Polyacrylate polymer assisted crystallization: Improved charge transport and performance consistency for solution-processable small-molecule semiconductor based organic thin film

Journal of Science: Advanced Materials and Devices 4 (2019) 467e472

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Original Article

Polyacrylate polymer assisted crystallization: Improved charge
transport and performance consistency for solution-processable
small-molecule semiconductor based organic thin film transistors
Zhengran He a, *, Ziyang Zhang b, Sheng Bi c
a

Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, AL, 35487, USA
Department of Electrical Engineering, Columbia University, New York City, NY, 10027, USA
Key Laboratory for Precision and Non-traditional Machining Technology of the Ministry of Education, Institute of Photoelectric Nanoscience and
Nanotechnology, Dalian University of Technology, Dalian, Liaoning 116024, China
b
c

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 16 January 2019
Received in revised form
5 February 2019
Accepted 8 February 2019

Available online 15 February 2019

In this study, we report on an effective approach to modulate crystallization, control charge transport and
enhance performance consistency of small-molecule semiconductor based organic thin film transistors
(OTFTs) with the addition of a polyacrylate polymer additive poly(2-ethylhexyl acrylate) (P2EHA). 6,13bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a benchmark semiconductor to
blend with the P2EHA additive, leading to a vertical phase separation between these two components.
The resultant TIPS pentacene film exhibited greatly reduced crystal misorientation, enlarged grain width
and enhanced film coverage. Bottom-gate, bottom-contact OTFTs based on the TIPS pentacene/P2EHA
blends were fabricated and showed an increased average hole mobility of 0.317 ± 0.047 cm2/V, as well as
a performance consistency factor of 6.72, which is defined as the ratio of the average hole mobility to the
standard deviation of mobility. Notably, it leads to a 10-fold and 7-fold enhancement of average mobility
and performance consistency as compared to the pristine TIPS pentacene OTFTs. This great improvement
of device performance can be attributed to the reduced crystal misorientation, less defects and trap
centers at the grain boundaries as a result of the enlarged grain width, as well as increased film coverage,
due to the addition of the P2EHA polyacrylate polymer additive.
© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
Small-molecule semiconductor
Polymer additive
Charge transport
Organic thin film transistors
Organic electronics

1. Introduction
In recent years, organic electronics has attracted numerous
attention due to its compatibility with high-performance, solutionprocessable, and low-cost applications on flexible substrates [1,2].
Particularly, significant progress has been achieved in the study of
charge transport, air stability and solvent choices of various highperformance, small-molecule organic semiconductors, such as
N,N0 -1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIFCN2) [3e5], 5,11-bis(triethylgermylethynyl) anthradithiophene

(diF-TEG-ADT) [6,7], and 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene) [8,9]. Despite these advances, the crystallization of small-molecule organic semiconductors in solution is still
anisotropic by nature, and the organic thin film transistors (OTFTs)

* Corresponding author.
E-mail address: (Z. He).
Peer review under responsibility of Vietnam National University, Hanoi.

based on such misoriented crystals exhibit severe performance
variations [10], which has largely restricted the application for
high-performance organic electronics devices [11].
In order to address the crystal misorientation issue, different
efforts which involve capillary force [12,13], substrate patterning
[14,15], and solution-shearing [16,17] based external crystal alignment techniques, have been made to align the small-molecule
organic semiconductor crystals. On the other hand, various polymer additives have been studied in order to control the crystallization of the small-molecule semiconductors [18e20]. These
polymer addition methods, which take advantages of the uniformity property of polymers and high mobility of semiconductors,
can work independently to tune the semiconductor crystallization
or be applied along with those external alignment techniques as
mentioned above. For example, Chen et al. reported the control of
TIPS pentacene thin film morphology by blending with conjugated
polymer additives, including a bidodecylthiophene copolymer
(PnBT-RRa) and poly(3-hexylthiophene) (P3HT) [21]. It was found

/>2468-2179/© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license
( />

468

Z. He et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 467e472

that different polymer additives contributed to distinctively

different polymorphism patterns, intermolecular interactions, and
charge transport modes of TIPS pentacene based OTFTs. AsareYeboah et al. demonstrated the addition of poly(a-methyl styrene) (PaMS) polymer, along with a temperature gradient technique, to align TIPS pentacene crystals [22]. The addition of the
PaMS polymer was shown to greatly enhance the crystal orientation, eliminate thermal cracks and improve hole mobility of TIPS
pentacene based OTFTs. More recently, Bi et al. reported that a P3HT
polymer additive was combined with a “controlled evaporative
self-assembly” (CESA) method to grow well-oriented 2,5-di-(2ethylhexyl)-3,6-bis(500 -n-hexyl-2,20 ,50 ,200 ]terthiophen-5-yl)-pyrrolo [3,4-c]pyrrole-1,4-dione (SMDPPEH) semiconductor crystals,
leading to controlled crystallization and enhanced device performance of SMDPPEH based OTFTs [23].
In this work, we report the addition of a polyacrylate polymer
additive poly(2-ethylhexyl acrylate) (P2EHA) in order to tune the
crystallization of small-molecule organic semiconductors. TIPS
pentacene was chosen as a benchmark semiconductor to blend with
P2EHA, which leads to a vertical phase separation between these
two components. By varying the loading ratios of the P2EHA polymer additive, we demonstrate to effectively modulate crystallization, control grain width and reduce crystal misorientation of TIPS
pentacene crystals. Since defects are located at the grain boundaries,
increased grain width essentially contributes to less trap centers of
the charge carriers. The bottom-gate, bottom-contact OTFTs based
on the TIPS pentacene/P2EHA blends demonstrated a great
enhancement of average mobility of up to 0.317 ± 0.047 cm2/V, and
an excellent performance consistency factor of 6.72, which is
defined as the ratio of average hole mobility to the standard deviation. The effective crystal alignment approach by using the P2EHA
additive can be applied to other solution-processed, small-molecule
semiconductors and shed light on the high-performance organic
electronics applications on flexible substrate.
2. Experimental
TIPS pentacene and P2EHA were purchased from Sigma Aldrich
and were used as purchased. Toluene was purchased from VWR
and was used without further purification. Bottom-gate, bottomcontact OTFTs were fabricated to test charge transport in the TIPS
pentacene/P2EHA blends. Optical photolithography was utilized to
pattern the substrate with source and drain contact electrodes.
Specifically, the 3-inch silicon wafer substrate with a 100 nm

thickness of thermally grown silicon dioxide (SiO2) was patterned
with a thin layer of photoresist, which served as a mask for the
following lift-off process. Then, 50 nm of gold was deposited using
electron-beam evaporation as source and drain electrodes, followed by lift-off in acetone with ultrasonication. After patterning,
each wafer contained a total of 10 bottom-gate, bottom-contact
transistor devices, which had a channel width of 500 microns
and 1000 microns, and a varied channel length from 5 microns to
50 microns.
Prior to the growth of semiconductor crystals, both pentafluorobenzenethiol (PFBT) and hexamethyldisilazane (HMDS)
treatments were conducted on the patterned bottom-gate, bottomcontact transistor substrate. In particular, HMDS self-assembled
monolayers (SAMs) were formed to passivate the hydrophilic SiO2
surface via vapor deposition at 140  C, followed by rinsing with
isopropyl alcohol (IPA). PFBT treatment was aimed at the gold
source and drain electrodes to modify the electrode surface energy,
by sinking the patterned substrate in a PFBT/toluene solution with a
concentration of 10 mM for 2 h and rinsing with toluene [24].
TIPS pentacene and P2EHA were dissolved in toluene at a concentration of 5 mg/ml, and were then mixed in solution at different

weight ratios of 5%, 10%, 20% and 60%. Then the TIPS pentacene/
P2EHA blends in toluene were drop casted onto the surface-treated
substrate, which was located in a petri-dish and covered with a cap,
and TIPS pentacene crystals were formed under a confined solvent
evaporation configuration at room temperature. Since there was no
external gas applied to navigate the crystal growth, the effect of gas
flow on the crystallization of TIPS pentacene can be neglected. A
small tilting angle (~5 ) was added to the substrate in order to
facilitate the formation of oriented TIPS pentacene crystals. The
substrate tilting orientation was parallel to the direction from
source to drain electrodes of the charge transport channel. After
solvent evaporation, the film thickness of the semiconductor layer

was approximately 150e200 nm.
To test charge transport in the TIPS pentacene/P2EHA blends,
electrical characterization of the bottom-gate, bottom-contact
OTFTs was conducted using an Agilent B1500A semiconductor
parameter analyzer. From the saturation regime of the slope of the
transfer curve ((IDS)1/2eVGS), the field-effect hole mobility was
extracted. All devices were measured for a total of five times to
ensure consistency of the extracted field-effect mobilities. Optical
micrographs of TIPS pentacene/P2EHA thin films were taken by
using a Zeiss Axioplan optical microscope with a built-in camera.
3. Results and discussion
The molecular structures of TIPS pentacene, P2EHA and toluene
are shown in Fig. 1(aec), respectively. TIPS pentacene was chosen
as a benchmark semiconductor to blend with P2EHA in this work
because of its high mobility, improved solution solubility and air
stability [25e28]. As shown in its molecular structure in Fig. 1(a),
the attachment of the two bulky side groups to the aromatic rings
of TIPS pentacene disrupts the herringbone packing, which improves its solubility in common solvents [29e31]. In addition, the
enhanced face-to-face interaction (p-p stacking) leads to improved
charge transport [32]. P2EHA is a polyacrylate polymer which has a
weight-average molecular weight MW of 100e120 k and a polydispersity index (PDI) of 3. The hydrophobic side group of P2EHA
has eight carbon atoms (Fig. 1(b)).
In order to modulate crystallization and tune charge transport of
TIPS pentacene, P2EHA was blended as a polymer additive at
different weight ratios of 5%, 10%, 20% and 60% which leads to
distinctive TIPS pentacene thin film morphologyies as shown in the
optical images of Fig. 2. While the pristine TIPS pentacene film
exhibited severe crystal misorientation and poor film coverage [33],
the loading of P2EHA polymer at 5% weigh ratio lightly improved
the coverage, although crystal misorientation still existed. At 10%

loading, the TIPS pentacene crystals were aligned along the tiled
orientation of the substrate (Fig. 2(b)). Furthermore, the addition of
the P2EHA additive at 20% dramatically improved both crystal
orientation and film coverage, as shown in Fig. 2(c). Finally, when
the P2EHA weight ratio increased to 60%, all TIPS pentacene crystals
were aligned along a uniform orientation and the film coverage
reached to nearly 100%, leading to a thin film morphology of TIPS
pentacene as demonstrated in Fig. 2(d).
In order to quantitatively characterize the change of crystal
orientation with the addition of the P2EHA polymer, we measured
the misorientation angle as a function the P2EHA loading ratio. As
shown in the inset of Fig. 3(a), the misorientation angle (q) is
defined as the angle between the long axis of a TIPS pentacene
crystal and a baseline crystal. While the pristine TIPS pentacene
film exhibited randomly-oriented crystals with a misorientation
angle of 41.4 ± 27.1 [34], the loading of P2EHA at 5%, 10%, 20% and
60% reduced the misorientation angle to 40.2 ± 33 , 12.5 ± 4.3 ,
11.5 ± 2.9 , and 4.1 ± 1.8 , respectively. It is noted that the loading
of P2EHA at 60% has greatly reduced the misorientation angle to


Z. He et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 467e472

469

Fig. 1. Molecular structure of (a) small-molecule organic semiconductor 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS pentacene), (b) polyacrylate polymer additive poly(2ethylhexyl acrylate) (P2EHA) and (c) toluene.

Fig. 2. Polarized optical images of TIPS pentacene/P2EHA blend films with different ratios of the P2EHA additive: (a) 5%, (b) 10%, (c) 20% and (d) 60%. The tilted blue rods represent
the TIPS pentacene molecule backbones. The yellow arrows imply the long axis direction of TIPS pentacene. The white triangles mark the bare substrate. Image (aed) share the same
scale bar of 100 mm as shown in (d).

Fig. 3. Plot of the average misorientation angle and grain width of the TIPS pentacene film as a function of the loading ratio of the P2EHA polymer additive. (a) The misorientation
angle (q) is defined as the angle between the long axis of TIPS pentacene crystals. (b) The grain width WG is defined as the domain width along the short axis of the TIPS pentacene
crystals. The standard deviation of both average misorientation angle and grain width are based on 8 measurements.


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Z. He et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 467e472

below 5 , which indicates a significant improvement of crystal
orientation as compared to pristine TIPS pentacene film.
In addition to the reduction of crystal misorientation, the
loading of P2EHA polymer additive also modulated the grain width
of the TIPS pentacene crystals, as shown in the optical images of
Fig. 2. The average grain width was measured as a function of the
P2EHA loading ratio, and the result was presented in Fig. 3(b). Here
we define grain width, or WG, as the domain width along the short
axis of the TIPS pentacene crystals, as shown in the inset. While
pristine TIPS pentacene exhibited an average grain width of
91 ± 20 mm [34], the loading of P2EHA at 5%, 10%, 20% and 60% led to
an average grain width of 62.17 ± 40.26 mm, 60.51 ± 61.28 mm,
40.59 ± 9.18 mm, and 140.44 ± 19.82 mm, respectively. Specially, it is
noted that the average grain width was significantly increased at
the loading ratio of 60%. Since defects are located at the grain
boundaries, the enlarged grain width indicates a reduction of grain
boundaries, and consequently, less trap centers of charge carriers,
which is beneficial for charge transport and device performance of
the TIPS pentacene based OTFTs [35].
Bottom-gate, bottom-contact OTFTs were fabricated to test

charge transport in the TIPS pentacene crystals. The representative output and transfer characteristics of OTFTs are shown in
Fig. 4(a, b), respectively, and the device configuration of OTFTs is
illustrated in Fig. 4(c). The mobility was calculated from the square
root curve of the transfer characteristic in the saturation region,
which is based on the following traditional MOSFET equation (1):

IDS ¼ mCi

W
ðV À VT Þ2
2L GS

(1)

where m is the mobility, Ci is the capacitance of SiO2 gate dielectrics,
W and L are the width and length of the semiconducting channel,
respectively, and VT is the threshold voltage.
The average mobility of OTFTs based on pristine TIPS pentacene
crystals and TIPS pentacene/P2EHA blends (with 60% loading ratio)
is compared in Fig. 4(d). Without the addition of P2EHA additive,
the pristine TIPS pentacene based OTFTs exhibited an average hole
mobility of 0.03 ± 0.03 cm2/V [34], which indicates great variations

in the charge carrier mobilities and performance consistency. In
comparison, the devices with 60% loading of P2EHA showed an
average hole mobility of 0.317 ± 0.047 cm2/V. This clearly indicates
that loading of the P2EHA polymer additive has greatly enhanced
the charge carrier mobility. It's worth noting that, as compared to
the pristine TIPS pentacene based OTFTs, the addition of P2EHA at
the weight ratio of 60% remarkably contributed to a 10-fold

enhancement of the average mobility of TIPS pentacene based
OTFTs, which can be attributed to the combination effect of
improved crystal orientation, reduced defects and charge trap
centers as a result of enlarged grain width, as well as an enhanced
film coverage.
In addition to the enhancement of average hole mobility, the
loading of the P2EHA polymer additive also significantly reduced
the variations in the charge carrier mobility of the TIPS pentacene
based OTFTs. To better demonstrate mobility variation, we define
the ratio of average mobility to standard deviation of mobility
(mAVE/mSTDEV), as a metric to quantitatively evaluate the device
performance consistency. While pristine TIPS pentacene based
OTFTs exhibited a performance consistency factor mAVE/mSTDEV of 1,
the loading of P2EHA polymer additive at 60% weight ratio
increased the mAVE/mSTDEV factor to 6.72, as shown in Fig. 4(d).
Particularly, it resulted in a 7-fold enhancement of the performance consistency factor, as compared to that of the pristine TIPS
pentacene based OTFTs.
Finally, we use a schematic picture to better illustrate the
change of the thin film morphology of TIPS pentacene as a result
of the addition of P2EHA polyacrylate polymer additive. As shown
in Fig. 5(a), the pristine TIPS pentacene film exhibited severe
crystal misorientation and poor coverage, which is responsible for
anisotropic charge transport and severe variations of device performance consistency of OTFTs. On the other hand, when P2EHA
was added (i.e. at a weight ratio of 60%) to tune the crystallization
and thin film morphology, as shown in Fig. 5(b), the TIPS pentacene crystal misorientation was significantly reduced, and both
the grain width and film coverage were greatly enhanced, favoring charge transport and device performance of TIPS pentacene
based OTFTs.

Fig. 4. Representative (a) output and (b) transfer curves of bottom-gate, bottom-contact OTFTs. (c) Device configuration of the bottom-gate, bottom-contact transistors studied in
this work. “S” and “D” represent the “source” and “drain” electrodes, respectively. “TP” represents “TIPS pentacene”. (d) Comparison of average mobility and performance consistency of OTFTs based on pristine TIPS pentacene crystals and TIPS pentacene/P2EHA blends (at 60% loading ratio). Performance consistency is defined as the ratio of average

mobility to standard deviation (mAVE/mSTDEV).


Z. He et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 467e472

471

Fig. 5. A schematic picture showing: (a) TIPS pentacene film with crystal misorientation and poor film coverage; (b) TIPS pentacene/P2EHA film with well-aligned crystals and
enhanced film coverage. “TP” stands for “TIPS pentacene”. The tilted light blue rods represent the direction of the TIPS pentacene molecule backbone. The yellow arrows imply the
long axis direction of TIPS pentacene.

Based on the Flory-Huggins theory [36], the Gibbs free energy of
mixing (DG) for a binary blend of solute and solvent is expressed by
equation (2):

DG
RT

¼

2
X

ni ln4i þ n1 42 c

(2)

i¼1

where ni is the molar number of each component in the binary

system, fi the volume fraction parameter, and c the interaction
parameter of the two components. On the right side of equation (2),
2
P
the first term
ni ln4i represents the combinatorial entropy
i¼1

change, whereas the second term n1 42 c is associated with contact
dissimilarity. Similarly, DG for a ternary blend system is expressed
by equations (3) and (4) [37]:

DG
RT

¼

3
X

ni ln4i þ GðT; 4; NÞ

(3)

i¼1

GðT; 4; NÞ ¼ n1 42 g12 þ n1 43 g13 þ n2 43 g23 þ n1 42 43 g123
where

3

P

(4)

plays a critical kinetic part in phase separation, a layer rich of the
semiconducting TIPS pentacene is firstly formed on the top. The residual solution with a greatly increased P2EHA concentration facilitates the formation of a middle layer rich of P2EHA. At last, the
remaining TIPS pentacene forms a bottom layer as the toluene solvent dries out. Therefore, the vertical phase separation between TIPS
pentacene and P2EHA through the kinetic interplay provides an
important confinement of the anisotropic crystallization. This leads
to the growth of well-aligned TIPS pentacene crystals with enhanced
crystal orientation, as illustrated in Fig. 5(b).
Since an effective vertical phase separation between TIPS pentacene and P2EHA depends on the remaining concentration of
P2EHA in solution after a top TIPS pentacene-rich layer is formed,
blending only a small amount of P2EHA with TIPS pentacene (i.e. at
5% weight ratio) resulted in a weak vertical phase separation, which
provided very limited confinement of TIPS pentacene crystallization, and consequently, negligible alignment of randomly-oriented
crystals. In contrast, as the loading ratio of P2EHA increased to 10%,
20% and 60%, stronger vertical phase separation occurred as a result
of the elevated P2EHA concentration in solution, leading to more
effective confinement of crystallization and greater enhancement
of crystal orientation, as evidenced by the reduction of misorientation angles as presented in Fig. 3(a).

ni ln4i in equation (3) accounts for the combinatorial

i¼1

entropy, GðT; 4; NÞ represents the non-combinatorial entropy and
enthalpy changes, g12, g13, and g23 are the interaction parameter of a
composition-related binary system, and g123 is the interaction
parameter in a ternary system. G is considered to be related to the

degree of polymerization (N) in a ternary blend system that contains a polymer component (i.e. P2EHA).
Based on the thermodynamics point of view, equations (3) and (4)
provide an insight into the kinetic interaction among the components existing in a ternary blend system (i.e. TIPS pentacene, P2EHA
and toluene as in this work). The long side group of P2EHA polymer
additive with eight carbon atoms is anticipated to contribute to a
3
P
moderate decrease of the combinatorial entropy ( ni ln4i )
i¼1

[36e38], but to a great reduction of GðT; 4; NÞ of the TIPS pentacene/
P2EHA blends, presumably due to a large enthalpy change. This facilitates the ternary system to form a phase separation during the
initial stage of crystallization (when DG < 0). Since crystallization

4. Conclusion
In summary, we have demonstrated an effective approach to
control crystallization, tune charge transport and improve performance consistency of TIPS pentacene based OTFTs by blending
P2EHA as a polyacrylate polymer additive. Vertical phase separation occurred between TIPS pentacene and P2EHA, resulting in an
effective confinement of the anisotropic crystallization and charge
transport of the semiconductor. At a loading ratio of 60%, an average
misorientation angle of 4.1 ± 1.8 , and grain width of
140.44 ± 19.82 mm were obtained. Increased grain width indicates
reduced grain boundaries and, correspondingly, less defects and
trap centers of charge carriers. Bottom-gate, bottom-contact OTFTs
based on the TIPS pentacene/P2EHA blends were fabricated and
exhibited an improved average mobility of 0.317 ± 0.047 cm2/V and
performance consistency of 6.72 (defined as the ratio of average
mobility to standard deviation of mobility), which is a 10-fold and
7-fold enhancement as compared to the pristine TIPS pentacene

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Z. He et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 467e472

OTFTs. Such a remarkable improvement in charge transport and
device performance of OTFTs can be attributed to the combination
effect of reduced crystal misorientation, enlarged grain width, and
enhanced film coverage of the TIPS pentacene film as a result of
P2EHA polyacrylate polymer additive.

Acknowledgments
S. Bi would like to thank Dalian University of Technology, China
DUT16RC(3)051.

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