Photovoltaic performance of an ultrasmall band gap polymer

Article abstract of DOI:10.1021/ol802839z

A conjugated polymer (PBTTQ) that consists of alternating electron-rich bithiophene and electron-deficient thiadiazoloquinoxaline units was synthesized via Yamamoto polymerization with Ni(cod)₂ and provides a band gap of 0.94 eV. This represents one of the smallest band gaps obtained for a soluble conjugated polymer. When applied in a bulk heterojunction solar cell together with [84]PCBM as the electron acceptor, the polymer affords a response up to 1.3μm.

Full text of DOI:10.1021/ol802839z

ORGANIC
LETTERS
2009
Vol. 11, No. 4
903-906
Photovoltaic Performance of an
Ultrasmall Band Gap Polymer
Arjan P. Zoombelt,†,‡ Marta Fonrodona,†,‡ Martijn M. Wienk,† Alexander
B. Sieval,§ Jan C. Hummelen,| and Rene´ A. J. Janssen†,*
Molecular Materials and Nanosystems, EindhoVen UniVersity of Technology,
P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Dutch Polymer Institute (DPI),
P.O. Box 902, 5600 AX EindhoVen, The Netherlands, Solenne BV, Zernikepark 6-8,
9747 AN Groningen, The Netherlands, and Molecular Electronics, Zernike Institute for
AdVanced Materials & Stratingh Institute for Chemistry, UniVersity of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
r.a.j.janssen@tue.nl
Received December 11, 2008
ABSTRACT
A conjugated polymer (PBTTQ) that consists of alternating electron-rich bithiophene and electron-deficient thiadiazoloquinoxaline units was
synthesized via Yamamoto polymerization with Ni(cod)2 and provides a band gap of 0.94 eV. This represents one of the smallest band gaps
obtained for a soluble conjugated polymer. When applied in a bulk heterojunction solar cell together with [84]PCBM as the electron acceptor,
the polymer affords a response up to 1.3 µm.
Recently various small band gap polymers have been
synthesized for application in polymer solar cells.1-10 The
motivation for developing polymers with an optical absorp-
tion that extends into the near-infrared is to exploit the low
energy photons available in the solar spectrum. Traditionally
polymers like poly(p-phenylene vinylene) and poly(3-hexyl-
thiophene) (P3HT) have been used in organic solar cells11,12
but their optical band gaps close to 2 eV significantly limit
the fraction of photons that can be absorbed from sunlight.
A proven strategy to reduce the band gap of these π-con-
jugated materials and possibly enhance the photocurrent is
to incorporate electron-rich and electron-deficient units in
an alternating fashion in a polymer chain.13-15 Virtually all
† Eindhoven University of Technology.
‡ Dutch Polymer Institute.
§ Solenne BV.
| University of Groningen.
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Andersson, M. R. AdV. Mater. 2007, 19, 3308
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10.1021/ol802839z CCC: $40.75
Published on Web 01/26/2009
2009 American Chemical Society

small band gap polymers for use in solar cells employ this
strategy to reach band gaps in the 1.2-1.6 eV range.1-10
For a copolymer of cyclopentadithiophene and benzothia-
diazole (PCPDTBT) that has Eg ) 1.4 eV, the energy
conversion efficiency of 5.5% is among the highest reported
for polymer solar cells.6,8 A further attractive approach to
improve overall efficiencies involves stacking multiple solar
cells that cover different parts of the solar spectrum.16-18
Hadipour et al. were able to cover a large part of the solar
spectrum by using high and low band gap solution process-
able polymers.18 An efficiency of 6.5% was reported by Kim
et al. when PCPDTBT was applied in tandem solar cells
together with P3HT.19 Especially for multijunction polymer
solar cells it is interesting to develop polymers that absorb
near-infrared (NIR) light.5,10
The synthesis of the PBTTQ is depicted in Scheme 1. A
Williamson ether synthesis of methyl-3,5-dihydroxybenzoate
and 1-bromo-2-ethylhexane afforded 1 in 92% yield, which
was reacted in a sodium-induced acyloin coupling to R-di-
ketone 2 in moderate yield. Next, bromination of 2,1,3-
benzothiadiazole (3), with bromine in aqueous HBr, gave 4
as white needles after recrystallization from methanol.
Nitration of 4 in fuming nitric acid and concentrated sulfuric
acid resulted in 5 that was reacted with 2-tributylstannyl-
thiophene to 6 in a Stille coupling with Pd(PPh3)2Cl2 as a
catalyst. Reduction of 6 by iron dust in acetic acid resulted
in diamine 7. At this point in the route, the bromine atoms
must be incorporated to allow a polymerization of 9 via
Yamamoto coupling. We found that, after condensation
coupling of 7 with 2, the R-position of the thiophene rings
became considerably less susceptible toward electrophilic
aromatic substitution. The thiadiazoloquinoxaline unit is
strongly electron deficient and reduces the reactivity of the
thiophenes with respect to bromination. Diamine 7 could
easily be halogenated by applying N-bromosuccinimide
(NBS) in THF and afforded 8 in 61% yield. The electron-
donating character of the amine groups compensates the
electron-withdrawing nature of the thiadiazole ring and
facilitates the electrophilic substitution. The condensation
coupling of 8 with R-diketone 2 did result in some Br-
cleavage, but purification by column chromatography gave
pure 9. Polymerization through Yamamoto coupling with
bis(1,5-cyclooctadiene)nickel(0) (Ni(cod)2) resulted in a
green/yellow polymer PBTTQ with Mw ) 58500 g/mol and
Mn ) 9300 g/mol.
In our search for such attractive NIR materials we report
here the synthesis of an ultrasmall band gap polymer
(PBTTQ, Scheme 1) via Yamamoto coupling using Ni(cod)2.
The polymer is based on alternating electron- rich bithiophene
and electron-deficient thiadiazoloquinoxaline units. The
optical and electrochemical properties of PBTTQ are de-
scribed and the performance in solar cells is evaluated.
Scheme 1. Synthesis of PBTTQ
Incorporating the electron-deficient thiadiazoloquinoxaline
unit in an alternating fashion with bithiophene results in a
strong absorption in the near-infrared (Figure 1a). Dissolved
in o-dichlorobenzene, the wavelength of maximum absorp-
tion (λmax) is 1165 nm with a shoulder at about 1020 nm. In
solution, the optical band gap is 0.94 eV as determined from
the onset of absorption. This represents one of the smallest
band gaps obtained for soluble conjugated polymers,5,10
although for electrochemically prepared conjugated polymer
films gaps as small as 0.36 eV have been established.13f
The small band gap of PBTTQ is also evidenced by the
redox potentials determined by cyclic voltammetry. The
onsets of the oxidation and reduction waves of PBTTQ in
o-dichlorobenzene are at -0.09 and -1.06 V vs Fc/Fc+,
(13) (a) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1993,
55, 299. (b) Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Chem. Soc., Chem.
Commun. 1994, 1585. (c) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem.
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Y. J. Am. Chem. Soc. 1995, 117, 6791. (e) Akoudad, S.; Roncali, J. Chem.
Commun. 1998, 2081. (f) Akoudad, S.; Roncali, J. J. Chem. Soc., Chem.
Commun. 1994, 1585.
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R.; Sariciftci, N. S. Appl. Phys. Lett. 2006, 89, 073502/1.
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P. W. M. AdV. Funct. Mater. 2006, 16, 1897.
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M.; Heeger, A. J. Science 2007, 317, 222.
904
Org. Lett., Vol. 11, No. 4, 2009

Figure 1. Absorption spectra of PBTTQ and [84]PCBM in
o-dichlorobenzene.
respectively, and provide an electrochemical band gap of 0.97
eV, in excellent agreement with the band gap determined
from optical absorption.
By incorporating the thiadiazoloquinoxaline unit the
electron affinity increases and the reduction potential of
PBTTQ is very close to the value of -1.07 V for [6,6]-
phenyl-C61-butyric acid methyl ester ([60]PCBM), which is
commonly used as n-type material in bulk-heterojunction
polymer solar cells. The negligible offset between the LUMO
levels of PBTTQ and [60]PCBM likely precludes electron
transfer between the two compounds in the excited state of
PBTTQ because it is generally considered that a minimum
offset of ∼0.4 eV between the LUMO levels of electron
donor and acceptor is required for efficient photoinduced
charge transfer.20,21 The offset between HOMO levels is
much larger: 0.7-0.8 eV when estimating the HOMO level
of [60]PCBM at -1.07 eV plus its optical band gap of 1.75
eV.
Bulk heterojunction solar cells with a PBTTQ:[60]PCBM
blend in a 1:4 ratio (by weight) as active layer (thickness 72
nm) were fabricated by spin coating the mixture from
chlorobenzene (total concentration 25 mg/mL) on a ITO/
PEDOT:PSS bottom electrode on a glass substrate and
subsequently evaporating a LiF/Al top electrode. The current
density-voltage (J-V) characteristics of this device under
white light illumination are shown in Figure 2a and the
relevant parameters are collected in Table 1. Both the open-
circuit voltage (Voc ) 0.37 V) and short-circuit current
(Jsc(SR) ) 0.45 mA/cm2)22 are relatively low. The mono-
chromatic external quantum efficiency (EQE) of the cell
based on PBTTQ and [60]PCBM is virtually zero above 700
nm, which shows that only [60]PCBM contributes to the
photocurrent. The selective photoresponse of [60]PCBM is
tentatively explained by considering that charges can be
Figure 2. Performance of PBTTQ:[60]PCBM and PBTTQ:
[84]PCBM solar cells: (a) J-V characteristics under 75 mW/cm2
white light illumination and (b) monochromatic EQE. Device
configuration: glass/ITO/PEDOT:PSS/PBTTQ:PCBM/LiF/Al.
generated by hole transfer from [60]PCBM to PBTTQ if this
process is faster than the competing energy transfer to the
small band gap polymer but not from photoexcited PBTTQ
because of the negligible offset of its LUMO level with that
of [60]PCBM.
[84]PCBM,23 the C84 analogue of [60]PCBM, was used
to increase the offsets between the LUMO levels of donor
and acceptor and create more favorable conditions for
photoinduced charge transfer from PBTTQ in the excited
state. [84]PCBM has a reduction potential of -0.73 V vs
Fc/Fc+ and, hence, the electron affinity is 0.35 V larger
than that of [60]PCBM.23 This creates a driving force for
photoinduced charge separation when combined with
PBTTQ. The optical gap of [84]PCBM is at ∼1.2 eV as
determined from the onset of the spectrum in ODCB.
The photovoltaic performance of the cells based on
PBTTQ:[84]PCBM as the active layer is shown in Figure
2a. The blend was spin coated from o-dichlorobenzene in a
1:4 ratio (by weight) at a total concentration of 36 mg/mL.
The EQE (Figure 2b) shows that PBTTQ combined with
(20) Scharber, M. C.; Muehlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
Table 1. Photovoltaic Performance of PBTTQ with PCBM
Derivatives
C.; Heeger, A. J.; Brabec, C. J. AdV. Mater. 2006, 18, 789
(21) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys.
Lett. 2006, 88, 093511
.
.
d
(nm)
Voc
(V)
Jsc(WL)
Jsc(SR)
(22) Jsc(SR) represents the short circuit current estimated by convulting
the spectral response (SR) of the cell with the AM1.5G spectrum and
generaly provides a more accuarte estimate of the performance of the cell
under solar light than the Jsc(WL) measured under white light conditions.
The differences between the two are due to spectral mismatch of the white
light source and the AM1.5 spectrum.
fullerene
(mA/cm2) (mA/cm2)
FF
η (%)
[60]PCBM
[84]PCBM
72
90
0.37
0.10
0.31
0.34
0.45
0.28
0.46
0.35
0.08
0.01
Org. Lett., Vol. 11, No. 4, 2009
905

[84]PCBM gives rise to charge separation. The cell shows a
photoresponse up to about 1.3 µm. This is one of the furthest
red-shifted responses reported for polymer bulk heterojunc-
tion solar cells.5,10 The overall performance of the cell,
however, is very modest and the EQE in the 800-1400 nm
region does not exceed 0.01. Compared to the cell with
[60]PCBM, the response in the visible region is reduced,
possibly because hole transfer from [84]PCBM to PBTTQ
is slower because of the reduced offsets of the HOMO levels.
The low Voc of 0.10 V is not unexpected. Voc is often found
to be ∼0.4 eV less than the difference between the HOMO
and LUMO levels of donor and acceptor, respectively and
is expected at 0.24 V considering the levels of PBTTQ and
[84]PCBM. The extra loss is due to the limited number of
photogenerated charges that causes an incomplete quasi-
Fermi level splitting.24 The low charge density further
reduces the energy difference (and hence Voc) between the
occupied hole and electron levels via the energetic disorder
associated with these materials.
The morphology of the active layer in a bulk heterojunc-
tion solar cell is also important to the overall photovoltaic
performance.25 Spin coating from different solvents and
thermal or solvent annealing affect charge separation ef-
ficiency and charge carrier mobility. For the present study,
the use of chlorobenzene with [60]PCBM and o-dichloro-
benzene with [84]PCBM proved to give the best results,
consistent with earlier observations with [60]PCBM and
[70]PCBM.26
Figure 3. AFM height images of (a) PBTTQ:[60]PCBM spin coated
from chlorobenzene and (b) PBTTQ:[84]PCBM spin coated from
o-dichlorobenzene.
PSS is substantial. Nevertheless, for a similar NIR absorbing
device Perzon et al. and Wen et al. have obtained higher
EQEs up to 0.12 and 0.03 at ∼950 nm, respectively.5,10
Therefore, specific properties of PBTTQ are expected to play
an important role as well. Two factors can be noticed. First,
the LUMO offset of ∼0.33 eV might be just sufficient for
photoinduced charge separation but does not ensure a
quantitative formation of free charge carriers. Second, the
low hole mobility (µhole ) 1.7 × 10-6 cm2/Vs in a FET)
will reduce charge separation efficiency and enhance charge
recombination.
In conclusion, by alternating bithiophene and thiadiazolo-
quinoxaline units along the chain PBTTQ has been obtained
as a soluble polymer that exhibits an ultrasmall band gap of
∼0.94 eV. Together with a new fullerene derivative,
[84]PCBM, as electron acceptor, the PBTTQ electron donor
provides a photoresponse in bulk heterojunction solar cells
up to 1.3 µm, one of the most red-shifted responses for this
type of photovoltaic devices at present. The overall limited
photocurrent, however, illustrates that creating NIR absorbing
organic photoactive layers that provide efficient charge
generation and collection remains a challenging task.
The morphology of the active layers was investigated by
atomic force microscopy (AFM) (Figure 3). Both blends
formed smooth and uniform films when spin coated from
chlorobenzene and o-dichlorobenzene, respectively, which
suggests that a relative intimate mixing of the two compo-
nents exists that itself would not inhibit charge generation.
Several other causes, however, may contribute to the low
photocurrent densities. Because of optical interference the
absorption of light with wavelengths above 1 µm is limited
in an active layer of about 100 nm thickness. Moreover at
these long wavelengths the absorption by ITO and PEDOT:
Acknowledgment. This work is part of the research
program of the Dutch Polymer Institute (DPI nos. 325 and
524)
(23) Kooistra, F. B.; Mihailetchi, V. D.; Popescu, L. M.; Kronholm,
D.; Blom, P. W. M.; Hummelen, J. C. Chem. Mater. 2006, 18, 3068.
(24) Koster, L. J. A.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M.
Appl. Phys. Lett. 2005, 86, 123509.
Supporting Information Available: Detailed synthetic
procedures and characterization. This material is available
free of charge via the Internet at http://pubs.acs.org.
(25) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45.
(26) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.;
Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed.
2003, 42, 3371.
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