Morphology Driven by Molecular Structure of Thiazole-Based Polymers for Use in Field-Effect Transistors and Solar Cells

Article abstract of DOI:10.1002/chem.201804803

The effects of the molecular structure of thiazole-based polymers on the active layer morphologies and performances of electronic and photovoltaic devices were studied. Thus, thiazole-based conjugated polymers with a novel thiazole-vinylene-thiazole (TzVTz) structure were designed and synthesized. The TzVTz structure was introduced to extend the π conjugation and coplanarity of the polymer chains. By combining alkylthienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) or dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) electron-donating units and a TzVTz electron-accepting unit, enhanced intermolecular interactions and charge transport were obtained in the novel polymers BDT-TzVTz and DTBDT-TzVTz. With a view to using the polymers in transistor and photovoltaic applications, the molecular self-assembly in and their nanoscale morphologies of the active layers were controlled by thermal annealing to enhance the molecular packing and by introducing a diphenyl ether solvent additive to improve the miscibility between polymer donors and [6,6]phenyl-C71-butyric acid methyl ester (PC₇₁BM) acceptors, respectively. The morphological characterization of the photoactive layers showed that a higher degree of π-electron delocalization and more favorable molecular packing in DTBDT-TzVTz compared with in BDT-TzVTz leads to distinctly higher performances in transistor and photovoltaic devices. The superior performance of a photovoltaic device incorporating DTBDT-TzVTz was achieved through the superior miscibility of DTBDT-TzVTz with PC₇₁BM and the improved crystallinity of DTBDT-TzVTz in the nanofibrillar structure.

Full text of DOI:10.1002/chem.201804803

A Journal of
Accepted Article
Title: Morphology Driven by Molecular Structure of Thiazole-Based
Polymers for Use in Field-Effect Transistors and Solar Cells
Authors: Jisu Hong, Canjie Wang, Hyojung Cha, Hyung Nam Kim,
Yebyeol Kim, Chan Eon Park, Tae Kyu An, Soon-Ki Kwon,
and Yun-Hi Kim
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201804803
Link to VoR: http://dx.doi.org/10.1002/chem.201804803
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Morphology Driven by Molecular Structure of Thiazole-Based
Polymers for Use in Field-Effect Transistors and Solar Cells
†
†
Jisu Hong,[a] Canjie Wang,[b] Hyojung Cha,[a] Hyung Nam Kim,[b] Yebyeol Kim,[a] Chan Eon Park,[a]
Tae Kyu An,*[c] Soon-Ki Kwon,*[d] and Yun-Hi Kim*[b]
†
Jisu Hong and Canjie Wang contributed equally to this work.
Abstract: We study the effects of molecular structure in thiazole- Introduction
based polymers on active layer morphologies and device
performances of electronic and photovoltaic devices. Therefore,
thiazole-based conjugated polymers with a novel thiazole-vinylene-
thiazole (TzVTz) structure are designed and synthesized. We
introduce the TzVTz structure to extend the π-conjugation and
coplanarity of the polymer chains. By combining alkylthienyl-
substituted BDT or DTBDT electron-donating units and a TzVTz
electron-accepting unit, enhanced intermolecular interactions and
charge transport are obtained from the novel polymers, BDT-TzVTz
and DTBDT-TzVTz. With a view to using the polymers in transistor
and photovoltaic applications, molecular self-assembly in and the
nanoscale morphologies of the BDT-TzVTz and DTBDT-TzVTz thin
films are controlled via thermal annealing to enhance molecular
packing and by introducing a diphenyl ether solvent additive to
improve miscibility between polymer donors and PCBM acceptors,
respectively. The morphology characterization on the photoactive
layers reflects that a higher degree of π-electron delocalization and
more favorable molecular packing of DTBDT-TzVTz than those of
BDT-TzVTz lead to distinctly higher performances in transistor and
photovoltaic devices. The superior performance of photovoltaic
device incorporating DTBDT-TzVTz is obtained from superior
miscibility of DTBDT-TzVTz with PCBM and improved crystallinity of
DTBDT-TzVTz in nano-fibrillar structure.
Extensive research has examined solution-processed organic
electronic devices in view of their advantages, which include light
weight, flexibility, and semi-transparency, as well as low
fabrication costs in mass production.[1-4] A variety of organic
semiconductors have been designed and synthesized, and their
optical, electrical, and chemical properties have been investigated
in an effort to realize efficient organic electronic devices.[5-8]
Significant research efforts have been devoted toward developing
donor–acceptor (D-A)-type copolymers with
a broad light
absorption spectrum and a high charge carrier mobility.[5,7,9,10]
Benzo[1,2-b:4,5-b′]dithiophene (BDT) and dithieno[2,3-d:2’,3’-
d’]-benzo[1,2-b:4,5-b’]dithiophene (DTBDT) derivatives have
been widely used as electron-donating units in polymer
semiconductors.[7,8,10-12] Composed of fused aromatic and planar
core units, these derivatives display high charge charier
mobilities.[10,11,13] Copolymers consisting of BDT and DTBDT have
been synthesized with a variety of electron-accepting units,
including thieno[3,4-b]thiophene (TT), diketopyrrolopyrrole (DPP),
thienopyrroledione (TPD), thazole, and benzodithiazole (BT).[10,12-
16]
The thiazole acceptor unit, featuring an electron-withdrawing
nitrogen on the imine, displayed a high planarity and efficient π-π
stacking.[17-19] Therefore, the superior charge transport properties
of copolymers that incorporate a thiazole unit have been reported
previously in the context of organic photovoltaic (OPV) device
applications.[20-22] However, bithiazole-based polymers have
struggled to show high performance due to the large-scale phase
separation between polymer donors and [6,6]phenyl-C71-butyric
acid methyl ester (PC71BM) acceptors.[15,23] Therefore, designing
thiazole-based polymers having better miscibility with PC71BM
and along with high crystallinity is required.
[a]
[b]
Prof. C. E. Park, J. Hong, H. Cha, Y. Kim
Department of Chemical Engineering
Pohang University of Science and Technology (POSTECH)
Pohang 790-784, Republic of Korea
Prof. Y.-H. Kim, C. Wang, H. N. Kim
Department of Chemistry and RINS
Gyeongsang National University
Jinju 660-701, Republic of Korea
E-mail: ykim@gnu.ac.kr
Here, we designed two copolymers, poly[(E)-5-(4,8-bis(4,5-
didecylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophen-2-yl)-2-(2-
(thiazol-2-yl)vinyl)thiazole] (BDT-TzVTz) or poly[(E)-5-(5,10-
bis(4,5-didecylthiophen-2-yl)benzo[1,2-b:4,5-b’]
dithieno[3,2-
[c]
[d]
Prof. T. K. An
b]thiophene)-2-(2-(thiazol-2- yl)vinyl)thiazole] (DTBDT-TzVTz),
for use in organic field-effect transistor (OFET) and OPV devices.
These two copolymers have alkylthienyl-substituted BDT or
DTBDT units as electron-donating units to improve miscibility with
PC71BM, and thiazole-vinylene-thiazole (TzVTz) as the electron-
accepting unit. We used alkylthienyl -substituted BDT or DTBDT
units, because the two-dimensional (2D) conjugated structure can
enhance light absorption and charge carrier transport
properties.[24] The molecular structures of BDT-TzVTz and
DTBDT-TzVTz are depicted in Scheme 1. The bithiazole-based
polymer device performances were improved by introducing a
novel TzVTz structure that provided appropriate electron-
Department of Polymer Science & Engineering and Department of
IT Convergence
Korea National University of Transportation
Chungju, 380-702, Republic of Korea
E-mail: taekyu1985@ut.ac.kr
Prof. S.-K. Kwon
Department of Materials Engineering and Convergence Technology
and ERI
Gyeongsang National University
Jinju 660-701, Republic of Korea
E-mail: skwon@gnu.ac.kr
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201804803
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withdrawing properties and long planar conjugated systems. This
structure, which featured vinylene groups between two thiazole
units, could extend the π-conjugation and coplanarity of the
polymer chains because the vinylene bonds between the two
thiazole units reduced steric hindrance on the conformation of the
rest of the polymer chain. By combining alkylthienyl-substituted
BDT or DTBDT electron-donating units and a TzVTz electron-
accepting unit, intermolecular interactions and charge transport in
copolymers and miscibility with PC71BM acceptor were expected
to improve.
For transistor and photovoltaic applications, molecular self-
assembly in and the nanoscale morphologies of the TzVTz-based
polymer or TzVTz-based polymer:PC71BM thin films were
controlled via thermal annealing and by introducing a diphenyl
ether (DPE) solvent additive, respectively. The self-assembly
behaviors and active layer morphologies of films prepared from
the TzVTz-based polymers were investigated to understand the
relationship between the molecular structure and device
performance. In OFET and OPV devices, distinctly higher
performances were obtained from DTBDT-TzVTz, which
displayed a higher degree of π-electron delocalization, stronger
crystallinity and better miscibility with PC71BM.
Scheme 1. Synthetic routes to BDT-TzVTz and DTBDT-TzVTz.
The UV-vis absorption spectra of the polymers in dilute chloroform
solutions and as thin films were recorded, as shown in Figures 1a
and 1b, respectively, to investigate the optical properties, as
summarized in Table 1. The maximum absorption bands
attributed to intramolecular charge transfer were observed at 535
and 571 nm for BDT- TzVTz in chloroform solution and at 552 and
588 nm for DTBDT-TzVTz, respectively. In the film state, the
absorption maxima were observed at 541 and 580 nm for BDT-
TzVTz and 548 and 587 nm for DTBDT-TzVTz, reflecting that
both polymers could form aggregates in solutions and displayed
good intermolecular interactions in the solution state as well as in
the solid state.[25-27] The optical band gaps of BDT-TzVTz and
DTBDT-TzVTz, estimated from the absorption onset, were 1.95
and 1.94 eV, respectively. The electrochemical properties were
studied using cyclic voltammetry (CV) in a chloroform solution to
determine the oxidation potentials and highest occupied
molecular orbital (HOMO) energy levels (Figure 1c). The HOMO
levels were calculated to be –5.28 eV for BDT-TzVTz and –5.29
eV for DTBDT-TzVTz, respectively. The lowest unoccupied
molecular orbital (LUMO) energy levels were estimated from the
HOMO levels and the optical band gaps.[28,29] The BDT-TzVTz
and DTBDT-TzVTz polymers provided LUMO levels of –3.33 and
–3.35 eV, respectively. The effects of introducing a vinylene group
between the thiazole units were investigated by obtaining the
HOMO and LUMO surface plots through density functional theory
(DFT) calculations using the B3LYP 6-311G** basis set for two
repeating units of each of BDT-TzVTz, DTBDT-TzVTz, BDT-TzTz,
and DTBDT-TzTz (Figure S8). The theoretical results predicted
that the new acceptor TzVTz-based units showed deeper LUMO
level and narrower band gaps than those in the TzTz-based units
because the presence of TzVTz favored the formation of a planar
conjugated structure and extended π-conjugation..
Results and Discussion
Materials Synthesis
The synthetic schemes used to produce the monomer and
polymers are depicted in Scheme 1. The new acceptor (E)-1,2-
bis(5-bromothiazol-2-yl)ethene was obtained by bromination of
TzVTz which was synthesized through a Stille coupling reaction
of (2-tributylstannyl)thiazole and 1,2-dichloroethene. The
polymerization was carried out through a Stille coupling reaction
in dichlorobenzene using Pd2(dba)3 and P(oTol)3. The
synthesized BDT-TzVTz and DTBDT-TzVTz polymers were
purified by Soxhlet extraction with methanol, acetone, hexane,
toluene, and chloroform. The extracted chloroform solution was
condensed and precipitated in methanol. The structures of the
monomers were characterized by H-NMR, 13C-NMR, and mass
spectroscopies (Figure S1–S6).
Material Properties
The polymers were soluble in common organic solvents, such as
chloroform and dichlorobenzene, after slightly heating the
polymer solutions. The molecular weights of the polymers were
measured by gel permeation chromatography (GPC) using a
polystyrene standard in tetrahydrofuran solvent. The weight
average molecular weights (Mw) of the BDT-TzVTz and DTBDT-
TzVTz polymers were 86.9k and 104k, with polydispersity indices
(PDIs) of 2.53 and 1.56, respectively. The thermal properties of
BDT-TzVTz and DTBDT-TzVTz were evaluated using
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC), as shown in Figure S7. BDT- TzVTz and
DTBDT-TzVTz were found to exhibit good thermal stabilities,
losing less than 5% of their weight at 431 °C, and 455 °C,
respectively, as determined by TGA under a nitrogen atmosphere.
The DSC plots of both polymers did not reveal melting or
crystalline transitions under 300 °C.
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(a)
(c)
interface between the BDT-TzVTz donor and the PC71BM
acceptor.[34-36] To decrease the domain size, DPE processing
additive was introduced to polymer:PC71BM blend solutions.
Previous studies reported that the DPE additive prevents severe
polymer and PC71BM aggregation and forms a bicontinuous
network in the blend systems.[37,38] Processing the BDT-
TzVTz:PC71BM blend films with the DPE additive decreased the
domain size and surface roughness of the active films, as
intended. The DTBDT-TzVTz:PC71BM blend films exhibited more
smooth and homogeneous surface morphologies compared to
the BDT-TzVTz:PC71BM blend films, although similar trends were
observed in the presence of the additive. The surface roughness
values decreased from 3.10 to 1.44 nm in the blend films prepared
without or with the additive, respectively.
(b)
Solution
Film
BDT-TzVTz
DTBDT-TzVTz
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
BDT-TzVTz
DTBDT-TzVTz
BDT-TzVTz
DTBDT-TzVTz
300
400
500
600
700
800
300
400
500
600
700
800
0.0
0.5
1.0
1.5
2.0
Wavelength (nm)
Wavelength (nm)
Potential (V) vs Ag/AgCl
Figure 1. Normalized UV-vis-NIR absorption spectra of the TzVTz-based
polymers (a) in a CHCl3 solution, and (b) in a thin film. (c) Cyclic voltammograms
of the polymers.
Table 1. Optical and electrochemical properties of the TzVTz-based
polymers.
λ
(nm)
solutio
n
λ
max
λ
HOMO[
LUMO[
max
opt[a]
Polym
er
Eg
onset
(nm)
film
b]
c]
(nm)
film
(eV)
(eV)
(eV)
BDT-
TzVTz
535
541
634
637
1.95
1.94
-5.28
-5.29
-3.33
-3.35
571(s)[d]
580 (s)[d]
DTBDT 552
-TzVTz
588(s)[d]
548
587 (s)[d]
[a] Estimated from the absorption edge in the film state (Egopt=1240/ λonset).
[b] Estimated from the onset oxidation potential. [c] Determined by the
opt
HOMO + Eg
.
[26,27]. [d] Peak positions of shoulders in absorption spectra.
Active Layer Morphology
Morphological analyses were conducted to understand the
relationship between molecular structure and aggregation
behavior of TzVTz-based polymers. The surface morphology of
the TzVTz-based polymer thin film was characterized using
atomic force microscopy (AFM) as shown in Figure S9. Although
the two polymers gave similar morphological features, DTBDT-
TzVTz displayed a more aggregated surface morphology with a
higher surface roughness. The root-mean-square (RMS)
roughness values of the BDT-TzVTz and DTBDT-TzVTz thin films
were 0.54 and 1.18 nm, respectively. Greater aggregation within
the DTBDT-TzVTz polymer was thought to originate from the
stronger intermolecular interactions in DTBDT-TzVTz,[30,31] which
featured extended conjugation and a high planarity among the
DTBDT units.[32,33] The longer conjugation length and enhanced
molecular packing of DTBDT-TzVTz are expected to result in a
more efficient channel for charge carrier transport.
The aggregation and phase separation behaviors of the polymers
in polymer:PC71BM bulk heterojunction (BHJ) blend films were
characterized using AFM and transmission electron microscopy
(TEM). The AFM images shown in Figure 2 reveal strong
aggregation in the TzVTz-based copolymers blended with
PC71BM. The strong self-assembly properties of the TzVTz-based
polymers induced the formation of large domains with a high
roughness upon blending with PC71BM. In the BDT-
TzVTz:PC71BM blend film, large domains with domain sizes of
hundreds of nanometers and rough surfaces with an RMS surface
roughness of 17.6 nm were observed. This result indicated
inefficient exciton dissociation in the blend films due to the limited
Figure 2. AFM images of polymer:PC71BM blend films spin-coated (a,c) without
or (b,d) with the 3% DPE additive.
The TEM measurements revealed the bulk morphologies of the
blend films (Figure 3). The large domains observed in AFM
images of the BDT-TzVTz:PC71BM film were also observed in
TEM images and indicated PC71BM-rich domains. The large
domain size and unconnected pathways in the PC71BM-rich
domains suggested the presence of unfavorable exciton
dissociation and charge transport properties of the BDT-
TzVTZ:PC71BM blend film prepared without DPE, which can lead
to low photovoltaic performances.[34,37,39] The addition of DPE to
the BDT-TzVTz:PC71BM blends decreased the domain size,
thereby increasing the chance of exciton dissociation.[34-36] The
DTBDT-TzVTz:PC71BM blend films displayed homogeneous bulk
morphologies compared to that of BDT-TzVTz:PC71BM. Although
the DTBDT-TzVTz copolymer displayed a stronger crystallinity, it
provided superior miscibility with the PC71BM acceptor compared
to BDT-TzVTz. These results may be interpreted in terms of the
molecular structures of the two TzVTz-based copolymers.
Compared to BDT-TzVTz, the bulky donor units in DTBDT-TzVTz
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are separated by a larger space, and this structure provides
adequate space to allow PC71BM intercalation into the polymers.
The facile intercalation of PC71BM appears to have contributed to
the homogeneous morphology.[40,41] The addition of DPE to the
DTBDT-TzVTz:PC71BM blend film induced the formation of small
domains and a nano-fibrillar structure. Within the DPE-containing
DTBDT-TzVTz:PC71BM blend film, the photogenerated excitons
profiles in the in-plane direction were extracted to compare the
peak position and intensity (Figure S12). The amorphous halo at
q ≈ 1.31 Å–1 was attributed to the PC71BM domains.[45,46] The
(h00) peaks in the scattering patterns of the BDT-TzVTz:PC71BM
blend films prepared with or without the DPE additive appeared at
q ≈ 0.32 and 0.65 Å–1, giving a d-spacing of 1.93 nm. The (010)
π-π stacking peak was observed at q ≈ 1.86 Å–1, and the π-π
stacking distance was 3.38 Å. In the DTBDT-TzVTz:PC71BM
blend films prepared with or without the DPE additive, the (h00)
peaks were observed at q ≈ 0.29 and 0.59 Å–1, giving a d-spacing
of 2.13 nm. The increased d-spacing of DTBDT-TzVTz seems to
originate from the larger core of DTBDT than BDT. The π-π
stacking peak was observed at q ≈ 1.84 Å–1, indicating a π-π
stacking distance of 3.41 Å.
can have
a greater chance of dissociating due to the
homogeneously blended morphologies, and the crystalline
interpenetrating structure can facilitate charge transport.[34,37,42]
The (h00) peaks displayed an slightly enhanced intensity in the
case of the DTBDT-TzVTz:PC71BM films compared to the BDT-
TzVTz:PC71BM films. The maintenance of crystallinity in the
DTBDT-TzVTzPC71BM blends which exhibited better miscibility
with PC71BM compared to those of BDT-TzVTz:PC71BM blends
was attributed to the copolymer design strategy. The extended
conjugated core and improved planarity of the backbone in the
DTBDT-TzVTz polymer facilitated crystallization. These results
agreed with the higher photovoltaic performances measured in
the DTBDT-TzVTz:PC71BM photovoltaic devices compared to the
performances in the BDT-TzVTz-based devices, in that the
improved miscibility and maintained crystallinity improve the
photovoltaic properties.[3,34,37] In addition, the (100) and (200)
polymer peaks in the scattering patterns of the blend films
processed with the DPE additive were slightly stronger in intensity,
indicating a stronger degree of crystallinity. The enhanced
crystallinity of the BDT-TzVTz and DTBDT-TzVTz polymers in the
DPE-containing blend films may improve the charge transport
properties of the DPE-containing device.
A
morphological analysis revealed the superior miscibility
between DTBDT-TzVTz and PC71BM and the greater crystallinity
in the DTBDT-TzVTz film. The polymer:PC71BM active layers
optimized with the DPE additive exhibited a better miscibility and
stronger crystallinity, which can result in efficient exciton
dissociation and charge carrier transport. The small domains and
nano-fibrillar crystalline structures of the DPE-added DTBDT-
TzVTz:PC71BM device accounted for the promising photovoltaic
performance of the TzVTz-based polymer discussed below.
Figure 3. TEM images of polymer:PC71BM blend films processed (a,c) without
or (b,d) with the 3% DPE additive.
Photoluminescence (PL) measurements were carried out to
measure the exciton dissociation properties in the four
polymer:PC71BM blend films (Figure S10). The PL spectra of the
DTBDT-TzVTz:PC71BM blends exhibit higher photoluminescence
quenching efficiencies (PLQE) with lower PL intensities compared
to the BDT-TzVTz blends and indicate that the excitons were well-
dissociated into charge carriers in the blend films as the emission
was efficiently quenched due to the larger interface between the
DTBDT-TzVTz and PC71BM.[43,44] The PL spectra of the DPE-
containing blend films, which displayed low PL intensities,
showed that exciton dissociation was more efficient in the blend
films prepared with DPE, as expected from the AFM and TEM
measurements.
The crystalline structures of the TzVTz-based polymers in the
polymer:PC71BM blend films were investigated in search of a
deeper understanding of the structural effects on the blend films
morphologies and photovoltaic characteristics. Grazing incidence
wide angle x-ray scattering (GIWAXS) studies were performed,
as shown in Figure 4. The TzVTz-based polymer peaks in the 2D-
GIWAXS patterns indicated that the edge-on orientation was
dominant for the polymers in blend films, whereas randomly
oriented crystals were evident in the scattering patterns. Line-cut
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-1.2x10-6
-8.0x10-7
-6.0x10-6
-4.0x10-6
(b)
(a)
VG
(a) BDT-TzVTz
(w/o DPE)
(b) BDT-TzVTz
(w DPE)
0
-20
-40
-60
-80
-100
-4.0x10-7
0.0
-2.0x10-6
0.0
0
-20 -40 -60 -80 -100
0
-20 -40 -60 -80 -100
VDS [ V ]
VDS [ V ]
(d) DTBDT-TzVTz
(w DPE)
(c) DTBDT-TzVTz
(w/o DPE)
1.5x10-3
1.0x10-3
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
10-13
6.0x10-3
4.0x10-3
10-4
10-5
10-6
10-7
10-8
10-9
(c)
(d)
2.0x10-3
0.0
5.0x10-4
0.0
qxy (Å-1)
qxy (Å-1)
-90 -60 -30
0
30
-90 -60 -30
0
30
VG [ V ]
VG [ V ]
Figure 4. 2D-GIWAXS patterns obtained from the polymer:PC71BM blend films
processed (a,c) without or (b,d) with the DPE additive.
Figure 5. (a, b) Output, and (c, d) transfer characteristics of the best performed
OFET devices incorporating (a, c) BDT-TzVTz and (b, d) DTBDT-TzVTz.
Organic Field-Effect Transistor Properties
The charge transport properties of the BDT-TzVTz and DTBDT-
TzVTz polymers and the potential utility of the TzVTz-based
polymers as active materials in OFET devices were explored by
fabricating BDT-TzVTz and DTBDT-TzVTz-based OFETs using a
spin-coating method. The extracted OFET parameters are
summarized in Table 2. As shown in Figure 5 and Figure S11, the
devices displayed typical p-channel transfer characteristics.
Initially, the field-effect mobility (μh) of the as-spun BDT-TzVTz
film was 1.2 × 10–3 cm2 V–1 s–1 (Ion/Ioff = 1.0 × 105) in the saturation
regime. Its hole mobility increased to 2.5 × 10–3 cm2 V–1 s–1 (Ion/Ioff
= 9 × 104) as the annealing temperature increased to 200 °C. For
the DTBDT-TzVTz-based OFET, the mobility values of the as-
spun film and the annealed film at 150 °C were 2.58 × 10–2 cm2
V–1 s–1 (Ion/Ioff = 1 × 104) and 3.68 × 10–2 cm2 V–1 s–1 (Ion/Ioff = 2 ×
103), respectively. These OFET performances of BDT-TzVTz and
DTBDT-TzVTz dovetailed with the UV-vis absorption spectra and
AFM data. The longer conjugation length of the DTBDT unit
compared to the BDT unit provided a higher degree of π-electron
delocalization and more favorable molecular packing in the
DTBDT-TzVTz film than in the BDT-TzVTz film.[33,47]
Table 2. OFET device characteristics of the TzVTz-based polymers.
µavg
µmax
Annealing
temp. (°C)
on/off
ratio
Vth
(V)
Polymer
(cm2 V–1
(cm2 V–1
s–1
)
s–1
)
As cast
100
0.0011
0.0012
0.0018
0.0022
0.0215
0.0234
0.0315
0.0153
0.0012
0.0013
0.0019
0.0025
0.0258
0.0295
0.0368
0.0201
1 × 105
2 × 104
3 × 104
9 × 104
1 × 104
4 × 103
2 × 103
7 × 103
-14
-9
BDT-
TzVTz
150
-10
-10
8
200
As cast
100
3
DTBDT-
TzVTz
150
0
200
29
Photovoltaic Characteristics
The photovoltaic properties of the TzVTz-based polymers were
explored in conventional solar cells using a device configuration
comprising
ethylenedioxythiophene):poly(styrene-sulfonate)
(PEDOT:PSS)/polymer:PC71BM/LiF/Al. The
glass/indium
tin
oxide
(ITO)/poly(3,4-
photovoltaic
performances are summarized in Figure 6 and Table 3. An active
layer composed of TzVTz-based polymers and PC71BM was
optimized at a blend ratio of 1:4 (w/w) as shown in Figure S13 and
Table S1 and a concentration of 30 mg ml–1 in chlorobenzene.
The photovoltaic devices processed with 3% (v/v) DPE additive
provided improved PCEs with a higher short-circuit current (Jsc)
and fill factor (FF). The BDT-TzVTz:PC71BM device prepared
without DPE showed an average PCE of 1.31% with an open-
circuit voltage (Voc) of 0.78 V, a Jsc of 3.93 mA cm–2, and a FF of
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10.1002/chem.201804803
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Figure 6. (a) J-V characteristics of the polymer:PC71BM BHJ solar cells
processed without or with the DPE additive, and (b) EQE spectra obtained from
the corresponding devices.
0.43. The BDT-TzVTz photovoltaic device prepared with DPE
exhibited an average PCE of 1.99% with a Voc of 0.77 V, a Jsc of
5.53 mA cm–2, and a FF of 0.47. The higher Jsc and FF values
measured in the photovoltaic devices prepared with DPE agreed
well with the improved morphologies of the BDT-TzVTz:PC71BM
blend films discussed above. As the DPE additive alleviate the
severe phase separation between polymer donor and PC71BM
acceptor, which can limit the exciton diffusion to the interface, the
higher device performances were achieved. The DTBDT-TzVTz
device prepared without DPE showed an average PCE of 4.27%
with a Voc of 0.79 V, a Jsc of 8.74 mA cm–2, and a FF of 0.62. As
with the BDT-TzVTz device, a distinctly higher performance was
achieved in the DTBDT-TzVTz device processed with the DPE
additive. This device displayed an average PCE of 6.58%, a Voc
of 0.79, a Jsc of 13.17 mA cm–2, and a FF of 0.64. The highest
PCE obtained from the DTBDT-TzVTz:PC71BM device was
6.73%. This PCE value is among the highest yet recorded in
OPVs that incorporate thiazole-based copolymers. The superior
photovoltaic performances of DTBDT-TzVTz compared to those
of BDT-TzVTz, particularly the higher Jsc and slightly increased
FF, appeared to derive from the active layer morphology and
higher hole mobility of DTBDT-TzVTz, considering that the two
TzVTz-based copolymers displayed comparable light absorption
properties and energy levels. The homogeneous morphology of
DTBDT-TzVTz:PC71BM blend films efficient for exciton
dissociation is considered to be the main factor underlying the
superior performance of the DTBDT-TzVTz:PC71BM photovoltaic
device. The moderate domain size and nano-fibrallar structure in
DTBDT-TzVTz:PC71BM devices processed with DPE are in
agreement with photovoltaic characteristics. The external
quantum efficiency (EQE) curves of TzVTz-based photovoltaic
devices shown in Figure 6b exhibited photoresponse from the
wave length of 300 to 700 nm. The photovoltaic devices showed
balanced photoresponse to the wavelength of 650 nm, but the
limited photoresponse in longer wavelength region can be
responsible for the limited overall EQE and Jsc values of the
devices. As the EQE curves were obtained from the devices with
polymer:PC71BM blend ratio of 1:4, the photoresponse in light
absorption region of PC71BM were slightly prominent. The two
TzVTz-based polymers showed similar trend of EQE curves as a
function of the wavelength, which indicates similar light absorption
properties of TzVTz-based devices. The distinctly increased
intensity in EQE curves of DTBDT-TzVTz-based devices reflects
that the charge carrier generation was more efficient in these
devices as discussed above. The EQE also increased as a result
of introducing DPE, and the curves according to the wavelength
revealed that the DPE additive affected charge transfer or
transport rather than light absorption characteristics.
Table 3. Photovoltaic characteristics of the TzVTz-based polymer:PC71BM
BHJ solar cells.
Jsc
(mA
cm–2
PCEavg
(%)
PCEmax
(%)
Polymer
Additive
Voc (V)
FF
)
0.78
± 0.01
3.93
± 0.18
0.43
± 0.01
1.31
± 0.10
w/o DPE
w DPE
1.41
2.11
BDT-
TzVTz
0.77
± 0.01
5.53
± 0.22
0.47
± 0.01
1.99
± 0.11
0.79
± 0.01
8.74
± 0.14
0.62
± 0.01
4.27
± 0.08
w/o DPE
w DPE
4.40
6.73
DTBDT-
TzVTz
0.79
± 0.01
13.17
± 0.21
0.64
± 0.01
6.58
± 0.18
The average values and standard deviations were obtained from 8 devices.
Conclusions
To investigate the effects of molecular structure in thiazole-based
polymers on the crystalline behavior and device characteristics, a
series of copolymers with thiazole-vinylene-thiazole as the
electron-accepting unit, combined with BDT or DTBDT electron-
donating units, was synthesized. The extended coplanarity of the
TzVTz-based copolymers enhanced the intermolecular
interactions and charge transport properties investigated in OFET
and OPV devices. As DTBDT-TzVTz showed higher degree of π-
electron delocalization and better molecular packing than BDT-
TzVTz, higher hole mobilities were obtained from DTBDT-TzVTz.
The BDT-TzVTz and DTBDT-TzVTz polymers exhibited charge
carrier mobilities of 2.5 × 10–3 cm2 V–1 s–1 and 3.7 × 10–2 cm2 V–1
s–1 in OFET devices, respectively. The active layer of OPV
devices prepared with the TzVTz-based polymers and PC71BM
were optimized by adding the DPE solvent additive. The DPE
additive prevented the formation of large domains favored by the
planar structure and strong aggregation behaviors of the
polymers, thereby improving the device performances. The
TzVTz-based OPV devices gave the highest PCEs of 2.11% for
BDT-TzVTz and 6.73% for DTBDT-TzVTz. The morphological
characteristics suggested that the distinctly better performance of
the DTBDT-TzVTz film arose from its stronger crystallinity and
superior miscibility with PC71BM in the active layer.
3
0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(a)
(b)
BDT-TzvTz (w/o DPE)
BDT-TzvTz (w DPE)
DTBDT-TzvTz (w/o DPE)
DTBDT-TzvTz (w DPE)
BDT-TzVTz
w/o DPE
w DPE
DTBDT-TzVTz
w/o DPE
Experimental Section
-3
w DPE
-6
OFET Device Fabrication and Mobility Measurements
-9
-12
OFETs based on BDT-TzVTz and DTBDT-TzVTz were fabricated using
heavily N-doped silicon with a 300 nm thick thermally grown SiO2 dielectric
layer. All OFET devices employed a bottom-gate/top-contact configuration,
and polymer films were deposited onto ODTS-modified Si/SiO2 substrates
along with gold source/drain electrodes (with a channel region 50 μm in
-15
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
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10.1002/chem.201804803
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length and 1000 μm in width). Before modifying the silicon oxide surface
with ODTS, the silicon oxide surface was cleaned with piranha solution
(H2O2 (40 mL)/ concentrated H2SO4 (60 mL)) for 20 min at 250 °C, rinsed
with distilled water several times, and ozone-treated for 20 min. BDT-
TzVTz and DTBDT-TzVTz polymers were deposited onto the chemically
modified SiO2 surfaces by spin-coating at 2000 rpm from 0.2 wt% 1,2-
dichlorobenzene solutions. The OFET devices were annealed for 10 min
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF)
funded
by
the
Ministry
of
Education
(2018R1A6A1A03023788) and supported by a grant from the
Center for Advanced Soft Electronics under the Global Frontier
Research Program of the Ministry of Education, Science, and
Technology (2013M3A6A5073175). This research was also
supported by the NRF grant funded by the Korean government
(MSIP) (2015R1A2A1A10055620) and by the New & Renewable
Energy Program of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) and MOTIE of the Republic of
Korea (20173010013000).
under
a nitrogen atmosphere. The electrical characteristics of the
fabricated OFETs were measured in air using Keithley 2400 and 236
source/measure units. Field-effect mobility values were extracted in the
saturation regime from the slope of the source–drain current.
Photovoltaic Device Fabrication and Characterization
The photovoltaic devices were fabricated to have
a structure of
Keywords: Thiazole-based copolymer • Intermolecular
interaction • Bulk heterojunction morphology • Organic field-
effect transistor • Polymer solar cell
glass/ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al. The patterned ITO
substrates were pre-cleaned with detergent, deionized water, acetone,
and isopropyl alcohol in an ultrasonic bath. After residual solvents were
removed using pressurized nitrogen, the substrates were exposed to UV-
ozone for 30 min. A PEDOT:PSS layer was then deposited onto the
substrate by spin-coating at 4000 rpm for 60 s and baking at 120 °C for 30
min. The active solutions were prepared at a weight ratio of 1:4 and a
concentration of 30 mg mL–1 in chlorobenzene with 3% (v/v) DPE. The
solutions were stirred overnight in nitrogen-filled glove box, filtered through
a 5 μm-polytetrafluoroethylene (PTFE) filter, and spin-coated onto the
PEDOT:PSS layer to have a film thickness of ca. 100 nm. Subsequently,
0.8 nm LiF and 100 nm Al layers were deposited as the top electrode by
thermal evaporation under 3×10–6 Torr. The J-V characteristics were
measured using a Keithley Model 2400 source measurement unit, and a
450W Xenon lamp (Oriel) with an AM 1.5 G filter was used as the solar
simulator. The light intensity was calibrated to 100 mW cm–2 using a silicon
cell calibrated with a KG5 filter, which was traced to the National
Renewable Energy Laboratory (NREL). The external quantum efficiency
(EQE) curves were obtained using a photomodulation spectroscopy setup
(model Merlin, Oriel).
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Entry for the Table of Contents
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Jisu Hong, Canjie Wang, Hyojung Cha,
Hyung Nam Kim, Yebyeol Kim, Chan
Eon Park, Tae Kyu An,* Soon-Ki Kwon,*
and Yun-Hi Kim*
Page No. – Page No.
Morphology Driven by Molecular
Structure of Thiazole-Based Polymers
for Use in Field-Effect Transistors
and Solar Cells
We studied the effects of molecular structure in thiazole-based polymers on active
layer morphologies and device performances of electronic and photovoltaic devices
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