Novel 4,8-benzobisthiazole copolymers and their field-effect transistor and photovoltaic applications

Article abstract of DOI:10.1039/c7tc03959j

A series of copolymers containing the benzo[1,2-d:4,5-d′]bis(thiazole) (BBT) unit has been designed and synthesised with bisthienyl-diketopyrrolopyrrole (DPP), dithienopyrrole (DTP), benzothiadiazole (BT), benzodithiophene (BDT) or 4,4′-dialkoxybithiazole (BTz) comonomers. The resulting polymers possess a conjugation pathway that is orthogonal to the more usual substitution pathway through the 2,6-positions of the BBT unit, facilitating intramolecular non-covalent interactions between strategically placed heteroatoms of neighbouring monomer units. Such interactions enable a control over the degree of planarity through altering their number and strength, in turn allowing for tuning of the band gap. The resulting 4,8-BBT materials gave enhanced mobility in p-type organic field-effect transistors of up to 2.16 × 10^-2 cm² V^-1 s^-1 for pDPP2ThBBT and good solar cell performance of up to 4.45% power conversion efficiency for pBT2ThBBT.

Full text of DOI:10.1039/c7tc03959j

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Materials Chemistry C
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Novel 4,8-benzobisthiazole copolymers and
their field-effect transistor and photovoltaic
Cite this:DOI: 10.1039/c7tc03959j
applications†
a
Gary Conboy,a Rupert G. D. Taylor,
Neil J. Findlay,a Alexander L. Kanibolotsky,ab
Anto R. Inigo,a Sanjay S. Ghosh,c Bernd Ebenhoch,c Lethy Krishnan Jagadamma,
c
Gopala Krishna V. V. Thalluri,c Muhammad T. Sajjad,
Ifor D. W. Samuel
and
c
c
a
Peter J. Skabara
*
A series of copolymers containing the benzo[1,2-d:4,5-d0]bis(thiazole) (BBT) unit has been designed and
synthesised with bisthienyl-diketopyrrolopyrrole (DPP), dithienopyrrole (DTP), benzothiadiazole (BT),
benzodithiophene (BDT) or 4,40-dialkoxybithiazole (BTz) comonomers. The resulting polymers possess
a
conjugation pathway that is orthogonal to the more usual substitution pathway through the
2,6-positions of the BBT unit, facilitating intramolecular non-covalent interactions between strategically
placed heteroatoms of neighbouring monomer units. Such interactions enable a control over the degree
of planarity through altering their number and strength, in turn allowing for tuning of the band gap. The
resulting 4,8-BBT materials gave enhanced mobility in p-type organic field-effect transistors of up to
2.16 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 for pDPP2ThBBT and good solar cell performance of up to 4.45% power
conversion efficiency for pBT2ThBBT.
Received 30th August 2017,
Accepted 15th October 2017
DOI: 10.1039/c7tc03959j
rsc.li/materials-c
organic materials that have values m 4 10 cm2 Vꢁ1 sꢁ1 8–15
Many of these leading candidates currently require pre-treatment
.
Introduction
The use of organic materials as the active components of of the SiO2 substrate to control polymer chain self-assembly12
electronic devices has been an area of much research over the or increase wettability,2 and/or thermal annealing at high
past few decades. Such interest over inorganic materials (e.g. temperature16 in order to achieve a suitable morphology capable
silicon based), is largely due to their inherent tunability, of affording high mobility.17 Such processes add to the time,
solubility and flexibility, allowing for devices to be made from cost and complexity of fabrication and, as such, materials which
organic materials with finely tuned properties that can be show high mobility with minimal processing steps are highly
processed from solution onto flexible substrates.1–3 The develop- desired for industrial use.
ment of new materials for use in organic field-effect transistors
Organic semiconductors are also often studied for use as
(OFETs) is facilitating great advancements in charge carrier functional active layer materials in organic photovoltaic (OPV)
mobility, with these materials finding many applications in devices, as they can be engineered to have a broad absorption
modern technology such as biosensors,4 addressing in active across the solar spectrum. When combined with a suitable
matrix light-emitting diodes (AMOLEDs)5 and as a low cost, acceptor species, resultant organic solar cells can have power
flexible alternative to amorphous silicon in radiofrequency conversion efficiencies (PCEs) of over 12%.18,19 In order to
identification tagging (RFIDs).6 In order for organic materials minimise the band gap, as well as to increase the carrier
to find useful applications, the mobility should be at least mobility and self-assembly within the bulk phase, it is beneficial
comparable to amorphous silicon, which has a hole mobility (m) to place alternating conjugated donor and acceptor units into
of 0.1 r m r 1 cm2 Vꢁ1
s
ꢁ1.7 There are now many reports of the backbone of the conductive material to facilitate a push–pull
effect.20,21 Such a structure is readily achievable in polymeric
a WestCHEM, Department of Pure and Applied Chemistry, Thomas Graham
Building, University of Strathclyde, Glasgow, G1 1XL, UK.
form through the copolymerisation of suitably functionalised
donor and acceptor units. Many of these units contain multiple
heteroatoms that offer further advantages, such as planarisation
and strong interchain packing due to a combination of p–p
stacking and non-covalent heteroatom/weak hydrogen bonding
E-mail: peter.skabara@glasgow.ac.uk
b Institute of Physical-Organic Chemistry and Coal Chemistry, 02160 Kyiv, Ukraine
c Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University
of St. Andrews, North Haugh, St. Andrews, KY16 9SS, UK
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc03959j interactions in the bulk material.22–25 For example, the use of
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intramolecular Sꢂ ꢂ ꢂN non-covalent interactions and a highly
planarised structure (maximum torsion angle of 5.11 across the
C–C bond connecting the BBT unit and heterocycle), and the use
of furan (Fig. 2: X = CH, Y = O) leads to a similarly highly
planarised structure (maximum torsion angle of 4.11) through
non-covalent SꢂꢂꢂO interactions.25 These, and other results, have
shown that non-covalent SꢂꢂꢂN and SꢂꢂꢂO contacts offer favourable
interactions of comparable strength, whilst SꢂꢂꢂS interactions are
repulsive and NꢂꢂꢂO interactions are weak/negligible. It is important
to note that additional intramolecular hydrogen bonding inter-
actions with the BBT nitrogen atoms may be playing a role when
X = CH (Fig. 2), however it has been unequivocally shown
that heteroatom-heteroatom interactions are influential on the
structure of such molecules.25
In this work we describe the synthesis of a series of BBT
copolymers conjugated along the 4,8-substitution pathway (Fig. 1)
and report their semiconducting properties in OFET and OPV
devices. Bisthienyl-diketopyrrolopyrrole (DPP), dithienopyrrole
(DTP), benzothiadiazole (BT), benzodithiophene (BDT) and 4,40-
dialkoxybithiazole (BTz) were selected as comonomers due to
their reported behaviour in high performance OFET32,39 and
OPV40 devices, but also to allow for band gap variation through
HOMO and LUMO energy level tuning and to provide varying
degrees of planarity through non-covalent heteroatom inter-
actions (as discussed above).
Fig. 1 Typical 2,6-BBT conjugation pathway (left) and alternate 4,8-BBT
conjugation pathway (right).
thiazole (rather than thiophene) facilitates these advantages in
combination with deeper highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels26 to give increased device performance in both
OFETs27,28 and OPVs.29,30 Additionally, intermolecular non-
covalent interactions in the bulk are evident, further contributing
to improved charge carrier properties.26,31–33
Recently we highlighted the use of a thiazole-containing
benzobisthiazole (BBT) unit with an orthogonal (4,8- vs. traditional
2,6-) conjugation pathway (Fig. 1).25 Whilst BBT units possessing
the 2,6-conjugation pathway are common and well-studied,
those with 4,8-substitution have been underexplored in
comparison,25,34–38 despite offering a better template with
which to facilitate planarising intramolecular non-covalent
interactions with neighbouring heterocycles.25 By carefully
selecting the type and location of heteroatoms in the flanking
heterocycles (Fig. 2), the planarity of the resultant molecule or
polymer can be tuned and, in turn, their solubility and energy
gap can be modified.
Non-covalent interactions are defined as contact distances
between two atoms (often heteroatoms) which are shorter than
the sum of the van der Waals radii of the two corresponding
atoms. Through a combination of X-ray crystallography and
computational simulations we have previously demonstrated
that the inclusion of thiophene units either side of the BBT
heterocycle (Fig. 2: X = CH, Y = S) gives a twisted, non-rigid
structure. In the orientation depicted in Fig. 2, C–Hꢂ ꢂ ꢂN hydrogen
bonding interactions are offset by the repulsive Sꢂ ꢂ ꢂS interactions,
whilst Sꢂ ꢂ ꢂN interactions (when the flanking heterocycles are
flipped 1801 relative to Fig. 2) are deterred by steric hindrance
between the C–H and S groups.25 Conversely, utilising thiazole
moieties in place of thiophene (Fig. 2: X = S, Y = N) results in four
Synthesis
The targeted BBT-containing polymers were realised via Stille
or Suzuki cross-coupling mediated polymerisations between
alkylated 4,8-dibromo-BBT and suitably functionalised bisthienyl-
diketopyrrolopyrrole (DPP),41,42 dithienopyrrole (DTP), benzothia-
diazole (BT),43 benzodithiophene (BDT)44 or 4,40-dialkoxybithiazole
(BTz)32 monomers. Each polymer was then end-capped with thio-
phene units via subsequent Stille or Suzuki cross-coupling reactions
with commercial tributyl(thiophen-2-yl)stannane (or thiophen-
2-ylboronic acid in the case of pDPPThBBT) then 2-bromo-
thiophene. Polymers containing a biheterocyclic bridge between
donor/acceptor units were prepared as random copolymers
using 2,5-bis(trimethylstannyl)thiophene45 (pDPP2ThBBT and
pBT2ThBBT) or 2,5-bis(trimethylstannyl)furan45 (pDPPThFBBT)
as a third monomer. As a result, their structures shown in
Scheme 1 represent an idealised structure in each case. Detailed
synthetic procedures for all monomers and polymers can be
found in the ESI.†
Molecular weights (Table 1) of the resultant polymers were
determined by gel permeation chromatography (GPC) in chloro-
form or o-dichlorobenzene solution and show a range of different
values (14.4–96.0 kg molꢁ1). The limiting factor in molecular
weight for each polymer is solubility – all polymers precipitated
from solution during their respective polymerisations. Despite
their very different molecular weights, pDPP2ThBBT and
pDPPThFBBT have very similar solubility, suggesting that there
is reduced rotational freedom in pDPPThFBBT due to intra-
molecular Sꢂ ꢂ ꢂO and C–Hꢂ ꢂ ꢂN interactions,25 facilitated by the
Fig. 2 Non-covalent intramolecular interactions between S and N atoms
of a BBT core and flanking heterocycles.
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Scheme 1 Synthesis of BBT-containing copolymers. Yields calculated from repeat units as drawn. Reagents and conditions: (i) Pd2(dba)3, P(o-tol)3,
chlorobenzene, 160 1C, mW. (ii) Pd2(dba)3, P(o-tol)3, toluene, reflux. (iii) Pd2(dba)3, P(o-tol)3, K3PO4, THF, reflux. End-capping reagents:
(A) tributyl(thiophen-2-yl)stannane. (B) 2-Bromothiophene. (C) Thiophen-2-ylboronic acid.
Table 1 GPC, optical, electrochemical and thermal decomposition (Td) data for the 4,8-BBT copolymers. GPC data not obtained for pBTzBBT and
pDTPBBT due to incomplete polymer solubility
Solutione
)
Mw (kg molꢁ1 PDI Egopt c (eV) Eeglect d (eV) lmax (nm) lonset (nm) lmax (nm) lonset (nm) HOMOd (eV) LUMOd (eV) Td (1C)
Film
Polymer
pBTzBBT
—
—
1.53
1.69
1.39
1.43
1.26
—
2.24
2.11
685, 740 f 795
657, 731 f 808
ꢁ4.69
ꢁ4.90
ꢁ5.10
ꢁ4.86
ꢁ5.20
ꢁ5.29
ꢁ4.80
ꢁ3.00
ꢁ3.51
ꢁ3.67
ꢁ3.60
—
360
395
411
365
451
332
401
pDPPThBBT 14.4a
pDPP2ThBBT 96.0a
pDPPThFBBT 18.0a
1.70 1.36
2.75 1.39
2.04 1.43
1.80 1.58
1.90 2.00
790
765
875
865
728
748
910
895
716 f, 751 855
705, 756 870
pBT2ThBBT
pBDTBBT
pDTPBBT
17a
61b
—
548
493
530
690
555
620
620
567
536
785
620
675
ꢁ3.05
—
1.84
ꢁ2.69
a
b
Calculated from GPC using 0.5 mg mlꢁ1 solutions in chlorobenzene at 80 1C. Calculated from GPC using 1 mg mlꢁ1 solutions in chloroform at
c
d
22 1C. Calculated from the onset of the longest solid state wavelength absorption peak. Found from CV, using the onset of redox activity and
referenced to Fc/Fc+ (ꢁ4.8 eV). Absorption spectra obtained from o-dichlorobenzene solutions. Shoulder.
e
f
furan rings flanking the BBT unit. In contrast, the thiophene feature thiophene units flanking the BBT units and hence
moieties flanking the BBT unit in pDPP2ThBBT result in contain unfavourable Sꢂ ꢂ ꢂS interactions) is likely due to a
unfavourable Sꢂ ꢂ ꢂS interactions and hence a more twisted combination of different alkyl chain lengths (leading to different
structure, allowing the growing polymer chain to remain in solubility limits of the growing polymer chains during poly-
solution longer before precipitating. Molecular weight variation merisation) and the variety of polymerisation conditions used
between pDPPThBBT, pBT2ThBBT and pBDTBBT (which all across the series.
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Optical properties
Comparison of the optical band gaps reveals that the
DPP containing polymers (pDDPThBBT, pDDP2ThBBT and
pDPPThFBBT) have very similar optical properties, and the
smallest optical band gaps (1.36–1.43 eV) of all the 4,8-BBT
copolymers. This suggests that the strong electron-accepting
nature of the DPP unit causes it to dominate the optical
properties of these copolymers, resulting in red-shifted absorption.
Copolymers featuring weaker acceptor units (BTz and BT) show
slightly wider optical band gaps (1.53 and 1.58 eV for pBTzBBT and
pBT2ThBBT, respectively), whilst those containing electron
donating units have the widest optical band gaps (1.84 and
2.00 eV for pDTPBBT and pBDTBBT, respectively).
The optical properties of pBT2ThBBT, pBDTBBT and pDTPBBT
reveal comparable absorption profiles to their equivalent 2,6-BBT
copolymers (PBBTzBT-HD,46 PBTHDDT47 and PBTDTP,47
respectively, Fig. 4) when measured in solution, with either
similar or slightly hypsochromically shifted absorption onsets.
Absorption spectra of the 4,8-BBT copolymers were obtained in
o-dichlorobenzene solution and as thin films. The resulting
spectra (Fig. 3a–f) all show broad absorption, extending up
to 900 nm and resulting in optical band gaps in the range
1.36–2.00 eV (as calculated from the onset of the longest wave-
length solid state absorption peak). All of the 4,8-BBT copolymers
show a red shift in their longest wavelength absorbance onset
when moving from solutions to thin films due to increased
intermolecular interactions. However, this red shift is very small
(r35 nm) for the three DPP containing copolymers (pDDPThBBT,
pDDP2ThBBT and pDPPThFBBT) and pBTzBBT, indicating that
these materials possess a rigid, aggregated structure even in
solution. In contrast, pBT2ThBBT, pDTPBBT and pBDTBBT
show a more significant red shift (up to 95 nm) in their film
form, suggesting enhanced order due to aggregation and
molecular packing in the solid state. This is in part due to As thin films, the absorption properties of pDTPBBT and its
2,6-substituted counterpart (PBTDTP)47 are also similar, and
these polymers possessing thiophene groups flanking the BBT
result in essentially identical optical band gaps (1.84 and
1.85 eV, respectively). However, the solid state absorption
unit, allowing for a more twisted and hence less conjugated
structure in solution.
Fig. 3 Absorption spectra of (a) pDPPThBBT, (b) pDPP2ThBBT/pDPPThFBBT, (c) pBT2ThBBT, (d) pBTzBBT, (e) pDTPBBT and (f) pBDTBBT in
o-dichlorobenzene solution (red) and as thin films drop cast from o-dichlorobenzene (a–e) or chloroform (f) (black).
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Fig. 4 Previously reported 2,6-BBT copolymers featuring DTP (top left),47 BDT (top right)47 and BT (bottom) units.46 acalculated from photoelectron
yield spectroscopy, bcalculated using (Eg (opt) + EHOMO).
profiles of pBT2ThBBT and pBDTBBT are bathochromically band gaps (2.11 and 2.24 eV, respectively). In comparison to its
shifted compared to their 2,6-analogues, resulting in lower optical 2,6-analogue, pBDTBBT has a smaller electrochemical band gap
band gaps (1.58 vs. 1.7 eV46 for the BT-containing polymers and (2.24 eV vs. 2.37 eV for PBTHDDT),47 which is in agreement to
2.00 vs. 2.13 eV47 for the BDT-containing polymers). Moreover, the the bathochromic shift seen in the absorption measurements.
absorption window of pBT2ThBBT is significantly broader than
that of its 2,6-analogue (PBBTzBT-HD),46 allowing for increased
photon absorption.
Organic field-effect transistor (OFET)
fabrication
Electrochemical properties
Bottom gate/bottom contact (BGBC) OFETs were prepared from
commercially available (Fraunhofer Institute fu¨r Photonische
To establish their electronic characteristics, cyclic voltammetry
Mikrosysteme IPMS, Dresden) n-doped silicon substrates with a
(CV) was performed on solutions and thin films (cast on
200 nm layer of thermally grown SiO2 dielectric and prefabricated
platinum disc electrodes) of the 4,8-BBT copolymers. Electronic
interdigitated gold source/drain electrodes (channel lengths: 2.5,
band gaps (Table 1) were found to be between 1.26–2.24 eV and
5, 10 and 20 mm, channel width: 1 cm). Unless otherwise stated,
broadly consistent with the trend in optical values. Cyclic
solutions were pre-stirred at 50 1C for at least three hours, then
voltammograms for all 4,8-BBT copolymers in solution and
spin-coated whilst hot at 2000 rpm. Following any required
solid state are shown in Fig. S8–S20 (ESI†), however pBT2ThBBT
annealing, the devices were dried under vacuum (5 ꢀ 10ꢁ2 mbar),
did not reveal a reduction peak at potentials as low as ꢁ2.0 V,
then their performance was measured on a Keithley 4200 semi-
preventing electrochemical determination of the polymer’s
conductor characterisation system, all whilst in a dry, nitrogen-
LUMO level.
filled atmosphere. Mobilities were calculated in the saturation
pDPPThBBT and pDPP2ThBBT exhibit relatively similar
region via the standard method using the following equation:
HOMO and LUMO energy levels, and by extension closely
pffiffiffiffiffi
ꢀ
ꢁ
2
matching electrochemical band gaps (1.39 and 1.43 eV, respectively).
pDPPThFBBT shows a slightly lower electrochemical band gap of
1.26 eV, which is likely due to a combination of the lower resonance
2L
@
Ids
msat
¼
ꢀ
WCi
@Vgs
stabilisation energy of the furan groups flanking the BBT unit and where Ids is the source–drain current, m is the carrier mobility, Vgs
increased conjugation through planarising intramolecular S–O is the gate voltage, L is the channel length, W is the channel width
interactions. The electrochemical data of the DPP-containing and Ci is the capacitance per unit area of the insulator material.
4,8-BBT copolymers is consistent with the optical data in
Device optimisation studies were carried out for pDPP2ThBBT
demonstrating that the strongly electron-accepting nature of (selected for its solubility and relatively high molecular weight,
the DPP unit results in it dominating the materials electronic which has been shown to result in improved charge carrier
behaviour, although thiophene or furan flanking units can fine mobility)48,49 through variation of the annealing temperature
tune this behaviour further.
(as cast, 60, 100, 150 and 200 1C) and solvent (chloroform,
In agreement with the optical data and comparing to those chlorobenzene or o-dichlorobenzene). Solutions of 10 mg mlꢁ1
copolymers containing DPP, copolymers featuring weaker electron concentration were prepared and deposited onto the prefabricated
acceptor units have slightly wider electrochemical band gaps substrates in accordance with the previously stated procedure,
(1.69 eV for pBTzBBT), whilst those featuring electron-donating with the same device tested at each annealing temperature
units (pDTPBBT and pBDTBBT) have much wider electrochemical increment. To facilitate accurate comparison of the materials
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Table 2 Device characteristics for pDPP2ThBBT based OFETs, averaged
across 3 devices
Table 3 OFET device data for the 4,8-BBT copolymers. Device para-
meters: BGBC, spin coated from 10 mg mlꢁ1 o-dichlorobenzene solution,
annealed at 100 1C. Averaged across 3 devices
Annealing
temperature (1C) mh (cm2 Vꢁ1 sꢁ1
) Ion/Ioff Vth (V)
Polymer
mh (cm2 Vꢁ1 sꢁ1
)
Ion/Ioff
Vth (V)
Solvent
2.16 ꢀ 10ꢁ3
3.23 ꢀ 10ꢁ6
2.16 ꢀ 10ꢁ2
2.03 ꢀ 10ꢁ3
3.60 ꢀ 10ꢁ3
8.69 ꢀ 10ꢁ5
2.11 ꢀ 10ꢁ5
105
105
103
105
103
103
103
ꢁ2
ꢁ10
5
ꢁ2
ꢁ5
ꢁ2
11
o-Dichlorobenzene As cast
1.17 ꢀ 10ꢁ2
1.63 ꢀ 10ꢁ2
2.16 ꢀ 10ꢁ2
1.20 ꢀ 10ꢁ2
8.56 ꢀ 10ꢁ3
3.97 ꢀ 10ꢁ3
6.35 ꢀ 10ꢁ3
7.89 ꢀ 10ꢁ3
7.76 ꢀ 10ꢁ3
—
103
103
103
103
102
12
14
5
0
ꢁ6
pBTzBBT
60
pDPPThBBT
pDPP2ThBBT
pDPPThFBBT
pBT2ThBBT
pBDTBBT
100
150
200
Chlorobenzene
As cast
60
103
103
103
103
—
13
14
6
1
—
pDTPBBT
100
150
200a
solution and annealed at 100 1C; data obtained from these
OFETs are shown in Table 3, with output and transfer char-
acteristics in Fig. S34–S39 (ESI†).
Chloroform
As cast
60
8.98 ꢀ 10ꢁ3
1.18 ꢀ 10ꢁ2
1.28 ꢀ 10ꢁ2
8.76 ꢀ 10ꢁ3
—
102
103
103
103
—
12
11
1
100
The hole mobility values obtained show a large variation,
spanning four orders of magnitude. pDPP2ThBBT based OFETs
150
ꢁ1
200a
—
gave the highest hole mobility (2.16 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1
)
a
Devices gave no response due to poor film morphology.
in combination with a moderate Ion/Ioff ratio (103) and low
threshold voltage (5 V) for a p-type device, but suffer from
and limit further processing steps, the use of self-assembled increased hysteresis in both output and transfer characteristics
monolayers (e.g. pentafluorobenzenethiol)50 or processing additives (Fig. S21–S33, ESI†). This could be indicative of charge trapping
were avoided. Summarised data for devices based on pDPP2ThBBT in the bulk film or non-optimal device structure. pDPPThFBBT
are shown in Table 2 and Fig. 5, with full device data in Fig. S21–S33 exhibits a slightly lower hole mobility (2.03 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1),
(ESI†).
possibly due to its lower molecular weight48,49 and larger thresh-
Fig. 5 shows that o-dichlorobenzene is the best solvent for old voltage for p-type devices (ꢁ2 V). This is likely due to a non-
preparing OFETs from pDPP2ThBBT, with the devices out- ohmic contact between the gold electrode (work function ꢁ5.0
performing analogous devices prepared from chlorobenzene to ꢁ5.1 eV) and the shallow HOMO of pDPPThFBBT (ꢁ4.86 eV)
or chloroform solutions at all annealing temperatures. However, resulting in poor charge injection. AFM images of the thin films
annealing was only beneficial up to 100 1C, after which point of pDPP2ThBBT and pDPPThFBBT cast on OFET substrates
further annealing resulted in a drop-off in device performance (Fig. 6a and b) show very smooth uniform films with a root
due to visibly poor film morphology. This is likely a result of the mean square (RMS) roughness of 0.55 and 0.34 nm, respectively,
polymer exhibiting significant rigidity, meaning that modest and grain boundaries on the order of 0.1 mm or less. Similarly
annealing temperatures are enough to force the as cast film acquired images of pDPPThBBT (Fig. 6c) show a much rougher
towards the thermodynamically most stable (crystalline) state film (RMS roughness of 2.82 nm) with domains extending up to
resulting in grain boundaries. Accordingly, OFETs made from other 5 mm, and grain boundaries greater than 1 mm wide. This poor
4,8-BBT copolymers were processed from o-dichlorobenzene film morphology combined with the relatively low molecular
weight of pDPPThBBT is likely the cause of the significantly
inferior hole mobility (3.23 ꢀ 10ꢁ6 cm2 Vꢁ1
s
ꢁ1).49
To overcome the poor solubility of pBTzBBT, all solutions
used for OFET fabrication were pre-stirred at 100 1C for three
hours then spin-coated whilst hot to prevent precipitation of
the material. In spite of this, pBTzBBT produced the most ideal
OFETs of those presented in this work, with modest hole
mobility (2.16 ꢀ 10ꢁ3 cm2 Vꢁ1
s
ꢁ1), but crucially a high Ion/Ioff
ratio (105) and a low driving voltage (ꢁ2 V). AFM (Fig. 6d)
revealed that the film morphology of pBTzBBT was very rough
(RMS roughness = 15.63 nm), which is likely due to the poor
solubility of the polymer (despite the extra pre-treatment),
meaning that further optimisation of the device preparation,
or increasing the solubility of the polymer through the use of
longer alkyl chains, could lead to a higher-performing solution-
processed OFET.
An additional example of the impact of the BBT unit on
OFET performance is evident in pBT2ThBBT. OFETs featuring
pBT2ThBBT showed modest mobility (3.60 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1),
Fig. 5 Hole mobilities of OFETs based on pDPP2ThBBT as a function of
annealing temperature.
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Fig. 6 AFM images of OFETs after annealing at 100 1C fabricated using (a) pDPP2ThBBT (RMS roughness = 0.55 nm), (b) pDDPThFBBT
(RMS roughness = 0.34 nm), (c) pDPPThBBT (RMS roughness = 2.82 nm) and (d) pBTzBBT (RMS roughness = 15.63 nm).
but this is over two orders of magnitude higher than a literature concentrations suitable for OPV devices could not be obtained.
example featuring only BT units,51 highlighting the enhanced The remaining materials were employed as donor materials,
crystallinity afforded by incorporation of BBT units into the blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)
polymer backbone through increased potential for non-covalent and fabricated into OPV devices using the conventional architecture
interactions and 2D conjugation.
of ITO/PEDOT:PSS/BHJ/Ca/Al unless otherwise stated. The
results are summarised in Table 4.
pDPP2ThBBT and pDPPThFBBT exhibit very similar device
performances across all measurements (PCEs of 0.89 and 0.87%,
respectively) representing a small improvement over the simpler
pDPPThBBT material (PCE 0.50%). The increased PCEs are
attributed to the higher FF of 60% and 55% respectively,
compared to 50% for pDPPThBBT. The FF obtained for each
of the DPP-containing BBT copolymers increases proportionally
with increasing hole mobility, which is in good agreement with
previously published data.52
Organic photovoltaic (OPV) devices
Due to the absorption characteristics in the visible spectral
range, narrow band gap and efficient charge carrier transport
properties of many of these 4,8-BBT copolymers, they were
investigated for use as donor molecules in bulk heterojunction
(BHJ) OPV devices. Unfortunately, due to the very poor solubility
of pBTzBBT, blend solutions with fullerene acceptors of
Table 4 OPV device data for the 4,8-BBT copolymers
PCE
avg. (%)
PCE
best (%)
Polymer
VOC (V)
JSC (mA cmꢁ2
)
FF (%)
pBTzBBT
—
—
—
—
pDPPThBBT
pDPP2ThBBT
pDPPThFBBT
pBT2ThBBT
pBDTBBT
0.5 ꢃ 0.03a
0.9 ꢃ 0.3
0.90 ꢃ 0.02
4.33 ꢃ 0.12
0.82 ꢃ 0.02
0.57 ꢃ 0.04
0.53a
1.2
0.92
4.45
0.84
0.62
0.62 ꢃ 0.03a
0.6 ꢃ 0.02
0.64 ꢃ 0.03
0.65 ꢃ 0.02
0.81 ꢃ 0.02
0.69 ꢃ 0.19
1.60 ꢃ 0.05a
2.40 ꢃ 0.50
2.50 ꢃ 0.12
14.3 ꢃ 0.39
2.71 ꢃ 0.05
2.80 ꢃ 0.06
50.2 ꢃ 1.0a
60.0 ꢃ 5.0
55.0 ꢃ 1.1
48.0 ꢃ 1.3
39 ꢃ 0.78
30.8 ꢃ 7.7
pDTPBBT
a
Inverted architecture: ITO/Cs2CO3/BHJ/MoO3/Ag. The data are for the active layer blend ratios (donor/acceptor, w/w) giving the best and average
(over 4 to 8 OPV devices) power conversion efficiency, namely pDPPThBBT : PC71BM, 1 : 3. pDPP2ThBBT : PC71BM, 1 : 2. pDPPThFBBT : PC71BM,
1 : 3. pBT2ThBBT : PC71BM, 1 : 1 (3% DIO). pBDTBBT : PC71BM, 1 : 1 (3% DIO). pDTPBBT : PC71BM, 1 : 1. Optimisation details of these devices are
included in the ESI. The error bars (ꢃ) are standard deviations of the measured data.
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Fig. 7 (a) J–V characteristics and (b) EQE spectra for the best performing pBT2ThBBT:PC71BM OPV devices.
The highest PCE (4.45%) was obtained from pBT2ThBBT (1.26–2.24 eV). The novel materials were fabricated into BGBC
using a 1 : 1 blend with PC71BM and 3% diiodooctane (DIO) as OFET devices resulting in record hole mobilities amongst
an additive. The high PCE can be largely attributed to a similar 2,6-BBT or 4,8-BBT-containing polymers (up to 2.16 ꢀ
very high JSC (14.32 mA cmꢁ2) as well as moderate VOC and FF 10ꢁ2 cm2 Vꢁ1 sꢁ1 for pDPP2ThBBT). OFETs fabricated from
(0.65 V, 48%). The impressive JSC generated from this device, pBTzBBT gave a highly optimal Ion/Ioff ratio (105) and a low
comparable to that of PTB7,53 indicates that this material is a driving voltage (ꢁ2 V), despite poor film quality (established by
promising candidate for high-efficiency OPV devices. The AFM). Optimisation of this material/device structure is
J–V characteristics, external quantum efficiency (EQE) and expected to result in efficient, low cost, solution-processed
absorption spectra of the active layer blend components OFETs. OPV devices utilising the 4,8-BBT copolymers as donor
(pBT2ThBBT and PC71BM) corresponding to the best OPV materials with fullerene acceptors yielded (for the best material,
device are shown in Fig. 7. The higher photocurrent of this pBT2ThBBT) a high short-circuit current of over 14 mA cmꢁ2
polymer compared to others can be due to its broad absorption and a respectable power conversion efficiency of 4.45%. Further
(as shown in Fig. 3) and the higher extinction coefficient improvements in device performance for pBT2ThBBT are
(Fig. S44, ESI†) compared to other BBT polymers. Moreover, expected to be realised from an inverted device structure and
pffiffiffiffiffiffiffiffi
the use of a ZnO electron transport layer.55
the exciton diffusion length 2Dt of pBT2ThBBT determined
by time resolved fluorescence studies is found to be B10 nm
which is higher than many donor–acceptor polymers. For
Conflicts of interest
pffiffiffiffiffiffi
example, the reported value of Dt for PTB7 of 4–5 nm54
pffiffiffiffiffiffiffiffi
There are no conflicts to declare.
would give 2Dt of 6–7 nm. This enhanced exciton diffusion
length of the pBT2ThBBT polymer can also contribute towards the
increased photon harvesting of the pBT2ThBBT:PC71BM blend.
The experimental details and calculations of exciton diffusion
length are included in the ESI.†
Acknowledgements
We are grateful to the EPSRC for funding through grants
EP/H004157/1, EP/L012294/1, EP/L017008/1 and EP/L012200/1
and to the European Research Council for funding from Grant
321305. We are grateful to the research groups of Prof. Iain
McCulloch (ICL/KAUST) and Prof. Neil McKeown (Edinburgh)
for generously performing GPC measurements on the polymers.
PJS and IDWS are Royal Society Wolfson Research Merit Award
holders. Sanjay Ghosh thanks North Maharashtra University,
India, for providing study leave to do post-doctoral research at
the University of St Andrews, UK. Supporting data are accessible
from 10.15129/9b457e8c-12bc-4a3a-9af3-7f53474f4e5c.
In comparison to a 2,6-BBT analogue (PBBTzBT-DT),46
pBT2ThBBT gives a higher base PCE (4.45 vs. 2.37%) using
the same conventional device structure. However, PCEs of
PBBTzBT-DT were shown to improve to 3.84%46 and 6.53%55
by utilising an inverted device structure and a ZnO electron
transport layer respectively, suggesting further improvements
in pBT2ThBBT based devices are possible.
Summary
Copolymers of 4,8-benzobisthiazole (BBT) with bisthienyl-
diketopyrrolopyrrole (DPP), dithienopyrrole (DTP), benzothia-
diazole (BT), benzodithiophene (BDT) and 4,40-dialkoxybithiazole
(BTz) units have been synthesised for the first time. The resultant
copolymers were found to possess optical band gaps (1.36–2.00 eV),
equal to or narrower than their 2,6-BBT copolymer counter-
parts, with good agreement to their electrochemical band gaps
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