Processing temperature control of a diketopyrrolopyrrole-alt-thieno[2,3-b]thiophene polymer for high-mobility thin-film transistors and polymer solar cells with high open-circuit voltages

Article abstract of DOI:10.1016/j.polymer.2016.10.024

We synthesized a planar pDPPTTi-OD polymer based on diketopyrrolopyrrole (DPP) and thieno [2,3-b]thiophene (TTi) and investigated the electrical properties of the pDPPTTi-OD polymer. pDPPTTi-OD films displayed a low optical bandgap of 1.57?eV, and HOMO and LUMO levels of??5.40 and??3.74?eV, respectively. The 150°C-annealed pDPPTTi-OD films showed a high hole mobility of 0.16?cm²V^?1s^?1 in organic thin-film transistor (OTFT) devices. The photovoltaic properties of polymer solar cells (PSCs) incorporating the pDPPTTi-OD were also measured. A pDPPTTi-OD:PC₇₁BM blend film was spin-coated at 25, 70 and 90?°C. High-temperature processing significantly improved the power conversion efficiency of PSCs by effectively reducing the PC₇₁BM domain sizes, which improved the miscibility between pDPPTTi-OD and PC₇₁BM. This work demonstrated that the TTi moiety is a useful donor building block for high- performance D–A type polymers in OTFTs and PSCs, and that processing temperatures should be controlled to fully realize the materials’ beneficial intrinsic properties.

Full text of DOI:10.1016/j.polymer.2016.10.024

Polymer 105 (2016) 79e87
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Processing temperature control of a diketopyrrolopyrrole-alt-thieno
[2,3-b]thiophene polymer for high-mobility thin-film transistors and
polymer solar cells with high open-circuit voltages
Hyo-Sang Lee , Joong Suk Lee , A-Ra Jung , Wonsuk Cha f, Hyunjung Kim e,
a
,
b
c
d
e,
a,
b
c
d, *
Hae Jung Son , Jeong Ho Cho , BongSoo Kim
a
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea
Green School (School of Energy and Environment), Korea University, Seoul 02841, Republic of Korea
SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), Department of Chemical Engineering,
b
c
Sungkyunkwan University, Suwon 440-746, Republic of Korea
d
Department of Science Education, Ewha Womans University, Seoul 03760, Republic of Korea
Department of Physics, Sogang University, Seoul 04107, Republic of Korea
Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA
e
f
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2016
Received in revised form
We synthesized a planar pDPPTTi-OD polymer based on diketopyrrolopyrrole (DPP) and thieno [2,3-b]
thiophene (TTi) and investigated the electrical properties of the pDPPTTi-OD polymer. pDPPTTi-OD films
displayed a low optical bandgap of 1.57 eV, and HOMO and LUMO levels of ꢀ5.40 and ꢀ3.74 eV,
4
October 2016
ꢁ
2 ꢀ1 ꢀ1
respectively. The 150 C-annealed pDPPTTi-OD films showed a high hole mobility of 0.16 cm V s in
Accepted 11 October 2016
Available online 13 October 2016
organic thin-film transistor (OTFT) devices. The photovoltaic properties of polymer solar cells (PSCs)
incorporating the pDPPTTi-OD were also measured. A pDPPTTi-OD:PC71BM blend film was spin-coated at
ꢁ
2
5, 70 and 90 C. High-temperature processing significantly improved the power conversion efficiency of
Keywords:
Processing temperature
Carrier mobility
PSCs by effectively reducing the PC71BM domain sizes, which improved the miscibility between pDPPTTi-
OD and PC71BM. This work demonstrated that the TTi moiety is a useful donor building block for high-
performance DeA type polymers in OTFTs and PSCs, and that processing temperatures should be
controlled to fully realize the materials' beneficial intrinsic properties.
Photovoltaic performance
©
2016 Elsevier Ltd. All rights reserved.
1. Introduction
high open-circuit voltage (Voc). While oligothiophenes or benzo
1,2-b:4,5-b’]dithiophenes have frequently been used as donors,
[
Conjugated polymers have been developed for use in high-
performance polymer solar cells (PSCs). The photovoltaic proper-
ties of such PSCs have great potential in low-cost, light-weight
flexible applications such as future energy sources [1,2]. Intensive
synthetic efforts to synthesize donor (D)-acceptor (A)-type poly-
mers have significantly improved power conversion efficiencies
diketopyrrolopyrroles (DPPs) [3,6e8] and benzothiadiazole (BTs)
[4,9] have been used as acceptors. Although BT-based DeA-type
polymers currently outperform DPP-based polymers, the latter are
particularly advantageous in terms of absorbing low-energy pho-
tons. Hence, they are good candidates for visibly transparent solar
cells and tandem solar cells [10e13]. A limiting photovoltaic
parameter in most DPP-based photovoltaic devices is the low Voc
value (typically < 0.7 V) [6,14e17]. This is mainly because DPPs as
acceptor moieties were often connected alternately with thieno
[3,2-b]thiophenes or dialkoxybenzo [1,2-b:4,5-b’]dithiophenes,
which provided planar polymer backbones. These structured
polymers displayed low bandgaps of 1.2e1.31 eV, high-lying HOMO
levels ranging from ꢀ5.10 to ꢀ5.16 eV with high carrier mobilities
(
PCEs) [2e5]. The connection of electron-rich donor moieties with
electron-deficient acceptor moieties can lower an optical bandgap
and optimally position the highest occupied molecular orbital
(
HOMO) and lowest unoccupied molecular orbital (LUMO) levels
for broader absorption of sunlight, efficient charge separation and a
2
ꢀ1 ꢀ1
over 1 cm V
s
[6,7,14,18,19]. To approach a predicted high-PCE
*
Corresponding author.
zone, DPPs were polymerized with phenyl (a weak donor) linkages.
E-mail address: bongsoo@ewha.ac.kr (B. Kim).
http://dx.doi.org/10.1016/j.polymer.2016.10.024
032-3861/© 2016 Elsevier Ltd. All rights reserved.
0

80
H.-S. Lee et al. / Polymer 105 (2016) 79e87
This connection resulted in tuning the bandgap and HOMO level of
the DPP-based polymers positively for high photovoltaic properties
pyrrole-1,4(2H,5H)-dione) (pDPPTTi-OD): To a 100 mL flame-dried
reaction flask with a magnetic bar, M1 (0.6564 g, 0.6440 mmol), M2
(0.30 g, 0.6440 mmol), 10 mL of anhydrous toluene, and 1 mL of
anhydrous DMF were added. The reaction mixture was degassed by
freeze-pump-thaw three cyclings. Tetrakis (triphenylphosphine)
[19e22]. However, the phenyl linkage may disfavor efficient charge
transport because of increased dihedral angles along the polymer
backbone [23,24].
In searching for new and appropriate donor moieties that would
provide both high backbone planarity and deep-lying HOMO levels,
the thieno [2,3-b]thiophene (TTi) moiety is an interesting donor
candidate and it has been rarely used in the synthesis of DeA-type
polymers. The literature indicates that the TTi moiety is a less
electronically communicating moiety compared with thieno [3,2-b]
thiophene, which can keep the HOMO levels of DPP polymers low
palladium (0) (Pd(PPh
to the reaction mixture, which was stirred at 110 C for 20 h under
an argon atmosphere. The reaction mixture was poured into a
3
)
4
) (0.0223 g, 0.0193 mmol) was then added
ꢁ
2
methanol/H O (4/1 v/v%) solution. The precipitated polymer was
redissolved in chloroform and reprecipitated in MeOH. The
collected polymer was further purified by Soxhlet extraction using
methanol, acetone, hexane, and chloroform. The chloroform frac-
tion was collected, reprecipitated in methanol, and then filtered.
[25,26]. Moreover, the linkage of DPPs with TTi units in the polymer
1
backbone would provide planarity, which could in turn provide
The polymer was dried under vacuum to yield 0.38 g (60%). H NMR
good charge transport behavior in organic thin-film transistors
3
(CDCl , 400 MHz) d (ppm) 9.32e8.69 (s, 2H), 7.63e6.48 (m, 4H),
(
OTFTs) [17,18].
4.37e3.22 (m, 4H), 2.28e0.59 (m, 78H); GPC M
w
¼ 22,500 g/mol,
1
Herein we report the synthesis and electrical properties of an
PDI ¼ 2.31. ( H NMR and GPC data of pDPPTTi-OD polymer are
alternating polymer of poly (2,5-bis(2-octyldodecyl)-3-(5-(thieno
shown in the supplementary information.)
[
[
2,3-b]thiophen-2-yl)thiophen-2-yl)-6-(thiophen-2-yl)pyrrolo
3,4-c]pyrrole-1,4(2H,5H)-dione) (pDPPTTi-OD) based on TTi and
2.3. Material characterization
DPP units. The synthesized pDPPTTi-OD polymer had a low optical
bandgap of 1.57 eV and a HOMO level of ꢀ5.40 eV. A good hole
1H NMR spectra were recorded on a Bruker advance 400 spec-
trometer (400 MHz). The molecular weights of the polymers were
2
ꢀ1 ꢀ1
s was measured for the OTFT device and
mobility of 0.16 cm V
a PCE of 2.9% was achieved via control of the processing
temperature.
measured by gel permeation chromatography (GPC, Waters) at
ꢁ
80 C using o-dichlorobenzene as an eluent and polystyrene as a
standard. UVevisible absorption spectra were obtained on a Perkin
Elmer Lambda 9 UVeVIS spectrophotometer. Differential scanning
calorimetry (DSC) was taken on a Perkin-Elmer Pyris 1 DSC in-
2
. Experimental section
ꢁ
ꢁ
2.1. Materials
strument at a rate of 10 C/min between 30 and 300 C under ni-
trogen. Thermogravimetric analysis was conducted on a TA TGA
2100 thermogravimetric analyzer under nitrogen at a scan rate of
Thieno [2,3-b]thiophene (TTi), 1.6 M n-butyllithium (n-BuLi) in
0
ꢁ
hexane, N,N,N ,N'-tetramethylethylenediamine (TMEDA), 1.0 M
10 C/min. Cyclic voltammetry (CV) was taken on a CH instruments
trimethyltin chloride solution in tetrahydrofuran (THF), tetrakis
electrochemical analyzer, and a degassed acetonitrile solution
(
triphenylphosphine)palladium (0) (Pd(PPh
3
)
4
) were purchased
containing 0.1
M tetrabutylammonium hexafluorophosphate
from Sigma-Aldrich, Acros, and TCI. 3,6-bis(5-bromothiophen-2-
yl)-2,5-bis(2-octyldodecyl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione
(TBAPF , Sigma-Aldrich) as the electrolyte was used. The potential
sweep rate was 50 mVs . A Pt wire electrode coated with a thin
6
ꢀ1
(
M2) was synthesized as reported previously [18]. Common organic
film of pDPPTTi-OD polymer was used as the working electrode,
another Pt wire was used as the counter electrode, Ag/Ag was the
þ
solvents were purchased from Daejung, J. T. Baker, and Sigma-
Aldrich. THF was dried over sodium and benzophenone prior to
use. All other reagents were used as received without further
purification.
reference electrode, and ferrocene was used as the internal stan-
dard for potential calibration (ꢀ4.8 eV). The crystalline structures of
the ambipolar polymer films were characterized by grazing inci-
dence X-ray diffraction (GIXD) experiments at the Pohang Light
Source II (9A beamline, X-ray wavelength 0.1115 nm, X-ray inci-
2
.2. Synthesis
ꢁ
dence angle 0.13 ) in Korea. The morphology of polymer:PC71BM
Synthesis of 2,5-bis(trimethylstannyl)thieno [2,3-b]thiophene:
thieno [2,3-b]thiophene (0.950 g, 6.775 mmol) was added to a
50 mL round-bottomed flask. Anhydrous THF (60 mL) was added
blend films examined by transmission electron microscopy (TEM)
(Philips CM-30, 200 kV) and the film surface was probed by tapping
mode atomic force microscopy (AFM) (D3100 Nanoscope V, Veeco).
2
to the 250 mL round-bottomed flask, which was then stirred
ꢁ
at ꢀ78 C using a dry ice/acetone bath. 1.6 M n-BuLi solution in
2.4. OTFT fabrication and characterization
hexane (1.585 mL, 16.936 mmol) dropped dropwise and TMEDA
(
1.9680 g, 16.936 mmol) was added. The dry ice/acetone bath was
To examine electrical property of pDPPTTi-OD polymer films,
bottom-gate top-contact OTFT devices were fabricated. Heavily
doped n-type Silicon (Si) wafers having a thermally grown 300 nm
removed and the reaction flask was warmed up to room temper-
ature slowly and stirred for 2 h. The reaction flask was cooled again
ꢁ
to ꢀ78 C and trimethyltin chloride solution in THF (3.3071 g,
thick silicon oxide (SiO
2
) layer were employed as the substrate,
1
6.936 mmol) was added dropwise. This reaction mixture was
where the doped Si wafer part acted as a OTFT gate and the 300 nm
warmed up to room temperature and stirred for 4 h. After reaction
completion, 50 mL of H O was added to the reaction mixture, which
was extracted with 200 mL of CHCl three times. The combined
organic layer was dried with anhydrous MgSO . The chloroform
SiO
2
layer worked as a gate insulator. The SiO
2
surface was cleaned
ꢁ
2
in piranha solution for 30 min at 100 C and washed with distilled
water. A SiO layer was then modified using octadecyltri-
2
3
4
chlorosilane (ODTS, Gelest, Inc) to be hydrophobic. A 90 nm thick
layer of the ambipolar semiconducting polymer was formed by
spin-coating a 1.0 wt% chloroform solution onto the ODTS-treated
Si substrates. The spin-coated polymer films were dried in a vac-
uum chamber (10 Torr) for 1 d, and heated for 1 h at 150 C. The
50 nm thick Au source/drain electrodes were evaporated under
high vacuum (2 ꢂ 10ꢀ Torr) through a shadow mask onto the
fraction was collected and recrystallized in methanol. White crystal
products were formed and then filtered to yield 1.37 g of M1
(
43.5%). 1H NMR (CDCl
3
, 400 MHz) d (ppm) 7.24 (s, 2H), 0.41 (s,
ꢀ5
ꢁ
1
8H).
Synthesis of poly (2,5-bis(2-octyldodecyl)-3-(5-(thieno [2,3-b]
thiophen-2-yl)thiophen-2-yl)-6-(thiophen-2-yl)pyrrolo [3,4-c]
6

H.-S. Lee et al. / Polymer 105 (2016) 79e87
81
polymer film, with the channel length and width of 100 and
00 m, respectively. Transistor currentevoltage characteristics
were recorded on Keithley 2400 and 236 source/measure units at
room temperature under vacuum conditions (10 Torr) in dark
condition.
3. Results and discussion
8
m
A new pDPPTTi-OD polymer was synthesized using Stille
coupling of 2,5-bis(trimethylstannyl)thieno [2,3-b]thiophene with
3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo
ꢀ
5
[3,4-c]pyrrole-1,4(2H,5H)-dione in toluene/DMF (10/1, v/v) solution
using Pd(PPh as a catalyst (Scheme 1). The weight-average mo-
3 4
)
lecular weight of pDPPTTi-OD, as determined by gel permeation
2
.5. Organic solar cell fabrication and characterization
chromatography (GPC) using o-dichlorobenzene as an eluent and
ꢀ
1
2
polystyrene as a standard, was M
w
¼ 22,500 g mol with a poly-
Patterned ITO-coated glass substrates (15
U
/sq., 2.5 ꢂ 2.5 cm )
dispersity index of 2.31. The thermal stability of the synthesized
pDPPTTi-OD polymer was characterized by thermogravimetric
analysis (TGA). The pDPPTTi-OD was thermally stable with 5%
were cleaned ultrasonically in isopropyl alcohol, acetone, and iso-
propyl alcohol sequentially for each 10 min. The ITO glass sub-
strates were then cleaned by UV/ozone treatment for 20 min. A
ꢁ
degradation at 390 C (Fig. 1a). Differential scanning calorimetry
poly
(3,4-ethylenedioxythiophene):poly
(styrenesulfonate)
(
DSC) established that there were no notable thermal transitions up
(
PEDOT:PSS, Clevios™ P VP AI 4083, Heraeus) solution diluted with
ꢁ
to 260 C (Fig. 1b).
methanol (1/1, v/v) was spincoated on the clean ITO substrate and
further dried at 80 C for 10 min, forming 30 nm thick PEDOT:PSS
layer. Photoactive layers were formed on top of the PEDOT:PSS layer
by spincoating a chlorobenzene/1,8-diiodooctane solution (97/3, v/
v) containing pDPPTTi-OD and [6,6]-phenyl-C71-butyric acid
ꢁ
The optical characteristics of the pDPPTTi-OD polymer were
investigated by UVevisible absorption spectroscopy. Fig. 2a shows
the absorption spectra of the polymer in chloroform solution and in
the spin-coated film state. The spectrum of the polymer in solution
had a broad absorption band ranging from 320 to 770 nm. Ab-
sorption peaks observed at 360, 438, 655 and 706 nm were
methyl ester (PC71BM) at
a ratio of 1:1.5 (wt/wt) (total
concentration ¼ 24 mg/mL); the thickness of the photoactive layer
was ~100 nm. Finally, a 100 nm thick Al layer was thermally
evaporated through a shadow mask at a pressure of less than
attributed to
pep* and sep* transitions, including a HOMO-to-
LUMO transition [27,28]. A well-developed vibronic peak was
observed at 706 nm even for the dilute solution, which implied that
the pDPPTTi-OD assumed a planar structure similar to other planar
polymers [29e31]; the vibronic peak was more intense in the solid
state. The UVevisible absorptions of the polymer film were slightly
red-shifted, indicating that good intermolecular interactions
occurred in the solid state. An optical bandgap of 1.57 eV was
calculated using the absorption edge of the polymer film at 792 nm
ꢀ
6
2
ꢂ 10 Torr with a deposition rate of ~5 Å/s. The active area of
2
each cell was 0.20 cm . Current density versus voltage character-
istics of PSC cells were recorded on a Keithley model 2400 source
measuring unit. A class-A solar simulator with a 1000 W Xenon
lamp (Yamashita denso) equipped with a KG-5 filter served as a
light source. Its light intensity was adjusted to AM 1.5 G 1 sun light
2
intensity (100 mW/cm ) using a NREL-calibrated mono Si solar cell.
(
Table 1). This optical bandgap is larger than that of poly (2,5-bis(2-
octyldodecyl)-3-(5-(thieno [3,2-b]thiophen-2-yl)thiophen-2-yl)-6-
thiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione) (DT-
External quantum efficiency (EQE) was measured as a function of
wavelength from 300 to 1000 nm on incident photon-to-current
conversion equipment (PV measurement Inc.). Light intensity was
calibrated by using a silicon photodiode G425 which is NIST-
calibrated as a standard.
(
TDPP2T-TT), which is structurally very similar [6] and is close to
that of poly (2,5-bis(2-octyldodecyl)-3-(5-phenylthiophen-2-yl)-6-
Scheme 1. Synthetic scheme for pDPPTTi-OD.

82
H.-S. Lee et al. / Polymer 105 (2016) 79e87
Fig. 1. TGA and DSC curves of pDPPTTi-OD polymer.
Fig. 2. (a) UVevisible spectra of pDPPTTi-OD in chloroform solution and in firm state. (b) Cyclic voltammogram of pDPPTTi-OD film.
(
(
thiophen-2-yl)
PDPPTPT) [23].
pyrrolo
[3,4-c]pyrrole-1,4(2H,
5H)-dione)
and geometric models of the structure. The molecular backbone is
almost planar. All of the dihedral angles are less than 23.6 , which
ꢁ
The electrochemical properties of the pDPPTTi-OD film were
evaluated by cyclic voltammetry (CV). A three-electrode cell was
used with a degassed 0.1 M TBAPF acetonitrile solution as the
forms between the thiophene and TTi rings. The calculated HOMO
and LUMO levels were ꢀ5.04 and ꢀ3.06 eV, respectively. The
electronic density was well-distributed along the two repeating
units in the HOMO level, whereas the middle DPP unit electroni-
cally contributed to the LUMO level. This fact may be associated
with carrier transport behavior in the OTFT measurements, where
the HOMO is a major transporting channel (see below). Addition-
ally, time-dependent (TD) DFT was conducted to investigate the
origin of the electronic transitions upon light absorption. A strong
absorption at 703 nm corresponding to the HOMO / LUMO
transition was predicted, which is closely related to the strong
absorption at ca. 700 nm in the UVevisible absorption spectrum
(Fig. 2a).
6
supporting electrolyte. Fig. 2b shows the CV curve of the pDPPTTi-
OD polymer film. The onset oxidation potential was 0.60 V and the
onset reduction potential was ꢀ1.06 V. From these values, the
HOMO and LUMO levels of the polymer film were estimated to
be ꢀ5.40 and ꢀ3.74 eV, respectively, and its electrochemical
bandgap was determined to be 1.66 eV (Table 1). The HOMO level is
slightly deeper than the HOMO levels of typical DPP-based poly-
mers, including the DT-TDPP2T-TT polymer [6,32].
The torsional angle between the backbone aromatic rings and
the energy-minimized structure of pDPPTTi-OD was determined by
density functional theory (DFT) calculations. To save calculation
cost and time, three repeating units of the pDPPTTi backbone were
used and the attached long OD groups were replaced with methyl
Charge transport behavior was characterized for bottom-gate
top-contact (BG/TC) OTFTs using 50-nm-thick pDPPTTi-OD films
as the channel semiconductor. The polymer thin films were spin-
ꢁ
groups to form the (DPPTTi)
3
model molecule. The (DPPTTi)
3
ge-
coated onto the ODTS-treated SiO
2
/Si wafer and heated at 150
C
ometry was optimized to an energy minimum using Gaussian 09
software at the DFT B3LYP level with the 6e31 þ G (d, p) basis set.
Fig. 3 shows the energy levels of the frontier orbitals, surface plots
in a vacuum chamber. Gold source/drain electrodes were then
vacuum-deposited through a shadow mask. Electrical measure-
ments of the devices were performed under vacuum. Fig. 4a shows
Table 1
Optical and Electrochemical Characteristics of pDPPTTi-OD.
UVevisible absorption
Solution Film
CV data
TGA
Opt
EC
ꢁ
d
Onset (nm)
792
E
g
(eV)
Oxidation
Potential (V)
HOMO (eV)
Reduction
Potential (V)
LUMO (eV)
E
g
(eV)
T ( C)
(
lmax,nm)
(
lmax,nm)
655, 706
653, 723
1.57
0.60
ꢀ5.40
ꢀ1.06
ꢀ3.74
1.66
387

H.-S. Lee et al. / Polymer 105 (2016) 79e87
83
3
Fig. 3. Density functional theory calculation of (DPPTTi) . (a) Molecular geometry, (b) surface plots and electronic structure of frontier orbitals. and (c) UVevisible spectrum
predicted by DT-DFT.
Fig. 4. (a) Transfer and (b) output characteristics of a BG/TC OTFT device based on the pDPPTTi-OD polymer as the semiconductor. (c) GIXD image of the pDPPTTi-OD film and (d)
linecut profiles in the in-plane and out-of-plane directions.

84
H.-S. Lee et al. / Polymer 105 (2016) 79e87
the transfer characteristics of the BG/TC OTFTs based on the as-
coated and 150 C-annealed pDPPTTi-OD films. The pDPPTTi-OD
films displayed ambipolar characteristics. The field-effect mobil-
direction, a clear indication of
p
e
p
stacking of the polymer chains
ꢁ
ꢀ1
was observed at 1.63 Å , with a corresponding spacing of 3.85 Å.
These features implied that the pDPPTTi polymer chains mainly
assumed an edge-on orientation, thereby allowing two-
dimensional charge transport in the OTFT device. Considering the
polymer chemical structure, film crystallinity and carrier transport
results, the TTi moiety would be a good construction unit for DeA
polymers to achieve highly crystalline polymer films and high
carrier mobilities in OTFT devices.
The photovoltaic performance of the polymer was investigated
by fabricating PSC devices with the ITO/PEDOT:PSS/photoactive/Al
and ITO/PEDOT:PSS/photoactive/TiO
properties were measured under 100 mW cm , AM 1.5G sunlight
illumination. The photoactive materials with a blend ratio of
pDPPTTi-OD:PC71BM (1:1.5, w/w) were dissolved in the mixed
solvent of chlorobenzene/1,8-diiodooctane (DIO) (97/3, v/v). The
ities were calculated in the respective saturation regimes of the
2
transfer curves according to the relationship I
2
D
¼ C
i
mW(V
G
e Vth) /
L, where W and L are the channel width and length, respectively, C
is the specific capacitance of the gate dielectric (11 nF cm ), Vth is
i
ꢀ
2
the threshold voltage and
m is the field-effect mobility. The hole and
electron mobilities of the pristine pDPPTTi-OD film were 0.015
ꢀ
5
2
ꢀ1 ꢀ1
(
±0.003) and 3.08 (±1.04) ꢂ 10 cm V
s
, respectively. After the
ꢁ
pDPPTTi-OD film was heated at 150 C for 60 min, the mobilities
ꢀ3
greatly improved to 0.16 (±0.04) and 2.41 (±0.89) ꢂ 10
2
/Al structures. Their electrical
2
ꢀ1 ꢀ1
ꢀ2
cm V
s
, respectively. The mobility enhancements suggested
that the thermal annealing promoted planar pDPPTTi-OD inter-
chain interactions. The higher mobility of the holes compared with
the electrons was primarily attributed to different electronic
coupling features in the polymer backbone for carrier transport:
electronic overlapping at the HOMO level was well-spread along
the chain, whereas the LUMO orbital was mainly localized on one of
the DPP units. A second reason is that the Fermi level of gold is
closer to the HOMO level, hence hole injection is easier than elec-
tron injection. The on/off current ratios of the OFETs based on the
ꢁ
pDPPTTi-OD:PC71BM blends were dissolved at 25, 70 and 90 C for
12 h. After filtering the solutions through an Acrodisc syringe filter
®
(0.45
mm pore size), the photoactive layer was formed by spin-
coating the polymer solutions on the ITO/PEDOT:PSS surface at
each dissolving temperature. The current densityevoltage (JeV)
characteristics are shown in Fig. 5a,b and the corresponding
photovoltaic results are summarized in Table 2. Several important
ꢁ
4
1
50 C-annealed pDPPTTi-OD film were ca. 10 for hole transport.
Fig. 4b shows the output characteristics of the devices made with
the 150 C-annealed pDPPTTi-OD film, which displayed saturation
observations can be made. First, the use of the TiO
layer improved the PCE. This was attributed to the efficient hole-
blocking role of the TiO layer, which reduced free carrier recom-
bination at the interface between the photoactive layer and the Al
electrode [33]. Second, the device performance improved with
2
nanoparticle
ꢁ
behavior at low gate voltages and diode-like behavior at high gate
voltages. The good transport characteristics were also associated
with the crystallinity of the pDPPTTi-OD polymer. Grazing inci-
dence X-ray diffraction (GIXD) experiments revealed a high-order
crystalline lamellar organization of polymer chains (Fig. 4c).
Fig. 4d shows that the (100), (200), (300) and (400) peaks appeared
2
increasing solution temperature; with the TiO
2
layer, the PCEs of
ꢁ
the devices processed at 25, 70 and 90 C were 2.05, 2.30 and 2.90%,
respectively. The PCE improvement mainly resulted from the in-
ꢁ
ꢁ
in the out-of-plane (q
which suggested that the long alkyl chains of the polymer were
interdigitated in the crystalline domains. In the in-plane (q
z
) direction with a lamellar spacing of 19.6 Å,
crease in Jsc. The 90 C-devices had a 27%-higher Jsc than the 25 C-
devices. The Jsc results were consistent with the external quantum
efficiency (EQE) spectra (Fig. 5c,d). High-temperature-processed
y
)
Fig. 5. (a, b) JeV curves and (c, d) EQE spectra of pDPPTTi-OD:PC71BM-based BHJ solar cells showing different treatment conditions under 1 sun illumination. Device structures of
a,c) ITO/PEDOT:PSS/Photoactive/Al and (b,d) ITO/PEDOT:PSS/Photoactive/TiO /Al.
(
2

H.-S. Lee et al. / Polymer 105 (2016) 79e87
85
Table 2
Photovoltaic properties of pDPPTTi-OD:PC70BM-based BHJ solar cell under 1 sunlight illumination.
Photoactive layer (processing temperature)
Voc (V)
J
sc (mA/cm2)
FF
PCE (%)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
w/o TiO
2
pDPPTTi-OD:PC71BM (25 C)
0.75
0.76
0.75
0.77
0.76
0.79
5.83
6.78
6.93
5.65
6.55
7.17
0.43
0.44
0.45
0.47
0.46
0.51
1.90
2.27
2.35
2.05
2.30
2.90
pDPPTTi-OD:PC71BM (70 C)
pDPPTTi-OD:PC71BM (90 C)
w/TiO
2
pDPPTTi-OD:PC71BM (25 C)
pDPPTTi-OD:PC71BM (70 C)
pDPPTTi-OD:PC71BM (90 C)
highest Voc values reported for DPP-based polymers such as
PDPPDTP [23]. The high Voc was ascribed to the low-lying HOMO
level, which illustrates that the TTi moiety can be a good donor unit
to achieve high Voc values in PSC devices.
The UVevisible absorption properties of pDPPTTi-OD:PC71BM
ꢁ
blend films processed at 25, 70 and 90 C, were measured (Fig. 6) to
understand the interchain interaction between polymer chains as a
ꢁ
function of the processing temperature. The 25 C-processed blend
film absorbed up to 1000 nm, which is significantly longer than that
of pDPPTTi-OD films cast from chloroform solution (Fig. 2). This
unusually low wavelength absorption might be caused by severe
aggregation of pDPPTTi-OD polymer fibers that were identified by
GIXD, transmission electron microscopy (TEM) and atomic force
ꢁ
microscopy (AFM) (discussed below). The 70- or 90 C-processed
blend films, however, had normal light absorption features. This
result suggested that processing at elevated temperatures could be
used to take advantage of the favorable intrinsic characteristics of
the pDPPTTi-OD polymer.
The crystallinity of pDPPTTi-OD and the blend with PC71BM was
examined again by GIXD experiments. Fig. 7 shows GIXD images of
the pDPPTTi-OD:PC71BM blend films. Highly crystalline peaks of
pDPPTTi polymer lamellar domains were observed for the polymer-
ꢁ
Fig. 6. UVevisible spectra of pDPPTTi-OD:PC71BM films processed at 25, 70, and 90 C.
ꢁ
devices showed enhanced EQE values. More specifically, the 70 C-
devices displayed more improved EQE values for all absorption
ꢁ
ꢁ
only film. In the out-of-plane (q
peaks appeared at 0.336, 0.649 and 0.969 Å , respectively, for a
corresponding lamellar spacing of ca. 19.6 Å. In the in-plane (q
direction, weak signals for stacking of the polymer chains
z
) direction, (100), (200) and (300)
wavelengths compared with the 25 C-devices. The 90 C-devices
exhibited even further improved EQE values, and the more im-
provements occurred in the PC71BM absorption range than in the
pDPPTTi-OD absorption range. These enhanced EQE behaviors
appeared to be strongly related to morphological features (see
below). Third, the Voc of 0.79 V was obtained, which is one of the
ꢀ
1
y
)
pep
ꢀ1
were observed at 1.63 E , corresponding to a distance of 3.85 Å.
These features implied that the pDPPTTi polymer chains still
ꢁ
Fig. 7. GIXD images of pDPPTTi-OD:PC71BM films processed at (a) 25, (b) 70, and (c) 90 C, and the corresponding line-cut profiles of GIXD images in the in-plane (q
of-plane (q ) (e) directions.
y
) (d) and out-
z

86
H.-S. Lee et al. / Polymer 105 (2016) 79e87
ꢁ
ꢁ
ꢁ
Fig. 8. TEM images of pDPPTTi-OD:PC71BM blend films. Films were prepared at temperatures of (a, b) 25 C, (c, d) 70 C, and (e, f) 90 C.
maintained their crystalline features and edge-on orientation just
as in the pDPPTTi-OD polymer-only film, even after blending with
PC71BM. Notably, while the polymer's crystalline features were less
dependent on the processing temperatures, the crystalline semi-
More detailed morphological features of the pDPPTTi-
OD:PC71BM blend films were revealed by TEM (Fig. 8) and AFM
imaging. Clear morphological differences were observed that
ꢁ
depended on the processing temperature. The 25 C-solution blend
ꢀ1
circles of the PC71BM domains at q ¼ 1.33 Å
significantly
film with photoactive films exhibited large PC71BM aggregates
between randomly distributed fibrous crystalline pDPPTTi-OD
polymer chains (Fig. 8a,d). With increasing solution temperature,
the aggregation of PC71BM molecules was significantly reduced and
decreased with increasing processing temperature. This observa-
tion indicated that highly crystalline PC71BM molecules dis-
aggregated at high temperatures.
ꢁ
ꢁ
ꢁ
Fig. 9. AFM images of pDPPTTi-OD:PC70BM blend films. Films were prepared at temperatures of (a, b) 25 C, (c, d) 70 C, and (e, f) 90 C.

H.-S. Lee et al. / Polymer 105 (2016) 79e87
87
the fibril polymers were better-dispersed. Consequently, more
dx.doi.org/10.1016/j.polymer.2016.10.024.
desirable morphological features were obtained with increasing
solution temperature by providing more intimate interactions be-
tween PC71BM molecules and the pDPPTTi-OD chains. This resulted
in the higher Jsc values at the higher temperatures. Fig. 9 shows the
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Supplementary data related to this article can be found at http://