Molecular engineering of push-pull chromophore for efficient bulk-heterojunction morphology in solution processed small molecule organic photovoltaics

Article abstract of DOI:10.1039/c1jm10667h

The synthesis and characterization of new push-pull chromophores, bisDMFA-diTh-CA, bisDMFA-diTh-MMN, and bisDMFA-diTh-MIMN from 5′-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)aniline (bisDMFA) electron donating, dithiophene bridging, and various electron withdrawing moieties, cyanoacrylic acid (CA), methylene malononitrile (MMN), and methylene indenylidene malononitrile (MIMN), were demonstrated for efficient solution processed BHJ solar cells. The photophysical properties, the hole mobilities from the space charge limitation of current (SCLC) J-V characteristics, and surface morphologies of these materials/PCBM bulk-heterojunction films were also investigated. The best power conversion efficiency of 3.66% (±0.12) with J_sc = 11.82 mA/cm², FF = 0.35, and V_oc = 0.90 V was observed in the BHJ film fabricated with the bisDMFA-diTh-MMN/C ₇₁-PCBM composite with 3% 1-chloronaphthalene (CN), which is an efficiency ～82% and ～49% higher than before and after post-annealing without treatment of CN, respectively.

Full text of DOI:10.1039/c1jm10667h

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PAPER
Molecular engineering of push-pull chromophore for efficient bulk-
heterojunction morphology in solution processed small molecule organic
photovoltaics†
Haye Min Ko,a Hyunbong Choi,a Sanghyun Paek,a Kyungjun Kim,a Kihyung Song,b Jae Kwan Leec
a
*
and Jaejung Ko
Received 15th February 2011, Accepted 17th March 2011
DOI: 10.1039/c1jm10667h
The synthesis and characterization of new push-pull chromophores, bisDMFA-diTh-CA, bisDMFA-
diTh-MMN, and bisDMFA-diTh-MIMN from 50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)aniline
(bisDMFA) electron donating, dithiophene bridging, and various electron withdrawing moieties,
cyanoacrylic acid (CA), methylene malononitrile (MMN), and methylene indenylidene malononitrile
(MIMN), were demonstrated for efficient solution processed BHJ solar cells. The photophysical
properties, the hole mobilities from the space charge limitation of current (SCLC) J-V characteristics,
and surface morphologies of these materials/PCBM bulk-heterojunction films were also investigated.
The best power conversion efficiency of 3.66% (ꢀ0.12) with Jsc ¼ 11.82 mA/cm2, FF ¼ 0.35, and
Voc ¼ 0.90 V was observed in the BHJ film fabricated with the bisDMFA-diTh-MMN/C71-PCBM
composite with 3% 1-chloronaphthalene (CN), which is an efficiency ꢁ82% and ꢁ49% higher than
before and after post-annealing without treatment of CN, respectively.
moiety can strongly induce intermolecular p–p interactions and
Introduction
a band-shift of longer wavelength absorption by intramolecular
charge transfer from the thiophene chain to the DPP core.3,7–9
And DPP-thiophene based small molecule donors have exhibited
good phase-separating networks with fullerene derivative
acceptors.
Solution processed organic solar cells based on bulk-hetero-
junction (BHJ)1,2 materials comprising small molecule chromo-
phores and fullerene derivatives have offered special
opportunities showing their promising power-conversion effi-
ciencies (PCE) of above 4% as attractive alternatives to
p-conjugated (semiconducting) polymers.3,4 Although the BHJ
devices fabricated by small molecule chromophore present lower
efficiency than ꢁ7% that by polymers,5,6 these approaches by
small molecules seem to fascinate more than the polymer ones
from the viewpoint of mass production for commercial appli-
cation due to their low reproducibility for characteristics such as
average molecular weight (Mw) and polydispersity index (PDI) as
well as difficulty in purification.
On the other hand, we have also focused on the development of
new chromophores for small molecule BHJ solar cells, which are
motivated by organic dyes in dye-sensitized solar cells (DSSC)11,12
and push-pull chromophores in nonlinear optics (NLO) due to
a strong structural similarity with small molecule chromophores
reported in BHJ solar cells.13–21 In this work, we have attempted
the fabrication of a BHJ device comprising a fullerene acceptor
and a 2-cyanoacrylic acid-4-(bisdimethylfluoreneaniline)dithio-
phene (bisDMFA-diTh-CA) donor,22,23 which is well-known as
JK-2 organic dye in DSSC dye and provides good performance
and stability for dye-sensitized solar cells.24,25 And we have
synthesized the new push-pull chromophores, 2-((50-(4-(bis(9,9-
dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-bithiophen-5-yl)
methylene)malononitrile (bisDMFA-diTh-MMN) and 2-(2-((50-
(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-bithio-
phen-5-yl)methylene)-3-oxo-2,3-dihydro-1H-inden-1-ylidene)
malononitrile (bisDMFA-diTh-MIMN), which have a more
electron withdrawing moiety, similarly to the structure of
bisDMFA-diTh-CA.
Recently, materials based on a diketopyrrolopyrrole (DPP)
core have been demonstrated, showing a comparable PCE of
above 4% to polymer BHJ solar cells.3,7,8 Since the DPP core unit
has a uniquely planar conjugated bicyclic structure and electron-
withdrawing properties,9,10 its derivatives comprising a thiophene
aDepartment of Advanced Material Chemistry, Korea University,
Chungnam, 330-700, Korea. E-mail: jko@korea.ac.kr; Fax: +82-41-860-
5396; Tel: +82-41-860-1337
bDepartment of Chemistry, Korea National University of Education,
Chungbuk, 363-791, Korea
cDepartment of Green Energy Engineering
& Research Center for
Herein, we report the synthesis and characteristics of the above
push-pull chromophores (bisDMFA-diTh-CA, bisDMFA-diTh-
MMN, and bisDMFA-diTh-MIMN)/PCBM BHJ films and their
Convergence Technology, Hoseo University, Chungnam, 336-795, Korea
† Electronic supplementary information (ESI) available: AFM pictures.
See DOI: 10.1039/c1jm10667h
7248 | J. Mater. Chem., 2011, 21, 7248–7253
This journal is ª The Royal Society of Chemistry 2011

View Online
application in solution processed BHJ solar cell. We have also
demonstrated the efficient morphology of these BHJ films and
PCBM using a processing additive, resulting in a significantly
increased efficiency of the device (Scheme 1).
donor compounds, a platinum wire and an Ag/AgNO3 (0.10 M)
electrode were used as the working electrode, as the counter
electrode and as the reference electrode, respectively. The
measurements were done using a scan rate of 100 mV/s and
ferrocenium/ferrocene redox couple was used as an internal
reference. AFM measurements were performed with a Digital
Instruments NanoScope IV in the tapping mode.
Experimental
Materials
2-((50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-
bithiophen-5-yl)methylene)malononitrile
50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-bitho-
phene-5-carbaldehyde22,23 and 2-(3-oxo-2,3-dihydro-1H-inden-1-
ylidene)malononitrile26,27 were synthesized according to the
procedures in the literature.
Malononitrile (8.5 mg, 0.13 mmol) and piperidine (3ꢁ5 drops)
were added to a solution of 50-(4-(bis(9,9-dimethyl-9H-fluoren-2-
yl)amino)phenyl)-2,20-bithophene-5-carbaldehyde (84 mg, 0.13
mmol) in CH2Cl2 (20 ml) at room temperature. The reaction
mixture was heated under reflux for 12 h. Concentration and
purification of the residue by flash column chromatography (Hex-
EtOAc, 5 : 1) gave 2-((50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)
amino)phenyl)-2,20-bithiophen-5-yl)methylene)malononitrile
Instruments and measurements
1H- and 13C-NMR spectra were obtained on a Varian Mercury
300 spectrometer, a Varian/Oxford As-500 (500 MHz) spec-
trometer. Chemical shift values were recorded as parts per
million relative to tetramethylsilane as an internal standard
unless otherwise indicated, and coupling constants in Hertz.
Optimized structures were calculated by TD-DFT using the
B3LYP functional and the 3–21G* basis set. The highest occu-
pied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) energies were determined using
minimized singlet geometries to approximate the ground state.
UV-vis data was measured with a Perkin-Elmer Lambda 2S UV-
visible spectrometer. Photoluminescence spectra were recorded
on a Perkin LS fluorescence Spectrometer. Cyclic voltammetry
(CV) was performed using VersaSTAT 3 (Princeton Applied
Research). Methylene chloride was used as the solvent and 0.1 M
of tetrabutyl ammonium hexafluorophosphate was used as the
supporting electrolyte. A platinum rod electrode coated with the
1
(50 mg, 55%). Rf 0.5 (Hex-EtOAc, 5 : 1). H-NMR (500 MHz,
CDCl3): d 7.75 (s, 1H), 7.68–7.62 (m, 5H), 7.51 (d, 2H, J ¼ 8.5 Hz),
7.43–7.39 (m, 3H), 7.35–7.24 (m, 8H), 7.20 (d, 2H, J ¼ 8.5 Hz),
7.14 (dd, 2H, J ¼ 8.0, 2.0 Hz), 1.43 (s, 12H) 13C-NMR (75 MHz,
CDCl3): d 155.4, 153.9, 150.1, 149.9, 148.7, 148.1, 146.8, 140.4,
138.9, 135.0, 133.2, 128.7, 127.2, 126.9, 124.2, 123.8, 123.6, 123.3,
122.7, 120.9, 119.7, 119.3, 115.6, 114.6, 113.7, 47.0, 27.2.
2-(2-((50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-
bithiophen-5-yl)methylene)-3-oxo-2,3-dihydro-1H-inden-1-ylidene)
malononitrile
2-(3-Oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (23 mg,
0.11 mmol) was added to a solution of 50-(4-(bis(9,9-dimethyl-
9H-fluoren-2-yl)amino)phenyl)-2,20-bithophene-5-carbaldehyde
(76 mg, 0.11 mmol) in EtOH/THF (10 ml/6 ml) at room
temperature. The reaction mixture was stirred for 12 h. The
precipitate was filtered and washed repeatedly with ethanol to
remove any remaining reactants. The product (90 mg, 94%) was
1
recrystallized from ethanol. H-NMR (500 MHz, DMSO-d6):
d 8.67 (s, 1H), 8.50 (d, 1H, J ¼ 12.5 Hz), 7.95–7.90 (m, 4H), 7.80–
7.75 (m, 5H), 7.70–7.66 (m, 3H), 7.54 (dd, 3H, J ¼ 21.0, 9.0 Hz),
7.35–7.28 (m, 6H), 7.11–7.07 (m, 4H), 1.38 (s, 12H) 13C-NMR
(125 MHz, DMSO-d6): d 187.5, 160.0, 154.9, 153.2, 152.2, 147.8,
146.9, 146.8, 146.2, 139.3, 138.2, 136.2, 135.5, 134.9, 134.7, 134.4,
133.4, 129.5, 127.1, 126.8, 126.6, 126.2, 125.3, 124.4, 123.5, 122.7,
122.5, 121.7, 121.2, 119.7, 119.0, 114.8, 114.6, 68.6, 46.5, 26.7.
Fabrication and characterization of solar cell devices
The BHJ films were prepared under optimized conditions
according to the following procedure reported previously:29 The
indium tin oxide (ITO)-coated glass substrate was first cleaned
with detergent, ultrasonicated in acetone and isopropyl alcohol,
and subsequently dried overnight in an oven. Poly(3,4-ethyl-
enedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS) in
aqueous solution was spun-cast to form a film with a thickness of
approximately 40 nm. The substrate was dried for 10 min at
Scheme 1 Schematic depiction of the molecular structure of the new
push-pull chromophores, bisDMFA-diTh-CA, bisDMFA-diTh-MMN,
bisDMFA-diTh-MIMN and the structure of the solar cell device fabri-
cated using these materials/fullerene BHJ active layer.
ꢂ
140 C in air, then transferred into a glove-box to spin-cast the
photoactive layer. The donor/PCBM BHJ blend solutions
(weight ratio, 1 : 2 or 1 : 3) were prepared in chlorobenzene at
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7248–7253 | 7249

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a concentration of 30 mg/mL, and then spun-cast on top of the
PEDOT layer. Then, the device was pumped down to lower than
10ꢃ7 torr and a ꢁ100 nm thick Al electrode was deposited on top.
The solar cell’s efficiencies were characterized under simulated
100 mW/cm2 AM 1.5G irradiation from a 1000 W Xe arc lamp
(Oriel 91193). The light intensity was adjusted with a Si solar cell
that was double-checked with an NREL-calibrated Si solar
cell (PV Measurement Inc.). The applied potential and measured
cell current were measured using a Keithley model 2400 digital
source meter. The current–voltage characteristics of the cell
under these conditions were determined by biasing the cell
externally and measuring the generated photocurrent. This
process was fully automated using Wavemetrics software. The
incident photon-to-current conversion efficiency (IPCE) spectra
for the cells were measured on an IPCE measuring system
(PV measurements).
Fig.
1
Absorption spectra of bisDMFA-diTh-CA (black line),
bisDMFA-diTh-MMN (red line) and bisDMFA-diTh-MIMN (blue line)
in ClBz (solid line) and thin film (dot line).
intramolecular p–p* transition bands at 400ꢁ800 nm and
a molar absorption coefficient of 20 791 Mꢃ1 cmꢃ1 at 472 nm,
32 174 Mꢃ1 cmꢃ1 at 536 nm, and 40 198 Mꢃ1 cmꢃ1 at 636 nm,
respectively. This indicates that the chromophore having the
stronger electron withdrawing moiety exhibits a significantly red-
shifted absorption spectrum and enhanced molar extinction
coefficient due to its strong intramolecular charge transfer. In
solid-state thin films, the absorption spectra of chromophores
showed ꢁ30 nm red-shift and broadened bands compared to
those in solution.
Results and discussion
The synthetic procedures of bisDMFA-diTh-CA, bisDMFA-
diTh-MMN, and bisDMFA-diTh-MIMN are shown in Scheme 2.
BisDMFA-diTh-CA,
50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)
amino)phenyl)-2,20-bithophene-5-carbaldehyde22,23 (1) and 2-(3-
oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile26,27 (2) were
prepared according to
a procedure reported previously.
BisDMFA-diTh-MMN and bisDMFA-diTh-MIMN were
synthesized by Knoevenagel condensation reaction using 50-(4-
(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20
-bitho-
Fig. 2 shows the cyclic voltammograms of bisDMFA-diTh-CA,
bisDMFA-diTh-MMN, and bisDMFA-diTh-MIMN in methy-
lene chloride solution. The corresponding electrochemical
properties are also summarized in Table 1. The energy levels of
the HOMO and LUMO of these materials were determined from
these cyclic voltammetry (CV) spectra. A platinum rod electrode,
a platinum wire and an Ag/AgNO3 (0.10 M) electrode were used
as the working electrode, as the counter electrode and as the
reference electrode, respectively. The analyses were performed in
an electrolyte consisting of a solution of 0.1 M tetrabuty-
lammonium hexafluorophosphate (TBAPF6) in methylene
chloride at room temperature under nitrogen with a scan rate of
100 mV/s and ferrocenium/ferrocene redox couple was used as an
internal reference. The HOMO and LUMO levels can be
deduced from the oxidation and reduction onsets with the
assumption that the energy level of ferrocene (Fc) is 4.8 eV below
vacuum level. The CV of these materials in solid-state thin films
could not be measured due to the stripping of the film on the
electrode. Therefore, we determined the optical bandgap from
calculations of the absorption thresholds from the absorption
spectra of bisDMFA-diTh series in chlorobenzene. The HOMO
level of the bisDMFA-diTh-CA, bisDMFA-diTh-MMN, and
bisDMFA-diTh-MIMN determined in solution by CV are
calculated as 4.883 eV, 4.916 eV, and 4.924 eV, respectively.
From these results, we can roughly guess that the Voc of the
devices fabricated using these materials/PCBM BHJ active layers
may be ꢁ0.6 eV, respectively,28 but the Voc estimated in experi-
ments was ꢁ0.3 eV higher. These results will be discussed in more
details below.
phene-5-carbaldehyde (1) and an electron withdrawing group
such as malononitrile, 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)
malononitrile (2).
Fig. 1 shows the UV-vis absorption spectra of the bisDMFA-
diTh-CA, bisDMFA-diTh-MMN, and bisDMFA-diTh-MIMN in
chlorobenzene solution and thin films. The corresponding optical
properties are summarized in Table 1. As shown in Fig. 1, the
absorption spectrum of bisDMFA-diTh-CA, bisDMFA-diTh-
MMN, and bisDMFA-diTh-MIMN in solution shows
The performances of bisDMFA-diTh-CA, bisDMFA-diTh-
MMN, and bisDMFA-diTh-MIMN were investigated through
the application of photovoltaic devices fabricated with these
Scheme 2 Synthesis of the new push-pull chromophores, bisDMFA-
diTh-CA, bisDMFA-diTh-MMN, bisDMFA-diTh-MIMN.
7250 | J. Mater. Chem., 2011, 21, 7248–7253
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Table 1 Optical and redox parameters of the compounds
Eonset, ox(V)/HOMO (eV)b
LUMO (eV)
E0–0 (eV)c
a
Compounds
labs /nm (3/Mꢃ1cmꢃ1
)
lPLa/nm
bisDMFA-diTh-CA
bisDMFA-diTh-MMN
bisDMFA-diTh-MIMN
472 (20 791)
536 (32 174)
636 (40 198)
739, 745
734
710
0.083/ꢃ4.883
0.116/ꢃ4.916
0.124/ꢃ4.924
ꢃ2.803
ꢃ2.946
ꢃ3.094
2.08
1.97
1.83
Absorption spectra were measured in ClBz solution. b Redox potentials of the compounds were measured in CH2Cl2 with 0.1M (n-C4H9)4NPF6 with
a
c
a scan rate of 100 mVsꢃ1 (vs. Fc/Fc+). E0–0 was calculated from the absorption thresholds from absorption spectra of bisDMFA-diTh series in
chlorobenzene.
On the other hand, Fig. 4 shows the optimized structure of
bisDMFA-diTh-CA, bisDMFA-diTh-MMN, and bisDMFA-
diTh-MIMN, which were calculated by the time dependent-
density functional theory (TD-DFT) using the B3LYP func-
tional/3–21G* basis set. The orbital density of the HOMO of
bisDMFA-diTh-CA, bisDMFA-diTh-MMN, and bisDMFA-
diTh-MIMN are similarly observed on both bisDMFA and the
dithiophene bridge. And the LUMO is located predominantly in
the electron accepting core and dithiophene bridge, showing
a general orbital distribution like push-pull chromophores. This
calculation reveals that bisDMFA can partially play a role in
stabilizing separated holes from excitons and improve the
transport properties of the hole carrier.
Donor/PCBM BHJ films were prepared under optimized
conditions according to the following procedure reported
previously:29 PEDOT : PSS (Baytron PH) was spun-cast on an
Fig. 2 Electrochemical characterization of bisDMFA-diTh-EWG series
in dichloromethane/TBAHFP (0.1 M), scan speed 100 mV/s, potentials
vs. Fc/Fc+.
materials/C61(or 71)-PCBM BHJ films. In the course of studying
the characteristics of over 600 solar cells, the most efficient
photovoltaic cells fabricated using bisDMFA-diTh-CA (or
bisDMFA-diTh-MMN)/C71-PCBM were optimized at a ratio of
1 : 2 approximately and bisDMFA-diTh-MIMN/C61-PCBM was
optimized at ratio of 1 : 3 approximately. These BHJ films were
cast on top of a PEDOT : PSS layer by a spin speed of 1 500 rpm,
and a 1.0 wt% of donor concentration in chlorobenzene. The
optimum thickness of the bisDMFA-diTh-CA/C71-PCBM,
bisDMFA-diTh-MMN/C71-PCBM, and bisDMFA-diTh-MIMN/
C61-PCBM BHJ films obtained under these conditions was
approximately 80 nm, 73 nm, and 76 nm, respectively. The
UV-visible absorption spectra of these BHJ films are shown in
Fig. 3a.
The hole mobility was extracted the space charge limitation of
current (SCLC) J-V characteristics obtained in the dark for hole-
only ITO/PEDOT:PSS/Donor:PCBM/Au devices (Fig. 3b). The
hole mobilities of bisDMFA-diTh-CA, bisDMFA-diTh-MMN,
and bisDMFA-diTh-MIMN evaluated using the above
Mott-Gurney Law (3 ¼ 330) were 1.06 ꢄ 10ꢃ5 cm2/V$s, 4.86 ꢄ
10ꢃ5 cm2/V$s, and 3.57 ꢄ 10ꢃ5 cm2/V$s, respectively. The newly
synthesized push-pull chromophores, bisDMFA-diTh-MMN,
and bisDMFA-diTh-MIMN exhibited above 3.5 times higher
Fig.
3 (a) UV-Visible absorption spectra of bisDMFA-diTh-CA/
mobility than bisDMFA-diTh-CA. These results note that the
modification of the small molecular sensitizer by the carboxylic
acid may negatively affect the transport properties of the hole
carrier.
C
71-PCBMfilms (blacksolid line), bisDMFA-diTh-MMN/C71-PCBM films
(redsolidline), and bisDMFA-diTh-MIMN/C61-PCBMfilms(blue line). (b)
Hole-only devices (ITO/PEDOT:PSS/bisDMFA-diTh-EWG:PCBM/Au)
with a thickness L ¼ 90 ꢀ 5 nm.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7248–7253 | 7251

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on top. The photovoltaic performances of the device were
obtained under white light AM1.5G illumination from a cali-
brated solar simulator with an irradiation intensity of
100 mW/cm2.
Fig. 5 shows the current (J)-voltage (V) curves under AM 1.5
conditions (100 mW per cm2) of bisDMFA-diTh-CA/C71-PCBM
(or bisDMFA-diTh-MMN/C71-PCBM, bisDMFA-diTh-MIMN/
C61-PCBM) BHJ solar cells with post-annealing at 100 ꢂC, and the
corresponding values are summarized in Table 2. As shown in
Table 2, the devices fabricated with the bisDMFA-diTh-MMN/
C71-PCBM film exhibited two times better performances than
with the bisDMFA-diTh-CA/C71-PCBM and bisDMFA-diTh-
MIMN/C61-PCBM films. The most efficient devices have an PCE
of 2.46% (ꢀ0.11), with a short-circuit current (Jsc) ¼ 8.91 mA/cm2,
fill factor (FF) ¼0.36, and open-circuit voltage (Voc) ¼ 0.77 Vafter
post-annealing at 100 ꢂC for 10 min, which is a ꢁ22% higher
efficiency than that before post-annealing. These results may be
caused by the higher hole-mobility of bisDMFA-diTh-MMN and
the morphology of the bisDMFA-diTh-MMN/C71-PCBM film
investigated by atomic force microscopy (AFM), resulting in the
better phase-separated bicontinuous network with finer grains
than those of the bisDMFA-diTh-CA/C71-PCBM and bisDMFA-
diTh-MIMN/C61-PCBM films (see ESI).†
Fig. 4 Isodensity surface plots of bisDMFA-diTh-CA, bisDMFA-diTh-
MMN, and bisDMFA-diTh-MIMN.
We have also attempted a more effective phase-separated
morphology of BHJ composite using a processing additive,
1-chloronaphthalene (CN),30,31 to provide an increased efficiency
of the bisDMFA-diTh-MMN/C71-PCBM BHJ device. Fig. 6a
shows the current (J)-voltage (V) curves under AM 1.5 condi-
tions (100 mW per cm2) of the device with a BHJ film fabricated
from the bisDMFA-diTh-MMN/C71-PCBM composite with 3%
CN. Indeed, the bisDMFA-diTh-MMN/C71-PCBM BHJ device
exhibited the significantly ꢁ82% enhanced efficiency after
addition of 3% CN in solution compared to that without
additive, resulting in the best PCE of 3.66% (ꢀ0.13), with
Jsc ¼ 11.82 mA/cm2, FF ¼ 0.35, and Voc ¼ 0.90 V. Also this is
ꢁ49% higher than that after post-annealing in Table 2. This
enhanced efficiency is mainly caused by the increase of Jsc and
Voc. The more phase-segregated morphology of the BHJ film
fabricated with the bisDMFA-diTh-MMN/C71-PCBM composite
with 3% CN could increase the photocurrent of device (Fig. 6c).
The incident-photon-to-current efficiency (IPCE) spectra as
shown in Fig. 6b exhibit well-matched curves with their optical
absorptions, resulting in the close correlation with their
Fig. 5 Current (J)-voltage (V) curves under AM 1.5 conditions of device
fabricated using bisDMFA-diTh-CA/C71-PCBM BHJ active layer (black
line), bisDMFA-diTh-MMN/C71-PCBM films active layer (red line) and
bisDMFA-diTh-MIMN/C61-PCBM films (blue line) and annealed
(triangle line).
indium tin oxide (ITO)-coated glass substrate from aqueous
solution to form a film of a thickness around 40 nm. The
substrate was dried for 10 min at 140 ꢂC in air, then transferred
into a glove-box to spin-cast the active layer. A solution con-
taining the bisDMFA-diTh-CA/C71-PCBM (or bisDMFA-diTh-
MMN/C71-PCBM, bisDMFA-diTh-MIMN/C61-PCBM) in chlo-
robenzene was then spun-cast on top of the PEDOT : PSS layer.
Subsequently, after the film was dried at room temperature, the
device was pumped down in vacuum (< 10ꢃ6 torr;
1 torr ꢁ 133 Pa), and a ꢁ100 nm thick Al electrode was deposited
Table 2 Photovoltaic performances of the BHJ polymer solar cells made of bisDMFA-diTh-CA or bisDMFA-diTh-MMN/C71-PCBM (1 : 2) &
bisDMFA-diTh-MIMN/C61-PCBM (1 : 3)
Jsc
(mA cmꢃ2
Componentsa
)
Voc (V)
FF (%)
h (%)
bisDMFA-diTh-CA
bisDMFA-diTh-CA (annealed)
bisDMFA-diTh-MMN
bisDMFA-diTh-MMN (annealed)
bisDMFA-diTh-MIMN
bisDMFA-diTh-MIMN (annealed)
4.11
3.16
8.54
8.91
5.64
5.77
0.749
0.780
0.705
0.774
0.628
0.600
34.78
34.69
33.35
35.74
32.08
33.55
1.07
0.85
2.01
2.46
1.14
1.16
a
The optimum devices were fabricated with bisDMFA-diTh-CA or bisDMFA-diTh-MMN/C71-PCBM (1 : 2) & bisDMFA-diTh-MIMN/C61-PCBM
(1 : 3) films which were spin-cast at 1500 rpm from a chlorobenzene solution. The performances are determined under simulated 100 mW/cm2 AM
1.5G illumination. The light intensity using calibrated standard silicon solar cells with a proactive window made from KG5 filter glass traced to the
National Renewable Energy Laboratory (NREL). The active area of the device is 4 mm2.
7252 | J. Mater. Chem., 2011, 21, 7248–7253
This journal is ª The Royal Society of Chemistry 2011

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ITRC program (IITA 2008 C1090 0904 0013), and ERC
(the Korean government [MEST]) program (No. R11-2009- 088-
02001-0).
Notes and references
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Fig. 6 (a) Current (J)-voltage (V) curves and (b) IPCE spectra under
AM 1.5 conditions of device fabricated using bisDMFA-diTh-MMN/C71
-
PCBM BHJ active layer with 3% CN (red line), and without 3% CN
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3% CN.
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Conclusions
In conclusion, we have demonstrated the synthesis and charac-
terization of new push-pull chromophores, which comprise
bisDMFA electron donating, dithiophene bridging, and various
electron withdrawing moieties, for their application in solution
processed BHJ solar cells. The chromophore having the stronger
electron withdrawing moiety exhibits a significantly red-shifted
absorption spectrum and enhanced molar extinction efficiency
due to its strong intramolecular charge transfer in solution as
well as in solid-state. A remarkable PCE of 3.66% (ꢀ0.12) was
observed in the BHJ film fabricated with the bisDMFA-diTh-
MMN/C71-PCBM composite with 3% CN, showing ꢁ82% and
ꢁ49% higher efficiencies compared to without treatment of CN
and after post-annealing, respectively. These results obtained
from the molecular engineering approaches shown in this study
can give an important guide for developing new materials in
solution-processed small molecule BHJ solar cells.
€
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Acknowledgements
The research was supported by the WCU (the Ministry of Educa-
tion and Science) program (Grant No. R 31-2008-0001- 10035-0),
This journal is ª The Royal Society of Chemistry 2011
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