Synthesis and characterization of push-pull organic semiconductors with various acceptors for solution-processed small molecule organic solar cells

Article abstract of DOI:10.1016/j.tet.2012.03.061

New efficient push-pull organic semiconductors comprising of the bis(9,9-dimethyl-9H-fluoren-2-yl)aniline (bisDMFA) donor and the various acceptors such as NO₂, DCBP, and TCF, which were linked with bithiophene or vinyl bithiophene π-conjugatio

Full text of DOI:10.1016/j.tet.2012.03.061

Tetrahedron 68 (2012) 4029e4036
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and characterization of pushepull organic semiconductors with various
acceptors for solution-processed small molecule organic solar cells
,
a,
Nara Cho a, Jooyoung Kim a, Kihyung Song b, Jae Kwan Lee c , Jaejung Ko
*
*
a Department of New Material Chemistry, Korea University, Chungnam 330-700, Republic of Korea
b Department of Chemistry, Korea National University of Education, Chungbuk 363-791, Republic of Korea
c Department of Chemistry Education, Chosun University, Gwangju 501-759, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New efficient pushepull organic semiconductors comprising of the bis(9,9-dimethyl-9H-fluoren-2-yl)
aniline (bisDMFA) donor and the various acceptors such as NO2, DCBP, and TCF, which were linked with
Received 25 January 2012
Received in revised form 14 March 2012
Accepted 17 March 2012
bithiophene or vinyl bithiophene
p-conjugation bridges, were synthesized, and their photovoltaic
characteristics were investigated in solution-processed small molecule organic solar cells (SMOSCs). The
intramolecular charge transfers of these materials were effectively appeared in between bisDMFA donor
and acceptors, depending on the electron-withdrawing strength of acceptors. The organic semi-
conductors having NO2 and DCBP acceptors exhibited the most efficient photovoltaic performance,
showing power conversion efficiency (PCE) of 1.98% (ꢀ0.17) and 2.01% (ꢀ0.21), respectively. When the
TiOx thin layer was treated on photoactive layer, the organic semiconductor having NO2 showed the best
PCE of 2.70% with short circuit current of 8.19 mA/cm2, fill factor of 0.40, and open circuit voltage of
0.83 V in SMOSC devices.
Available online 23 March 2012
Keywords:
Bulk heterojunction
Intramolecular charge transfer
Electron-withdrawing acceptor
Organic solar cell
Organic semiconductor
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
for characteristics such as average molecular weight (Mw) and
polydispersity index (PDI) as well as difficulty in purification. Moti-
Solution-processed organic solar cells (OSCs) fabricated by means
of versatile printing techniques such as doctor blade, inkjet, and roll-
to-roll can provide the most attractive advantages of device such as
low cost, light weight, solution processability. Over the last few
years, enormous efforts have intensively focused on improving the
device performances toward power conversion efficiency (PCE) of
vated by this, considerable research effort has been focused on de-
veloping efficient small molecule materials for improved device
performance, with the near-term goal being a power conversion
efficiency (PCE) comparable to polymer-based solar cells (PSCs).4
Indeed, recent breakthroughs in realizing PCEs of above 6% have
placed solution-processed small molecule organic solar cells
(SMOSCs) in competition with PSCs.5
10% though developments of photoactive materials such as
p-con-
jugated (semiconducting) polymer and fullerene derivatives, or
functional layers such as buffering, charge transporting, and optical
spacing.1 And recently great achievement of promising PCEs of above
8% in OSC has offered special attention as a strong candidate on next
generation solar cell with threatening inorganic thin film solar cell as
well as dye-sensitized solar cells (DSSCs).2 Most of high efficiencies
have been reported in OSC fabricated with bulk-heterojunction (BHJ)
materials comprising of low-bandgap semiconducting polymers and
[6,6]-phenyl-C61(or 71)-butyric acid methyl ester (C61(or 71)-PCBM).3
Nevertheless, small molecule organic semiconductor seems to fas-
cinate more than these polymers from the viewpoint of mass pro-
duction for commercial application due to their low reproducibility
The efficient organic semiconductors reported in SMOSC often
have the structural symmetric motifs containing the electron-
withdrawing cores such as benzothiadiazole,6 squaraine,7 or dike-
topyrrolopyrrole,4c which were motivated by low-bandgap semi-
conducting polymers or pushepull molecular structures in
nonlinear optics (NLO) and DSSC due to their superior optoelectronic
properties. Recently, we have also reported various symmetric
pushepull organic semiconductors comprising triphenylamine
(TPA) donor and squaraine (or diketopyrrolopyrrole) acceptor for
solution-processed SMOSC.8 These pushepull structures in SMOSC
enable an efficient intramolecular charge transfer (ICT) to give the
better molar absorptivity. And an TPA electron-donating unit can
play an important role of stabilizing the separated hole from exciton
and improving the transporting property of hole carrier.9
On the other hand, we have interestingly found that the
unsymmetric pushepull organic semiconductor having the differ-
ent end groups of TPA donor and squaraine acceptor presented the
* Corresponding authors. Tel.: þ82 62 230 7319; fax: þ82 62 232 8122 (J.K.L.);
tel.: þ82 41 860 1337; fax: þ82 41 860 5396 (J.K.); e-mail addresses: chemedujk@
chosun.ac.kr (J.K. Lee), jko@korea.ac.kr (J. Ko).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.03.061

4030
N. Cho et al. / Tetrahedron 68 (2012) 4029e4036
better PCE than the symmetric structures of these donor and ac-
ceptor,10 and that the new organic semiconductor, bisDM-
FAediTheMMN comprising bis(9,9-dimethyl-9H-fluoren-2-yl)
aniline (bisDMFA) donor, methylene-malononitrile (MMN) accep-
tor, and bithiophene bridge exhibited the superior photovoltaic
performance in solution-processed BHJ SMOSC even its small mo-
lecular size, which should be a obstacle for forming the self-
network domains in BHJ composite with PCBM, showing the PCE
of 3.6%.11 In this study, we also tried to develop new semi-
conducting pushepull type organic small molecules having various
acceptor motifs, especially 4-dicyanomethylene-6-tert-butyl-4H-
malononitrile (iv), and 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-
ylidene)malononitrile (v) could be effectively synthesized through
modifying the procedures reported previously.12 BisDM-
FAediTheDCBP (2) (and bisDMFAediTheTCF (3)) were readily
prepared through Knoevenagel condensation reaction with i and
iii (and iv). And the borate compound of ii, which was produced
from reaction of ii and pinacol borate using the n-BuLi, success-
fully performed the palladium-catalyzed Suzuki coupling reaction
with 2-bromo-5-nitrothiophene using phase-transfer catalyst,
Aliquat 336 mixture in biphase solution of anhydrous toluene and
deionized water, producing the bisDMFAediTheNO2 (1) with good
yield above 85%. The chemical structure of the synthesized prod-
ucts, bisDMFAediTheacceptor series, (1, 2, and 3) were verified
with 1H NMR, 13C NMR, ATR-FTIR, and MALDI-TOF mass analysis.
And the corresponding further characteristic data of these mate-
rials were summarized in Fig. S1 and Table S1 (see Supplementary
data). These materials have good solubility in common organic
solvents, such as methylene chloride, chloroform, chlorobenzene,
and toluene.
Fig. 1 shows the UVevis absorption spectra of the bisDM-
FAediTheacceptor series, (1, 2, and 3) in chlorobenzene solution and
thin films, and the corresponding optical properties are summarized
in Table 1. As shown in Fig. 1, the absorption spectra of 1 (black) and 2
(red) in chlorobenzene (solid line) showed typical two transition
bands with the moderate molar absorptivities in wavelength region
of 300e700 nm13: (1) 38,000 Mꢁ1 cmꢁ1 at367 nm, 22,000 Mꢁ1 cmꢁ1
at 493 nm and (2) 66,500 Mꢁ1 cmꢁ1 at 356 nm, 34,000 Mꢁ1 cmꢁ1 at
510 nm. The first peak at shorter wavelength could be assigned to the
pyran
(DCBP),
2-dicyanomethylene-3-cyano-5-dimethyl-2,5-
dihydrofuran (TCF), and nitrogen dioxide (NO2). To the best our
knowledge, the electron accepting features can significantly affect
the energy bandgap and molar absorptivity as well as the ICT
strength of pushepull organic materials. Beside, the structural
variation of acceptor motifs often afford its unique features that
when integrated into bulk heterojunction composite with PCBM
can critically affect the performance characteristics of BHJ OSC.
Herein, we report the synthesis and photovoltaic character-
istics of new efficient pushepull organic semiconductors
comprising of on the bisDMFA donor and the various
acceptors,
4-(bis((9,9-dimethyl-9H-fluoren-2-yl)amino)phe-
nyl)-2-(50-nitrothiophen-20-yl)thiophen (bisDMFAediTheNO2,
1), (E)-2-(2-(2-(50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)
phenyl)-2,20-bithiophen-5-yl)vinyl)-6-tert-butyl-4H-pyran-4-ylidene)
malononitrile (bisDMFAediTheDCBP, 2), and (E)-2-(4-(2-(50-(4-
(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-bithiophen-
5-yl)vinyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malononi-
trile (bisDMFAediTheTCF, 3), which were linked with bithiophene
pep
*
transition and the second peak at longer wavelength should
result from an ICT between TPA donor and 2,20-bithiophene (BT) ac-
ceptor. These and ICT transitions originate in the highest oc-
or vinyl bithiophene
p-conjugation bridges, in solution-processed
pep
*
SMOSC. Scheme 1 shows the structures of the synthesized organic
semiconductors and device architecture of solution-processed
SMOSC.
cupied molecular orbital (HOMO)ꢁ1/lowest unoccupied molecular
orbital (LUMO) and the HOMO/LUMO monoexcitations, re-
spectively. The ICT band between bisDMFA donor and DCBP acceptor
Scheme 1. Molecular structures of the bisDMFAediTheacceptors and device architecture of solution-processed small molecule organic solar cell.
2. Results and discussion
of 2 showed w17 nm red-shift and w1.6 times higher intensity than
that of 1 due to the more elongated -conjugation length and the
p
The synthetic methods are outlined in Scheme 2. All reactions
were carried out under a nitrogen atmosphere. 50-(4-(Bis(9,9-
dimethyl-9H-fluoren-2-yl)amino)phenyl)-2,20-bithiophene-5-car-
baldehyde (i), N-(4-bromophenyl)-N-(9,9-dimethyl-9H-fluoren-2-
yl)-9,9-dimethyl-9H-fluoren-2-amine (ii), 5-iodo-50-nitro-2,20-
bithiophene (iii), 2-(2-tert-butyl-6-methyl-4H-pyran-4-ylidene)
higher electron-withdrawing strengths of DCBP acceptor compared
tothatofNO2 acceptor. Meanwhile, 3 (blue)exhibitedthreetransition
bands, showing the absorption in whole visible region. The three
peaks at 365 nm, 489 nm, and 630 nm could be also assigned to the
transition conducted by the HOMOꢁ2/LUMO (365 nm), HOMO-
ꢁ1/LUMO, (489 nm) and the HOMO/LUMO (630 nm)

N. Cho et al. / Tetrahedron 68 (2012) 4029e4036
4031
Scheme 2. Schematic diagram for the synthesis of the bisDMFAediTheacceptors (NO2 (1), DCBP (2), TCF (3)).
intensive and significantly w137 nm and w120 nm red-shifted ab-
sorption compared to those of 1 and 2, respectively, because of the
better electron-withdrawing characteristics by three CN groups of
TCF acceptor.
In solid-state thin film, the
show the red-shifted absorption bands and broaden spectra by the
intermolecular
packing interaction than those in solution.15
p-conjugated organic materials often
pep
But, 1 in solid-state exhibited a little bit red-shifted absorption
spectrum and 2 and 3 in solid-state exhibited only broaden spectra
without significant band-shifts to longer wavelength region. On the
basis of these observations, it seems that the intermolecular
packing interactions in solid-state film could be interrupted by the
amorphous non-planar bisDMFA and affected by the spatial size of
acceptors.
Fig. 2 shows the optimized structure of bisDMFAediTheacceptor
series, (1, 2, and 3), which were calculated by the time dependent-
density functional theory (TD-DFT) using the B3LYP functional/6-
31G basis set. The orbital density of highest occupied molecular
*
Fig. 1. UVevis absorption spectra of the bisDMFAediTheacceptors [1 (black), 2 (red),
and 3 (blue)] in chlorobenzene solution (solid line) and thin films (dashed line).
orbital (HOMO) of 1, 2, and 3 were evenly distributed on bisDM-
FAebithopheneeacceptor, but the orbital density of lowest un-
occupied molecular orbital (LUMO) of these materials was
predominantly located on the electron accepting core and thiophene
bridge, showing a general orbital distribution like pushepull organic
semiconductors. These calculations reveal that the ICT from bisDMFA
to acceptor core can effectively occur in 1, 2, and 3, absorbed the
photon energy, and the bisDMFA can especially play a role for sta-
bilizing of hole separated from exciton and improve the transport
property of hole carrier.
monoexcitations, respectively.14 Also, the ICT transition observed as
*
third peak of 3 was more intensive than the other pep transitions,
which provide the ICT transition state between TPA donor and TCF
acceptoras dominantexcitation, even though the ICT bands of 1 and 2
showed the less intensive than their
pep transition bands. More-
*
over, the ICT band of 3 exhibited the w2 times and w1.4 times more

4032
N. Cho et al. / Tetrahedron 68 (2012) 4029e4036
Table 1
Optical, redox parameters of the bisDMFAediTheacceptors [1, 2, and 3]
/Mꢁ1 cmꢁ1
)
Eonset,ox (V)/HOMO (eV)b
Eonset,red (V)/LUMO (eV)
Eonsetc (eV)
E0e0 (eV)
d
Compounds
labsa/nm (
3
1
2
3
367 (38,000), 493 (22,000)
356 (66,500), 510 (34,000)
365 (40,500), 489 (25,000), 630 (46,000)
0.308/ꢁ5.108
0.197/ꢁ4.997
0.221/ꢁ5.021
ꢁ1.202/ꢁ3.51
ꢁ1.287/ꢁ3.513
ꢁ1.000/ꢁ3.800
2.08
2.05
1.69
1.97
1.99
1.58
a
Absorption spectra were measured in chlorobenzene solution.
b
c
Redox potential of the compounds were measured in CH2Cl2 with 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mVsꢁ1
Eonset was calculated from the onset from absorption spectra in chlorobenzene solution.
.
d
E0e0 was calculated from the absorption thresholds from absorption spectra in chlorobenzene solution.
Fig. 2. Isodensity surface plots of bisDMFAediTheacceptors [1, 2, and 3], calculated by the time dependent-density functional theory (TD-DFT) using the B3LYP functional/6-31G
*
basis set.
Fig.
3
shows the cyclic voltammograms of the bisDM-
summarized in Table 1. The energy levels of HOMO and LUMO of
FAediTheacceptor series, (1, 2, and 3) in methylene chloride so-
lution. And the corresponding electrochemical properties are also
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 analysis were performed in an electrolyte consisting of a solu-
tion of 0.1 M tetrabutylammonium 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 external reference. The HOMO and LUMO levels can
be deduced from the oxidation and reduction onsets with the as-
sumption that the energy level of ferrocene (Fc) is 4.8 eV below
vacuum level. The CV of these materials in solid-state thin film
could not be measured due to the stripping of film on electrode.
Therefore, we determined the optical bandgap from calculation of
the absorption thresholds from absorption spectra of bisDM-
FAediTheacceptor series in chlorobenzene. The HOMO levels of 1,
2, and 3 determined in solution by CV are calculated as 5.108 eV,
4.997 eV, and 5.021 eV, respectively. From these results, we can
roughly guess that the Voc of devices fabricated using these mate-
rials/PCBM BHJ active layers may be in 0.7e0.9 eV, respectively.16
The photophysical characteristics of bisDMFAediTheacceptor
series, (1, 2, and 3) were investigated through the application of
Fig. 3. Electrochemical characterization of the bisDMFAediTheacceptors [1 (black), 2
(red), and
potentials.
3 (blue)] in dichloromethane/TBAHFP (0.1 M), scan speed 100 mV/s,

N. Cho et al. / Tetrahedron 68 (2012) 4029e4036
4033
ꢀ
ꢁ
photovoltaic device fabricated with these materials/C61(or 71)-PCBM
BHJ films. In the course of studying the characteristics of over 200
solar cells, the most efficient photovoltaic cells fabricated using of
[bisDMFAediTheacceptor/C71-PCBM were optimized at ratio of 1:3
(1 and 3) or 1:2 (2) approximately. These BHJ films were cast on top
of PEDOT:PSS (Heraeus, AI 4083) layer. The optimum thicknesses of
these materials, 1, 2, and 3 based BHJ films obtained under these
conditions were approximately 90 nm, 95 nm, and 80 nm, re-
spectively. Fig. 4a shows the UVevis absorption spectra of the
J ¼ ð9=8Þ
where
3
$
m
V2=L3
(1)
3
is the static dielectric constant of the medium and
m
is
carrier mobility. The hole mobilities of bisDMFAediTheacceptor
series, [1 (black), 2 (red), and 3 (blue)] evaluated using the above
MotteGurney
Law
(
3 ¼3
3 0),
were
4.03ꢂ10ꢁ5 cm2/V s,
2.00ꢂ10ꢁ5 cm2/V s, and 1.28ꢂ10ꢁ5 cm2/V s, respectively. Com-
pound 1 exhibited about 2 times and 3.1 times higher hole mobility
than those of 2 and 3, respectively. It was interestingly found that
these hole mobilities were contrary to the electron-withdrawing
strength of acceptors. The hole mobilities of p-type organic semi-
conductors observed in these BHJ films might be closely related to
the charge stability separated from excition as well as the in-
termolecular packing interaction, leading to the increase of trans-
porting property of hole carrier. The bisDFMA donor can stabilize
the separated hole from exciton. Therefore, we note that the bulk
substituents such as tert-butyl group of DCBP acceptor or dimethyl
group of TCF acceptor might spatially interrupt the intermolecular
packing interaction in BHJ system with PCBM. These BHJ films of
C71-PCBM and 2 (or 3) containing the bulk DCBP (or TCF) also
exhibited the rougher surface morphology than that of 1, showing
higher root mean square (rms) values, which were investigated
with atomic force microscopy (AFM) analysis (see Fig. S3 of
Supplementary data).
bisDMFAediTheacceptor [1 (black), 2 (red), and 3 (blue)]/C71
-
PCBM (1:3) films. As shown in Fig. 4a, the absorption bands of these
materials in BHJ composites with C71-PCBM were red-shifted
w10 nm and broaden to the longer wavelength compared to that
of its pristine films shown in Fig. 1.
To investigate the space-charge effects, we extracted the hole
mobilities of these organic semiconductors from the space-charge
limitation of current (SCLC) JeV characteristics obtained in the dark
for hole-only devices. We measured the average of hole mobility for
20 cells at three times per one cell in order to compensate for
causable error. Fig. 4b shows the dark-current characteristics of
hole-only ITO/PEDOT:PSS/bisDMFAediTheacceptor:C71-PCBM/Au
devices as a function of the bias corrected by the built-in voltage
determined from the difference in work function between Au and
the HOMO level of these materials. The Ohm’s law can be observed
at low voltages as an effect of thermal free carriers. For the presence
of carrier traps in the active layer, there is a trap-filled-limit (TFL)
region between the ohmic and the trap-free SCLC regions. The SCLC
behavior in the trap-free region can be characterized using the
MotteGurney square law (Eq. 1).17
Fig. 5 shows the current (J)evoltage (V) curves under AM 1.5
conditions (100 mW/cm2) and incident-photon-to-current effi-
ciency (IPCE) spectra of bisDMFAediTheacceptors [1 (black), 2
(red), and 3 (blue)]/C71-PCBM BHJ solar cells fabricated under
Fig. 5. (a) Current (J)evoltage (V) curves under AM 1.5 conditions (100 mW/cm2) and
(b) IPCE spectra of the bisDMFAediTheacceptors/C71-PCBM [1 (black), 2 (red), and 3
(blue)] BHJ solar cells fabricated under optimized processing condition with (solid
line)/without (dashed line) insertion of TiOx layer.
Fig. 4. (a) UVevis absorption spectra and (b) space-charge limitation of current JeV
characteristics of the bisDMFAediTheacceptors/C71-PCBM [1 (black), 2 (red), and 3
(blue)] BHJ films, which hole-only devices (ITO/PEDOT:PSS/Donor:C71-PCBM/Au).

4034
N. Cho et al. / Tetrahedron 68 (2012) 4029e4036
optimized processing condition. The corresponding values are
summarized in Table 2. The IPCE spectra of these devices as shown
in Fig. 5b exhibit well-matched curves with their optical absorp-
tions, resulting in the close correlation with their photocurrents in
JeV curves.
semiconductor shown in this study can give an important guide for
developing new materials in solution-processed small molecule
BHJ solar cell.
3. Experimental section
3.1. General methods
Table 2
Photovoltaic performances of the devices fabricated with the bisDM-
FAediTheacceptors [1, 2, and 3]/C71-PCBM BHJ filmsa
Solvents were distilled from appropriate reagents. All reagents
were purchased from SigmaeAldrich, TCI, and Alfa Aesar. All re-
actions were carried out under a nitrogen atmosphere. And [6,6]-
phenyl-C71-butyric acid methyl ester (C71-PCBM) was obtained
from Nano-C. 1H NMR and 13C NMR spectra were recorded on
a Varian Mercury 300 spectrometer. Chemical shift values were
recorded as parts per million relative to tetramethylsilane as an
internal standard unless otherwise indicated, and coupling con-
stants in hertz. Elemental analyses were performed with a Carlo
Elba Instruments CHNS-O EA 1108 analyzer. Mass spectra were
recorded on a JEOL JMS-SX102A instrument. UVevis data were
measured with a PerkineElmer Lambda 2S UVevisible spectrom-
eter. Optimized structures calculated by TD-DFT using the B3LYP
Organic
semiconductor layer
Functional Jsc (mA cmꢁ2
)
Voc (V)
FF (%)
h (%)
1
None
TiOx
None
None
7.40 ꢀ 0.1
8.19 ꢀ 0.3
6.80 ꢀ 0.13
6.26 ꢀ 0.2
0.77 ꢀ 0.1 35 ꢀ 2.0 1.98 ꢀ 0.25
0.83 ꢀ 0.1 40 ꢀ 2.2 2.70 ꢀ 0.3
0.85 ꢀ 0.08 35 ꢀ 1.8 2.01 ꢀ 0.21
0.80 ꢀ 0.09 33 ꢀ 1.7 1.68 ꢀ 0.18
1
2b
3b
a
The photovoltaic characteristics are performed 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 Re-
newable Energy Laboratory (NREL). The masked active area of device is 4 mm2.
b
The devices fabricated with this BHJ films were optimized after post-annealing
at 100 ꢃC for 10 min.
As shown in Fig. 5 and Table 2, the devices conventionally
fabricated with 1/C71-PCBM, 2/C71-PCBM, and 3/C71-PCBM
exhibited a PCE of 1.98% (ꢀ0.17) with short circuit current (Jsc)¼
7.40 mA/cm2, fill factor (FF)¼0.35, and open circuit voltage (Voc)¼
0.77 V, a PCE of 2.01% (ꢀ0.21) with Jsc¼6.80 mA/cm2, FF¼0.35,
and Voc¼0.84 V, and a PCE of 1.68% (ꢀ0.23) with Jsc¼6.26 mA/
cm2, FF¼0.33, and Voc¼0.80 V, respectively. Although 3 showed
the absorption band covering the whole visible region, the so-
lution-processed SMOSCs fabricated with 3/C71-PCBM exhibited
the less Jsc value than those of 1/C71-PCBM or 2/C71-PCBM BHJ
film. This might be mainly originated from the low IPCE value in
longer wavelength region, indicating the facile recombination of
separated charges from the excitons conducted by mono-
excitation in ICT transition band especially. These results were
consistent with their hole mobilities obtained by SCLC JeV
characteristics shown in Fig. 4b. For these results, in order to
increase the efficiency of the device with especially 1 due to its
highest photocurrent, we used the TiOx thin layer that can play
the effective roles of optical spacer and buffer layer through the
insertion between 1/C71-PCBM BHJ layer and Al electrode.18 The
best PCE of 2.70% (ꢀ0.25) in devices fabricated using 1/C71-PCBM
film with TiOx layer was observed with Jsc¼8.19 mA/cm2, FF¼0.40,
and Voc¼0.83 V, which showed the further improvements of all
photovoltaic parameters, resulting in w36% higher efficiency than
that without TiOx layer.
functional and the 6-31G basis set. The highest occupied molecular
*
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energies were determined using minimized singlet ge-
ometries to approximate the ground state. Cyclic voltammetry was
carried out with a BAS 100B (Bioanalytical System, Inc.). A three-
electrode system was used and consisted of non-aqueous reference
electrode (0.1 M Ag/Agþ acetonitrile solution; MF-2062, Bio-
analytical System, Inc.), platinum working electrode (MF-2013,
Bioanalytical System, Inc.), and a platinum wire (diam. 1.0 mm,
99.9% trace metals basis, SigmaeAldrich) as counter electrode.
Redox potentials of materials were measured in CH2Cl2 with 0.1 M
(n-C4H9)4NePF6 with a scan rate between 100 mVsꢁ1 (vs Fc/Fcþ).
Atomic force microscope (AFM) measurements were performed
with a Digital Instruments NanoScope IV in the tapping mode. All
FTIR measurements were performed using a Nicolet 6700 FT-IR
(Thermo
Scientific)
spectrometer
operating
in
the
4000e400 cmꢁ1 wavenumber range (mid-infrared). Melting point
€
was measured by Melting Point B-540 (BUCHI).
3.2. Fabrication of solar cell devices
The BHJ films were prepared under optimized conditions
according to the following procedure reported previously:19 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. PEDOT:PSS (Heraeus,
Clevios P VP.AI 4083) in aqueous solution was spin-cast to form
a film with thickness of approximately 35 nm. The substrate was
dried for 10 min at 140 ꢃC in air, then transferred into a glove box to
spin-cast the photoactive layer. The optimized bisDM-
FAediTheacceptor series:C71-PCBM BHJ materials were blended
with 1:2 or 1:3 weight ratio in chlorobenzene at a concentration of
30 mg/mL. These solutions were then spin-cast on top of the PEDOT
layer. The substrate was dried for 10 min at 80 ꢃC in air. Then, the
device was pumped down to lower than 10ꢁ7 Torr and a w100 nm
thick Al electrode was deposited on top.
In conclusion, we have demonstrated the synthesis and photo-
voltaic characteristics of new efficient pushepull organic semi-
conductors comprising of on the bis(9,9-dimethyl-9H-fluoren-2-yl)
aniline (bisDMFA) donor and the various acceptors, bisDM-
FAediTheNO2 (1), bisDMFA-diTh-DCBP (2), and bisDMFA-diTh-TCF
(3), which were linked with bithiophene or vinyl bithiophene
p-
conjugation bridges, in solution-processed SMOSC. These materials
showed the strong intramolecular charge transfer between
bisDMFA donor and acceptors (NO2, DCBP, or TCF). Especially, the
ICT band of 3 was significantly red-shifted above 120 nm compared
to those of 1 and 2 because of the better electron-withdrawing
characteristics by three CN groups of TCF acceptor. We also found
that the bulk substituents such as tert-butyl group of DCPB acceptor
or dimethyl group of TCF acceptor might spatially interrupt the
intermolecular packing interaction in BHJ system with PCBM. The
best performances were observed in the devices fabricated with
1/C71-PCBM BHJ material that could be significantly improved by
means of insertion of TiOx thin layer between photoactive layer and
Al electrode. These results obtained from the facile structural
modifications using various accepting units of organic
3.3. Characterization of solar cell devices
Solar cells efficiencies were characterized under simulated
100 mW/cm2 AM 1.5G irradiation from a Xe arc lamp with an AM
1.5 global filter. 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

N. Cho et al. / Tetrahedron 68 (2012) 4029e4036
4035
source meter. The currentevoltage characteristics of the cell un-
3.6. Synthesis of (E)-2-(4-(2-(50-(4-(bis(9,9-dimethyl-9H-
fluoren-2-yl)amino)phenyl)-2,20-bithiophen-5-yl)vinyl)-3-
cyano-5,5-dimethylfuran-2(5H)-ylidene)malononitrile
(bisDMFAediTheTCF, 3)
der these conditions were determined by biasing the cell exter-
nally and measuring the generated photocurrent. This process
was fully automated using Wavemetrics software. The IPCE
spectra for the cells were measured on an IPCE measuring system
(PV measurements).
2-(3-Cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile
(iv, 35.9 mg, 0.18 mmol) and piperidine (3e5 drops) were added to
a solution of 50-(4-(bis(9,9-dimethyl-9H-fluoren-2-yl)amino)phe-
nyl)-2,20-bithiophene-5-carbaldehyde (ii, 80 mg, 0.12 mmol) in
CHCl3/CH3CN (1:1, 10 ml:10 ml) at room temperature. The reaction
mixture was heated under reflux for 24 h. Concentration and pu-
rification of the residue by flash column chromatography (hexane/
ethyl acetate¼2:1, Rf¼0.2) gave the product (dark blue solid, 68 mg,
69%). Mp: 317e319 ꢃC. Mass: m/z 850.18 [Mþ]. IR: 2225 cmꢁ1 (CN).
3.4. Synthesis of 4-(bis((9,9-dimethyl-9H-fluoren-2-yl)amino)
phenyl)-2-(50-nitrothiophen-20-yl)thiophen
(bisDMFAediTheNO2, 1)
Under nitrogen atmosphere and at ꢁ10 ꢃC, n-BuLi (0.68 ml,
1.6 M in hexane, 1.09 mmol) was added drop-wise to a dry THF
solution containing N-(4-bromophenyl)-N-(9,9-dimethyl-9H-fluo-
ren-2-yl)-9,9-dimethyl-9H-fluoren-2-amine (i, 0.5 g, 0.89 mmol).
After 30 min stirring at ꢁ10 ꢃC, 0.3 ml (1.47 mmol) of 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added slowly to the
reaction solution at ꢁ10 ꢃC. The temperature of the solution was
warmed to room temperature and the reaction mixture was stirred
for 1 h. Then, quench the reaction with deionized water. The so-
lution was extracted with dichloromethane, dried with MgSO4, and
then a solvent was evaporated with a rotary evaporator. A light
yellow solid (silica gel TLC plate, ethyl acetate/hexane¼1:10,
Rf¼0.2) was obtained. Without further purification, this compound
(0.2 g, 0.33 mmol) was mixed with 5-iodo-50-nitro-2,20-bithio-
phene (E, 0.1 g, 0.296 mmol), Pd(PPh3)4 (0.02 g, 0.017 mmol), K2CO3
(0.18 g, 1.3 mmol), a drop of Aliquat 336, and degassed water (2 M)
in dry toluene (25 ml), then refluxed under nitrogen atmosphere
for overnight. After cooling to room temperature, the solution was
extracted with dichloromethane, dried with MgSO4, and subjected
to column chromatography (silica gel, dichloromethane/
hexane¼1:4, Rf¼0.1). A purple solid was obtained. Yield: 85%
1H NMR (300 MHz, DMSO-d6):
d
8.14 (d, 1H J¼15.9 Hz), 7.83e7.75
(m, 6H), 7.68e7.51 (m, 7H), 7.31e7.27 (m, 6H), 7.12e7.06 (m, 3H),
6.76 (d, 1H, J¼15.9 Hz), 1.79 (s, 6H), 1.38 (s, 12H). 13C NMR (75 MHz,
DMSO-d6): d 176.3, 174.1, 154.7, 153.0, 147.5, 146.0, 145.1, 144.1, 139.4,
138.2, 137.9, 137.2, 134.1, 133.3, 127.8, 126.7, 126.4, 126.3, 125.5,
124.0, 123.2, 122.4, 122.2, 120.7, 119.2, 118.5, 112.7, 112.3, 111.5, 110.5,
98.3, 55.7, 46.1, 26.4, 25.1, 18.1. Anal. Calcd for C56H42N4OS2: C,
79.03; H, 4.97. Found: C, 78.85; H, 4.88.
Acknowledgements
This research was supported by World Class University program
funded by the Ministry of Education, Science and Technology
through the National Research Foundation of Korea (R31-2011-
000-10035-0) and the New & Renewable Energy of the Korea In-
stitute of Energy Technology Evaluation ad Planning (KETEP) grant
funded by the Korea Government Ministry of Knowledge Economy
(no. 20103060010020).
(0.17 g). Mp: 136e138 ꢃC. Mass: m/z 668.50 [Mþ]. IR: 1322 cmꢁ1
1446 cmꢁ1 (NO2). 1H NMR (300 MHz, CDCl3):
7.86 (d, 1H,
,
d
J¼4.5 Hz), 7.68 (d, 2H, J¼6.9 Hz), 7.64 (d, 2H, J¼7.8 Hz), 7.51 (d, 2H,
J¼8.4 Hz), 7.41 (d, 2H, J¼7.8 Hz), 7.35e7.18 (m, 10H), 7.15 (dd, 2H,
J¼7.8, 1.8 Hz), 7.08 (d, 1H J¼4.2 Hz), 1.43 (s, 12H). 13C NMR (75 MHz,
Supplementary data
Supplementary data related to this article can be found in the
online version, at doi:10.1016/j.tet.2012.03.061.
CDCl3):
d 155.31, 153.63, 148.55, 147.41, 146.79, 145.65, 138.89,
134.93, 132.93, 130.01, 129.58, 127.87, 127.16, 126.81, 123.74, 123.38,
123.27, 122.64, 121.94, 120.83, 119.64, 119.24, 47.06, 27.24. Anal.
Calcd for C44H34N2O2S2: C, 76.94; H, 4.99. Found: C, 76.77; H, 4.90.
References and notes
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1H NMR (300 MHz, DMSO-d6):
d 7.79e7.71 (m, 5H), 7.65e7.62 (m,
2H), 7.54e7.40 (m, 6H), 7.34e7.28 (m, 6H), 7.12e7.05 (m, 5H), 6.97
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