A straightforward route to electron transporting conjugated polymers

Article abstract of DOI:10.1039/c2jm32217j

We report a straightforward synthetic method to generate solution processable electron transporting polymers with low band gap and wide absorption range from readily available acceptor monomers. We show the efficacy of this approach using widely used elec

Full text of DOI:10.1039/c2jm32217j

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PAPER
A straightforward route to electron transporting conjugated polymers†
a
G. Nagarjuna, Akshay Kokil, Jayant Kumar and D. Venkataraman
b
b
a
*
*
Received 9th April 2012, Accepted 19th June 2012
DOI: 10.1039/c2jm32217j
We report a straightforward synthetic method to generate solution processable electron transporting
polymers with low band gap and wide absorption range from readily available acceptor monomers. We
show the efficacy of this approach using widely used electron acceptor 2,1,3-benzothiadiazole. The
polymers have absorption up to 750 nm with electron mobility comparable to PCBM.
Electron transporting polymers with wide absorption are needed
in all polymer solar cells to achieve maximum absorption in the
solar spectrum.1–9 Currently, the electron transporting conju-
gated polymers that are widely used in all polymer solar cells are
limited to poly(benzimidazobenzophenanthroline (BBL), poly[2-
methoxy-5-(20-ethylhexyloxy)-1,4-(1-cyanovinylene) phenylene]
(CN-PPV) and rylene diimide based polymers.1–6,10,11 In addition
to the wide absorption window, optimal frontier energy levels
offset and a balanced charge transport between hole and electron
transporters is also needed to achieve maximum photovoltaic
efficiency.5,6,12 However, at present time, there are not many p-
conjugated electron transporters to complement and exploit the
existing wide variety of hole transporting polymers for solar
energy conversion.1–3,13–15 Thus, there is a need for a synthetic
strategy to rapidly generate electron transporting p-conjugated
solution processable polymers from readily available monomers.
Recently, there has been a widespread interest in developing
donor–acceptor polymers for lowering the band gap and
widening the absorption window of hole transporting poly-
mers.1–3,13–19 This has resulted in a wide-array of acceptor moie-
ties. Conjugated homopolymers of an acceptor unit could be
electron transporting with interesting optoelectronic proper-
ties.20–24 But, not many of them are synthesized because the
synthesis is specific to a few monomers and cannot be readily
extended to other acceptor monomers. Moreover, most of the
conjugated homopolymers of an acceptor monomer have high
band gap, narrow absorption range and low solubility in
common organic solvents.22–24 Although a conjugated alter-
nating copolymer of two acceptor monomers was reported
recently,21 at the present time, there is no general strategy to
generate solution processable, low band gap conjugated poly-
mers of a single acceptor monomer. Herein, we report a
straightforward synthetic route via Stille coupling to generate
solution processable low band gap conjugated polymers from
readily available acceptor monomers. We demonstrate the effi-
cacy of our approach using widely used electron acceptor, 2,1,3-
benzothiadiazole and commercially available trans-1,2-bis(tri-n-
butylstannyl)ethylene. The polymer shows electron mobilities
that are comparable to that of phenyl-C61-butyric acid methyl
ester (PCBM),25,26 the widely used electron transporter in organic
solar cells. We believe that our method can be readily extended to
a wide array of acceptor monomers to rapidly generate electron
transporting polymers.
We chose 2,1,3-benzothiadiazole as our monomer and electron
accepting moiety because of the ready availability of the mono-
mer as it is widely used to synthesize donor–acceptor alternating
conjugated copolymers.2,3,16–19 PBTDV1 and PBTDV2 (poly-
(benzothiadiazole vinylene)) were synthesized through a Stille
coupling27 of dihalo 2,1,3-benzothiadiazole and trans-1,2-bis(tri-
n-butylstannyl)ethylene (Fig. 1 and 2). The synthesis of mono-
mers 1 and 3 is shown in the ESI.†18 The synthesis of 3 from 2 was
not successful under commonly employed iodinating conditions
(see ESI, Table S1†) and required mild and active iodinating
conditions.
After screening various conditions for polymerization (see
ESI, Tables S3 and S4†), we found that the polymers with desired
molecular weights can be obtained if Pd(PPh3)4 was a catalyst.
We found no appreciable differences in the molecular weight or
aDepartment of Chemistry, University of Massachusetts Amherst, 710
North Pleasant Street, Amherst, MA, 01003, USA. E-mail: dv@chem.
umass.edu; Fax: +1 413-545-4490
bCenter for Advanced Materials and Department of Physics, University of
Massachusetts Lowell, Lowell, MA, 01854, USA. E-mail: jayant_kumar@
uml.edu; Fax: +1 978-934-3687
† Electronic supplementary information (ESI) available: GPC,
cyclicvoltammetry, WAXS, photocurrent transients data and synthesis
of monomers and polymers along with the other attempted synthetic
conditions are presented. See DOI: 10.1039/c2jm32217j
Fig. 1 Chemical structures of the polymers synthesized in this study and
their solution in chloroform.
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Fig. 2 Key steps in the synthesis of polymers.
the reaction time if the monomer was an aryl bromide or an aryl
iodide. In order to probe the effect of the vinylene linker, we also
synthesized conjugated polymers, PBTD1 and PBTD2 (poly-
(benzothiadiazole)), containing 2,1,3-benzothiadiazole alone.
Conjugated polymers made of a single electron deficient unit are
rare and most of them are not solution processable.22,24 PBTD1
and PBTD2 are the first reported solution processable conju-
gated polymers containing 2,1,3-benzothiadiazole alone. These
polymers were synthesized using a palladium-mediated Suzuki
coupling of the benzothiadiazole dihalide monomer with the
commercially available diborate BTD (Fig. 2). The Suzuki
coupling, to yield polymers, required a binary or ternary solvent
mixture, the highly active Pd(tBu3P)2 catalyst, and a strong base
such as LiOH$H2O. The solvents for the polymerization were
chosen based on the activity of the catalyst and the solubility of
the polymer in that solvent. In general, palladium catalysts are
highly active in polar solvents such as di-n-butylether and
conjugated polymers are soluble in 1,2-dichlorobenzene. The
polymerization was not successful with less active but widely
used Pd(PPh3)4 catalyst for Suzuki coupling (see ESI, Table S2†
for other attempted polymerization conditions). Much like in
PBTDV1 and PBTDV2, we did not find any significant differ-
ence in the reaction time if the monomer was an aryl bromide or
an aryl iodide. The synthesis of PBTDV1 and PBTDV2 involves
the condensation of a dihalo derivative of 2,1,3-benzothiadiazole
with commercially available trans-1,2-bis(tri-n-butylstannyl)
ethylene. Since dihalo derivatives of many acceptor monomers
are already known, this strategy can be readily extended to a
wide range of acceptor monomers.
Fig. 3 Thin film UV-Vis absorption spectra of polymers.
polymer backbone. There is also an increase in the absorption
range of alternating polymers compared to their non-vinylene
counterparts. We found that there was no appreciable difference
between the solution and thin film absorption for all the poly-
mers (ESI, Fig. S1†). The vinylene polymers, PBTDV1 and
PBTDV2, exhibit a vibronic structure in the absorption spectra
of both solution and thin film. Since the presence of vibronic
structure in the absorption spectrum is an indication of inter-
chain p–p interactions,28–34 we believe that the vinylene polymer
may be aggregating in solution. On the other hand, no vibronic
structure was observed in the absorption spectra for PBTD1 and
PBTD2 indicating the absence of inter-chain p–p interactions.
This might be due to the twist in the polymer backbone caused by
the sterics. Interestingly, the side chains have negligible effect on
the optical properties of all the polymers.
The redox properties of the polymers are shown in Fig. S2†
and their frontier energy levels are determined using established
protocols.35,36 The frontier energy levels, electrical and optical
band gap values of the polymers are tabulated in Table 1. PBTD1
showed no oxidation peak thus, HOMOelec and LUMOopt cannot
be determined. The vinylene polymers PBTDV1 and PBTDV2
are much easier to oxidize and reduce compared to PBTD1 and
PBTD2. This is expected because the charges in the vinylene
polymers can be better stabilized because of the increased delo-
calization along the conjugated polymer backbone. This obser-
vation once again is consistent with the vinylic spacer unit
facilitating the p–p conjugation along the polymer backbone by
reducing the steric interactions between the neighboring repeat
units. PBTDV1 and PBTDV2 have optimal frontier energy levels
offset with respect to many low band gap hole conductors such as
P3HT, PCPDTBT,19 PBDTTT-CF,37 and PBnDT-FTAZ,17
making them promising candidates for use as electron conduc-
tors in all polymer solar cells (Fig. S6†). Wide angle X-ray
scattering (WAXS) was recorded for the polymers and the data
are shown in Fig. S3.† PBTD2 and PBTDV2 polymers with
octyloxy side chains exhibit peaks at 2.25 nm and 2.28 nm,
respectively. This distance correlates well with the expected
octyloxy–octyloxy inter-chain distance. On the other hand,
PBTD1 and PBTDV1 polymers exhibit peaks at 3.25 nm and
3.45 nm, respectively, which are smaller than the expected tet-
radecyloxy side chain packing distance of 4.3 nm. The simulation
The thin film UV-Vis absorption spectra of all the polymers
are shown in Fig. 3. The thin films absorption range for PBTD1
and PBTD2 is 250–400 nm. The large band gap and narrow
absorption range of these polymers might be due to the sterics
between the neighbouring repeat units, leading to a twist and
thereby disrupting the p–p conjugation along the polymer
backbone, consistent with our expectation. PBTDV1 and
PBTDV2 with the vinylene linkers have the absorption range of
250–735 nm with the absorption maximum around 580 nm and
shoulders at 630 nm and 680 nm. The optical band gap of
PBTDV1 and PBTDV2 was found to be 1.68 eV, which is 1.0 eV
less than for PBTD1 and PBTD2 (Table 1). The lowering of the
band gap clearly indicates that the vinylic spacer unit facilitates
the reduction in sterics between neighboring benzothiadiazole
units, and thus enhancing the p–p conjugation along the
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Table 1 Summary of physical properties of polymers
Mw(Mw/Mn)a (kDa)
Eogpt (eV)
l
(nm)
max
HOMOelec (eV)d
LUMOelec (eV)d
LUMOopt (eV)e
film
Polymer
PBTD1b
PBTD2b
PBTDV1
PBTDV2
79.1 (1.2)
16.2 (1.1)
19.2 (1.8)
15.0 (1.6)
2.69
2.69
1.68
1.68
411
396
—
ꢀ2.93
ꢀ3.03
ꢀ3.42
ꢀ3.45
—
ꢀ5.96
ꢀ5.45
ꢀ5.24
ꢀ3.27
ꢀ3.77
ꢀ3.59
584 (633, 686)c
579 (627 675)c
GPC (THF) with respect to polystyrene standards. b Shows a small molecular weight peak in GPC (see ESI, Fig. S5†). Eogpt: calculated from the onset of
a
thin film UV-Vis absorption spectra. c Shoulders in absorption spectra. d Calculated from cyclic voltammetry. Calculated from HOMOelec + Eogpt
.
e
of polymer packing on Cerius2 for the corresponding interchain
packing indicated partial interdigitation between side chains (see
Fig. 4). Side chain interdigitation is commonly observed in many
semiconducting conjugated polymer packing.1,31,32,34,38 Side chain
interdigitation decreases the distance between polymer back-
bones and can enhance the charge transport.
photovoltaic devices. The samples were prepared by drop casting
the polymer solutions onto ITO coated glass slides followed by
the thermal evaporation of aluminum on top of the polymer film.
In ToF, a short laser pulse incident on a sample generates a thin
sheet of charge carriers. Transit time (tTR), time to migrate the
generated charge carriers across the sample under the influence
of an applied electric field (E), is determined. The photocurrent
transient of polymers (PBTDV1 and PBTDV2) at various
applied fields is shown in Fig. S7† and the shape of photocurrent
transit is characteristic of dispersive transport.39 Charge
carrier mobility (m) was determined using the following equation:
m ¼ L/(EtTR), where L is the film thickness. Electron mobilities of
the polymers at various applied fields are shown in Fig. 5.
PBTDV1 and PBTDV2 exhibit high electron mobilities of
10ꢀ3 cm2 Vꢀ1 sꢀ1, comparable to those of PCBM and F8BT thin
films.25,26,40 A slight increase in the electron mobility was
observed with a decrease in inter-chain distance from PBTDV1
(3.45 nm) to PBTDV2 (2.25 nm). The dispersive charge transport
is explained by a variation of local transport rates due to a high
spatial and/or energetic disorder in the material.41 Each dataset is
averaged over 128 photocurrent transients generated by the light
pulses. In organic photovoltaics there should be a balance in
charge transport between hole and electron transporters.42 The
hole mobility of most of the conjugated polymers used in organic
photovoltaics is on the order of 10ꢀ2 to 10ꢀ5 cm2 Vꢀ1 sꢀ1 making
PBTDV1 and PBTDV2 excellent electron transporters for
organic photovoltaic devices. In order to determine the hole
mobilities, the polarity of the applied field was changed.
However, negligible photocurrent transients were observed in
this configuration, indicating that the hole mobility could be
several orders of magnitude lower than the electron mobility.
In conclusion, we have shown that a poly(arylene vinylene)
derivative of an acceptor unit such as 2,1,3-benzothiadiazole is a
good electron transporter (me ꢁ10ꢀ3 cm2 Vꢀ1 sꢀ1) with low band
gap and broad absorption range (250–735 nm). Electron trans-
porting properties of PBTDV1 and PBTDV2 indicate that the
vinyl spacer indeed maintains the electron affinity of 2,1,3-
benzothiadiazole unlike other donor–acceptor polymers2,3,16–19 of
2,1,3-benznothiadiazole that exhibit hole transporting proper-
ties. Moreover, the vinyl spacer also reduces the band gap and
widens the absorption range of PBTDV1 and PBTDV2
compared to the conjugated homopolymers of 2,1,3-benzothia-
diazole (PBTD1 and PBTD2). The synthesis of PBTDV1 and
PBTDV2 involves the condensation of a dihalo derivative of
2,1,3-benzothiadiazole with commercially available trans-1,2-
bis(tri-n-butylstannyl)ethylene. Since dihalo derivatives of many
acceptor monomers are already known, this strategy can be
readily extended to a wide range of acceptor monomers to
The electron mobilities of PBTDV1 and PBTDV2 are deter-
mined using time of flight (ToF) technique.39 We chose ToF to
measure the mobility because the charge transport occurs across
the polymer film in a device configuration similar to organic
Fig. 4 Illustration of a simulated packing (using Cerius2) of the side
chains in PBTDV1.
Fig. 5 Time of flight electron mobilities of polymers as a function of the
applied electric field.
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Acknowledgements
We thank Kedar G. Jadhav for recording high temperature
NMR. We thank PHaSE Energy Frontier Research Center
supported by the US Department of Energy, Office of Basic
Energy Sciences, through grant DE-SC0001087 and the National
Science Foundation Materials Research and Science Center on
Polymers (DMR 0820506) for the financial support of this work.
G. Nagarjuna (partial support) and Akshay Kokil were sup-
ported by PHaSE EFRC. G. Nagarujuna (partial support) was
supported by NSF MRSEC. The characterization of the poly-
mers was done at the facilities supported by PHaSE EFRC and
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