Development of bulk heterojunction morphology by the difference of intermolecular interaction behaviors

Article abstract of DOI:10.1016/j.orgel.2014.10.006

The morphology of a bulk heterojunction can be controlled by adding a processing additive in order to improve its power conversion efficiency (PCE) in photovoltaic devices. The phase-separated morphologies of blends of PONTBT or P3HT with fullerene derivatives are systematically examined in the presence of processing additives that possess various alkane alkyl chain lengths or end-group electronegativities. We determined the morphologies of the bulk heterojunction layers by using atomic force microscopy (AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). The photocurrent-voltage characteristics of the bulk heterojunction solar cells were found to be strongly dependent on the intermolecular interactions between the conjugated polymers, the fullerene derivatives, and the processing additives in the photoactive layer. The optimal PONTBT:fullerene derivative blend morphology was obtained with a processing additive, 1,3-diiodopropane (1,3-DIP), that possesses a short alkyl chain and an end group with weak electronegativity, and was found to exhibit a high fill factor (FF) and a high current density (J_SC). In contrast, in blends of P3HT with the fullerene derivative, PCEs with higher FF and J_SC values were achieved by incorporating the processing additive, 1,8-dibromooctane (1,8-DBrO), which has a long alkyl chain and a strong electronegative end group. Thus the selection of the processing additive with the aim of enhancing photovoltaic performance needs to take into account the intermolecular interaction of the conjugated polymer.

Full text of DOI:10.1016/j.orgel.2014.10.006

Organic Electronics 15 (2014) 3558–3567
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Development of bulk heterojunction morphology by the
difference of intermolecular interaction behaviors
Hyojung Cha a, Jang Yeol Baek b, Tae Kyu An a, Seul-Ong Kim b, Soon-Ki Kwon b,
,
⇑
a,
Yun-Hi Kim c, , Chan Eon Park
⇑
⇑
a Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
b School of Materials Science & Engineering, Engineering Research Institute (ERI), Gyeongsang National University, Jinju 660-701, Republic of Korea
c Department of Chemistry, Engineering Research Institute (ERI), Gyeongsang National University, Jinju 660-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2014
Received in revised form 3 October 2014
Accepted 3 October 2014
Available online 18 October 2014
The morphology of a bulk heterojunction can be controlled by adding a processing additive
in order to improve its power conversion efficiency (PCE) in photovoltaic devices. The
phase-separated morphologies of blends of PONTBT or P3HT with fullerene derivatives
are systematically examined in the presence of processing additives that possess various
alkane alkyl chain lengths or end-group electronegativities. We determined the morphol-
ogies of the bulk heterojunction layers by using atomic force microscopy (AFM) and
grazing incidence wide angle X-ray scattering (GIWAXS). The photocurrent–voltage char-
acteristics of the bulk heterojunction solar cells were found to be strongly dependent on
the intermolecular interactions between the conjugated polymers, the fullerene deriva-
tives, and the processing additives in the photoactive layer. The optimal PONTBT:fullerene
derivative blend morphology was obtained with a processing additive, 1,3-diiodopropane
(1,3-DIP), that possesses a short alkyl chain and an end group with weak electronegativity,
and was found to exhibit a high fill factor (FF) and a high current density (JSC). In contrast,
in blends of P3HT with the fullerene derivative, PCEs with higher FF and JSC values were
achieved by incorporating the processing additive, 1,8-dibromooctane (1,8-DBrO), which
has a long alkyl chain and a strong electronegative end group. Thus the selection of the
processing additive with the aim of enhancing photovoltaic performance needs to take into
account the intermolecular interaction of the conjugated polymer.
Keywords:
Bulk heterojunction morphology
Processing additive
Phase separation
Electronegativity
Crystallinity
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
properties of the D:A blend including the crystallinity of
the polymers [11–13], domain size [14–16], miscibility of
Over the last few years, high power conversion efficien-
cies (PCEs) have been achieved in bulk heterojunction solar
cells with bi-continuous interpenetrating donor:acceptor
(D:A) networks; in some recent studies, PCEs approaching
10% have even been reported [1–10]. In addition to the
intrinsic properties of the active layer, the morphological
the component materials [17], and molecular orientation
[18] are also important for the photovoltaic performance
of a given device. Several strategies, including slow growth
[19], solvent annealing [20,21], thermal annealing [22,23],
and selection of the solvent [24–29] or mixed solvent
[30,31], have been tested in the control of the morphology
of D:A blends. In particular, binary solvent mixtures con-
taining poor solvents for the conjugated polymers have
been successfully used in morphology control [32–42].
For example, dichlorobenzene (DCB) or chlorobenzene
⇑
Corresponding authors.
E-mail addresses: skwon@gnu.ac.kr (S.-K. Kwon), ykim@gnu.ac.kr
(Y.-H. Kim), cep@postech.ac.kr (C.E. Park).
http://dx.doi.org/10.1016/j.orgel.2014.10.006
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
3559
(CB):1,8-diiodooctane (1,8-DIO) binary solvent systems
have been widely applied in organic photovoltaic device
(OPV) fabrication processes. By mixing a few volume per-
cent of processing additives with the host solvent (DCB
or CB), the efficiencies of many polymer-based solar cells
can be improved dramatically [32–38]. The advantage of
the processing additive approach is that a phase-separated
morphology develops in the bulk heterojunction layer
through the use of a single-step spin-coating process with-
out post treatment [39,40]. According to these studies, the
crystallinity of the polymers and the domain sizes in D:A
blends can be tuned effectively by using binary solvent
mixtures, and thus binary solvent mixtures play a very
important role in the fabrication of high performance OPVs
[32–34].
Peet et al. showed that processing additives can be intro-
duced into D:A blends to control domain sizes and thus
maximize charge generation and collection in bulk hetero-
junction layers, which increases the efficiency of organic
photovoltaic devices [32,33]. Lee et al. concluded that pro-
cessing additives require good miscibility with the host sol-
vent, a higher boiling point than the host solvent, and a
good solubility with respect to the fullerene derivative,
and that these properties enable control of phase separa-
tion [34]. Furthermore, the effects on photovoltaic device
performance of a few other processing additives, such as
several alkanedithiols with various alkane chain lengths
or electronegative end groups [33,34,39,40], N-methyl-2-
pyrrolidinone [41], and 4-bromoanisole [42], have been
investigated. Despite this increasing interest in processing
additives, few studies have investigated how the intrinsic
intermolecular interaction of the conjugate polymer needs
to be taken into account in the selection of such processing
additives.
ester (PC71BM) [43]. PONTBT has an octyl side chain to
modify its energy levels and its UV–visible absorption
spectrum with respect to those of previous alkoxy naph-
thalene-based conjugated polymers. Blends of PONTBT,
or poly(3-hexylthiophene-2,5-diyl) (P3HT), respectively,
with fullerene derivatives and various processing additives
with various alkyl chain lengths and end-group electroneg-
ativities were systematically examined [43–45]. The
interpenetrating network morphologies of the bulk hetero-
junction layers were determined with atomic force micros-
copy (AFM) and grazing incidence wide angle X-ray
scattering (GIWAXS). The photovoltaic device perfor-
mances were found to be strongly dependent on the selec-
tion of the processing additive and the intrinsic
intermolecular interaction of the conjugated polymer.
The effects of processing additives on the blends contain-
ing highly crystalline P3HT are not as significant as in
the case of PONTBT. In other words, the introduction of
processing additives into PONTBT:PC71BM is more effec-
tive than for P3HT:PC61BM.
2. Material and methods
Organic solar cells were fabricated with the structure
ITO/PEDOT:PSS/conjugated polymer and fullerene deriva-
tive blend/LiF/Al. A glass substrate with a pre-patterned
ITO active layer with a thickness of 0.09 mm2 was ultraso-
nicated in detergent, D.I. water, CMOS-grade acetone, and
isopropanol, and the surface of the glass substrate was
modified with UV-ozone treatment for 20 min. PEDOT:PSS
(Bay P VP AI4083, Bayer AG) for hole injection was spin-
coated at 4000 rpm for 60 s for a thickness of 30–40 nm,
as measured with an alpha-stepper, on the cleaned
ITO-patterned glass substrate after filtration through a
In this study, we investigated the effects of the intermo-
lecular interactions of the conjugated polymers in bulk
heterojunction active layers on their phase-separation
behavior by selecting two conjugated polymers with dif-
ferent crystallinities, and testing the responses of their
blends with fullerene derivatives to the incorporation of
various processing additives. We synthesized a new alkoxy
naphthalene-based amorphous polymer, poly(4-(5-(1,5-
bis(octyloxy)naphthalen-2-yl)thiophen-2-yl)-7-(thiophen-
0.45 lm filter, followed by baking in an oven at 120 °C
for more than an hour.
The organic solar cells consisted of a conjugated poly-
mer as the electron donor and a fullerene derivative as
the electron acceptor. The conjugated polymer, PONTBT,
was synthesized via a palladium-catalyzed Stille coupling
reaction, as described in the supporting information. The
molecular weight of PONTBT was 16 kDa. The electron
donor conjugated polymer, P3HT, was obtained from Rieke
Metals. The PC61BM and PC71BM fullerene derivatives
(99.5%) were obtained from Nano-C. Solutions of the
conjugated polymer/fullerene derivative with 2 vol% of a
processing additive were prepared in dichlorobenzene to
2-yl)benzo[c][1,2,5]thiadiazole)
(PONTBT)
(Fig.
1).
Previously, we have reported an alkoxy naphthalene-based
conjugated polymer that exhibits amorphous behavior and
a high PCE of 4.2% in a bulk heterojunction active layer
upon mixing with [6,6]-phenyl-C71-butyric acid methyl
PONTBT
P3HT
Fig. 1. Structures of the amorphous polymer PONTBT and the highly ordered crystalline polymer P3HT.

3560
H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
yield 40 mg/mL solutions that were stirred in a glove box
under a nitrogen atmosphere overnight. The blended solu-
tions were spin-coated at 1000 rpm for 60 s for a thickness
of 100 nm on top of the PEDOT:PSS layer after filtration
electrochemical analysis showed that the highest occupied
molecular orbital (HOMO) energy level of PONTBT is
ꢀ5.34 eV, the lowest unoccupied molecular orbital (LUMO)
energy level of PONTBT is ꢀ3.60 eV, and its band gap is
1.74 eV. Table S1 provides a summary of the electrochem-
ical and electrical properties of PONTBT. The spectrum of
the PONTBT film contains a high energy absorption maxi-
mum at 595 nm, which indicates that PONTBT is a low
band gap polymer. Structural analysis with atomic force
microscopy (AFM), transmission electron microscopy
(TEM), and grazing incidence wide angle X-ray scattering
(GIWAXS) was performed to determine the molecular
ordering and the film morphologies of PONTBT and
PONTBT:PC71BM. The films were prepared by dissolving
PONTBT and PONTBT:PC71BM in DCB, followed by spin-
coating at 1000 rpm onto ITO glass coated with PEDOT:PSS.
An AFM image of a PONTBT film is shown in Fig. S3(a). The
PONTBT film has an r.m.s. roughness of 0.459 nm. The bulk
morphology of the PONTBT:PC71BM film was analyzed
with TEM. Fig. S3(b) shows a TEM image of a homogeneous
PONTBT:PC71BM domain. The dark areas are attributed to
PCBM-rich regions because their electron scattering den-
sity is higher than that of the polymer. Fig. S3(c) shows
GIWAXS patterns of PONTBT and PONTBT:PC71BM films.
Neither GIWAXS pattern contains any Bragg refraction
peak, which confirms that both films are amorphous.
The measured current–voltage (J–V) curves in Fig. S4
shows that the device performance is strongly dependent
on the ratio of PONTBT and PC71BM. The photovoltaic
results for all of the investigated blends under illumination
with an intensity of 100 mW cmꢀ2 are summarized in
Table S2. The device with a 1:3 weight ratio of conjugated
polymer to PC71BM was found to exhibit the best PCE.
with a 0.2 lm PTFE filter for all solvents, then annealed
at various temperatures for 20 min on a hot plate in the
glove box. In all the devices, the PONTBT:PC71BM layers
had similar thicknesses, on the order of 100 nm, and the
P3HT:PC61BM layers had similar thicknesses of 120 nm,
as measured with an alpha-stepper. LiF and Al cathodes
were thermally deposited to thicknesses of 1 and 100 nm
respectively onto the surface of each active layer.
The J–V characteristics were measured by using a Keith-
ley 4200 source measurement unit in the dark and under
AM 1.5G solar illumination (Oriel 1 kW solar simulator)
with respect to a reference cell PVM 132 calibrated at the
National Renewable Energy Laboratory at an intensity of
100 mW/cm2.
UV–visible (Cary, Varian Co.) and PL (FR 650, JASCO Co.)
measurements were used to analyze the optical properties
of the conjugated polymer/fullerene derivative blends.
AFM (Multimode IIIa, Digital Instruments) was performed
in tapping mode to acquire images of the surfaces of the
conjugated polymer/fullerene derivative blend films. In
the transmission electron microscopy (TEM) measure-
ments, the conjugated polymer/fullerene derivative layers
on a water-soluble PEDOT:PSS substrate were floated on
the surface of deionized water and picked up using
200-mesh copper TEM grids. TEM images were obtained
by using a HITACHI-7600 operated at 100 kV. GIWAXS data
were obtained at the 5A beamline (E = 11.57 keV) of the
Pohang Accelerator Laboratory (PAL) [46]. Total external
reflection angle of GIWAXS measurements is 0.14° to unify
the penetration depth of thin films. 6 Circle diffractometer
and scintillation detector are used to control external
reflection angle with no calibration standard. The exposure
time was 1 s at each measured angles.
3.2. Optical properties of the conjugated polymer/fullerene
derivative blends with various processing additives
Solar cell performance can be optimized by the addition
of processing additives to the conjugated polymer/fuller-
ene derivative blend layers. The conjugated polymer/
fullerene derivative ratios were optimized at 1:3 (wt:wt)
for PONTBT:PC71BM in DCB and 1:1 (wt:wt) for P3HT:
PC61BM in CB. In this study, processing additives with var-
ious alkyl chain lengths and end-group electronegativities
were incorporated into these optimized blends.
As shown in Fig. 2, the absorption maximum of the
PONTBT:PC71BM blends is at 550 nm. When 2 vol% of the
additives are added to the host solvent, DCB, the absorp-
tion intensity in the region 600–700 nm increases, which
is indicative of the close packing of PONTBT and a red-shift
3. Results and discussion
3.1. Properties of PONTBT and the optimization of the
PONTBT:PC71BM Solar Cells
The amorphous polymer, PONTBT, was synthesized
with the procedures shown in Scheme S1 and described
in the supporting information. The number average molec-
ular weight (Mn) of PONTBT is 16.2 kDa. The resulting
copolymer is soluble in toluene, chloroform, CB, and DCB.
The thermal properties of PONTBT were investigated with
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC), as shown in Fig. S1. This molecule
exhibits excellent thermal stability: the decomposition
temperature for 5% weight loss is approx. 344 °C. The
DSC traces for this compound contain a peak indicative of
phase transitions at 222 °C.
The electrochemical and optical properties of PONTBT
were determined in order to optimize the PONTBT:PC71BM
blend solar cells. Fig. S2 shows the UV–visible absorption
spectra of PONTBT in chloroform solution and in the film
state and the cyclic voltammogram for PONTBT. Our
in the p–p absorption band. The intensities of the intermo-
lecular interactions of PONTBT are increased by the addi-
tion of the processing additives. Moreover, the intensities
of the shoulders at 650 nm in the spectra of the PON-
TBT:PC71BM blends increase with increases in the alkyl
chain length (Fig. 2(a)) and in the electronegativity
(Cl > Br > I) of the end group (Fig. 2(b)) of the processing
additives. The incorporation of the processing additives
into the PONTBT:PC71BM (1:3, w/w) blend results in the
close packing of PONTBT and better phase separation of
PONTBT and the fullerene derivative.

H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
3561
(a)
(b)
Pristine
1,8-DClO
1,8-DBrO
1,8-DIO
1,3-DIP
Pristine
1,2-EDT
1,4-BDT
1,6-HDT
1,8-ODT
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV–visible spectra of PONTBT:PC71BM (1:3, w/w) films cast from dichlorobenzene (DCB) and PONTBT:PC71BM (1:3, w/w) films cast from DCB
containing 2% of the processing additives with (a) various alkyl chain lengths and (b) end groups with various electronegativities.
Similarly, by adding 2 vol% of the processing additives
into the P3HT:PC61BM (1:1, w/w) blend solutions, the
intensity of the absorption increases in the region 600–
700 nm (Fig. 3). The peaks in the absorption bands of
P3HT in the P3HT:PC61BM blends with processing additives
are red-shifted by 12 nm and have a stronger red shoulder
(600 nm) than annealed P3HT:PC61BM without any pro-
cessing additive. The absorption maxima of the annealed
P3HT:PC61BM film and the 1,9-NDT-treated P3HT:PC61BM
film are at 507 nm and 519 nm respectively. The additions
of processing additives to the PONTBT:PC71BM and
P3HT:PC61BM blend films increase the strengths of the
intermolecular interactions between the conjugated poly-
mers, the fullerene derivatives, and the processing
additives.
(1,3-PDT), 1,8-diiodooctane (1,3-DIO), and 1,3-diiodopro-
pane (1,3-DIP) do not contain Bragg reflection peaks
because PONTBT is an amorphous polymer (Fig. S5). For
P3HT:PC61BM, Bragg scattering signals due to the
p–p
spacing (approximately 4 Å) and the lamella spacing
(approximately 20 Å) in the conjugated polymer film are
present, as well as the higher order reflections of crystal-
line films. The intensity of the first Bragg reflection peak
for thin films of P3HT:PC61BM containing various process-
ing additives varies with the additive (Fig. 4). A comparison
of the d-spacings in the six systems (Fig. 4(a) and (b))
clearly shows that the addition of processing additives
with longer alkyl chains produces blends with shorter d-
spacings than the addition of additives with shorter alkyl
chains. The P3HT crystallites in P3HT:PC61BM are oriented
in the (100) plane, in the so-called ‘edge-on’ orientation.
The as-cast P3HT:PC61BM blend layer without processing
additives has a d-spacing of 1.83 nm at 3.36°, whereas
the P3HT:PC61BM blend layer with 1,9-nonanedithiol
(1,9-NDT) has a d-spacing of 1.58 nm at 3.88° (Fig. 4(b)).
The d-spacings of the P3HT:PC61BM blend layers with pro-
cessing additives with longer alkyl chains are clearly less
than those of the P3HT:PC61BM blends with additives with
3.3. Morphologies of the conjugated polymer/fullerene
derivative blends with various processing additives
The morphologies of the blends with and without the
processing additives were investigated by using GIWAXS
and AFM. The GIWAXS patterns for the PONTBT:PC71BM
blends containing the additives 1,3-propanedithiol
2.0
2.0
pristine
annealing
1,2-EDT
1,3-PDT
1,4-BDT
1,6-HDT
pristine
annealing
1,8-ODT
1,8-DClO
1,8-DBrO
1,8-DIO
(a)
(b)
1.5
1.5
1.0
1.0
1,8-ODT
1,3-DIP
1,9-NDT
0.5
0.5
0.0
0.0
300 400 500 600 700 800
Wavelength (nm)
300 400 500 600 700 800
Wavelength (nm)
Fig. 3. UV–visible spectra of P3HT:PC61BM (1:1, w/w) films cast from chlorobenzene (CB) and P3HT:PC61BM (1:1, w/w) films cast from CB containing 2% of
the processing additives with (a) various alkyl chain lengths and (b) end groups with various electronegativities.

3562
H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
(b)
(a)
1,9-NDT
1,8-ODT
1,6-HDT
1,4-BDT
1,3-PDT
1,2-EDT
Pristine
2
4
6
8
10
12
14
2
4
6
8
10
12
14
Two Theta
Two Theta
(d)
(c)
1,3-DIP
1,8-DIO
1,8-DBrO
1,8-DClO
Pristine
2
4
6
8
10
12
14
2
4
6
8
10
12
14
Two theta
Two Theta
Fig. 4. One-dimensional GIWAXS patterns for a P3HT:PC61BM blend film without any processing additive and for P3HT:PC61BM blend films cast from
chlorobenzene (CB) containing 2% of the processing additives with various alkyl chain lengths (a) on a linear scale and (b) on a log scale, and 2% of the
processing additives with end groups with various electronegativities (c) on a linear scale and (d) on a log scale.
shorter alkyl chains. Similarly, the comparison of the d-
spacings of the blends with processing additives with
end groups of three different electronegativities (Fig. 4(c)
and (d)) shows that the incorporation of additives with
end groups that have higher electronegativities results in
larger d-spacings than the incorporation of an additive
with an end group with a lower electronegativity.
the conjugated polymer and the fullerene derivative,
which enhances carrier transport [20].
The r.m.s. roughnesses of P3HT:PC61BM blends with
processing additives with longer alkyl chains or end groups
with stronger electronegativities (Fig. 6) are not signifi-
cantly higher than those of the PONTBT:PC71BM blends
(Fig. 5) because the intermolecular interactions of P3HT
and fullerene derivatives are stronger than those between
PONTBT and fullerene derivatives. The intermolecular
interactions between PONTBT:PC71BM and the processing
additives induce the phase separation of PONTBT and
PC71BM. In other words, PC71BM molecules, which are
selectively dissolved by the processing additives, increase
the aggregation of PONTBT.
Fig. 5 shows typical AFM height images of the blend
films processed with additives with various alkyl chain
lengths (the image size is 5
lm ꢁ 5
lm). As the alkyl chain
lengths of the processing additives in the PONTBT:PC71BM
blends increase from that of 1,2-ethanedithiol (1,2-EDT) to
that of 1,9-NDT, the films contain more connected blend
networks and larger domains, and the top surfaces become
rougher. As the electronegativities of the end groups of the
processing additives in the PONTBT:PC71BM blend layers
increase from that of 1,8-DIO to that of 1,8-dichlorooctane
(1,8-DClO), the phase separation is enhanced. Although
PONTBT:PC71BM is smooth in the AFM image in Fig. 5,
the addition of processing additives significantly modifies
its surface topography. Since PC71BM is soluble in the pro-
cessing additives, the phase separation of PONTBT and
PC71BM is increased. Decreasing the alkyl chain length
and reducing the electronegativity of the end group of
the processing additives increases the phase separation of
3.4. Photovoltaic performances of the conjugated polymer/
fullerene derivative blends with processing additives
The photovoltaic properties of the conjugated polymer/
fullerene derivative blends incorporating processing addi-
tives were investigated in bulk heterojunction solar cells
with the configuration glass/ITO/PEDOT:PSS/active layer/
LiF/Al.
Fig. S6 presents the J–V curves for devices prepared from
PONTBT:PC71BM blends with various processing additives.

H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
3563
(a) Pristine
(b) 1,2-EDT
(c) 1,3-PDT
(d) 1,4-BDT
20nm
20nm
20nm
20nm
Rr.m.s. = 1.05 nm
R
r.m.s. = 1.55 nm
Rr.m.s. = 1.48 nm
Rr.m.s. = 0.37 nm
(e) 1,6-HDT
(f) 1,8-ODT
(g) 1,9-NDT
(h) 1,3-DIP
50nm
100nm
100nm
20nm
Rr.m.s. = 6.06 nm
Rr.m.s. = 10.8 nm
Rr.m.s. = 11.6 nm
Rr.m.s. = 0.377 nm
(i) 1,8-DClO
(j) 1,8-DBrO
(k) 1,8-DIO
100nm
100nm
100nm
Rr.m.s. = 8.02 nm
Rr.m.s. = 9.81 nm
Rr.m.s. = 8.37 nm
Fig. 5. AFM topographies of films cast from PONTBT:PC71BM: (a) without additives; with the additives (b) 1,2-ethanedithiol, (c) 1,3-propanedithiol, (d) 1,4-
butanedithiol, (e) 1,6-hexanedithiol, (f) 1,8-octanedithiol, (g) 1,9-nonanedithiol, (h) 1,3-diiodopropane, (i) 1,8-dichlorooctane, (j) 1,8-dibromooctane, and
(k) 1,8-diiodooctane.
(a) Pristine
(b) 1,2-EDT
(c) 1,3-PDT
(d) 1,4-BDT
10nm
10nm
10nm
50nm
Rr.m.s. = 0.443 nm
Rr.m.s. = 0.455 nm
Rr.m.s. = 0.632 nm
Rr.m.s. = 6.22 nm
(e) 1,6-HDT
(f) 1,8-ODT
(g) 1,9-NDT
(h) Annealed
50nm
10nm
50nm
50nm
Rr.m.s. = 6.60 nm
Rr.m.s. = 6.71 nm
Rr.m.s. = 6.47 nm
Rr.m.s. = 0.692 nm
(i) 1,8-DClO
(j) 1,8-DBrO
(k) 1,8-DIO
70nm
70nm
70nm
Rr.m.s. = 8.71 nm
Rr.m.s. = 7.59 nm
Rr.m.s. = 9.25 nm
Fig. 6. AFM topographies of films cast from P3HT:PC61BM: (a) without additives; with the additives (b) 1,2-ethanedithiol, (c) 1,3-propanedithiol, (d) 1,4-
butanedithiol, (e) 1,6-hexanedithiol, (f) 1,8-octanedithiol, (g) 1,9-nonanedithiol, (i) 1,8-dichlorooctane, (j) 1,8-dibromooctane, (k) 1,8-diiodooctane; (h)
annealed at 150 °C without processing additives.
Table 1 shows the characteristics of the photovoltaic devices
based on PONTBT:PC71BM with processing additives,
namely current density (JSC), open circuit voltage (VOC), fill
factor (FF), and PCE. The cell based on PONTBT:PC71BM
incorporating 2 vol% of 1,2-EDT exhibits the best PCE (5%)
of the blends processed with additives with various alkyl

3564
H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
chain lengths. The cell based on PONTBT:PC71BM incorpo-
rating 2 vol% of 1,8-DIO exhibits the highest PCE (4.5%) of
the octane blends processed with additives with various
end-group electronegativities. 1,8-DIO has the end group
with the lowest electronegativity of the processing addi-
tives from 1,8-DClO to 1,8-DIO (Fig. S6(b)).
Table 1 and Fig. 8 summarize the VOC, JSC, FF, and PCE
values of the devices based on PONTBT:PC71BM (1:3)
blends. The VOC values are in the range 0.79–0.85 V and
are insensitive to variation of the processing additive,
whereas the JSC, FF, and PCE values are sensitive to this var-
iation. For the P3HT:PC61BM cells, as the lengths of the
However, the cell based on PONTBT:PC71BM incorpo-
rating 2 vol% of 1,3-diiodopropane (1,3-DIP) exhibits an
even better PCE (5.5%), as well an enhanced JSC and the
highest FF value (Table 1 and Fig. 7(a)). Note that 1,2-
diiodoethane (1,2-DIE) does not dissolve in dichloroben-
zene, therefore 1,3-DIP was used instead of 1,2-DIE. Thus
the solubility of PC71BM in processing additives with suffi-
ciently short alkyl chains or end groups with sufficiently
low electronegativity induces adequate phase separation
of PONTBT in PONTBT:PC71BM cells.
alkyl chains in the processing additives increase, the JSC,
FF, and PCE values increase (Fig. S6 and Table 1).
Fig. 7(b) and S7 displays the J–V curves for devices
based on P3HT:PC61BM with processing additives pos-
sessed end groups with various electronegativities. The
best device performance was obtained with a P3HT:PC61-
BM blend with 2 vol% of 1,8-DBrO (Table 1). For the
P3HT:PC61BM cells, the processing additives with long
alkyl chains or strongly electronegative end groups
increase the crystallinity of P3HT.
0
0
(a)
(b)
-2
-2
-4
-4
Pristine (0.76%)
-6
-6
Pristine (4.0%)
-8
Thermal treatment
(2.9%)
-8
-10
-10
-12
1,8-DBrO (4.3%)
-14
1,3-DIP (5.5%)
-12
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
Voltage (V)
Voltage (V)
Fig. 7. The J–V curves of photovoltaic devices based on (a) PONTBT:PC71BM (1:3, w/w) and (b) P3HT:PC61BM (1:1, w/w) with or without 2% processing
additive under AM 1.5G illumination at an intensity of 100 mW/cm2.
Table 1
Performances of OPVsa based on PONTBT:PC71BM and P3HT:PC61BM under AM 1.5G illumination at an intensity of 100 mW/cm2.
Electron donor
Alkyl lengthb
End groupc,d
VOC (V)
JSC (mA/cm2)
FF (%)
PCE (%)
PONTBT
w/oe
-SH
-SH
-SH
-SH
-SH
-SH
-SH
-Cl
0.91 0.01
0.83 0.01
0.85 0.01
0.80 0.01
0.82 0.01
0.82 0.01
0.82 0.02
0.80 0.02
0.79 0.01
0.82 0.01
0.83 0.01
10.2 0.7
13.9 0.3
12.6 0.3
12.7 0.2
12.0 0.5
11.6 0.8
7.2 2.4
11.2 1.2
13.5 0.6
11.8 0.3
11.1 0.2
43.3 1.5
43.1 1.3
42.9 1.0
37.8 1.1
33.8 1.5
31.9 2.0
21.7 4.0
29.5 3.0
39.2 2.5
46.9 2.0
59.8 1.2
4.0 0.49
5.0 0.30
4.6 0.27
3.8 0.26
3.3 0.36
3.0 0.49
1.3 0.77
2.6 0.70
4.2 0.50
4.5 0.41
5.5 0.29
Ethane
Propane
Butane
Hexane
Octane
Nonane
Octane
Octane
Octane
Propane
-Br
-I
-I
P3HT
w/o
-SH
-SH
-SH
-SH
-SH
-SH
-Cl
0.64 0.02
0.67 0.01
0.71 0.01
0.59 0.01
0.59 0.01
0.58 0.02
0.55 0.02
0.58 0.01
0.58 0.01
3.6 0.7
7.4 0.2
3.8 0.4
32.7 4.4
58.5 1.9
35.4 2.3
42.2 1.7
52.2 1.5
52.3 1.2
50.7 2.4
60.2 2.1
52.6 3.1
0.76 0.29
2.9 0.22
0.96 0.18
2.4 0.23
3.2 0.28
3.7 0.28
3.4 0.38
4.2 0.39
3.3 0.41
Annealingf
Ethane
Butane
Hexane
Nonane
Octane
Octane
Octane
9.5 0.5
10.4 0.4
12.2 0.2
12.3 0.2
12.3 0.2
11.0 0.3
-Br
-I
a
b
c
d
e
f
The data shown are the average values obtained from 20 devices with standard deviation.
The alkyl length of processing additives based on alkanedithiols or alkanedihalides.
The functional end group of processing additives based on alkanedithiols or alkanedihalides.
SH: -dithiol, -Cl: dichloro-, -Br: dibromo-, and - I: diiiodo-.
These films were fabricated without processing additives and thermal annealing process.
These films fabricated without processing additives were annealed at 150 °C for 20 min in a N2 glove box.

H. Cha et al. / Organic Electronics 15 (2014) 3558–3567
3565
(a) alkyl group modification
50
40
30
20
10
0
1.0
0.9
0.8
0.7
5
4
3
2
1
0
14
12
10
8
6
(o-DCB b.p. 180 C)
4
(b) end group electron negativity modification
5
4
3
2
1
0
60
1.00
0.95
0.90
0.85
0.80
0.75
14
50
40
30
20
13
12
11
10
9
Fig. 8. Summary of the photovoltaic performances of cells based on PONTBT:PC71BM (1:3, w/w) with or without 2% processing additives (a) with various
alkyl chain lengths and (b) with end groups with various electronegativities under AM 1.5G illumination at an intensity of 100 mW/cm2.
For the PONTBT:PC71BM blends, the optimum morphol-
ogies are obtained by incorporating processing additives
with short alkyl chains or end groups with low electroneg-
ativities. In contrast, the photovoltaic performances of the
P3HT:PC61BM cells are improved by incorporating process-
ing additives with long alkyl chains or strongly electroneg-
ative end groups.
enhance the p–p coupling of P3HT, which enhances JSC.
The incorporation of the additive 1,8-DBrO was found to
produce the highest PCE in the P3HT:PC61BM cells.
Acknowledgements
This work was supported by the National Research
Foundation of South Korea (NRF) grant funded by the
Korea government (MSIP) (2012R1A2A2A06047047) and
the Center for Advanced Soft Electronics under the Global
4. Conclusions
Frontier Research Program (Nos. 20110031639
&
We have studied the development of the bulk hetero-
junction morphologies of conjugated polymer/fullerene
derivative blends in the presence of processing additives;
PONTBT and P3HT were used as conjugated polymers.
UV–visible spectra show that the intermolecular interac-
tions between the two polymers and the processing
additives vary with their alkyl chain lengths and the
electronegativities of their end groups. Cells based on a
PONTBT:PC71BM blend incorporating 2 vol% of 1,3-DIP,
which has short alkyl chains and end group with lower
electronegativity exhibit the best performance, in particu-
lar enhanced JSC and FF values. The phase separation of the
PONTBT phase and the PC71BM phase increases for PON-
TBT:PC71BM blends incorporating longer alkyl chain and
high electronegative end group processing additives. For
the P3HT:PC61BM blend, processing additives with longer
alkyl chains and strongly electronegative end groups
2013M3A6A5073172) through the National Research
Foundation funded by the Ministry of Education, Science
and Technology (Korea).
Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.10.006.
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