Synthesis of isoindigo-based oligothiophenes for molecular bulk heterojunction solar cells

Article abstract of DOI:10.1021/ol902512x

(Figure Presented) Isoindigo, as a new electron acceptor unit for organic electronic materials, was integrated into two low-energy gap oligothiophenes. Optical and electrochemical studies of the newly synthesized oligomers demonstrate broad absorption through the visible spectrum, along with appropriate energy levels, as desired for light harvesting donors for organic solar cells when blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM). Molecular heterojunction solar cells were fabricated using these oligomers and exhibit a power conversion efficiency up to 1.76% with a V_oc of 0.74 V, l_SC of 6.3 mA/cm² and fill factor of 0.38.

Full text of DOI:10.1021/ol902512x

ORGANIC
LETTERS
2010
Vol. 12, No. 4
660-663
Synthesis of Isoindigo-Based
Oligothiophenes for Molecular Bulk
Heterojunction Solar Cells
Jianguo Mei, Kenneth R. Graham, Romain Stalder, and John R. Reynolds*
The George and Josephine Butler Polymer Research Laboratory, Department of
Chemistry and Center for Macromolecular Science and Engineering, UniVersity of
Florida, GainesVille, Florida 32611-7200
reynolds@chem.ufl.edu
Received November 2, 2009
ABSTRACT
Isoindigo, as a new electron acceptor unit for organic electronic materials, was integrated into two low-energy gap oligothiophenes. Optical
and electrochemical studies of the newly synthesized oligomers demonstrate broad absorption through the visible spectrum, along with
appropriate energy levels, as desired for light harvesting donors for organic solar cells when blended with [6,6]-phenyl-C61-butyric acid methyl
ester (PC60BM). Molecular heterojunction solar cells were fabricated using these oligomers and exhibit a power conversion efficiency up to
1.76% with a Voc of 0.74 V, Isc of 6.3 mA/cm2 and fill factor of 0.38.
Conjugated polymer-based bulk-heterojunction (BHJ) solar
cells have received considerable attention as potential
alternative renewable energy sources,1 due to their suitability
for low-cost, fast solution processing techniques and their
compatibility with flexible substrates. Significant efforts are
being put forth to develop conjugated polymers that meet
various criteria for high performance photovoltaic applica-
tions.2 Currently, state-of-the-art polymer-based BHJ solar
cells have reached power conversion efficiencies (PCE) of
6%.3 However, these numbers are still significantly lower
than the critical efficiencies of 10-12% many believe are
required for commercial utility.4 The strong motivation for
approaching such efficiencies is driving various research
efforts in the area of organic based BHJ solar cells.1-5
Molecular crystalline semiconductors as alternatives to
conjugated polymers offer several intrinsic advantages in
solution-processable BHJ solar cells. Owing to their mono-
disperse nature with well-defined chemical structures, to-
gether with no end-group contaminants or batch-to-batch
variations molecular organic semiconductors can be repro-
(3) (a) Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.;
Li, G. J. Am. Chem. Soc. 2009, 131, 15586–15587. (b) Park, S. H.; Roy,
A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.;
(1) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science
1992, 258, 1474–1476. (b) Thompson, B. C.; Fre´chet, J. M. J. Angew.
Chem., Int. Ed. 2008, 47, 58–77. (c) Gunes, S.; Neugebauer, H.; Sariciftci,
N. S. Chem. ReV. 2007, 107, 1324–1338. (c) Cheng, Y.; Yang, S.; Hsu, C.
Chem. ReV. 2009, 109, 5868–5923.
Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297–302
(4) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
C.; Heeger, A. J.; Brabec, C. L. AdV. Mater. 2006, 18, 789–794
(5) (a) Scherf, U.; Gutacker, A.; Koenen, N. Acc. Chem. Res. 2008, 41,
1086–1097. (b) Taranekar, P.; Qiao, Q.; Jiang, H.; Ghiviriga, I.; Schanze,
K. S.; Reynolds, J. R. J. Am. Chem. Soc. 2007, 129, 8958–8959. (c) Lee,
J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.;
Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619–3623. (d)
Kose, M. E.; Mitchell, W. J.; Kopidakis, N.; Chang, C. H.; Shaheen, S. E.;
.
.
(2) (a) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007,
91, 954–985. (a) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.;
Boer, B. D. Poly. ReV. 2008, 48, 531–582. (c) Liang, Y.; Feng, D.; Wu,
Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem. Soc. 2009, 131, 7792–
7799.
Kim, K.; Rumbles, G. J. Am. Chem. Soc. 2007, 129, 14257–14270
.
10.1021/ol902512x  2010 American Chemical Society
Published on Web 01/25/2010

ducibly prepared, functionalized and purified. Despite the
fact that the highest PCEs to date for small molecule based
solar cells remain lower than their polymer-based analogs,
the considerations above make molecular crystalline semi-
conductors attractive as active materials in BHJ solar cells.6
Encouraged by recent reports on diketopyrrolopyrrole
oligothiophenes as donor materials in molecular BHJ solar
cells, where PCEs of 2.2-4.4% have been demonstrated by
Nguyen and co-workers,7 we here report the synthesis and
characterization of isoindigo-based oligothiophenes and their
use as donor materials in molecular BHJ solar cells.
Isoindigo is a structural isomer of the famous pigment
indigo and is often found as an intermediate in drug
development. In this communication, the isoindigo unit is
used as an electron acceptor to form donor-acceptor-donor
(DAD) and acceptor-donor-acceptor (ADA) isoindigo-
based oligothiophenes in conjunction with bithiophene as an
electron donor.
in addition to the desired product 7, upon reaction of
6-bromoisatin and oxindole followed by alkylation. Fortu-
nately, the mixture can be easily separated by chromatog-
raphy. The unexpected formation of 8 and 4 is likely due to
acid- or base-promoted retro-adol/adol sequences that
scrambled the product distribution under the existing condi-
tions.9 To test this hypothesis, an equimolar mixture of 8
and 4 was mixed and subjected to the two-step reaction
conditions, respectively. The experiments revealed that
crossover was indeed observed and 7 was produced in DMF
with K2CO3 at 100 °C, whereas no crossover was observed
in the acidic condition. It was further found that use of
anhydrous DMF and fresh-dried base can minimize the
scrambling.
Scheme 2.
Synthesis of 6-Bromoisoindigoa
Scheme 1. Synthesis of 6,6′-Dibromoisoindigo
a Mole ratio of compound 7, 8 and 4 (∼2:1:1).
The work starts with the acid-catalyzed adol condensation
and dehydration of commercially available 6-bromoisatin 1
and 6-bromooxindole 2 in acetic acid under argon,8 yielding
6,6′-dibromoisoindigo 3 in almost quantitative yield (Scheme
1). Due to the existence of strong π-π interactions and
hydrogen bonding, compound 3 is slightly soluble in
common organic solvents. Subsequently shown is the N-
alkylation of 3 using branched alkyl bromide and efficient
formation of highly soluble 6,6′-dibromoisoindigo derivative
4 in 85% yield.
With 4 and 7 in hand, isoindigo-based DAD and ADA
oligothiophenes 9 and 10 were successfully obtained by
incorporating electron rich bithiophene units via Suzuki
coupling (Scheme 3).10 The structures of 9 and 10 were
1
confirmed by H- and 13C NMR, MALDI-TOF mass spec-
trometry, and elemental analysis. Differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA)
revealed that 9 and 10 melt at 181 and 230 °C, respectively,
and are both stable to greater than 350 °C.
UV-vis absorption spectra of the oligomers, both in THF
solution and as films on indium-tin oxide coated glass were
measured (Figure 1). Compound 9 and 10 broadly absorb at
wavelengths up to 688 and 655 nm in solution with molar
The same approach has also been utilized to prepare
alkylated 6-bromoisoindigo 7 (Scheme 2). Interestingly,
isoindigo 8 and 6,6′-dibromoisoindigo 4 were also observed,
(6) (a) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani,
G. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 17640–17641. (b) Zhang,
J.; Yang, Y.; He, C.; He, Y.; Zhao, G.; Li, Y. Macromolecules 2009, 42,
7619–7622. (c) Roquet, S.; Cravino, A.; Leriche, P.; Aleveque, O.; Frere,
P.; Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459–3466. (d) Ma, C.-Q.;
Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.; Bauerle,
P. AdV. Funct. Mater. 2008, 18, 3323–3331. (e) Roncali, J. Acc. Chem.
Res. 2009, 42, 1719–1730.
(9) (a) Komata, T.; Matsunaga, K.; Hirotsu, Y.; Akiba, S.; Ogura, K. J.
Fluorine Chem. 2007, 128, 902–909. (b) Overman, L. E.; Peterson, E. A.
Tetrahedron 2003, 59, 6905–6919.
(7) (a) Tamayo, A. B.; Walker, B.; Nguyen, T.-Q. J. Phys. Chem. C
2008, 112, 11545–11551. (b) Walker, B.; Tamayo, A. B.; Dang, X.-D.;
Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T.-Q. AdV. Funct.
Mater. 2009, 19, 3063–3069. (c) Tamayo, A. B.; Tantiwiwat, M.; Walker,
B.; Nguyen, T.-Q. J. Phys. Chem. C 2008, 112, 15543–15552.
(8) Christos, P.; Xaver, B. HelV. Chim. Acta 1988, 71, 1079–1083.
(10) Perzon, E.; Zhang, F.; Andersson, M.; Mammo, W.; Ingana¨s, O.;
Andersson, M. R. AdV. Mater. 2007, 19, 3308–3311.
(11) Jayakannan, M.; Paul, A. v. H.; Rene´, A. J. J. J. Polym. Sci., Part
A: Polym. Chem. 2002, 40, 251–261.
(12) Thompson, B. C.; Kim, Y.-G.; McCarley, T. D.; Reynolds, J. R.
J. Am. Chem. Soc. 2006, 128, 12714–12725.
Org. Lett., Vol. 12, No. 4, 2010
661

dichloromethane (DCM) solution (Figure 2). Both oligoth-
iophenes present two consecutive quasi-reversible reduction
peaks with E1/2red of -1.31 and -1.71 V for 9, and of -1.35
and -1.74 V for 10. In the oxidation region, two oxidation
waves appear in the CV of 9 with E1/2ox of 0.47 and 0.58 V;
while only one oxidation wave was observed for 10 with
E1/2ox of 0.52 V. This is in accordance with the more electron
donating contribution in the DAD molecule than the ADA
molecule. The LUMO and HOMO energies of 9 and 10,
estimated by CV,12 are -3.9 and -5.5 eV, and -3.8 and
-5.5 eV, respectively.
Scheme 3. Synthesis of Isoindigo-Based Oligothiophenes
Figure 2. Cyclic voltammetry of 9 (left) and 10 (right) measured
in a 0.1 M solution of TBAPF6/DCM (scan rate 25 mV/s) vs Fc/
Fc+.
absorptivities of 19 500 and 47 300 L M-1 cm-1 at 579 and
560 nm, and are, thus, strongly absorbing through most of
the visible spectrum as desired for potential solar cell
applications. Markedly different from other donor-acceptor
systems,7,11 there exists only a very shallow absorption valley
between two absorption bands for both molecules, which
makes them appear virtually black in solution. The spectra
broaden and extend to 744 and 703 nm in the solid state
with a peak emerging at 660 and 636 nm for 9 and 10,
To gain more insight into the energy levels in solid state,
differential pulse voltammetry (DPV) was performed on
drop-cast films of 9 and 10 on Pt button electrodes (see
Supporting Information). From the onset of oxidation in
DPVs, HOMO levels of -5.6 eV are estimated for both 9
and 10. Unfortunately, the first reduction wave was not
observed for both molecules. Therefore, the LUMO levels
were calculated using the values obtained from the solid state
optical band gap, giving - 3.9 and - 3.8 eV for 9 and 10.
The solid state results are very close to the ones in solution.
The appropriate energy levels and gaps, together with the
Figure 1. Normalized solution and solid state absorption spectra
of 9 (left) and 10 (right).
indicating strong π-π interactions in their crystalline
forms.7a Upon annealing at 100 °C for 20 min, a slight
decrease in absorption intensity was observed for both
oligothiophenes (see Supporting Information). From the
absorption onsets in the solid state, the optical energy gaps
are estimated to be 1.67 eV for the DAD molecule and 1.76
eV for the ADA molecule. Both 9 and 10 are fluorescent,
emitting with maxima at 780 and 749 nm in chloroform,
respectively (see Supporting Information).
Figure 3. Performance of 9/10:PC60BM solar cells. J-V charac-
teristics under 100 mW/cm2 white light illumination (9 in blue and
10 in red) annealed at 100 °C for 20 min.
The redox behavior of the oligomers was investigated by
cyclic voltammetry (CV) in a 0.1 M solution of TBAPF6/
662
Org. Lett., Vol. 12, No. 4, 2010

annealing temperature of 100 °C, and a total solution
concentration of 18 mg/mL.
The BHJ cells made from 9 performed significantly better
than devices made from 10. After annealing at 100 °C, solar
cells made from 9 showed a PCE of up to 1.76%, with a Voc
of 0.74 V, Jsc of 6.3 mA/cm2, and fill factor of 0.38. Annealed
photovoltaic devices (100 °C) of 10 had PCEs of up to
0.55%, with a Voc of 0.66 V, Jsc of 2.4 mA/cm2, and fill
factor of 0.36. Preannealed devices were less efficient with
PCEs of 0.72 and 0.11% for 9 and 10 respectively. The J-V
characteristics for annealed devices are shown in Figure 3.
The AFM height images of the blend films before and
after annealing are presented in Figure 4. These images show
that upon annealing the surface of the films become rougher
and more ordered domains appear to form. It is suspected
that 9 performs better than 10 in solar cells in part due to a
higher degree of ordering, and a lower energy gap.
In conclusion, we report the synthesis and characterization
of isoindigo-based oligothiophenes, and their utilization as
electron donors in molecular BHJ solar cells for the first time.
These devices exhibit very promising power conversion
efficiencies for solution-processed small molecule solar cells.
We are currently working to fine-tune the optical and
electronic properties of isoindigo-based donor-acceptor
materials through molecular design, to bridge chemical
structure, film morphology and photovoltaic response, as well
as to optimize processing conditions suitable for small
molecules.
Figure 4. AFM height images of 9:PC60BM (50:50) spin-coated
from chlorobenzene (a) as cast, and (b) annealed at 100 °C for 20
min; 10:PC60BM (60:40) spin-coated from chlorobenzene (c) as
cast, and (d) annealed at 100 °C for 20 min. All images are 1 × 1
µm with 5 nm height scales. RMS surface roughness values of the
AFM images are (a) 0.15 nm, (b) 0.98 nm, (c) 0.14 nm, and (d)
0.95 nm.
Acknowledgment. We gratefully acknowledge AFOSR
(FA9550-09-1-0320) for financial support. Thanks to Prof.
Aaron Aponick at University of Florida (UF) for useful
suggestions and to Dr. Quentin Bricaud at UF for his
assistance in device testing. K.R.G. and R.S. acknowledge
the University Alumni Awards Program for a fellowship.
high absorption coefficients, suggest the new isoindigo-based
oligothiophenes should be effective electron donors in BHJ
solar cells.
Molecular BHJ solar cells were fabricated by spin-coating
9/10:PC60BM blends from chlorobenzene onto a clean ITO/
PEDOT:PSS bottom electrode on a glass substrate. Optimi-
zation experiments exploring different blend ratios, annealing
temperatures, and solution concentrations resulted in the
highest power conversion efficiencies (PCEs) for 9/10:
PC60BM blend ratios of 50:50 (9) and 60:40 (10), an
Supporting Information Available: Detailed procedures,
characterization details, optical spectra of blends, and device
fabrication. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL902512X
Org. Lett., Vol. 12, No. 4, 2010
663