 to 7.37 nm when
the applied strain was from 0 to 100%. The larger crystal size
facilitates charge transport. This trend agrees with the improved
mobility and dichroic ratios with the increased strains. These
results indicate that the conjugated polymer embedded in the
photo-cross-link SBS network did not cause any degradation of
device performance. Instead, the improved stretchability and
charge transport properties were observed in the photo-cured
P4TDPP/SBS polymer blends.
3. Conclusion
This work demonstrates stretchable field-effect transistors using
elastic photo-cross-linkable conjugated polymer blends with a
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semi-interpenetrating polymer network. The photo-cross-linked
semi-interpenetrating polymer network provides high charge
transport properties, stretchability, and solvent resistance. The
feature size of the polymer semiconductor pattern ≈10 µm can
be achieved. Moreover, the P4TDPP/SBS blend films exhibit a
vertical phase separation and still offer great charge transport.
The cross-linked P4TDPP/SBS thin films showed the nanofiber
structure and could withstand the stretch of 100% without crack
formation. The mobility of P4TDPP/SBS can achieve a value of
up to 1.18 cm2 V−1 s−1. Moreover, the P4TDPP/SBS film still has
great stability for charge transport under 500 strain-and-release
cycles. Most importantly, this approach can obtain the desired
pattern without significantly sacrificing their charge mobilities.
Therefore, this work shows the great potential of thiol–ene chemistry for the mass production of stretchable transistor arrays.
4. Experimental Section
FET Device Fabrication and Characterization: The conjugated polymer
P4TDPP (Corning, Mw = 68k) and elastomer Poly(styrene-butadienestyrene) (SBS, Aldrich, Mw = 140k) solutions were prepared by dissolving
in p-xylene (5 mg mL−1) and heating overnight at 90 °C, respectively.
Photo-cross-linker Trimethylolpropane tris(3-mercapto propionate) (TRIS)
and photoinitiator solutions were also prepared by dissolving in p-xylene
(4 mg mL−1), respectively. The organic semiconductor thin films were
spun-cast on the highly doped n-type Si(100) wafers with 300 nm thick
thermal SiO2. The SiO2 surface was modified with OTS self-assembled
monolayer according to the reported method.[51] The neat and blend films
were spun-cast on the SiO2/Si substrates in the glove box at 1000 rpm
for 60 s from the solution, which was pre-heated at 90 °C. The UV curing
experiments were accomplished using UVACUBE 100 (Hönle, German).
The wavelength of UV light was in the range of 315 to 400 nm (UVA). The
calibrated power of the UV light was 5.4 mW cm−2. The P4TDPP/SBS film
required an energy of 324 mJ cm−2 for photo patterning. Therefore, the
blend films were under UV exposure for 1 min, then used PDMS to peel
off the blend films and transfer the films to the FET test bed after a strain
at each different strain ratio. Neither the neat films nor the blend films
were without annealing procedure. The FETs were bottom-gate top-contact
devices. The source and drain electrodes were thermally evaporated 80 nm
thick gold electrodes onto the polymer films through a shadow mask
with the channel length (L) and width (W) defined as 50 and 1000 µm,
respectively. The electrical properties of FETs were using a Keithley 2634B
semiconductor parameter analyzer in a nitrogen-filled glovebox.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
W.-Y. L. is thankful for the financial support from the Ministry of
Science and Technology in Taiwan (MOST 110-2221-E-027 -008 -MY3;
110-3116-F-011 -003), National Science and Technology Council (111-3116F-011 -006), the Research and Development Center for Smart Textile
Technology, and National Taipei University of Technology International
Joint Research Project (NTUT-IJRP-109-04; L7101101).
Conflict of Interest
The authors declare no conflict of interest.
Adv. Funct. Mater. 2023, 2211108
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Keywords
field-effect transistors, photolithography, polymer blends, polymer
semiconductors, stretchable electronics
Received: September 24, 2022
Revised: December 24, 2022
Published online:
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