Solution-processed interlayer of discotic-based small molecules for organic photovoltaic devices: Enhancement of both the open-circuit voltage and the fill factor

Article abstract of DOI:10.1016/j.dyepig.2014.08.007

A novel alcohol/water-soluble, discotic type derivative of pyrene was obtained using a simple synthesis process. The pyrene derivative was dissolved in organic solvents and highly polar solvents. The absorption spectrum of the pyrene derivative in a thin film was almost identical with that observed in solution compared with a fluorene derived co-polymer, the extensive planarity of the pyrene unit is invoked to rationalize this observation. According to an XRD measurement, a huge portion of the pyrene derivatives are arranged face-on in regard of the substrate due to their discotic structure. A photovoltaic device containing the pyrene derivative exhibited an open-circuit voltage of 0.73 V, current density of 14.6 mA/cm², fill factor of 65.1% and a power conversion efficiency of 7.0%. The photovoltaic device with the pyrene derivative exhibited an improved fill factor compared with that of the fluorene co-polymer (60.7%) due to the discotic structure. An inverted photovoltaic device containing pyrene derivative showed a power conversion efficiency of 8.3%.

Full text of DOI:10.1016/j.dyepig.2014.08.007

Dyes and Pigments 113 (2015) 210e218
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Solution-processed interlayer of discotic-based small molecules for
organic photovoltaic devices: Enhancement of both the open-circuit
voltage and the fill factor
,
Ho Jun Song a b, Eui Jin Lee a, Doo Hun Kim a, Tae Ho Lee a, Munju Goh c, Sangkug Lee b,
,
*
Doo Kyung Moon a
a Department of Materials Chemistry and Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Republic of Korea
b Cungcheong Regional Division IT Convergence Material R&D Group, Korea Institute of Industrial Technology, 89 Yangdaegiro-gil, Ipjang-myeon,
Seobuk-gu, Cheonan-si, Chungcheongnam-do 331-822, Republic of Korea
c Carbon Convergence Materials Research Center, Korea Institute of Science and Technology, Eunhari san 101, Bongdong-eup, Wanju-gun, Jeonbuk 565-905,
Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel alcohol/water-soluble, discotic type derivative of pyrene was obtained using a simple synthesis
process. The pyrene derivative was dissolved in organic solvents and highly polar solvents. The ab-
sorption spectrum of the pyrene derivative in a thin film was almost identical with that observed in
solution compared with a fluorene derived co-polymer, the extensive planarity of the pyrene unit is
invoked to rationalize this observation. According to an XRD measurement, a huge portion of the pyrene
derivatives are arranged face-on in regard of the substrate due to their discotic structure. A photovoltaic
device containing the pyrene derivative exhibited an open-circuit voltage of 0.73 V, current density of
14.6 mA/cm2, fill factor of 65.1% and a power conversion efficiency of 7.0%. The photovoltaic device with
the pyrene derivative exhibited an improved fill factor compared with that of the fluorene co-polymer
(60.7%) due to the discotic structure. An inverted photovoltaic device containing pyrene derivative
showed a power conversion efficiency of 8.3%.
Received 20 May 2014
Received in revised form
8 August 2014
Accepted 10 August 2014
Available online 19 August 2014
Keywords:
Small molecule
OPVs
Interlayer
DLC
CPEs
Semiconductor
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
molecular orbital (LUMO) energy levels, and 3) improve the fill
factor (FF) by controlling resistance at the interfaces. To achieve
Conjugated polymers have been widely used in organic light
emitting diodes (OLEDs) [1e4], organic photovoltaic cells (OPVs)
[5e11] and organic thin film transistors (OTFTs) [12,13] for several
decades. For some times, OPVs have drawn crucial concern for
these applications due to the universal technology tendency to-
ward economic potential and continuative growth coupled with
efforts to preserve the environment. However, the poor power
conversion efficiency (PCE) of these materials has been the greatest
barrier in organic photovoltaic development [6].
The next ideal conditions must be achieved in OPV materials
and devices to improve the PCE [8]: 1) increase of the short-circuit
current (JSC) through a wide absorption spectrum, 2) rise of the
open-circuit voltage (VOC) through optimization of the highest
occupied molecular orbital (HOMO) and lowest unoccupied
these ideal conditions, various efforts continue to be applied to
control the materials of the active layer and the device structure:
various donoreacceptor (DeA) type polymers, additives, thermal
treatments, and film thicknesses. However, there is still an inevi-
table loss at the interfaces because of the charge transport barriers
between the active layer and the metal cathode [14].
To improve the charge transport between the interfaces, a
number of investigations to study the effects of introducing an
interlayer have been reported recently. In particular, most research
efforts have focused on alcohol/water soluble conjugated polymer
electrolytes (CPEs) for the interlayer. The Maes group recently re-
ported that the PCE improved from 3.8% to 6.6% with imidazolium-
substituted polythiophene used as the CPE [15]. In the Cao group, a
PCE of 8.3% was reported with a device structure that introduced
poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-
2,7-(9,9-dioctylfluorene)] (PFN) between the active layer and the
cathode, and an inverted device configuration with PFN improved
the PCE to 9.2%, with increases of the JSC, VOC, and FF [16,17]. It has
* Corresponding author. Tel.: þ82 2 450 3498; fax: þ82 2 444 0765.
E-mail address: dkmoon@konkuk.ac.kr (D.K. Moon).
http://dx.doi.org/10.1016/j.dyepig.2014.08.007
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
211
been suggested that these interlayers introduce an enhanced built-
in potential across the device because of the presence of an inter-
face dipole, which is conducive to a refinement in the charge-
transport, an elimination of the built-up space charge, and a
diminution in the recombination losses of the charge carriers
[14,16].
Small molecules have various advantages compared with poly-
mers, such as straightforward synthesis and purification, mono-
dispersity and precise structures, residual end functionality and
good reproducibility [18,19]. A number of research studies on active
materials by small molecules have been reported [20e22], but only
a few investigations of interlayers about small molecules by
solution-process have been reported in the field of OLEDs and OPVs
[4,23].
spectra were collected by a Bruker ARX 400 spectrometer using
solutions in CDCl3 with chemical concentrations recorded in ppm
units using TMS as the internal standard. The elemental analyses
were measured with an EA1112 apparatus using a CE Instrument.
The electronic absorption spectra were measured in chloroform
using an HP Agilent 8453 UVeVis spectrophotometer. The cyclic
voltammetric waves were obtained using a Zahner IM6eX elec-
trochemical workstation with a 0.1 M acetonitrile (purged with
nitrogen for 20 min) solution containing tetrabutyl ammonium
hexafluorophosphate (Bu4NPF6) as the electrolyte at a constant
scan rate of 50 mV/s. ITO, a Pt wire, and silver/silver chloride [Ag in
0.1 M KCl] were used as the working, counter, and reference elec-
trodes, respectively. The electrochemical potential was calibrated
against Fc/Fcþ. The HOMO levels of the polymers were determined
using the oxidation onset value. The onset potentials are the values
obtained from the intersection of the two tangents drawn at the
rising current and the baseline changing current of the CV curves.
TGA measurements were performed on a NETZSCH TG 209 F3
thermogravimetric analyzer. Differential scanning calorimetry
(DSC) was used to determine phase-transition temperatures on a
Netzsch DSC 200 F3 maia differential scanning calorimeter with a
constant heating/cooling rate of 10 ꢀC/min. Texture observations by
polarizing optical microscopy (POM) were made with a Leica
DM2500M and DFC295. All GPC analyses were performed using
THF as an eluent and a polystyrene standard as a reference. Grazing
Incidence X-ray diffraction (GIXD) patterns were obtained using a
SmartLab 3 kW (40 kV 30 mA, Cu target, wavelength: 1.541871 Å)
instrument of Rigaku, Japan. Topographic images of the active
layers were obtained through atomic force microscopy (AFM) in a
tapping mode under ambient conditions using an XE-100 instru-
ment. Scanning Kelvin probe microscopy (SKPM) measurements
were carried out on AFM equipment, using the standard SKPM
mode. Theoretical analyses were performed using density func-
tional theory (DFT), as approximated by the B3 LYP functional and
employing the 6-31G* basis set in Gaussian09. The contact angle
measurements were performed using a Kruss DSA100. SEM mi-
croscopy was performed on a JEOL model JSM-6701F field-emission
scanning electron microscope operating at 10 kV. The melting point
was evaluated using a Carl Zeiss AXIO IMAGER MIM polarizing
optical microscope (POM) equipped with a Linkam TH-600PM and
L-600 heating and cooling stage with temperature control.
In this study, we synthesized novel alcohol/water-soluble dis-
cotic type small molecules. The discotic derivatives exhibit unique
material properties, such as effective charge-carrier mobilities by
one-dimensional structure and anisotropic mechanical and optical
properties [13,24,25]. Discotic liquid crystal (DLC) materials with
effective charge carrier mobility along columnar phases have the
possibility to be used as the active constituent in electronic appli-
cations. Moreover the adjustment of their molecular packing and
large fraction orientation exhibits a principal factor for the perfor-
mance of devices due to the design of the molecular structure [26].
Thus,
we
synthesized
3,30,300,3000-((pyrene-1,3,6,8-
tetrayltetrakis(benzene-4,1-diyl))tetrakis(oxy))tetrakis(N,N-dime-
thylpropan-1-amine) (PBPA) using a simple synthesis process, as
shown in Scheme 1. The dimethyl-amine derivative was introduced
as the end group of the pyrene derivative to provide good alcohol/
water-solubility. Due to their molecular packing and long-range
organization, improved FF and VOC values are expected in OPVs
using these discotic type pyrene derivatives.
2. Experimental section
2.1. Instruments and characterization
Unless otherwise specified, all reactions were performed under
a nitrogen atmosphere. The solvents were dried using the standard
procedures. All column chromatography was performed with silica
gel (230e400 mesh, Merck) as the stationary phase. 1H NMR
Scheme 1. Synthesis route of PBPA and chemical structure of PTB7, PFN.

212
H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
2.2. Fabrication and characterization of polymer solar cells
following modified literature procedures: 1,3,6,8-tetrabromopyrene
(M3) [20].
All of the bulk-heterojunction PV cells were prepared using the
following device fabrication procedure. The glass/indium tin oxide
2.3.1. 3-(4-Bromophenoxy)-N,N-dimethylpropan-1-amine (M1)
4-Bromophenol (10 g, 57.8 mmol), 3-dimethylaminopro-
pylchloride hydrochloride (27.4 g, 173.4 mmol), K2CO3 (23.9 g,
173.4 mmol) were placed in a round-bottomed flask, and under a
nitrogen atmosphere and dry DMF (20 mL) was added. The mixture
was placed in a microwave reactor and heated to 90 ꢀC using 300 W
of microwave power for 4 h. After reaction quenching, the reaction
mixture was purified by column chromatography on silica gel
(dichloromethane as eluent) to obtain the product as light-yellow
oil (6.1 g, yield: 40.8%). 1H NMR (400 MHz; CDCl3; Me4Si):
(ITO) substrates [Sanyo, Japan (10
U/g)] were sequentially litho-
graphically patterned, cleaned with detergent, and ultrasonicated
in deionized water, acetone, and isopropyl alcohol. The substrates
were then dried on a hot plate at 120 ꢀC for 10 min and treated
with oxygen plasma for 10 min to improve the contact angle
immediately before the film coating process. Poly(3,4-
ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS,
Baytron P 4083 Bayer AG) was passed through a 0.45-mm filter
before being deposited onto the ITO substrates at a thickness of ca.
32 nm by spin-coating at 4000 rpm in air and then dried at 120 ꢀC
for 20 min inside a glove box. Composite solutions with polymers
and PCBM were prepared using chlorobenzene (CB) with 1,8-
diiodooctane (DIO). The concentration was adequately controlled
in the 0.3e0.5 wt% range. The solutions were then filtered through
d
¼ 7.32 (d, 2H, J ¼ 8.8 Hz), 6.74 (d, 2H, J ¼ 8.8 Hz), 3.94 (t, 2H,
J ¼ 6.4 Hz), 2.41 (t, 2H, J ¼ 6.8 Hz), 2.23 (s, 6H), 1.92 (m, 2H,
J ¼ 6.8 Hz). 13C NMR (100 MHz; CDCl3; Me4Si): 158.12; 132.15;
116.28; 112.62; 66.39; 56.26; 45.52; 27.46. Anal. Calcd for:
C
11H16BrNO: C, 51.18; H, 6.25; N, 5.43. Found: C, 48.06; H, 5.90; N,
a 0.45-
m
m PTFE filter and spin-coated (500e2000 rpm, 30 s) on top
6.01. IR (KBr, cmꢁ1): 2946, 2861, 2766, 1590, 1488, 1285, 1241, 641.
of the PEDOT:PSS layer. The PFN solution in methanol and acetic
acid was spin-coated on the top of the obtained active layer at
4000 rpm for 30 s to form a thin interlayer of 8e10 nm. The device
fabrication was completed by depositing thin layers of Al (200 nm)
at pressures of less than 10ꢁ6 torr. The active area of the devices was
4.0 mm2. Finally, the cell was encapsulated using a UV-curing glue
(Nagase, Japan). In this study, all of the devices were fabricated with
the following structure: ITO glass/PEDOT:PSS/polymer:PCBM/with
or without interlayer/Al/encapsulation glass; ITO glass/ZnO/with or
without interlayer/polymer:PCBM/MoO3/Ag/encapsulation glass.
The illumination intensity used to test the OPVs was calibrated
using a standard a Si photodiode detector that was equipped with a
KG-5filter. The output photocurrent was adjusted to match the
photocurrent of the Si reference cell to obtain a power density of
100 mW/cm2. After the encapsulation, all of the devices were
operated under an ambient atmosphere at 25 ꢀC. The cur-
rentevoltage (IeV) curves of the photovoltaic devices were
2.3.2. N,N-dimethyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenoxy)propan-1-amine (M2)
To a 100-mL flame-dried two necked flask was added M1 (3.0 g,
11.6 mmol) and freshly distilled THF (80 mL). The resulting solution
was cooled at ꢁ78 ꢀC and 8.7 mL n-butyllithium (13.9 mmol, 1.6 M
in hexane) was added over 10 min under a nitrogen atmosphere.
The mixture was stirred at ꢁ78 ꢀC for 1 h. 2-Isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (2.6 g, 13.9 mmol) was added
rapidly to the solution, and the resulting mixture was slowly
warmed to room temperature for 24 h. The mixture was poured
into 50 mL water and extracted with CHCl3 (300 mL). The combined
organic layers were washed with brine and dried over anhydrous
MgSO4. The solvent was removed by rotary evaporation, and the
residue purified by column chromatography on silica gel (ethyl-
acetate:dichloromethane as eluent) to obtain the product as a light-
yellow oil (1.6 g, yield: 45.0%). 1H NMR (400 MHz; CDCl3; Me4Si):
measured using
a
computer-controlled Keithley 2400 source
d
¼ 7.77 (d, 2H, J ¼ 8.8 Hz), 6.90 (d, 2H, J ¼ 8.8 Hz), 3.94 (t, 2H,
measurement unit (SMU) that was equipped with a Peccell solar
simulator under an illumination of AM 1.5G (100 mW/cm2). The
thicknesses of the thin films were measured using elipsometer of
Elli-SE¼Uam12.
J ¼ 6.4 Hz), 2.46 (t, 2H, J ¼ 6.8 Hz), 2.26 (s, 6H), 1.88 (m, 2H,
J ¼ 6.8 Hz) 1.25 (s, 12H). 13C NMR (100 MHz; CDCl3; Me4Si): 161.57;
136.73; 113.41; 83.13; 74.40; 66.02; 54.09; 45.42; 27.43; 25.00.
Anal. Calcd for: C17H28BNO3: C, 66.90; H, 9.25; N, 4.59. Found: C,
66.73; H, 9.23; N, 4.63. IR (KBr, cmꢁ1): 2981, 2936, 2816, 2716, 1566,
1463, 1281, 1245.
The hole-only devices were fabricated with a diode configura-
tion of ITO (170 nm)/PEDOT:PSS (40 nm)/PTB7:PC71BM (50 nm)/
interlayer/MoO3 (30 nm)/Al (100 nm). The hole mobility of the
active layers was calculated from the SCLC using the JeV curves of
the hole-only devices in the dark as follows:
2.3.3. 3,30,300,3000-((Pyrene-1,3,6,8-tetrayltetrakis(benzene-4,1-diyl))
tetrakis(oxy))tetrakis(N,N-dimethylpropan-1-amine) (PBPA)
M2 (1.60 g, 5.24 mmol), 1,3,6,8-tetrabromopyrene (M3) (0.30 g,
0.87 mmol) and Pd(PPh3)4(0) (0.35 g, 0.30 mmol), were placed in a
Schlenk tube, purged with three nitrogen/vacuum cycles, and un-
der nitrogen atmosphere added 2 M degassed aqueous K2CO3
(15 mL) and dry 1,4-dioxane(30 mL). The mixture was heated to
90 ꢀC and stirred in the dark for 48 h. After reaction quenching, the
mixture was poured into 50 mL water and extracted with CHCl3
(100 mL). The combined organic layers were washed with brine and
dried over anhydrous MgSO4. The solvent was removed by rotary
evaporation, and the final product was obtained after drying in
vacuum. Off-white solid. (0.25 g, yield: 31.5%) 1H NMR (400 MHz;
sffiffiffiffiffiffiffi
!
9
8
hðeÞV2
V
E0L
J ¼ εrε0m
exp 0:89
L3
where ε0 is the permittivity of free space (8.85 ꢂ 10ꢁ14 C/V cm); εr is
the dielectric constant (assumed to be 3, which is a typical value for
conjugated polymers) of the polymer; mh(e) is the zero-field
mobility of holes (electrons);
L is the film thickness; and
V ¼ Vappl ꢁ (Vr þ Vbi), where Vappl is the applied voltage to the
device, Vr is the voltage drop due to series resistance across the
electrodes, and Vbi is the built-in voltage.
CDCl3; Me4Si):
d
¼ 8.15 (s, 1H), 7.94 (s, 4H), 7.57 (d, 8H, J ¼ 8.8 Hz),
7.07 (d, 8H, J ¼ 8.8 Hz), 4.12 (t, 8H, J ¼ 6.4 Hz), 2.51 (t, 8H, J ¼ 6.8 Hz),
2.29 (s, 24H), 2.02 (m, 8H, J ¼ 6.8 Hz). 13C NMR (100 MHz; CDCl3;
Me4Si): 158.66; 136.97; 133.69; 132.27; 131.89; 129.83; 128.21;
126.35; 125.31; 114.63; 66.61; 56.70; 45.80; 27.88. Anal. Calcd for:
2.3. Materials
All reagents were purchased from Aldrich, Acros or TCI com-
panies. All chemicals were used without further purification. PTB7
and PFN were purchased from Nano clean tech (Product NO.:
OS0737, OS0743). The following compounds were synthesized
C
60H70N4O4: C, 79.09; H, 7.74; N, 6.15. Found: C, 76.94; H, 7.42; N,
4.90. IR (KBr, cmꢁ1): 2944, 2857, 2814, 2762, 1513, 1494, 1458, 1285,
1244. Melting point: 192 ꢀC.

H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
213
3. Results and discussion
3.1. Synthesis and thermal properties
Scheme 1 exhibits the chemical structures of the materials and
the synthesis process. As shown in Scheme 1, M1 was synthesized
by the Williamson ether reaction under microwave heating, and
M2 was synthesized using M1 and a boronic ester. Finally, PBPA was
synthesized by a Suzuki-coupling reaction with monomer M2 and
1,3,6,8-tetrabromopyrene. The reaction mixture to form PBPA was
heated for 48 h at 110 ꢀC with a palladium(0) catalysts, a 2 M po-
tassium carbonate solution in 1,4-dioxane as a solvent. The syn-
thesized M3 was purified using several rounds of re-crystallization
with methanol/H2O, and the yield of PBPA was 31%. The obtained
PBPA was soluble in organic solvents (chlorobenzene, ortho-
dichlorobenzene, chloroform) and highly polar solvents (meth-
anol, ethanol).
The solubility of PBPA and PFN, a representative alcohol/water
soluble polymer was compared. We added acetic acid in MeOH to
improve solubility relative to protonation of the terminal dime-
thylamino groups. For the same weight ratio condition (5 mg of
material/cosolvent (Acetic acid 10
m
L þ MeOH 2.5 mL)), PFN was
soluble with heat treatment, but PBPA completely dissolved
without heat treatment at RT (See Fig. S3). Thus, PBPA exhibited
higher solubility in highly polar solvents compared with PFN. Using
a PBPA solution, a uniform and transparent film was formed by
spin-coating.
Fig. 1a) shows the thermal properties of the PBPA. PBPA expe-
rienced a 5% weight loss at a temperature of 358 ꢀC, indicating high
thermal stability due to the rigid pyrene structure. This result was
similar with the thermal stability of a conjugated polymer, which
makes PBPA applicable to OLED and OPVs which demand a thermal
stability above 300 ꢀC [6]. We investigated the thermal phenome-
non using differential scanning calorimetry (DSC) and polarizing
optical microscope (POM) analyses (Fig. 1b)). The DSC trace of the
PBPA revealed no obvious liquid-crystal phase transitions and
exhibited only melting to an isotropic phase upon heating at 192 ꢀC,
which is consistent with the POM result. Interestingly, in the crystal
phase at RT, PBPA formed large plate domains, which might be due
to the formation of a columnar phase by the discotic pyrene de-
rivative (POM and SEM image of Fig. 1b)).
Fig. 1. a) TGA graph of PBPA (inset: solubility test of PBPA and PFN for 5 mg of material/
cosolvent (Acetic acid 10
mL þ MeOH 2.5 mL)) b) DSC trace of PBPA (inset: polarized
optical micrograph (left) and scanning electron microscope (right) of PBPA cooling
form the isotropic phase at 200 ꢀC).
As shown in Fig. 3, the PBPA and PFN exhibited typical p-type
oxidation peaks. The oxidation (Eoonxset) of PBPA and PFN occurred
at þ0.93 V and 1.24 V, and the HOMO energy levels of PBPA and PFN
determined through calculation were ꢁ5.26 eV and ꢁ5.57 eV,
respectively. The LUMO energy levels were obtained from the gap
between the HOMO energy levels and the optical band gap en-
ergies. As a result, the LUMO levels of PBPA and PFN were ꢁ2.29
and ꢁ2.68 eV, respectively. As shown in Fig. 3b), the HOMO level of
PBPA (ꢁ5.26 eV) is similar with that of PEDOT:PSS (ꢁ5.0 eV).
3.2. Optical and electrochemical properties
Fig. 2a) shows the UVevisible spectra of PBPA and PFN. The
maximum absorption spectra of PBPA (lmax) were exhibited at
387 nm in solution (10ꢁ5 M in a co-solvent of acetic acid and
methanol) and at 384 nm in the thin film form. Especially, the
absorption spectrum of PBPA in its thin film form almost matched
up with that of solution, which means that the more planar pyrene
unit effectively decreases the steric hindrance in the thin film and
facilitates stronger association in solution [27]. The maximum ab-
sorption of PFN (lmax) were exhibited at 387 nm in solution (10ꢁ5 M
in cosolvent of acetic acid and methanol) and at 392 nm in the thin
film form. In thin film form, the absorption spectrum of PFN also
exhibited a broad spectrum in the section of 400e425 nm, which
significantly differed from that of PBPA. This broad spectral ab-
sorption is the result of the decrease in the dihedral angle between
fluorene derivatives in the thin film due to the stronger interdigi-
tate interactions [28]. The calculated optical band gaps of PBPA and
PFN by the measured value of UV onset for the film were 2.96 and
2.89 eV, respectively.
3.3. XRD analysis and DFT calculation
Fig. 4 exhibits the X-ray diffraction investigation of the films of
PBPA and PFN to analyze their ordering structures. The samples for
measurement were fabricated to the structure of spin-casted film
on the surface of a Si-wafer using solution with acetic acid and
MeOH. In the out-of-plane diffraction pattern of PBPA, as shown in
Fig. 4a), a prominent diffraction peak showed at 18.7ꢀ, which in-
dicates an out-of-plane peak (010) due to the arrangement of
a regular columnar structure by
distance of PBPA was 0.48 nm (
distance matched up with the stacking distance among the pyrene
units by formation of hexagonal columnar phase reported in the
p
l
e
p
stacking. The
p
pe
e
p
stacking
stacking
Fig. 3 exhibits the cyclic voltammograms of PBPA and PFN.
The cyclic voltammograms were recorded in 0.1
tetrabutylammonium-hexafluorophosphate acetonitrile solution.
¼ 2dsin ). This
q
p
M

214
H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
Fig. 2. Absorption spectra for a) PBPA and PFN in solution & thin film b) spin-coated
layers of PBPA and PFN on the top of the active layer (PTB7:PC71BM).
literature of pyrene-based molecules [24,29]. In contrast, in the
out-of-plane diffraction pattern of PFN, a very weak diffraction peak
was observed at 19.3ꢀ, which means (010) diffraction correspond-
Fig. 3. a) Cyclic voltammogram of PBPA and PFN b) DFT Gaussian simulation and band
diagram of PBPA, PFN, ITO, PC71BM, Al.
ing to
0.46 nm. The results of PBPA and PFN were comparable with the
results for the stacking distance of fluoreneebenzene
pep stacking. The pep stacking distance of PFN was
pep
(dp ¼ 4.0e4.4 Å) [30]. However, the diffraction peak of PBPA was
sharper compared with that of PFN, which indicates PBPA exhibited
a more strongly ordered orientation than that of PFN.
In the in-plane diffraction pattern of PBPA, as shown in Fig. 4b), a
diffraction pattern was rarely exhibited. When considering the re-
sults of the out-of-plane and in-plane diffraction for PBPA, these
results suggest that a huge fraction of the PBPA derivatives exhibit
SI). However, as shown in the XRD results, the
pep stacking dis-
tances of PBPA (0.48 nm) and PFN (0.46 nm) exhibited similar
values. Namely, the torsion angle of PBPA is 20ꢀ higher than that of
PFN, but the
pep stacking distances were similar. These results
suggest that PBPA exhibited effective
staggered stacking of PBPA [13].
pep stacking due to the
face-on orientation in regard of the substrate; namely, the
p-
stacking orientation is perpendicular to the substrate direction
[5,31]. The ordered orientation and oriented face-on structure of
PBPA reduce the series resistance in the electron transport direction
for OPVs, which may result in the improved FF in the PBPA-based
OPV device compared with that of the PFN-based OPV device
(see Table 1).
By calculating PBPA and PFN using a DFT calculation, the torsion
angle was comparatively analyzed. For PBPA, the torsion angle
between pyrene and benzene was 53e57ꢀ, while the torsion angle
between fluorene derivatives for PFN was 37ꢀ. Compared with
PBPA, the formation of a planar backbone was observed for PFN (see
3.4. Characterization of OPV devices
Fig. 5 and Table 2 exhibit the data for devise performance. Each
device was fabricated as follows: ITO (170 nm)/PEDOT:PSS (40 nm)/
active layer (50e80 nm)/Interlayer (~5 nm)/Al (100 nm). The donor
polymer in the active layer was thieno[3,4-b]-thiophene-benzodi-
thiophene (PTB7), and the acceptor derivative in the active layer
was phenyl-C71-butyric acid methyl ester (PC71BM). The blend
ratio for PTB7/PC71BM was 1:1.5 by weight, and the active layer was
spin-coated from a mixed solvent (chlorobenzene 97 vol%, 1,8-
diiodoctane 3 vol%). On the top of the active layer, either a PBPA

H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
215
Fig. 5. a) JeV characteristics b) EQE spectra of the BHJ solar cells with the device (a
Fig. 4. X-ray diffraction pattern in thin films of PBPA and PFN a) out-of-plane and b) in-
plane.
Conventional device structure: ITO/PEDOT:PSS/PTB7:PC71BM(1:1.5)/without or with
b
interlayer/Al.
Inverted device structure: ITO/ZnO/without or with interlayer/
PTB7:PC71BM(1:1.5)/MoO3/Ag.).
or a PFN solution formed a film (thickness of 5 nm) using the spin-
coating method, and an Al electrode (100 nm) was deposited on top
by evaporation through a shadow mask. We fabricated over 100
devices in order to confirm device reproducibility.
The OPV device without an interlayer exhibited VOC of 0.65 V, JSC
of 14.9 mA/cm2, FF of 50.0% and PCE of 4.9%, which is similar to the
results of PTB7 device reported in the literature [16,32]. For a device
with PFN, the VOC, JSC, FF and PCE were 0.73 V, 14.7 mA/cm2, 60.7%
and 6.6%, respectively. For a device with PBPA, the VOC, JSC, FF and
PCE were 0.73 V, 14.6 mA/cm2, 65.1% and 7.0%, respectively. The VOC
of the devices with either the PFN or the PBPA interlayer exhibited
increased values. To verify the interfacial dipole moment of the PFN
and PBPA, the surface potentials of active layers with and without
interlayers are investigated through Scanning Kelvin probe micro-
scopy (Fig. S7). It was noted that the potential of the films with
interlayers are þ200 mV higher than that of the film without
interlayer. This suggests that the polar amino groups of PBPA and
PFN contribute a powerful electrical field between the active layer
and cathode interface [14,16].
The devices with an interlayer (PFN, PBPA) exhibited substan-
tially increased FF values compared with the device without an
interlayer. In particular, the FF of device with PBPA (65.1%) was
higher than that with PFN (60.7%), which effected in the improved
PCE of the device with PBPA (PFN 6.6% vs. PBPA 7.0%). The series
Table 1
Optical & electrochemical properties of PBPA and PFN.
onset
Polymer Absorption, lmax (nm)
E
EHOMO ELUMO Eopt
Hole mobility
ox
(V)
(eV)c (eV)d (eV)e (cm2/V s)f
Solutiona
Filmb
PBPA
PFN
301, 387
387
304, 384
392
0.94 ꢁ5.26 ꢁ2.29 2.96 2.86*10ꢁ5
1.24 ꢁ5.57 ꢁ2.68 2.89 5.26*10ꢁ7
a
b
c
Absorption spectrum in CHCl3 solution (10ꢁ6 M).
Spin-coated thin film (50 nm).
Calculated from the oxidation onset potentials under the assumption that the
resistance for the device with PBPA, as shown Table 2, was 6.9
U
U
/
/
cm2, and the series resistance for the device with PFN was 8.0
cm2, i.e., the series resistance of the PBPA layer exhibited a lower
value compared with the PFN layer. This reduction of series resis-
tance could grow the FF. Thus, the device with PBPA exhibited a
higher FF than that of PFN because of the decrease of the series
absolute energy level of Fc/Fcþ was ꢁ4.8 eV below a vacuum.
d
HOMO e Eopt.
e
Estimated from the onset of UVevis absorption data of the thin film.
Structure of the hole-only devices (ITO (170 nm)/PEDOT:PSS (40 nm)/
f
PTB7:PC71BM (50 nm)/interlayer/MoO3 (30 nm)/Al (100 nm)).

216
H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
were both 13.5 mA/cm2. Although it makes a little gap, the EQE data
Table 2
Photovoltaic performance of the BHJ solar cells.
are similar with the cell currentedensity of AM 1.5 condition. In the
case of PFN device, EQE showed increased intensity around
350e400 nm, which is because PFN absorb more photons around
350e400 nm as shown in UV result (Fig. 1b & Fig. S4)).
We also fabricated an inverted device as follows: ITO (170 nm)/
ZnO (10 nm)/without or with PBPA (~5 nm)/active layer
(50e80 nm)/MoO3 (5 nm)/Ag (100 nm). For a device without PBPA,
VOC, JSC, FF and PCE were 0.73 V, 17.6 mA/cm2, 55.1% and 7.2%,
respectively, which is similar to the results of inverted device re-
ported in the literature [33]. The inverted OPV device with PBPA
showed VOC of 0.73 V, JSC of 17.7 mA/cm2, FF of 63.6% and PCE of
8.3%, which represent improved results over the PFN device re-
ported in the literature [33].
Device
structure
VOC (V)
JSC
FF (%)
PCE (%)
Rs
Rsh
(U
/cm2)
(mA/cm2)
(U
/cm2)
Al
0.65
0.73
0.73
0.73
0.73
14.9
14.7
14.6
17.6
17.7
50.0
60.7
65.1
55.1
63.6
4.9 (4.9)c
6.6 (6.5)c
7.0 (6.9)c
7.2 (7.1)c
8.3 (8.3)c
13.9
8.0
6.9
11.8
5.2
533
835
1044
808
PFN/Ala
PBPA/Ala
ZnOb
ZnO/PBPAb
808
a
Conventional device structure: ITO/PEDOT:PSS/PTB7:PC71BM(1:1.5)/without or
with interlayer/A.
b
Inverted
device
structure:
ITO/ZnO/without
or
with
interlayer/
PTB7:PC71BM(1:1.5)/MoO3/Ag.
c
Average PCE value for five devices.
resistance resulting from the ordered orientation and packing of
PBPA [6,19].
3.5. Morphology and contact angle analysis
To confirm the preciseness of the cell performances, the external
quantum efficiency (EQE) of each device was investigated, and the
EQE curves are shown in Fig. 5(b). The short-circuit current density
for the PFN- and PBPA-based devices obtained from the EQE data
To analyze the torpology of the interlayer, atomic force micro-
scopy (AFM) was investigated (Fig. 6). As shown in Fig. 6a, a smooth
morphology was observed between PTB7 and PC71BM, with a
roughness of 0.9 nm. For the PBPA film on the top of the
Fig. 6. Topographic AFM images of a) PTB7:PC71BM film b) PTB7:PC71BM/PBPA film c) PTB7:PC71BM/PFN film (3 ꢂ 3
m
m2).

H.J. Song et al. / Dyes and Pigments 113 (2015) 210e218
217
PTB7:PC71BM blend film (Fig. 6b)), a noticeable number of raised
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