From benzobisthiadiazole, thiadiazoloquinoxaline to pyrazinoquinoxaline based polymers: Effects of aromatic substituents on the performance of organic photovoltaics

Article abstract of DOI:10.1039/c2jm33317a

Here we report the syntheses of low bandgap polymers based on benzobisthiadiazole (BBT), thiadiazoloquinoxaline (TQ) and pyrazinoquinoxaline (PQ) core structures with different aromatic substituents. The effects of changing core structures from BBT to PQ

Full text of DOI:10.1039/c2jm33317a

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PAPER
From benzobisthiadiazole, thiadiazoloquinoxaline to pyrazinoquinoxaline
based polymers: effects of aromatic substituents on the performance of
organic photovoltaics†
Teck Lip Dexter Tam,ab Teddy Salim,ab Hairong Li,b Feng Zhou,b Subodh G. Mhaisalkar,ab Haibin Su,b
ab
ab
*
*
and Andrew C. Grimsdale
Yeng Ming Lam
Received 24th May 2012, Accepted 13th July 2012
DOI: 10.1039/c2jm33317a
Here we report the syntheses of low bandgap polymers based on benzobisthiadiazole (BBT),
thiadiazoloquinoxaline (TQ) and pyrazinoquinoxaline (PQ) core structures with different aromatic
substituents. The effects of changing core structures from BBT to PQ and also substituents from
biphenyl and bithienyl on the photophysical, electrochemical and morphology of the polymers were
studied. These polymers were incorporated into solar cell devices as donors, with PC[71]BM as
acceptors, and their device performances were correlated with their properties. It was found that the
effect of these structural changes has significant consequences on the overall device performances.
TQ and PQ molecules is a consequence of their possessing
Introduction
conjugated side chains. A number of low bandgap polymers
containing BBT, TQ and PQ units have been reported, some of
which have shown promising results in OPV devices.15–23 In this
paper, we report the synthesis of copolymers of these units with
fluorene and their testing as donor materials in BHJ OPV
devices. The effect of the conjugated side chains on these poly-
mers upon their performance in OPV devices is discussed.
Low bandgap polymers are of interest in current organic
photovoltaic (OPV) research as high power conversion efficiency
may be achieved from bulk heterojunction (BHJ) devices using
them as electron donors.1–4 Ideally, the materials used should
have broad absorption spectra to ensure effective harvesting of
the solar photons.2,5–7 In designing low bandgap polymers, the
more common strategies involve using (1) alternating donor–
acceptor units to induce intramolecular charge transfer and (2)
the introduction of pro-quinoid units which promote quinoid
character along the backbone.8,9 The main strategy used in
designing polymers with broad absorption spectra, on the other
hand, involves attaching conjugated side chains to the polymer
main chains.10–13 To date, there is no report on the combined use
of all three strategies to achieve low bandgap and broad
absorption polymers.
Results and discussion
Polymer syntheses
Compounds 1, 2, 3, 4, PQ1 and PQ2 were synthesized as
previously reported.14,24 1 was combined with the tin reagent 2 by
Stille coupling to yield the polymer pBBTF. For the syntheses of
TQ polymers, 1 was partially reduced using iron in acetic acid,
followed by condensation with diketone 3 or 4 to yield the
dibromo TQ derivatives 5 and 6 respectively. Stille coupling of
Previously, we have reported the synthesis of benzo[1,2-c;4,5-
c0]bis[1,2,5]thiadiazole (BBT), [1,2,5]thiadiazolo[3,4-g]quinoxa-
line (TQ) and pyrazino[2,3-g]quinoxaline (PQ) based small
molecules that show low bandgaps and broad absorption
ranges.14 While the low bandgap character of the BBT derivative
is due to the strong intramolecular charge transfer and quinoid
character within the molecule, the broad absorption range of the
aEnergy Research Institute @ NTU (ERI@N), Nanyang Technological
University, 50 Nanyang Drive, Singapore 637553. E-mail: ymlam@ntu.
edu.sg; ACGrimsdale@ntu.edu.sg
bSchool of Materials Science and Engineering, Nanyang Technological
University, 50 Nanyang Avenue, Singapore 639798
† Electronic supplementary information (ESI) available: Detailed
syntheses of all new compounds, structures of PQ1a and PQ2a, NMR
and MALDI-TOF of all new intermediates, and NMR, TGA, DSC
and CV of all polymers. See DOI: 10.1039/c2jm33317a
Fig. 1 Structures of the monomer units.
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Table 1 Characterization of the BBT, TQ and PQ polymers
HOMOa/
eV
LUMOa/
eV
CV Eg/
eV
Soln opt.
Egb/eV
Thin film
opt. Egc/eV
lmax
Solnb/nm
lmax Thin
filmc/nm
Mw ꢁ 104/
d
Polymer
g molꢂ1
PDI
pBBTF
pTQ1F
pTQ2F
pPQ1F
pPQ2F
ꢂ5.08
ꢂ5.19
ꢂ5.16
ꢂ5.32
ꢂ5.52
ꢂ4.00
ꢂ3.92
ꢂ3.98
ꢂ3.52
ꢂ3.72
1.08
1.27
1.18
1.80
1.80
1.27
1.33
1.30
1.55
1.45
1.06
1.28
1.23
1.44
1.37
359, 415, 759
305, 428, 788
362, 443, 817
308, 405, 498, 657
357, 424, 621, 754s
364, 422, 848
307, 437, 846
367, 445, 877
308, 418, 512, 719
363, 429, 641, 811s
1.41
1.57
2.08
2.08
2.16
1.89
1.65
1.72
1.36
1.34
Measured by CV. Measured in CHCl3. c Spincoated on glass. Measured by GPC against polystyrene standard.
a
b
d
these molecules with 2 yielded the two polymers pTQ1F and
pTQ2F. The dibromo derivatives of BBT, TQ1 and TQ2 were
found to have limited solubilities and hence are not used for
polymerization (Fig. 1). For the syntheses of PQ polymers,
bromination of PQ1 and PQ2 using NBS in chloroform first
yielded the monomers 7 and 8 respectively. Suzuki coupling of
these with 9,9-dioctylfluorene-2,7-bis(boronic acid pinacol ester)
yielded pPQ1F and pPQ2F, respectively.
calculated HOMOs and LUMOs, can be found in Fig. 3. The
calculated values for PQs have been previously reported by us,25
whereas the values for the TQs were derived in the current work.
Photophysical and electrochemical properties
All five polymers showed a low bandgap with absorption up to
ꢀ900 nm for pPQ1F and pPQ2F and ꢀ1000 nm for pBBTF,
pTQ1F and pTQ2F in thin films (Fig. 2b). As expected, the
bandgaps of the polymers are in the order BBT < TQ < PQ due
to stronger intramolecular charge transfer arising from stronger
electron accepting nature and quinoid forming ability.14,26,27
pTQ2F and pPQ2F also showed a slightly lower bandgap than
pTQ1F and pPQ1F respectively due to the higher conjugation of
the bithienyls as compared to the biphenyls.25 The different
substituents on TQ did not cause any significant change in the
energy levels for the TQ polymers. However, a more pronounced
The molecular weight of the polymers (Table 1) is modest and
may be capable of optimization, e.g. through use of different
catalysts or microwave heating. The syntheses of these polymers
are shown in Scheme 1, and details of their physical and elec-
trochemical characterization data can be found in Table 1.
Optical bandgaps were calculated from the edges of absorption
in the UV-Visible spectra which are shown in Fig. 2. There is
excellent agreement between the solid state optical and electro-
chemical bandgaps for the polymers with exceptions in the case
of polymers pPQ1F and pPQ2F. Geometry optimized structures
of the dithienyl BBT, TQs and PQs, together with their
Fig. 2 (a) Solution (chloroform) and (b) thin film (glass slide) UV-Vis-
Scheme 1 Syntheses of BBT, TQ and PQ polymers.
This journal is ª The Royal Society of Chemistry 2012
NIR absorption spectra of BBT, TQ and PQ polymers.
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Fig. 3 DFT modeling of the TQ and PQ units using the B3LYP functional and 6-31G* basis set under the geometry optimization conditions.
ꢀ
The TQ polymers displayed interchain distances of 16.61 A
effect on the absorption spectra was easily seen from the less
obvious valley at ꢀ600 nm for pTQ2F as compared to pTQ1F.
The same trend was observed for the absorption spectra of
pPQ2F and pPQ1F respectively. On the other hand, the different
substituents on PQ did cause significant change in the energy
levels for PQ polymers. This is because the bithienyls now induce
a larger torsional strain on the thiophenes at the quinoid posi-
tions than the biphenyls, and have larger contribution in the
HOMO and LUMO in PQ than TQ, as shown in the density
functional theory (DFT) in Fig. 3. Together with the inherent
weaker quinoid character of PQ as compared to TQ, these
resulted in a stronger substituent effect on the energy levels for
the PQ polymers.
ꢀ
and 10.26 A for pTQ1F and 21.76 A and 10.14 A for pTQ2Fwhile
ꢀ
ꢀ
ꢀ
the PQ polymers showed 22.77 A and 8.21 A for pPQ1F and
ꢀ
ꢀ
ꢀ
27.27 A and 10.09 A for pPQ2F. Due to the asymmetric nature of
TQ units, there are three ways where the TQ polymers can
arrange themselves: (1) head–head (thiadiazole–thiadiazole), (2)
head–tail (thiadiazole-side groups) and (3) tail–tail (side groups-
side groups). With reference to the polymers of symmetrical BBT
and PQ units, the largest interchain distances for the TQ poly-
mers should be a consequence of head–tail stacking since they are
smaller than that of the PQ polymers, while the almost constant
second interchain distance should be a result of head–head
interaction for the TQ polymers since they match well with that
of the BBT polymer. Such a head–head interaction has been
observed previously in a TQ small molecule.30
From solution to thin film, UV-Vis-NIR absorption spectra
(Fig. 2a and b) show insignificant bathochromic shifts for all the
polymers at short wavelengths while higher shifts were observed
at longer wavelengths. The BBT polymer showed the largest
bathochromic shift of 89 nm while TQ and PQ polymers showed
similar shifts from 57–62 nm. This higher bathochromic shift of
pBBTF indicates that a higher aggregation of the polymer occurs
in the solid state as compared to the TQ and PQ polymers.
Both TQ and PQ polymers showed a significant p–p stacking
signal as compared to BBT polymers, which might suggest larger
tilt or face-on configuration. This is in contrast with the TQ–
acetylene based polymers reported by Dallos et. al. in which no
clear indication of p–p stacking was noticeable.31 From TQ to
PQ polymers, the increasing torsional strain between the
Thin film XRD
The aromatic substituents, together with the alkyl groups, can
affect the packing of the polymers by changing the interchain and
p–p stacking distance. Thus XRD of the polymer thin films was
measured as shown in Fig. 4. For pBBTF, the lack of these side
groups and strong intermolecular S–N contacts allow individual
polymer chains to come close together. Thus pBBTF showed a
ꢀ
short interchain distance of 9.78 A. Interestingly, no significant
p–p stacking was observed, as one would expect highly planar
polymer chains of pBBTF to show strong p–p stacking. Such an
observation was previously reported by Yuen et. al. for other
BBT polymers.28,29 Though we did not manage to obtain in-plane
XRD for all of the polymers, we suspect that an edge-on
configuration was generally present in BBT polymers which
would account for their reported high charge mobilities.
Fig. 4 Out-of-plane thin film XRD of BBT, TQ and PQ polymers
dropcast from 1,2-dichlorobenzene on the SiO2/Si substrate. The data
were normalized at the baseline.
18530 | J. Mater. Chem., 2012, 22, 18528–18534
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Table 2 Interchain and p–p stacking distances and their intensities and
intensity ratio of p–p stacking/interchain
OPV device characterization
The optimum device results, together with the J–V and IPCE
curves of TQ and PQ polymers are shown in Table 3, Fig. 5 and
8, respectively. pBBTF exhibited poor solubility and due to the
low LUMO which reduces efficiency of electron transfer to the
Interchain
ꢀ
distance/A
(intensity, IID
p–p stacking
ꢀ
distance/A
(intensity, ISD
Polymer
)
)
ISD/IID
pBBTF
pTQ1F
pTQ2F
pPQ1F
pPQ2F
9.78 (14.11)
16.61 (11.11)
21.76 (15.87)
22.77 (12.34)
27.27 (22.46)
—
—
acceptor, showed very poor device performance (Jsc
¼
0.08 mA cmꢂ2, Voc ¼ 0.42 V, FF ¼ 0.25, PCE ¼ 0.0086%). As
noted above, pBBTF also may have a preference towards an
edge-on orientation on substrates which is not favorable for
vertical charge transport in OPV devices. On the other hand, the
TQ polymers showed much better device performance. pTQ1F
achieved a PCE of 1.05% at donor : acceptor (D : A) ratio of
1 : 4 while pTQ2F achieved a PCE of 0.51% at D : A ratio of
1 : 3. To the best of our knowledge, pTQ1F thus displays the
highest PCE yet recorded for TQ based alternating copolymers in
such standard device architectures without use of additives or of
an electron transport/hole blocking layer.
4.10 (5.24)
3.96 (5.64)
4.35 (6.27)
4.26 (3.83)
0.472
0.355
0.508
0.171
aromatic substituents and the thiophenes at the quinoidal posi-
tions (see side view images of the TQ and PQ units in Fig. 3)
prevents polymer chains from packing closely together, resulting
ꢀ
in larger p–p stacking distances for PQ (pPQ1F – 4.35 A, pPQ2F
ꢀ
ꢀ
– 4.26 A) as compared to TQ polymers (pTQ1F – 4.10 A, pTQ2F
ꢀ
– 3.96 A).
The XRD pattern of a PQ small molecule (PQ1a in the ESI,
Fig. S1†) that resembled a part of the monomer unit of pPQ1F
showed similarity when the X-ray incident beam was vertical to
the nanowires, with strong signals from the (001), and (210),
(223) and (213) planes.32 The (001) plane corresponded to the
intermolecular distance in the c-axis direction, while the (210),
(223) and (213) planes corresponded more or less to the pp
stacking directions. When the X-ray beam was parallel to the
nanowires, the XRD pattern showed the absence of these p–p
stacking signals, indicating that the PQ small molecule was
parallel to the X-ray beam. The vertical and parallel X-ray beam
configurations resemble that of the out-of-plane and in-plane
XRD. Inferring from these data, we can conclude that pPQ1F
adopts a predominant face-on configuration on SiO2/Si. The
crystal structure of PQ1a was said to be applicable to PQ2a32
which resembled a part of the monomer unit of pPQ2F (see ESI
Fig. S1†). Comparing with the XRD pattern of PQ2a (see ESI of
ref. 23), we thus conclude that pPQ2F adopts a predominant
edge-on configuration on SiO2/Si.
The intensity ratio of p–p stacking and interchain distance of
TQ and PQ polymers were calculated and tabulated in Table 2.
This ratio gives a qualitative picture of the polymer/molecular
orientation33,34 of the p–p stacking with respect to the interchain
direction. As shown, pPQ1F showed the highest ISD/IID
,
followed by pTQ1F, pTQ2F and pPQ2F. It seems that the
conjugated side groups have an effect on either the preferential
orientation of the polymer on the substrate or organization of the
polymer chains.
Table 3 Device performance of TQ polymers
Thickness Jsc/mA
Polymer D/A ratio /nm
cmꢂ2
Voc/V FF
PCE/%
pTQ1F
pTQ2F
1 : 1
1 : 2
1 : 3
1 : 4
1 : 1
1 : 2
1 : 3
1 : 4
ꢀ70
ꢀ70
ꢀ70
ꢀ60
ꢀ50
ꢀ50
ꢀ40
ꢀ40
ꢂ1.06
ꢂ2.53
ꢂ3.36
ꢂ3.83
ꢂ0.72
ꢂ1.89
ꢂ2.54
ꢂ2.47
0.71
0.71
0.72
0.74
0.58
0.62
0.59
0.66
0.32 0.24
0.33 0.59
0.35 0.85
0.37 1.05
0.27 0.11
0.30 0.35
0.31 0.47
0.28 0.45
Fig. 5 J–V curve and IPCE (inset) of (a) pTQ1F and (b) pTQ2F devices.
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Table 4 Device performance of PQ polymers
Thickness/ Jsc/
Polymer D/A ratio nm
mA cmꢂ2 Voc/V FF
PCE/%
pPQ1F
1 : 1
1 : 2
1 : 3
1 : 4
1 : 1
1 : 2
1 : 3
1 : 4
ꢀ100
ꢀ90
ꢀ90
ꢀ90
ꢀ50
ꢀ50
ꢀ50
ꢀ40
ꢂ4.06
ꢂ6.44
ꢂ7.35
ꢂ6.95
ꢂ0.35
ꢂ0.46
ꢂ0.51
ꢂ0.39
0.84
0.83
0.82
0.82
0.35
0.25
0.29
0.22
0.31 1.05
0.37 1.97
0.39 2.36
0.36 2.05
0.25 0.031
0.26 0.030
0.26 0.038
0.27 0.023
pPQ2F
PC[71]BM increases, the nanofibre-like morphology disappears,
replaced by the fine morphology. This suggests that the surface of
the blend may have changed from pTQ1F rich to PC[71]BM rich,
which results in better extraction of electrons and lower recom-
bination as suggested by the increasing Jsc and FF. Besides, the
higher PC[71]BM content also promotes more percolation
pathways for electron transport. This results in a more balanced
charge transport between holes and electrons, reducing series
resistance of the device (or higher FF). However, the width of the
nanofiber appeared to be around 100 nm, which is too large for
the exciton to diffuse to the donor–acceptor interface, thus
limiting the performance of pTQ1F. Proper morphological
control of the nanofibre dimensions (to 10–20 nm) may improve
exciton dissociation prior to recombination, generating more
free-carriers; hence, better solar cell devices.
Fig. 6 AFM images of non-annealed pTQ1F and PC[71]BM active layer
on an actual device. The scan area is 5 mm ꢁ 5 mm.
AFM images showed smooth surfaces for thin films of all D/A
ratios and annealing conditions for both TQ polymers, with
pTQ1F showing nanofibre-like morphology (see Fig. 6) and
pTQ2F showing fine morphology (see Fig. 7). pTQ1F showed
higher Jsc, Voc and FF with respect to pTQ2F and these likely
resulted from (1) higher mobility due to a shorter interchain
distance, shorter p–p stacking distance and higher ratio of face-
on configuration, (2) a better percolation pathway through a
nanofibre-like morphology, (3) a higher energy offset to over-
come the binding energy of the exciton due to the higher LUMO
and (4) the deeper HOMO resulting in higher Voc. As the ratio of
In the case of PQ polymers, there was a drastic contrast in the
device performance between pPQ1F and pPQ2F (Table. 4).
pPQ1F achieved a high PCE of 2.36% at D : A ratio of 1 : 3 with
a high Jsc of 7.35 mA cmꢂ2, a high Voc of 0.82 V and a low FF of
0.39 (Fig. 8). On the other hand, pPQ2F only managed to give a
PCE of 0.038% with a very low Jsc, Voc and FF after optimiza-
tion. Energetically speaking, pPQ2F has suitable HOMO and
LUMO levels to be used as a donor material with PC[71]BM.
The surprising device performance prompted a possible issue in
the morphology of the active layer and/or active layer–electrode
Fig. 7 AFM images of non-annealed pTQ2F and PC[71]BM active layer
on an actual device. The scan area is 5 mm ꢁ 5 mm.
Fig. 8 J–V curve and IPCE (inset) of pPQ1F devices.
This journal is ª The Royal Society of Chemistry 2012
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device. Likewise, the edge-on configuration of pPQ2F may have
contributed to the low Jsc. However, these do not fully explain
the low Voc of pPQ2F devices.
It was reported that the kinetics of recombination at the D–A
interfaces have a strong impact on the Voc.35 Materials with
sterically inaccessible p-systems and non-planar geometries can
kinetically suppress dark recombination processes, represented
by the reverse saturation current density (J0).35–37 Under open
circuit conditions, Voc is correlated with J0 from the following
relationship:37
ꢀ
ꢁ
nkT
q
Jsc
J0
Voc
z
ln
Therefore, D–A systems with suppressed J0 tend to have a
higher Voc. The dark J–V characteristics of pPQ1F : PC[71]BM
(1 : 3) and pPQ2F : PC[71]BM (1 : 3) were exponentially fitted
(Fig. 11) to extract J0 values of 3.6 ꢁ 10ꢂ9 mA cmꢂ2 and 8.6 ꢁ
10ꢂ4 mA cmꢂ2, respectively. The five orders of magnitude
difference in J0 is represented as the remarkably lower Voc in
pPQ2F : PC[71]BM devices. The shunt effect can also be
Fig. 9 AFM images of non-annealed pPQ1F and PC[71]BM active
layer. The scan area is 5 mm ꢁ 5 mm.
interface. However the AFM images show smooth surfaces for
both PQ polymers, with pPQ1F showing very fine features (see
Fig. 9) while pPQ2F showing very large features of more than
500 nm in the morphology (see Fig. 10). The fine features of
pPQ1F help to maximize exciton dissociation via nanoscale
phase separation between the donor and acceptor interface while
the large features of pPQ2F do not. The shorter interchain
distance, shorter p–p stacking distance and the higher ratio of
face-on configuration for pPQ1F are likely to produce better
charge transport which will further boast the efficiency of the
Fig. 11 Dark current density–voltage (J–V) characteristics of (a)
pTQ1F : PC[71]BM (1 : 4) and pTQ2F : PC[71]BM (1 : 3) and (b)
pPQ1F : PC[71]BM (1 : 3) and pPQ2F : PC[71]BM (1 : 3) devices. The
solid lines are the exponential fits to the J–V characteristics at the
intermediate voltages. The inset shows the same curves over a wider range
of voltages.
Fig. 10 AFM images of non-annealed pPQ2F and PC[71]BM active
layer. The scan area is 5 mm ꢁ 5 mm.
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of 3.1 ꢁ 10ꢂ2 and 5.7 ꢁ 10ꢂ2 U cm2. This also suggests that the
higher J0 indicates stronger interactions of the p-conjugated
molecules in the pPQ2F : PC[71]BM blend.37 Comparison of
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in pPQ2F devices. This is also supported by the presence of large
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used to explain why pTQ2F (J0 ¼ 9.4 ꢁ 10ꢂ5 mA cmꢂ2) shows
almost 0.1 V lower in Voc as compared to pTQ1F (J0 ¼ 2.2 ꢁ
10ꢂ8 mA cmꢂ2) when both of them have similar HOMO energy
levels.
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1665–1670.
18 E. Perzon, F. Zhang, M. Andersson, W. Mammo, O. Inganas and
€
In conclusion, we have demonstrated the effects of aromatic
substituents on the photophysical properties, morphology and
OPV device performance of BBT, TQ and PQ polymers. The
bithienyls in pTQ2F and pPQ2F offer a lower bandgap and
broader light absorption via higher conjugation as compared to
the biphenyls in pTQ1F and pPQ1F. Even though these prop-
erties are generally beneficial for device performance, they are at
the expense of a higher HOMO and a larger dark current which
result in a lower Voc, and lower LUMO which provides a lesser
driving force to overcome exciton binding energy. The lower
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This work was supported financially by Robert Bosch (SEA) Pte
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This journal is ª The Royal Society of Chemistry 2012