Dibenzo[f,h]thieno[3,4-b] quinoxaline-based small molecules for efficient bulk-heterojunction solar cells

Article abstract of DOI:10.1021/ol9019953

Two Isomeric compounds (1 and 2) containing a dibenzo[f,h)]thieno[3,4-b] quinoxaline core and two peripheral arylamines were synthesized. Solution-processed bulk heterojunction (BHJ) solar cells based on these sensitizers and [6,6]-phenyl-C61-butyric acid

Full text of DOI:10.1021/ol9019953

ORGANIC
LETTERS
2009
Vol. 11, No. 21
4898-4901
Dibenzo[f,h]thieno[3,4-b] quinoxaline-
Based Small Molecules for Efficient
Bulk-Heterojunction Solar Cells
Marappan Velusamy,† Jen-Hsien Huang,‡ Ying-Chan Hsu,† Hsien-Hsin Chou,†
Kuo-Chuan Ho,‡ Pei-Lun Wu,† Wei-Hau Chang,† Jiann T. Lin,*,† and
Chih-Wei Chu*,§
Institute of Chemistry and Research Center for Applied Sciences,
Academia Sinica, Nankang, Taiwan, and Department of Chemical Engineering,
National Taiwan UniVersity, Taiwan
jtlin@chem.sinica.edu.tw; gchu@gate.sinica.edu.tw
Received August 27, 2009
ABSTRACT
Two isomeric compounds (1 and 2) containing a dibenzo[f,h]thieno[3,4-b]quinoxaline core and two peripheral arylamines were synthesized.
Solution-processed bulk heterojunction (BHJ) solar cells based on these sensitizers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
are reported. The cell fabricated from 1/67 wt % of PCBM exhibited a high power conversion efficiency of 1.70% and an external quantum yield
of 55%. The film of the cell was found to have balanced electron and hole mobility and good film morphology.
Organic solar cells have been intensely explored in the past
decade because of the increasing demand of sustainable and
renewable energy supply arising from rapid depletion of
fossil fuels and disastrous environmental problems ac-
companying the combustion of these fuels.1 Among these,
dye-sensitized solar cells (DSSCs) have attracted consider-
able interest, and an efficiency record of ∼11% has been
achieved with ruthenium-based sensitizers.2 Since the seminal
report of Tang,3 organic photovoltaic (OPV) cells have also
made significant progress, and high efficiencies at 6% and
6.7% have been reported for single4 and tandem5 cells,
respectively. Organic semiconductors are attractive because
of their low cost, easy manufacture of thin film by vacuum
deposition, solution cast, and printing technologies. Conju-
(2) (a) Nazeeruddin, M. K.; DeAngelis, F.; Fantacci, S.; Selloni, A.;
Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc.
2005, 127, 16835. (b) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.;
Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (c) Gao, F.;
Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphrey-Baker, R.; Wang, P.;
Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Commun. 2008, 2629.
(3) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183.
† Institute of Chemistry, Academia Sinica.
‡ Department of Chemical Engineering, National Taiwan University.
§ Research Center for Applied Sciences, Academia Sinica.
(1) For recent reviews, see: (a) Grätzel, M. Inorg. Chem. 2005, 44, 6841.
(b) Durrant, J. R.; Haque, S. A.; Palomares, E. Chem. Commun. 2006, 3279.
(c) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324.
(d) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91,
954. (e) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58. (f) Mishra, A.; Fisher, M. K. R.; Bu¨erle, P. Angew. Chem., Int. Ed.
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10.1021/ol9019953 CCC: $40.75
Published on Web 09/25/2009
 2009 American Chemical Society

gated polymers have been widely used in OPV devices, with
either heterojunction (HJ) or bulk heterojunction (BHJ)
architecture, via solution cast. BHJ cells normally have better
energy conversion efficiencies than layered HJ cells because
of their limited exciton diffusion length (normally <20 nm).6
Prototype BHJ cells consist of an n-type [6,6]-phenyl C61-
butyric acid methyl ester (PCBM) and a p-type conjugated
organic polymer which form bicontinuous interpenetrating
phase structures with the blend films.1e In contrast, solar cells
based on small molecules did not receive much attention until
recently.7 Up to 3.4% and 5.0% efficiencies for vacuum-
deposited layered OPV cells based on small molecules were
reported by Pfeiffer8 and Forrest’s9 groups, respectively. A
high efficiency of 4.4% for a solution-processed BHJ solar
cell was also demonstrated by Nguyen.7j It is optimistic that
solution-processable BHJ solar cells based on small mol-
ecules will have efficiencies higher than 5% in future.
In this paper, we report small molecules which have an
electron-deficient dibenzo[f,h]thieno[3,4-b]quinoxaline entity
with two peripheral arylamine units. The molecules may have
several advantages: (1) dipolar characteristic due to charge
transfer from arylamine to quinoxaline may be beneficial to
red shifting the electronic absorption band; (2) the arylamine
units are capable of hole transport; (3) the nonlinear fashion
between the donor and the acceptor may facilitate separation
of the electron and the hole. Scheme 1 depicts the synthesis
with diketone in the presence of catalytic amount of
p-toluenesulfonic acid to form intermediate 1b or 2b, which
then underwent palladium-catalyzed Stille coupling reaction
with 9,9-dihexyl-N,N-diphenyl-7-(5-(tributylstannyl)thiophen-
2-yl)-9H-fluoren-2-amine10 to provide the desired products.
The absorption spectra of the dyes in CH2Cl2 are displayed
in Figure 1, and the data are collected in Table 1. Both
Figure 1. Absorption spectra of the dyes in dichloromethane. The
normalized absorption spectra of 1 as a solution in CH2Cl2 and as
a blend film coated from CHCl3 solution.
compounds exhibit two distinct absorption bands: the one
with very high molar extinction coefficient (∼ 1.2 × 105
M-1 cm-1) can be attributed to the π-π* transition of the
thienoquinoxaline core, and the weaker broad band at 580
nm (1) or 478 nm (2) is due to charge transfer from the
arylamine to the quinoxaline. The charge transfer band of 2
exhibits hypsochromical shifts compared to 1. This is
consistent with theoretical computation results using density
functional theory (DFT) at a B3LYP/6-31G* level of theory.
The results for the theoretical approach are summarized in
Table S1 (Supporting Information), and Figures S1 and S2
show selected frontier orbitals and calculated electronic
Scheme 1. Synthesis of the Compounds 1 and 2
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M.; Pron, A.; Demadrille, R. AdV. Funct. Mater. 2008, 18, 3444. (f) Silvestri,
F.; Irwin, M. D.; Beberina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J.
J. Am. Chem. Soc. 2008, 130, 17640. (g) Kronenberg, N. M.; Deppisch,
M.; Wu¨rthner, F.; Lademann, K. D.; Meerholz, K. Chem. Commun. 2008,
6489. (h) Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.;
Roncali, J. Chem. Commun. 2009, 1673. (i) Marrocchi, A.; Silvestri, F.;
Seri, M.; Facchetti, A.; Taticchi, A.; Marks, T. J. Chem. Commun. 2009,
1380. (j) Walker, B.; Tamayo, A. B.; Dang, X. D.; Jung, P. Z.; Seo, H.;
Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q. AdV. Funct. Mater. DOI: 10/
1002/adfm.200900832. (k) Wong, W. W. H.; Jones, D. J.; Yan, C.; Watkins,
S. E.; King, S.; Haque, S. A.; Wen, X.; Ghiggino, K. P.; Holmes, A. B.
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of two dipolar compounds 1 and 2. 2,5-Bis(4-tert-butylphe-
nyl)thiophene-3,4-diamine underwent condensation reaction
(10) Stille, J. K. Angew. Chem., Int. Ed. 1986, 25, 508.
Org. Lett., Vol. 11, No. 21, 2009
4899

Table 1. Electrooptical Parameters of the Dyesa
dye
λmax, nm (ε × 10-4 M-1 cm-1
)
Eox (mV)
Ered (mV)
HOMO (eV)
LUMO (eV)
1
2
580 (0.97), 417 (11.9), 352 (9.20), 317 (8.57)
478 (4.47), 393 (12.6), 314 (6.40)
368, 601, 820
369, 537, 831
-1484
-1486
-5.16
-5.16
-3.34
-2.93
a Absorption and electrochemical data were collected in CH2Cl2 solutions. Potentials are quoted with reference to the internal ferrocene standard (E1/2
+256 mV vs Ag/AgNO3).
)
transitions (Supporting Information), respectively. The higher
energy band for 1 stems from S0 f S5, whereas that of 2 is
higher in energy and consists mainly of S0 f S6 and S0 f
S7. This band indeed has strong π-π* transition character.
The lower energy bands for both 1 and 2 have prominent
charge-transfer character. In 1, this band mainly stems from
the S0 f S1 transition. In comparison, the same band with
much higher intensity in 2 consists of S0 f S2 and S0 f S3
transitions where charge transfer dominates. The same trend
was also observed for isomeric 2,7-bis(9′,9′-dioctyl)fluoren-
2′-yl)-9,10-phenathraquinone and 3,6-bis(9′,9′-dioctyl)fluo-
ren-2′-yl)-9,10-pheanathraquinone.11 The absorption spectrum
of the compounds as a film on ITO glass is very similar to that
in the solution except that the band is somewhat broadened in
the longer wavelength region (Figure 1), suggesting the presence
of J-aggregation of the molecules in the film state.
facilitate hole injection from the organic layer to ITO anode
and to alleviate the surface roughness of ITO. Chloroform,
chlorobenzene, and o-dichlorobenzene were chosen first to
cast the active layers. The performance parameters of the
cells are listed in Table 2. The photovoltage in each cell
Table 2. Device Parameters with Films Spin-Coated from 1:1
Weight Ratio of 1/PCBM Dissolved in Different Solvents
solvent
VOC (V)
JSC (mA cm-2
)
η (%)
FF
chlorobenzene
o-dichlorobenzene
chloroform
0.87
0.87
0.90
2.00
2.33
2.60
0.55
0.60
0.75
0.32
0.30
0.32
reached ∼0.9 V, close to the ideal value estimated from the
difference between the HOMO level of 1 and LUMO level
of PCBM. Moreover, the BHJ cell made from chloroform
has a rather high conversion efficiency as well as photocur-
rent. In viewing that the cell fabricated from chloroform has
the best performance among the three solvents used, BHJ
cells with three different weight ratios of 1/PCBM were also
fabricated from the blends in chloroform. The J-V curves
from different cells with postannealing of the blend films
are shown in Figure S5 (Supporting Information), and the
corresponding parameters are collected in Table 3. The cell
with a 67 wt % of PCBM had optimal efficiency at 1.70%.
The external quantum efficiency (EQE) in the region 350-
600 nm surpassed that of the BHJ cell based on pristine
P3HT/PCBM (P3HT ) poly(3-hexylthiophene)).14 In the
region 373-440 nm, the EQE is above 55%, which surpasses
that observed in the BHJ cell based on annealed P3HT/
PCBM.14 However, the EQE value is lower compared to
the annealed cell of P3HT/PCBM at longer wavelengths. BHJ
cells of 2 with PCBM at 50, 67, and 80 wt % were also
checked. The cell with a 67 wt % of PCBM exhibited the
best conversion efficiency. Though the cell had comparable
open-circuit voltage with that of the 1-based cell, the
significant drop in short-circuit resulted in a lower efficiency
at 0.90% which was ∼50% lower compared to the cell based
on 1 with the same wt % of PCBM. The decreased efficiency
is mainly due to the drop of short-circuit current in the cell
of 2. It is possible that there is inferior film morphology and/
or imbalanced carrier mobility for the film of 2.
The electrochemical data of the compounds are listed in
Table 1, and the cyclic voltammograms are shown in Figure
S3 (Supporting Information). Similar to our previous obser-
vations on bipolar compounds with a benzo[1,2,5]thiadiazole
core and two peripheral diarylamines,12 the oxidation of the
peripheral amines was detected as a quasireversible two-
electron redox wave, indicating no significant interaction
between the two amines. The redox waves at higher
potentials may be due to the oxidation of dibenzo[f,h]-
thieno[3,4-b]quinoxaline core and the oxidation of thie-
nylfluorene segment, respectively. A one-electron wave at
-1.48 V is attributed to the reduction of dibenzo[f,h]-
thieno[3,4-b]quinoxaline core. The HOMO energy level of
1 was calculated to be -5.16 eV from cyclic voltammetry
by comparison with ferrocene (4.8 eV).13 This together with
the HOMO/LUMO gap (1: 1.82 eV; 2, 2.23 eV) calculated
from the absorption band edge were used to obtain the
LUMO energy level (1: -3.34 eV; 2, -2.93 eV). Based on
the relative HOMO and LUMO energy levels of the compounds
and PCBM (HOMO ) -4.1; LUMO ) -6.1 eV; Figure S4,
Supporting Information), it is appropriate to use them and
PCBM in OPV cells as p- and n-type materials, respectively.
BHJ cells with a 1/PCBM blend in a 1:1 weight ratio as
the active layer were fabricated by spin coating on a PEDOT/
PSS/ITO substrate (PEDOT/PSS ) poly(3,4-ethylenedioxy-
thiophene)/poly(styrenesulfonate)). PEDOT/PSS was used to
(11) Gautrot, J. E.; Hodge, P.; Helliwell, M.; Raftery, J.; Cupertino, D.
J. Mater. Chem. 2009, 19, 4148.
The electron and hole mobilities for 1/PCBM film were
computed by analyzing their dark J-V curves which are
(12) Justin Thomas, K. R.; Lin, J. T.; Velusamy, M.; Tao, Y.-T.; Chuen,
C.-H. AdV. Funct. Mater. 2004, 14, 83.
(13) (a) Pommerehne, H.; Vestweber, H.; Guss, W.; Mahrt, R. F.;
Bässler, H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551. (b) Thelakkat,
M.; Schmidt, H. AdV. Mater. 1998, 10, 219.
(14) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, J. H.; Wienk, M. M.;
Kroon, J. M.; Michels, M. A. J.; Janssen, A. J. Nano. Lett. 2005, 5, 579.
4900
Org. Lett., Vol. 11, No. 21, 2009

Table 3. Summary of Device Parameters at Various 1/PCBM and 2/PCBM Compositions
sample
µe (m2 V-1 s-1
)
µh (m2 V-1 s-1
)
VOC (V)
JSC (mA cm-2
)
FF
η (%)
1/PCBM 50 wt %
1/PCBM 67 wt %
1/PCBM 80 wt %
2/PCBM 50 wt %
2/PCBM 67 wt %
2/PCBM 80 wt %
1.61 × 10-8
2.94 × 10-8
6.90 × 10-8
3.14 × 10-8
1.95 × 10-8
5.42 × 10-9
0.90
0.89
0.74
0.91
0.92
0.91
2.60
4.15
2.13
2.36
2.87
2.49
0.32
0.46
0.30
0.33
0.34
0.34
0.75
1.70
0.48
0.70
0.90
0.77
related by a relation J ) 9ε0εrµV2/8L3 where ε0εr is the
dielectric permittivity of the polymer, µ is the carrier
mobility, and L denotes the thickness of the active layer.15
The relevant data are collected in Table 3. Balanced electron
and hole mobility are beneficial to the device performance;
the one with the highest efficiency (67 wt % PCBM) has
the ratio of electron/hole mobility (µe/µh) at ∼1.5 and the
one with the poorest efficiency has electron/hole mobility at
∼13. It is also worth noting that the system with more
balanced charge transport had higher current density value.
The morphologies of the active layers with different weight
ratios of PCBM/1 were also investigated by atomic force
microscopy (AFM, Digital instrument NS 3a controller with
D3100 stage). The AFM images of these films are shown in
Figure S6 (Supporting Information). The blends formed
smooth films, and the root-mean-square (rms) roughness was
smaller than 3 nm. Film morphology was further examined
by transmission electron microscopy (TEM), and the TEM
micrograms of the annealed blend films16 of 1/67 wt % of
PCBM and 1/80 wt % of PCBM are shown in Figure 2.
Though no resolvable interpenetrating network frequently
observed in blend film of polymer/PCBM16 can be detected,
it is evident that the film of 1/67 wt % PCBM has smoother
morphology than that of 1/80 wt % PCBM. This gives further
support for the better performance of BHJ cell from 1/67 wt
% PCBM.
In conclusion, new bipolar compounds containing diben-
zo[f,h]thieno[3,4-b]quinoxaline and arylamine motifs were
synthesized and characterized. Besides intense π-π* transi-
tion, the charge-transfer transition in these compounds also
contributes to light harvesting. Because of good solubility
of the compound in common organic solvents, solution-
processable BHJ cells with good power conversion efficien-
cies can be achieved from the blends of the compound and
PCBM. The performance of the cells depends on the solvent
used for fabrication and the ratio of the compound vs PCBM.
The cell fabricated from a CHCl3 blend of one of the
compounds with 67 wt % of PCBM has the best perfor-
mance, with the conversion efficiency reaching 1.70% and
high EQE values close to 60% in the 380-420 nm region.
Further improvement of the device efficiency by tuning of
the molecular structure as well as device fabrication is
currently under investigation.
Acknowledgment. We would like to acknowledge the
Academia Sinica, National Taiwan University, and National
Science Council, Taiwan, for financial support. We thank
the Cryo-EM Common Facility of Scientific Institument
Center, Academia Sinica, for TEM service.
Supporting Information Available: Synthetic procedures
and characterization for new compounds. This materials is
available free of charge via the Internet at http://pubs.acs.org.
OL9019953
Figure 2. TEM images for the annealed blend films of 1/67 wt %
(15) Hertel, D. H.; Ba¨ssler, H. ChemPhysChem 2008, 9, 666.
(16) The films were prepared following the procedures reported in the
following paper: Sivula, K.; Luscombi, C. K.; Thompson, B. C.; Fréchet,
J. M. J. J. Am. Chem. Soc. 2006, 128, 13988.
PCBM and 1/80 wt % PCBM. The scale bars in both images are
200 nm.
Org. Lett., Vol. 11, No. 21, 2009
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