Facile synthesis of fluorine-substituted benzothiadiazole-based organic semiconductors and their use in solution-processed small-molecule organic solar cells

Article abstract of DOI:10.1002/chem.201201321

A facile new protocol for the synthesis of iodinated derivatives of fluorinated benzothiadiazoles is demonstrated for the production of p-type semiconducting materials. The newly synthesized small-molecule compounds bis[TPA-diTh]-MonoF-BT and bis[TPA-diTh]-DiF-BT exhibited a power conversion efficiency of 2.95 % and a high open-circuit voltage of 0.85 V in solution-processed small-molecule organic solar cells. A facile new protocol was developed for synthesis of iodinated derivatives of fluorinated benzothiadiazoles as precursors for fluorine-substituted benzothiadiazole-based organic semiconductors 2 and 3, which exhibit enhanced intramolecular charge transfer compared to unfluorinated analogue 1 and hence improved photovoltaic performance in small-molecule organic solar cells (see figure). Copyright

Full text of DOI:10.1002/chem.201201321

FULL PAPER
DOI: 10.1002/chem.201201321
Facile Synthesis of Fluorine-Substituted Benzothiadiazole-Based Organic
Semiconductors and Their Use in Solution-Processed Small-Molecule
Organic Solar Cells
Nara Cho,[a] Kihyung Song,[b] Jae Kwan Lee,*[c] and Jaejung Ko*[a]
Abstract: A facile new protocol for the synthesis of iodinated derivatives of fluori-
nated benzothiadiazoles is demonstrated for the production of p-type semiconduct-
Keywords: charge
halogenation
semiconductors · solar cells
transfer
heterocycles
·
·
ing materials. The newly synthesized small-molecule compounds bis
ACHTUNGTRENUN[NG TPA-diTh]-
·
MonoF-BT and bis[TPA-diTh]-DiF-BT exhibited a power conversion efficiency of
ACHTUNGTRENNUNG
2.95% and a high open-circuit voltage of 0.85 V in solution-processed small-mole-
cule organic solar cells.
Introduction
small-molecule chromophores have several advantages over
their polymeric counterparts from the viewpoint of mass
production for commercial application, on account of the
low reproducibility of the polymers in terms of weight aver-
age molecular weight and polydispersity index, in addition
to difficulties associated with purification. Hence, considera-
ble research effort has focused on developing efficient
small-molecule materials for improved device performance,
with the near-term goal being a PCE comparable to that of
polymer-based solar cells (PSCs).[4] Indeed, recent break-
throughs in achieving PCEs greater than 6% have placed
solution-processed small-molecule organic solar cells
(SMOSCs) squarely in competition with PSCs.[5]
Efficient organic semiconductors reported for use in
SMOSCs often have structural core motifs[4] such as oligo-
thiophene, bridged dithiophene, benzothiadiazole (BT),
squaraine, or diketopyrrolopyrrole. Their use is motivated
by low-bandgap semiconducting polymers or push–pull mo-
lecular structures in nonlinear optics and DSSCs owing to
their superior optoelectronic properties. Recently, we re-
ported various symmetric and unsymmetric push–pull chro-
mophores for solution-processed SMOSCs.[6] The use of
push–pull structures in SMOSCs enables efficient intramo-
lecular charge transfer (ICT), providing better molar ab-
sorptivity and the low bandgap required for an organic semi-
conductor. In addition, an electron-donating group such as
triphenylamine (TPA) can play an important role in stabiliz-
ing the separated hole from an exciton and improving the
hole-transporting properties of the hole carrier.[4,6] Recently,
fluorine-substituted benzothiadiazole motifs were reported
as effective electron-accepting units, producing a low-bandg-
ap semiconducting PSC with 5–7% efficiency.[7–9] The fluo-
rine atom often affords unique features that, when integrat-
ed into a BHJ composite with PCBM, boosts intermolecular
Solution-processed organic solar cells (OSCs) can be fabri-
cated by versatile printing techniques such as doctor blade,
inkjet, and roll-to-roll to provide inexpensive and light-
weight devices. Over the last few years, efforts have been in-
tensively focused on improving device performance toward
achievement of a power conversion efficiency (PCE) of
10%. Research has mainly been concerned with further de-
velopments in photoactive materials such as p-conjugated
(semiconducting) polymer and fullerene derivatives, or func-
tional layers such as buffering, charge transporting, and opti-
cal spacing layers.[1] Recently, the significant achievement of
promising PCEs above 8% for OSCs has provided opportu-
nities for them to become strong candidates for next-genera-
tion solar cells, over inorganic thin-film solar cells and dye-
sensitized solar cells (DSSCs).[2] Most of the reported high
efficiencies have been demonstrated for OSCs fabricated
with bulk-heterojunction (BHJ) materials comprising low-
bandgap semiconducting polymers and [6,6]-phenyl-C61(or 71)
-
butyric acid methyl ester (C61(or 71)-PCBM).[3,7] Nevertheless,
[a] N. Cho, Prof. J. Ko
Department of New Materials Chemistry
Korea University
Chungnam, 330-700 (Republic of Korea)
Fax : (+82)41-860-5396
E-mail: jko@korea.ac.kr
[b] Prof. K. Song
Department of Chemical Education
Korea National University of Education
Chungbuk, 363-791 (Republic of Korea)
[c] Prof. J. K. Lee
Department of Chemistry Education
Chosun University
Gwangju, 501-759 (Republic of Korea)
Fax : (+82)62-232-8122
ꢀ
charge transfer through C F···H interactions and thus af-
fects the performance characteristics. For these reasons, we
are interested in the more reinforced ICT from small-mole-
cule organic semiconductors through substitution of an elec-
E-mail: chemedujk@chosun.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201321.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

tron-withdrawing atom, specifi-
cally fluorine, into the efficient
push–pull semiconductor back-
bone for solution-processed
SMOSCs.
Herein, we report the synthe-
sis and photovoltaic characteris-
tics of novel fluorine-substitut-
ed
small-molecule compounds 2,6-
bis(5,5’-bis(N,N’-diphenylani-
line)-2,2’-bithiophenyl)-4,7-(5-
fluorobenzo[c][1,2,5]thiadi-
azole) (bis[TPA-diTh]-MonoF-
BT, 2) and bis[5,5’-bis(N,N’-di-
benzothiadiazole-based
ACHTUNGTRENNUNG
Scheme 1. Molecular structures of fluorinated bisACTHUNRTGNEUNG[TPA-diTh]-BT derivatives and device architecture of solu-
tion-processed small-molecule organic solar cell.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
phenylaniline)-2,2’-bithiophen-
yl]-4,7-(5,6-difluorobenzo[c]-
A
(bisACTHNGUTRENNU[G TPA-
diTh]-DiF-BT, 3) as efficient p-
type organic semiconductors in
solution-processed
SMOSCs.
These compounds, in which
mono- or difluorinated benzo-
thiadiazole cores are linked
with a electron-donating TPA
group through a p-conjugated
bithiophene bridge, gave PCEs
of 2.95% with a high open-cir-
cuit voltage (Voc) of 0.85 V.
They are compared with un-
fluorinated bis
phenylaniline)-2,2’-bithiophen-
yl]-4,7-benzo[c][1,2,5]thiadi-
azole (bis[TPA-diTh]-BT, 1)
ACHTUNGTRENNUNG[5,5’-bis(N,N’-di-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
(Scheme 1).
Results and Discussion
The synthetic methods are out-
lined
Difluorobenzo[c]ACHTUNGTRENNUNG
in
Scheme 2.
[1,2,5]thia-
5,6-
diazole (ii) and N,N-diphenyl-4-
(thiophen-2-yl)aniline were syn-
thesized according to a previ-
ously reported procedure.[7,10]
All reactions were carried out
under a nitrogen atmosphere.
Monofluorinated benzothia-
diazole MonoF-BT (i) was read-
Scheme 2. Synthesis of the fluorinated bisACTHNUTRGNEU[GN TPA-diTh]-BT derivatives.
ily prepared from its precursor by hydrogenation with Pd/C
and hydrazine hydrate in absolute ethanol and subsequent
cyclization with SOCl2. Next, halogenation of the 4- and 7-
positions of BT was carried out to facilitate the Stille cou-
pling reaction with a stannyl BT. Recently, Zhou et al.[7] re-
ported a synthetic approach for iodination of the 4- and 7-
positions of difluorinated BT by using fuming sulfuric acid.
Zhang et al. reported on bromination of mono- or difluori-
nated BT at the 4- and 7-positions using less hazardous re-
agents than Zhou et al., obtaining compounds i and ii in six
synthetic steps.[8] The obvious benefits of a synthetic strategy
using milder reagents and fewer steps than these methods
led us to develop a facile new protocol for producing mono-
or difluorinated BT iodinated at the 4- and 7-positions. The
&
2
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Benzothiadiazole-Based Organic Semiconductors
FULL PAPER
syntheses were performed successfully in a single-step reac-
tion by using the Barker–Waters procedure. This involved
the use of Ag2SO4 and I2 in concentrated H2SO4 at 1108C,[11]
giving products iii and iv in yields of more than 65%. The
strong I+ reagent is needed owing to the electron-deficient
character of fluorinated BT.[12] Compounds iii and iv were
converted to dithiophenyl compounds v and vi, respectively,
31G* basis set provided more information on these transi-
tion bands (see Table S1 in the Supporting Information).
Compounds 1–3 showed high molar absorption coeffi-
cients of 54000mꢀ1 cmꢀ1 at 546 nm, 58000mꢀ1 cmꢀ1 at
547 nm, and 57900mꢀ1 cmꢀ1 at 545 nm, respectively. Fluori-
nated bisACTHNUTRGNEUG[N TPA-diTh]-BT derivatives 2 and 3 exhibited
higher molar absorptivities than the unfluorinated counter-
part 1, which indicated a more strongly reinforced ICT from
TPA to the BT core. However, the reinforced ICT did not
induce a redshift of the transition bands of 2 or 3, perhaps
because fluorination of the BT core slightly increased the
energy bandgap between the HOMO and the LUMO by re-
by a Stille coupling reaction with [PdACHTNURTGNEUNG(PPh3)4] catalyst in
N,N-dimethylformamide. Final compounds 2 and 3 were
prepared by a Stille coupling reaction between electron-defi-
cient fluorinated BT compounds vii and viii and stannyl
TPA-thiophene. To clarify the effects of fluorine substitution
on the electron-accepting BT core of these p-type organic
semiconductors for SMOSCs, the unfluorinated BT deriva-
tive 1[13] was also prepared by modifying the procedure.
Figure 1 shows the UV/Vis absorption spectra of 1–3 in
chlorobenzene solution and thin films, and the correspond-
ing optical properties are summarized in Table 1. The ab-
sorption spectra of 1–3 in chlorobenzene showed two typical
ducing the HOMO levels of bis
2 and 3 (see below).
ACHTUNGTNER[NUNG TPA-diTh]-BT derivatives
In solid-state thin films, p-conjugated organic materials
often show redshifted absorption bands and broader spectra
relative to those in solution owing to intermolecular p–p
packing interactions.[14] However, Figure 1 shows that 1 ex-
hibited only spectral broadening, with no evident band shift
to a longer wavelength region. This is likely caused by the
amorphous nonplanar TPA, which may interrupt the inter-
molecular packing interactions in the solid-state film. Mean-
while, mono- or disubstitution with fluorine causes the ICT
bands of 2 and 3 to blueshift by about 18 nm. These results
indicate that fluorine substitution of the benzothiadiazole
core unit can affect the intermolecular packing interactions
more than the amorphous nonplanar TPA in the solid-state
film. Moreover, the p–p* transition bands of 2 and 3 were
significantly redshifted to by about 30 nm. These redshifts
ꢀ
ꢀ
may be due to intermolecular packing through C F···H, C
[15]
ꢀ
F···S, or C F···pF interactions in the thin films. To clarify
this, attenuated total reflectance (ATR) FTIR spectra of 1–3
were recorded in powder and film states.
The ATR-FTIR spectra of thin films might be expected to
provide information on the aggregation of film formation
relative to that of powder, as the amorphous solid state of
Figure 1. UV/Vis absorption spectra of 1–3 in chlorobenzene solution (1:
solid line, 2: dashed line, 3: short-dashed line) and thin films (1: dotted
line, 2: dash-dotted line, 3: short-dash-dotted line).
ꢀ
the materials. As shown in Figure 2, the C F vibrational
bands of fluorinated benzothia-
diazole could be assigned be-
Table 1. Optical and electrochemical properties of the bisACTHNUGRTENUNG[TPA-diTh]-BT series.
tween 1000 and 1200 cmꢀ1 by
comparison with unfluorinated
BT. Differences between the
ATR-FTIR spectra of the thin
films and those of films in the
powder state indicate packing
aggregation in the thin film.
Specifically, the fluorinated bis-
Compound
labs [nm][a]
[e mꢀ1 cmꢀ1
Eonset,ox
Eonset,red
Eeglectro
Eogpt
]
[V]/HOMO [eV][b]
[V]/LUMO [eV][b]
[eV][b]
[eV][c]
1
2
3
546 (54000)
547 (58000)
546 (57900)
0.252/ꢀ5.052
0.273/ꢀ5.073
0.288/ꢀ5.088
ꢀ1.114/ꢀ3.686
ꢀ1.116/ꢀ3.684
ꢀ1.119/ꢀ3.681
1.366
1.389
1.407
1.85
1.85
1.85
[a] Absorption spectra were measured in chlorobenzene solution. [b] Redox potentials were measured in
CH2Cl2 with 0.1m (n-C4H9)4NPF6 and a scan rate of 100 mVsꢀ1. [c] Egopt was calculated from the absorption
thresholds from absorption spectra in chlorobenzene solution.
AHCTUNGTREG[NNUN TPA-diTh]-BT derivatives 2
transition bands in the wavelength region of 300–700 nm.
The absorption bands observed at longer wavelengths result
from ICT originating in HOMO!LUMO monoexcitations.
The absorption bands observed at 300–500 nm could be as-
signed to the p–p* transition, which predominantly origi-
nates in HOMOꢀ2!LUMO excitation (f=0.06–0.10) with
minor HOMOꢀ1!LUMO monoexcitations (f=0.02–0.06),
showing very high oscillator strength (f) of transition. Time-
dependent DFT calculations with the B3LYP functional/6-
and 3 in thin film form showed somewhat broader peaks at
slightly higher wavenumbers compared to the powder state.
These results may indicate the state of aggregation, especial-
ly H-aggregation, which often results in blueshifts of absorp-
tion bands, during formation of the thin film.[16] Also, the
higher wavenumber observed in ATR-FTIR spectra of the
thin films may indirectly prove the blueshifted absorption
spectra in the film state, because the wavenumber is inverse-
ly proportional to wavelength.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

J. K. Lee, J. Ko et al.
Figure 4. Isodensity surface plots of 1 (a), 2 (b), and 3 (c), calculated by
TD-DFT with B3LYP functional/6-21G* basis set.
evenly distributed on BT–dithophene–TPA, but the orbital
density of the LUMO of these materials is predominantly
located on BT and fluorine. The calculations revealed that
ICT from TPA to BT core can effectively occur in 1–3 when
excited by light energy, and TPA can play a role in stabiliz-
ing separation of the hole from the exciton. In addition,
fluoro substituents may provide a more strongly reinforced
ICT from TPA to the BT core owing to their electron-with-
drawing properties, which results in increased hole mobility
due to TPA as a hole stabilizer (see below). Figure 5 shows
Figure 2. ATR-FTIR spectra of bis
A
ACHTUNGTRENNUNG
(short-dashed line) as amorphous solid (a) and spin-cast thin film (b). c)
and d) are enlarged regions of a) and b), respectively, from 1200 to
1000 cmꢀ1
.
It is also expected that the fluorine atom, which has high
electronegativity, will affect the direction and size of the
dipole moment of the bisACTHNUTRGNE[NUG TPA-diTh]-BT derivatives.
Figure 3 shows the dipole moments of 1–3, calculated by
TD-DFT with the B3LYP functional/6-31G* basis set. The
dipole moment of unfluorinated 1 is directed from thiadia-
zole (d+) to benzene ring (dꢀ), while this direction is re-
versed and the value significantly decreased in difluorinated
3. These results might offset the dipole moment between the
TPA donor and BT, resulting in partial H-aggregation, even
in an amorphous TPA moiety.
Figure 4 shows the optimized structures of 1–3, calculated
by TD-DFT with the B3LYP functional/6-31G* basis set.
The orbital densities of the HOMOs of 1, 2, and 3 are
Figure 5. Electrochemical characterization of 1 (top), 2 (middle), and 3
(bottom) in CH2Cl2 with 0.1m (n-C4H9)4NPF6, scan speed 100 mVsꢀ1
.
cyclic voltammograms (CVs) of 1–3 in methylene chloride
solution, and the corresponding electrochemical properties
are summarized in Table 1. The HOMO and the LUMO
energy levels of these materials were determined from these
CVs. A platinum rod electrode, a platinum wire, and an Ag/
AgNO3 (0.10m) electrode were used as working electrode,
counterelectrode, and reference electrode, respectively. The
analyses were performed in an electrolyte solution of 0.1m
tetrabutylammonium hexafluorophosphate (TBAPF6) in
methylene chloride at room temperature under nitrogen
with a scan rate of 100 mVsꢀ1, and ferrocenium/ferrocene
(Fc+/Fc) redox couple was used as external reference. The
HOMO and LUMO levels can be deduced from the oxida-
tion and reduction onsets under the assumption that the
energy level of Fc is 4.8 eV below vacuum level.[17] The CVs
of these materials in solid-state thin films could not be
measured, owing to film stripping on the electrode. There-
Figure 3. Direction and size of dipole moment of bisACTHUNRTGNEUNG[TPA-diTh]-BT de-
rivatives 1, 2, and 3, calculated by TD-DFT with the B3LYP functional/6-
31G* basis set.
&
4
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Benzothiadiazole-Based Organic Semiconductors
FULL PAPER
fore, we determined the bandgaps from solution-state CV.
The HOMO levels of 1, 2, and 3 were calculated as 5.052,
5.073, and 5.088 eV, and the LUMO levels as 3.686, 3.684,
and 3.681 eV, respectively. These results indicate that fluo-
rine substitution on BT slightly reduced the HOMO level of
(J–V) characteristics obtained in the dark for hole-only devi-
ces. Figure 6b shows the dark-current characteristics of ITO/
PEDOT:PSS/donor:C71-PCBM/Au devices as a function of
the bias, corrected for the built-in voltage determined from
the difference in work function between Au and the HOMO
level of these materials. Ohmꢁs law manifests at low voltages
as an effect of thermal free carriers. In the presence of carri-
er traps in the active layer, a trap-filled-limit (TFL) region
exists between the ohmic and trap-free SCLC regions. The
SCLC behavior in the trap-free region can be characterized
by using the Mott–Gurney square law [Eq. (1)][19]
bisACHTUNGTRENNUNG[TPA-diTh]-BT, which leads to an increase in Voc of devi-
ces fabricated by using these materials with PCBM BHJ
active layers.[18]
Figure 6a shows the UV/Vis absorption spectra of 1/C71-
PCBM (1/2), 2/C71-PCBM (1/4), and 3/C71-PCBM (1/4)
films, which exhibited the best performances in BHJ solar
J ¼ ð9=8Þe ꢁ mðV2Lꢀ3
Þ
ð1Þ
where e is the static dielectric constant of the medium and m
is the carrier mobility. The hole mobilities of 1, 2, and 3
evaluated using the above Mott–Gurney law (e=3e0) are
2.14ꢂ10ꢀ6, 6.62ꢂ10ꢀ6, and 4.45ꢂ10ꢀ6 cm2 Vꢀ1 sꢀ1, respective-
ly. The fluorinated materials 1 and 2 exhibited higher hole
mobilities than unfluorinated 1. Specifically, the hole mobili-
ty of monofluorinated 2 was approximately 3 and about 1.5
times higher than those of unfluorinated 1 and difluorinated
3, respectively. These results suggest that TPA can effective-
ly stabilize the separated hole from the exciton in a BHJ
system with PCBM. This reduces charge recombination be-
tween the donor and PCBM, and the reinforced ICT due to
mono- and difluoro substitution on the BT core may boost
the long-lived charge separation and lead to enhanced trans-
porting properties of the hole carrier.
These materials were used for the fabrication of photovol-
taic devices along with PCBM BHJ films, and their perform-
ances were assessed. In the course of studying the character-
istics of more than 500 solar cells, the most efficient photo-
voltaic cells were obtained from the BHJ system of 1, 2, or
3 with C71-PCBM, which were optimized at a ratio of 1:2,
1:4, or 1:4, respectively. These BHJ films were cast on top of
a PEDOT:PSS (Heraeus, AI 4083) layer. The optimum
thicknesses of 1/C71-PCBM, 2/C71-PCBM, and 3/C71-PCBM
BHJ films obtained under these conditions were found to be
approximately 80, 85, and 86 nm, respectively. Figure 7
shows the J–V curves under AM 1.5 conditions
(100 mWcmꢀ2) and incident photon-to-current efficiency
(IPCE) spectra of these material/C71-PCBM BHJ solar cells
fabricated under optimized processing conditions, with/with-
out insertion of TiOx, which can effectively act as an optical
spacer and buffer layer[20] between the photoactive layer and
the Al electrode. The corresponding values are summarized
in Table 2.
The IPCE spectra of these devices (Figure 7b) show
curves well matched with their optical absorptions, resulting
in close correlation with their photocurrents in the J–V
curves. As shown in Figure 7 and Table 2, the devices con-
ventionally fabricated with 1/C71-PCBM, 2/C71-PCBM, and
3/C71-PCBM exhibited a PCE of 1.62% (ꢂ0.2) with a short-
circuit current Jsc of 7.26 mAcmꢀ2, fill factor (FF) of 0.32,
and Voc of 0.69 V; a PCE of 2.22% (ꢂ0.13) with Jsc =
Figure 6. a) UV/Vis absorption spectra and b) space-charge limitation of
J–V characteristics of 1/C71-PCBM (1:2) (solid), 2/C71-PCBM (1:4)
(dashed), and 3/C71-PCBM (1:4) (short-dashed) BHJ films in hole-only
devices (ITO/PEDOT:PSS/donor:C71-PCBM/Au).
cell devices. The ICT bands of 1 and 2 (or 3) observed in
the 1/C71-PCBM BHJ (1/2) and 2 (or 3)/C71-PCBM BHJ (1/
4) films were similar and blueshifted by about 20 nm, rela-
tive to those in solid-state films. These results indicate that
the PCBM may significantly interrupt intermolecular p–p
packing interactions between p-type organic materials by in-
teracting with the fluorine substituent on the BT moieties of
2 and 3 in the BHJ film.
To investigate the space-charge effects, we extracted the
hole mobilities of these organic semiconductors from the
space charge limitation of current (SCLC) current–voltage
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

J. K. Lee, J. Ko et al.
though the absorption spectra of the 2 (or 3)/C71-PCBM
BHJ film showed maximum absorption bands blue-shifted
relative to that of 1/C71-PCBM, as shown in Figure 6. The
best PCE of 2.95% (ꢂ0.16) was obtained for devices fabri-
cated from a 2/C71-PCBM film with a TiOx layer (Jsc =
8.51 mAcmꢀ2, FF=0.41, and Voc =0.85 V), which has about
33% higher efficiency than that without a TiOx layer.
Furthermore, compound 2, containing a monofluorinated
BT core, exhibited better photovoltaic performances with
higher Jsc and Voc than 1 and 3, which resulted in approxi-
mately 24 and 8% higher efficiencies of SMOSCs, respec-
tively. We also investigated the morphologies of these BHJ
films by atomic force microscopy (AFM). Although the
BHJ film cast from 1 and PCBM exhibited a slightly higher
roughness than those cast from 2 and 3, there is no obvious
difference in the phase-separated morphologies between
these BHJ films, all of which show good bicontinuous net-
works[21] (see Figure S3 in the Supporting Information). As
shown in Figure 5, the better efficiency of organic semicon-
ductor with monofluorinated BT is governed by rather high
photocurrent compared with that of organic semiconductor
with difluorinated BT. Since the surface images of organic
semiconductors with mono- or difluorinated BT investigated
from AFM analysis both showed good phase-segregated
morphologies with similar roughness values, these results
may be attributed to the better hole-transporting properties
of 2, as shown in Figure 6.
Conclusion
Figure 7. a) J–V curves under AM 1.5 conditions (100 mWcmꢀ2) and
b) IPCE spectra of the 1/C71-PCBM (1:2) dotted line), 2/C71-PCBM (1:4)
(dash-dotted line), and 3/C71-PCBM (1:4) (short-dash-dotted line) BHJ
solar cells fabricated under optimized processing condition with (undot-
ted line)/without (dotted line) insertion of a TiOx layer.
We have demonstrated the synthesis and photovoltaic char-
acteristics of new fluorine-substituted benzothiadiazole-
based compounds bis
ACHTUNGTRNE[NUNG TPA-diTh]-MonoF-BT (2) and bis-
AHCTUNGTRENNUNG
ductors for solution-processed SMOSCs, which showed a
PCE of 2.95% with a Voc of 0.85 V. Moreover, we developed
a facile new protocol for the synthesis of mono- or difluori-
nated BT compounds iodinated at the 4- and 7-positions.
These compounds were produced in a single-step reaction
by means of a Barker–Waters procedure under relatively
Table 2. Photovoltaic performances of the devices fabricated with donor
(1, 2, or 3)/C71-PCBM BHJ film.[a]
Donor
Functional layer
Jsc [mAcmꢀ2
]
Voc [V]
FF [%]
E [%]
1
1
2
2
3
3
none
TiOx
none
TiOx
none
TiOx
7.26
7.65
8.02
8.51
7.52
8.03
0.69
0.81
0.78
0.85
0.79
0.81
32
38
36
41
36
42
1.62
2.38
2.22
2.95
2.14
2.74
mild conditions. These fluorinated bisACTHNUTRGNEUNG[TPA-diTh]-BT deriv-
atives, which are linked with a TPA electron-donating group
through a p-conjugated bithiophene bridge, exhibited rein-
forced ICT compared to the unfluorinated analogue, in ad-
dition to enhanced molar absorptivities.
[a] The optimized BHJ films were spin-cast at 2000 rpm for 60 s. The
photovoltaic characteristics were determined under simulated
100 mWcmꢀ2 AM 1.5G illumination. The light intensity was calibrated by
using calibrated standard silicon solar cells with proactive windows made
from KG5 filter glass traced to the National Renewable Energy Labora-
tory (NREL). The masked active area of the device was 4 mm2.
The unique electronic-repulsive interactions of the fluo-
rine atom affect the intermolecular packing interactions
more than the amorphous nonplanar TPA in solid-state film
form. Even though these fluorinated SMOSCs exhibited the
worst intermolecular interaction, the electron-donating TPA
unit could effectively stabilize the hole separated from the
exciton in the BHJ system with PCBM to reduce charge re-
combination between the donor and PCBM. The reinforce-
ment of ICT by mono- and difluoro substitution on the BT
core may boost the long-lived charge separation and thus
result in improved transporting properties of the hole carri-
8.02 mAcmꢀ2, FF=0.36, and Voc =0.78 V; and a PCE of
2.14% (ꢂ0.11) with Jsc =7.52 mAcmꢀ2, FF=0.36, and Voc =
0.79 V, respectively. The SMOSCs fabricated with 2/C71-
PCBM or 3/C71-PCBM BHJ film exhibited better Jsc values
than that fabricated with 1/C71-PCBM. This was revealed by
superior IPCE values in the long-wavelength region, even
&
6
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Benzothiadiazole-Based Organic Semiconductors
FULL PAPER
Nguyen, Appl. Phys. Lett. 2009, 94, 103301; d) C. Q. Ma, M. Fonro-
dona, M. C. Schikora, M. M. Wienk, R. A. J. Janssen, P. Bauele, Adv.
Funct. Mater. 2008, 18, 3323–3331; e) Z. E. Ooi, T. L. Tam, R. Y. C.
Shin, Z. K. Chen, T. Kietzke, A. Sellinger, M. Baumgarten, K.
Mullen, J. C. deMello, J. Mater. Chem. 2008, 18, 4619–4622; f) J.
Lee, M. H. Yun, J. Kim, J. Y. Kim, C. Yang, Macromol. Rapid
Commun. 2012, 33, 140–145; g) J. Lee, J. Kim, G. Kim, C. Yang, Tet-
rahedron 2010, 66, 9440–9444.
er. The molecular engineering approach demonstrated here
provides an important guide for the development of novel
materials for use in solution-processed small-molecule BHJ
solar cells.
Experimental Section
[5] Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, A. J.
Heeger, Nat. Mater. 2011, 11, 44–48.
The BHJ films were prepared under optimized conditions according to
the following procedure reported previously.[21] Indium tin oxide (ITO)-
coated glass substrate was first cleaned with detergent, ultrasonicated in
acetone and isopropyl alcohol, and subsequently dried overnight in an
[6] a) H. M. Ko, H. Choi, S. Paek, K. Kim, K. Song, J. K. Lee, J. Ko, J.
Mater. Chem. 2011, 21, 7248–7253; b) B. S. Jeong, H. Choi, N. Cho,
H. M. Ko, W. Lim, K. Song, J. K. Lee, J. Ko, Sol. Energy Mater. Sol.
Cells 2011, 95, 1731–1740; c) S. So, H. Choi, C. Kim, N. Cho, H. M.
Ko, J. K. Lee, J. Ko, Sol. Energy Mater. Sol. Cells 2011, 95, 3433–
3441; d) S. So, H. Choi, H. M. Ko, C. Kim, S. Paek, N. Cho, K. Song,
J. K. Lee, J. Ko, Sol. Energy Mater. Sol. Cells 2012, 98, 224–232;
e) H. Choi, S. Paek, J. Song, C. Kim, N. Cho, J. Ko, Chem. Commun.
2011, 47, 5509–5511.
oven.
Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate
(PE-
DOT:PSS) in aqueous solution was spun-cast to form a film with thick-
ness of approximately 40 nm. The substrate was dried for 10 min at
1408C in air, and then transferred into a glove box to spin-cast the photo-
active layer. The donor/PCBM BHJ blend solutions were prepared in
chlorobenzene at a concentration of 30 mgmLꢀ1, and then spun-cast on
top of the PEDOT layer. Then, the device was pumped down to less than
10ꢀ7 torr and an approximately 100 nm thick Al electrode was deposited
on top. Solar cell efficiencies were characterized under simulated
100 mWcmꢀ2 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
with 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 by using Wavemetrics software. The
IPCE spectra for the cells were measured on an IPCE measuring system
(PV measurements).
[7] H. Zhou, L. Yang, A. C. Stuart, S. C. Price, S. Liu, W. You, Angew.
Chem. 2011, 123, 3051–3054; Angew. Chem. Int. Ed. 2011, 50, 2995–
2998.
[8] Y. Zhang, S. C. Chien, K. S. Chen, H. L. Yip, Y. Sun, J. A. Davices,
F. C. Chen, A. K. Y. Jen, Chem. Commun. 2011, 47, 11026–11028.
[9] a) Y. Y. Liang, D. Q. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray, L. P. Yu,
J. Am. Chem. Soc. 2009, 131, 7792–7799; b) Q. Wei, T. Nishizawa,
K. Tajima, K. Hashimoto, Adv. Mater. 2008, 20, 2211–2216.
[10] S.-J. Moon, J.-H. Yum, R. Humphry-Baker, K. M. Karlsson, D. P.
Hagberg, T. Marinado, A. Hagfeldt, L. Sun, M. Gratzel, Md. K. Na-
zeeruddin, J. Phys. Chem. C 2009, 113, 16816–16820.
[11] I. R. L. Barker, W. A. Waters, J. Chem. Soc. 1952, 150–153.
[12] J. Bouffard, T. M. Swager, Macromolecules 2008, 41, 5559–5562.
[13] W. Li, C. Du, F. Li, Y. Zhou, M. Fahlman, Z. Bo, F. Zhang, Chem.
Mater. 2009, 21, 5327–5334.
[14] a) R. C. Coffin, J. Peet, J. Rogers, G. C. Bazan, Nat. Chem. 2009, 1,
657–661; b) A. Leliꢄge, P. Blanchard, T. Rousseau, J. Roncali, Org.
Lett. 2011, 13, 3098–3101.
Acknowledgements
[15] Y. Wang, S. R. Parkin, J. Gierschner, M. D. Watson, Org. Lett. 2008,
10, 3307–3310.
[16] F. Nꢅetsch, J. E. Moser, V. Shklover, M. Grꢆtzel, J. Am. Chem. Soc.
1996, 118, 5420–5431.
This research was supported by World Class University program funded
by the Ministry of Education, Science and Technology through the Na-
tional Research Foundation of Korea (R31-2011-000-10035-0), the Con-
verging Research Center Program through the Ministry of Education,
Science and Technology (No. 2011K000596) and the New & Renewable
Energy of the Korea Institute of Energy Technology Evaluation ad Plan-
ning (KETEP) grant funded by the Korea government Ministry of
Knowledge Economy (no. 20103060010020).
[17] a) J. Pommerehene, H. Vestweber, W. Guss, R. F. Mahrt, H. Bꢆssler,
M. Porsch, J. Daub, Adv. Mater. 1995, 7, 551–554; b) Z. Chen, M. G.
¨
Debije, T. Debaerdemaeker, P. Osswald, F. Wurthner, ChemPhy-
sChem 2004, 5, 137–140.
[18] M. C. Scharber, D. Mꢅhlbacher, M. Koppe, P. Denk, C. Waldauf,
A. J. Heeger, C. J. Brabec, Adv. Mater. 2006, 18, 789–794.
[19] V. D. Mihailetchi, H. Xie, B. Boer, L. J. A. Koster, P. W. M. Blom,
Adv. Funct. Mater. 2006, 16, 699–708.
[20] K. Lee, J. Y. Kim, S. H. Park, S. H. Kim, S. Cho, A. J. Heeger, Adv.
Mater. 2007, 19, 2445–2449.
[21] J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moo, J. Y. Kim, K.
Lee, G. C. Bazan, A. J. Heeger, J. Am. Chem. Soc. 2008, 130, 3619–
3623.
[1] a) D. Demeter, T. Rousseau, P. Leriche, T. Cauchy, R. Po, J. Roncali,
Adv. Funct. Mater. 2011, 21, 4379–4387; b) B. Walker, C. Kim, T.-Q.
Nguyen, Chem. Mater. 2011, 23, 470–482.
[2] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop,
Prog. Photovoltaics 2011, 19, 565–572.
[3] S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W. You, J. Am. Chem.
Soc. 2011, 133, 4625–4631.
[4] a) S. Roquet, A. Cravino, P. Leriche, O. AlꢃvÞque, P. Frꢄre, J. Ron-
cali, J. Am. Chem. Soc. 2006, 128, 3459–3466; b) A. Cravino, P. Leri-
che, O. Aleveque, S. Roquet, J. Roncali, Adv. Mater. 2006, 18, 3033–
3037; c) A. B. Tamayo, X. D. Dang, B. Walker, J. Seo, T. Kent, T. Q.
Received: April 19, 2012
Revised: May 25, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

J. K. Lee, J. Ko et al.
A facile new protocol was developed
for synthesis of iodinated derivatives
of fluorinated benzothiadiazoles as
precursors for fluorine-substituted ben-
zothiadiazole-based organic semicon-
ductors 2 and 3, which exhibit
enhanced intramolecular charge trans-
fer compared to unfluorinated ana-
logue 1 and hence improved photovol-
taic performance in small-molecule
organic solar cells (see figure).
Organic Semiconductors
N. Cho, K. Song, J. K. Lee,*
J. Ko* .......................... &&&& — &&&&
Facile Synthesis of Fluorine-Substi-
tuted Benzothiadiazole-Based Organic
Semiconductors and Their Use in
Solution-Processed Small-Molecule
Organic Solar Cells
&
8
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!