Triisopropylsilylacetylene-functionalised anthracene-alt-benzothiadiazole copolymers for application in bulk heterojunction solar cells

Article abstract of DOI:10.1039/c5ra20927g

Three triisopropylsilylacetylene-functionalised anthracene (TIPSAnt) based polymers were synthesised by copolymerising TIPSAnt with either dithienyl-5,6-difluoro-benzo[c]-[1,2,5]thiadiazole, dithienyl-benzo[c]-[1,2,5]thiadiazole or dibithiophenyl-benzo[c]-[1,2,5]thiadiazole to yield PTATffBT, PTATBT-8 and PTAT2BT-8, respectively. PTAT2BT-8 demonstrated a reduced optical and electrochemical band gap, relative to PTATffBT and PTATBT-8. The HOMO level of PTAT2BT-8 (-5.32 eV) is significantly shallower compared to its counterparts. This can be attributed to increased intramolecular charge transfer along the polymer backbone; a consequence of the incorporation of additional thiophene spacer units. The photovoltaic properties of the polymers were investigated by fabricating bulk heterojunction (BHJ) polymer solar cells using PC₇₀BM as the electron acceptor. PTATffBT displayed limited solubility in common organic solvents and could not be used for the fabrication of photovoltaic cells. Optimised photovoltaic devices fabricated from PTATBT-8 and PTAT2BT-8 demonstrated power conversion efficiencies of 2.36% and 3.15%, respectively. PTAT2BT-8 provided better efficiencies chiefly as a result of better J_sc and FF values.

Full text of DOI:10.1039/c5ra20927g

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Triisopropylsilylacetylene-Functionalised Anthracene-alt-
Benzothiadiazole Copolymers for Application in Bulk
Heterojunction Solar Cells
Received 00th January 20xx,
Accepted 00th January 20xx
Luke Cartwright,a Lois. J. Taylor,a Hunan Yi,a Ahmed Iraqi,*a Yiwei Zhang,b Nicholas. W. Scarratt,b
Tao Wang,c and David. G. Lidzey*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Three triisopropylsilylacetylene-functionalised anthracene (TIPSAnt) based polymers were synthesised by copolymerising
TIPSAnt with either dithienyl-5,6-difluoro-benzo[c]-[1,2,5]thiadiazole, dithienyl-benzo[c]-[1,2,5]thiadiazole or
dibithiophenyl-benzo[c]-[1,2,5]thiadiazole to yield PTATffBT, PTATBT-8 and PTAT2BT-8, respectively. PTAT2BT-8
demonstrated a reduced optical and electrochemical band gap, relative to PTATffBT and PTATBT-8. The HOMO level of
PTAT2BT-8 (-5.32 eV) is significantly shallower compared to its counterparts. This can be attributed to increased
intramolecular charge transfer along the polymer backbone; a consequence of the incorporation of additional thiophene
spacer units. The photovoltaic properties of the polymers were investigated by fabricating bulk heterojunction (BHJ)
polymer solar cells using PC70BM as the electron acceptor. PTATffBT displayed limited solubility in common organic
solvents and could not be used for the fabrication of photovoltaic cells. Optimised photovoltaic devices fabricated from
PTATBT-8 and PTAT2BT-8 demonstrated power conversion efficiencies of 2.36 % and 3.15 %, respectively. PTAT2BT-8
provided
better
efficiencies
chiefly
as
a
result
of
better
Jsc
and
FF
values.
frontier energy levels and high hole mobilities.12 Such
conjugated polymers must ideally have excellent solubility in
common organic solvents to ensure the photovoltaic device
can be fabricated using low-cost solution based processing.13,14
Kallmann and Pope were the first to investigate the
photovoltaic effect in organic anthracene crystals.15 Since
then, the body of work covering anthracene and its
optoelectronic properties has been extended. Anthracene and
the acene family in general, possess extended π-conjugated
systems resulting in planar molecules with a high degree of
crystallinity.16 π-conjugated systems aid electron transfer
along the polymer backbone resulting in high charge mobility.
Furthermore, anthracene is an extremely versatile molecule
and can be polymerised through its 2,6-positions or 9,10-
positions.17–22 Providing that the polymerisation is conducted
through the 2,6-positions, the 9,10-positions can be
functionalised with substituents; affording a way to tune the
optoelectronic properties of the resulting polymer.19
Consequently, anthracene and its derivatives have found
widespread use in organic-field effect transistors and organic
light emitting diodes.23–27
Introduction
The combustion of fossil fuels releases carbon dioxide; a gas
that accentuates the greenhouse effect and climate change.
This has prompted the scientific community to develop new
forms of renewable energy. Polymer solar cells (PSC) have the
potential to provide a clean source of energy owing to their
ease of fabrication and low production costs.1,2
The most successful PSCs to date are devices fabricated using a
bulk heterojunction (BHJ) active layer architecture.3–6 An n-
type material, commonly a fullerene derivative, is blended
with
a conjugated polymer, p-type material to form a
nanoscale, bicontinous interpenetrating network.7 Fullerene
derivatives find wide-scale application as near ideal n-type
materials for PSCs.8–10 However, the development of new
donor-acceptor (D-A) alternating conjugated copolymers is
essential if the efficiencies of PSCs are to rival inorganic
devices.11 It is important that the conjugated polymer has a
low optical band gap, high absorption coefficients, finely tuned
The use of anthracene in PSC is limited, which is surprising
considering that benzo[1,2-b:4,5-b’]dithiophene (BDT), which
has an analogous molecular framework, has found widespread
use in high performance PSC devices.28–32 Park et al reported a
new class of anthracene-thiophene based copolymers
incorporating bulky triisopropylsilylacetylene (TIPS) groups at
the 9,10-positions of the anthracene moiety.32 It was reported
that the incorporation of TIPS groups overcame the poor

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charge transportation properties and high crystallinity that are PTAT2BT-8 display power conversion efficiencies (PCEs) of
generally associated with polythiophene-based copolymers. 2.36 % and 3.15 %, respectively, when blended with PC70BM in
The polymer yielded efficiencies of 1.70 % when blended with BHJ photovoltaic devices.
PC60BM and used as the active layer in a BHJ photovoltaic
device.32 Iraqi and co-workers presented the preparation of
Results and Discussion
donor-acceptor polymers with alternating 2,6-linked
anthracene units with aryloxy substituents at their 9,10-
Monomer and Polymer Synthesis
positions and various benzothiadiazole alternate repeat units.
The preparation of monomers and polymers are outlined in
Efficiencies ranging from 1.93 to 4.17 % were measured for
Scheme 1. Anthracene dibromide (3) was prepared according
these polymers.33 More recently, Jo and co-workers
synthesised a copolymer composed of thienyl substituted
anthracene and diketopyrrolopyrrole. The resulting polymer
exhibited an efficiency of 7.02 % and 4.23 % when blended
with PC70BM and di-perylene bisimide, respectively.18
to literature procedures.34 It was then converted to the bis-
boronate ester M1 upon reaction with
bis(pinacolato)diboron in the presence of Pd(dppf)Cl2.
M4 was then reacted with M2 M3 and M4, via Suzuki
coupling, in the presence of Pd(OAc)2 and tri(o-
tolyl)phosphine, to yield PTATffBT PTATBT-8 and PTAT2BT-8
(
)
,
,
,
respectively. The polymers were fractionated via Soxhlet
extraction using the solvents; methanol, acetone, hexane and
toluene. All polymers were obtained as deep purple powders.
The yield of PTAT2BT-8 was significantly lower than PTATffBT
and PTATBT-8 and large amounts of insoluble polymer
remained in the thimble after Soxhlet extraction. This suggests
that the incorporation of additional thiophene units together
with the small solubilising TIPSsubstituents positioned on the
anthracene unit were not able to facilitate the processability of
the high molecular mass fraction of the polymer. Despite large
quantities of PTATffBT being extracted in the toluene fraction,
it should be noted that PTATffBT exhibited poor solubility in
common organic solvents such as chloroform, chlorobenzene,
o-dichlorobenzene and 1,2,4-trichlorobenzene at elevated
temperatures. Consequently, photovoltaic devices could not
be fabricated from PTATffBT. The chemical structures of
PTATffBT,
PTATBT-8 and PTAT2BT-8 were confirmed via 1H-
NMR and elemental analysis. The number average molecular
weight (Mn) and weight average molecular weight (Mw) of all
polymers were measured using high-temperature (140oC) gel
permeation
chromatography
(GPC)
using
1,2,4-
trichlorobenzene as the eluent and polystyrene as the internal
standard (Table 1). PTATBT-8 displayed a Mn of 13,600 and a
Mw of 38,700 Da. PTAT2BT-8 displayed a reduced Mn (6,600
Da) and a Mw (13,200 Da) relative to PTATBT-8, which can be
attributed to the incorporation of additional thiophene-spacer
units without solubilising groups. PTATffBT displayed
significantlyreduced Mn and Mw values of 2,100 Da and 2,500
Da, respectively as a result of the absence of solubilising
substituents on the TffBT units. Furthermore, previous studies
have also indicated that incorporation of fluorine on the
benzothiadiazole-moiety facilitates stronger π-π stacking and
aggregation of polymer chains, which severely limits
accessibility to high molecular weight materials.35,36
Scheme
1. Synthesis of 2,6-bis-4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,10-bis(triisopropylsilylacetylene)
anthracene (4): (a) CuBr2, t-BuONO, CH3CN; (bi) n-BuLi, TIPS,
THF, 24 hours; (bii) SnCl2, HCl (10
%
aq); (c)
bis(pinacolato)diboron, KOAc, Pd(dppf)Cl2, DMF; (d) Pd(OAc)2,
P(o-tolyl)3, NEt4OH, toluene.
Here, we report the synthesis and optoelectronic properties of
three
anthracene-alt-benzothiadiazole
donor-acceptor
copolymers, PTATffBT
,
PTATBT-8 and PTAT2BT-8. The study
Optical Properties
focusses on incorporating TIPS groups at the 9,10-positions of
the anthracene unit and the impact of various
benzothiadiazole acceptors. PTATffBT was highly insoluble and
photovoltaic devices could not be fabricated from this
polymer. Organic photovoltaic devices based on PTATBT-8 and
The absorption spectra of the polymers were recorded both in
solution (chloroform) and as drop-cast films (Figure 1). The
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Table 1.GPC data, UV-Vis data, energy levels and energy gaps of the polymers
Polymer
Mn (Da)a
2,100
Mw (Da)a
2,500
λmax Solution (nm)
357, 455, 520
344, 524
λmax Film (nm)
328, 469, 523
369, 555
Egopt (eV)b
1.92
HOMO (eV)c
-5.48
LUMO (eV)d
Egelec (eV)e
2.32
PTATffBT
PTATBT-8
PTAT2BT-8
-3.16
-3.42
-3.47
13,600
38,700
1.85
-5.46
2.04
6,600
13,200
395, 540
402, 566
1.81
-5.31
1.84
aMeasurements conducted on the toluene fractions of polymers using a differential refractive index (DRI) detection method.
bOptical energy gap determined from the onset of the absorption band in thin film.cHOMO position (vs vacuum) determined
from the onset of oxidation. dLUMO position (vs vacuum) determined from the onset of reduction. eElectrochemical energy gap.
transitions of the polymer. The optical band gap of PTATffBT
was calculated to be 1.92 eV. The large optical band gap can
be attributed to the low molecular weight of PTATffBT
.
PTATBT-8 displays absorption bands in the visible region at
524 and 556 nm, in chloroform solution and film states,
respectively. The bathochromic shift that is observed from
solution to film can be ascribed to a more coplanar structure in
solid state. PTAT2BT-8 displays a maximum at 540 nm in
chloroform solution. The absorption maximum of PTAT2BT-8 is
red-shifted to 566 nm when cast into a thin-film. The
intramolecular charge transfer (ICT) band in the solid-state
optical spectrum of PTAT2BT-8 is more pronounced relative to
the π-π* band. This phenomenon is not evident in PTATBT-8
.
Furthermore, the optical band gap of PTAT2BT-8 (1.81 eV) is
lower than that of PTATBT-8 (1.85 eV). We speculate that the
incorporation of
a
bithiophene spacer-unit facilitates
intramolecular charge transfer along the polymer backbone
resulting in increased electronic delocalisation and a reduced
optical band gap. The optical band gaps of PTATBT-8 and
PTAT2BT-8 are located close to the maximum solar flux.
PTATBT-8 and PTAT2BT-8 are analogous to two polymers
previously synthesised by Iraqi and co-workers (PPATBT-8 and
PPAT2BT-8).33 In PPATBT-8 and PPAT2BT-8 the anthracene
moiety is functionalised with 4-dodecyloxybenzene groups,
instead of TIPS. PPATBT-8 and PPAT2BT-8 have optical band
gaps of 1.96 and 1.86 eV, respectively.33 We speculate that the
acetylene group and the long C-Si bond present in TIPS locate
the solubilising alkyl chains further away from the conjugated
backbone, which facilitates planarization of the polymer
backbone and improves π-π interchain stacking. This result is
consistent with the reduced Mn and Mw values obtained for
PTATBT-8 and PTAT2BT-8, relative to PPATBT-8 and PPAT2BT-
Figure 1.Normalised absorption spectra of PTATffBT
and PTAT2BT-8 in: (a) chloroform solutions; and (b) thin
films.
, PTATBT-
8
optical band gap and absorption value maxima are shown in
Table 1. PTATffBT displays absorptions maxima at 357, 455 and
520 nm in chloroform solution and at 328, 469 and 523 nm in
films. The lack of a significant bathochromic shift from solution
to film indicates that the polymers adopt similar energetic
conformations in both solution and film states. The additional
peaks in the UV-vis spectra of PTATffBT are attributed to the
inhomogeneously-broadened electronic and vibrational
8.
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PTATffBT has the deepest HOMO level of all polymers
synthesised within this report; a consequence of substituting
octyloxy groups on the benzothiadiazole-moiety for fluorine.
This result is consistent with previous literature that shows
that fluorination of acceptor moieties yields deeper HOMO
levels.35–38 However, the LUMO level of PTATffBT is
significantly shallower relative to both PTATBT-8 and
PTAT2BT-8. This is not consistent with previous literature
findings. We speculate that the low molecular weight of
PTATffBT is responsible for this observation. Previous
literature has shown that the LUMO level of a polymer
decrease more rapidly relative to the HOMO level as the
molecular weight increases.39 This is attributed to molecular
orbital hybridisation of each monomer resulting in localisation
of the LUMO level on the acceptor moiety as the Mw of the
polymer increases.39 We speculate that the low Mw of
PTATffBT results in the polymer displaying poor charge
transport properties owing to a higher concentration of
polymer chain ends and decreased packing density.
Figure 2. Cyclic voltammograms of PTATffBT
PTAT2BT-8 on platinum disc electrodes (area 0.031 cm2)
recorded at
scan rate of 100 mV s-1 in
acetonitrile/tetrabutylammonium perchlorate (0.1 mol dm-3).
, PTATBT-8 and
a
PPATBT-8 and PPAT2BT-8 synthesised by Iraqi et al had
HOMO/LUMO levels of -5.48/-3.14 eV and -5.35/-3.11 eV,
respectively.33 Clearly, the electronic properties of the
resulting polymer are changed significantly when the
substituents attached at the 9,10-positions of anthracene are
modified. The TIPS group is fully conjugated to the anthracene
π-system (resonance effect) and the sp hybridised acetylene
carbons are more electron withdrawing than the sp2
hybridised orbitals carbons to which they are attached
(inductive effect). Thus, polymers that incorporate TIPS groups
induce efficient intramolecular charge separation resulting in a
notable decrease in the LUMO level.
Thermal Properties
The thermal properties of all conjugated polymers were
investigated using thermograviemtric analysis (TGA). The
resulting thermograms of the polymers are plotted in Figure 3.
All polymers possess good thermal stability with degradation
temperatures (5 % weight loss) in excess of 300oC; a desirable
property for the fabrication of organic photovoltaic devices.
Figure 3. TGA plots of PTATffBT, PTATBT-8 and PTAT2BT-8with
a heating rate of 10 oC min-1 under an inert atmosphere of
nitrogen.
PTATffBT, PTATBT-8 and PTAT2BT-8 display degradation
temperatures of 352, 334 and 341oC, respectively. We
speculate that the degradation temperatures of PTATBT-8 and
PTAT2BT-8 are lower than that of PTATffBT as a result of
fluorine substitution.35,40,41 It Is speculated the octyloxy
substituents attached to the 5,6-positions of benzothiadiazole
in PTATBT-8 and PTAT2BT-8 are more thermally labile relative
to fluorine.
Electrochemical Properties
The frontier energy levels (vs vacuum) of the polymers were
determined from the onsets of oxidation and reduction (Table
1). The onsets were determined via cyclic voltammetry
measurements, which were conducted on drop-cast polymer
films in acetonitrile with tetrabutylammonium perchlorate as
the electrolyte (Figure 2). The HOMO/LUMO levels of
PTATffBT, PTATBT-8 and PTAT2BT-8 were estimated to be -
Powder X-Ray Diffraction Studies
5.48/-3.16 eV, -5.46/-3.42 eV and -5.31/-3.47 eV, respectively.
The shallower HOMO level of PTAT2BT-8 relative to that of
PTATBT-8 occurs as a result of increased intramolecular charge
transfer along the polymer backbone. Such effects are a
consequence of incorporating bithiophene spacer-units into
the polymer backbone. However, the LUMO level of PTAT2BT-
Powder X-ray diffraction (XRD) studies were conducted on the
three polymers (Figure 4). The XRD patterns PTATBT-8 and
PTAT2BT-8 display broad diffraction peaks at the angle of ca.
22o and is associated with π-π stacking. This x-ray diffraction
pattern is consistent with many other amorphous donor-
acceptor conjugated copolymers, for example PCDTBT.42
8
is at a similar energy level to that of PTATBT-8.
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Table 2.Performance of glass/ITO/PEDOT:PSS/polymer : PC70BM/Ca/Al BHJ polymer solar cells under a simulated photovoltaic
light with 1000 Wm-2 illumination (AM 1.5). PCE values given represent the highest and average values obtained.
Polymer
Polymer : PC70BMa (w/w)
Solvent
CFb
CBc
CB + DIOd
CFb
CBc
Jsc (mA cm-2)
-6.66
Voc (V)
0.88
0.90
0.86
0.74
0.78
0.78
FF
PCE (%)
Thickness(nm)
PTATBT-8
1 :3
1 :3
1 :3
1 :3
1 :3
1 :3
37.5
37.8
49.4
50.0
41.8
65.3
2.20 (2.11 ± 0.10)
2.36 (2.28 ± 0.07)
1.80 (1.69 ± 0.10)
3.15 (2.99 ± 0.11)
2.72(2.56±0.10)
2.19 (2.08 ± 0.08)
70
70
75
75
85
85
-6.88
-4.26
PTAT2BT-8
-8.37
-8.30
CB + DIOd
-4.27
aPolymer:PC70BM weight ratio. bCF = chloroform. cCB = chlorobenzene. dCB + DIO = chlorobenzene + diiodooctane (3 % additive).
Figure 4.Powder XRD patterns of PTATffBT, PTATBT-8 and
PTAT2BT-8
.
The XRD pattern of PTATffBT displays multiple diffraction
peaks ranging from 5.27o to 25.7o. Previous literature has
shown that the sharp intense peak at 5.27o corresponds to the
distance between polymer main chains, where the solubilising
alkyl chains organise in an interdigitated manner.43,44 The peak
at the wide angle of 25.7o corresponds to a π-π stacking
distance of 3.7 Å. The smaller π-π stacking peak exhibited by
PTATffBT, relative to PTATBT-8 and PTAT2BT-8, can be
attributed to the different substituents attached to the
benzothiadiazole moiety with octyloxy substituents being
larger and more sterically demanding than fluorine. The
presence of sharp peaks in both the wide-angle and small-
angle region suggests that PTATffBT adopts a more crystalline
(possibly microcrystalline or semicrystalline) arrangement in
solid state as a result of self-organisation of polymer chains
forming more ordered domains in the solid state. This will
reduce the solubility of PTATffBT in organic solvents, as well as
limit its miscibility with the electron acceptor PC70BM.
Compared with that of PTATffBT, the miscibility of PC70BM
with amorphous PTATBT-8 and PTAT2BT-8 is superior,
Figure 5.(a) J-V characteristic curves of photovoltaic devices
fabricated from PTATBT-8 and PTAT2BT-8; and (b) External
quantum efficiency (EQE) spectra of corresponding devices
measured using a monochromatic light source.
and therefore enables moderate power conversion
efficiencies.
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EQEs in excess of 45% between a wider range of wavelengths
(380 – 576 nm) with a peak value of 52 % at 384 nm. In
contrast, PTATBT-8 demonstrated EQEs in excess of 45 %
between a shorter range of wavelengths (380 – 388 nm) with a
peak value of 46 % at 380 nm. The higher EQE corresponds to
improved charge carrier separation and transportation
properties, which is consistent with the higher Jsc values
Photovoltaic Properties
Photovoltaic measurements were made on
a series of
glass/ITO/PEDOT:PSS/polymer : PC70BM/Ca/Al devices using
blends of polymers : PC70BM in weight ratios from 1 : 1 to 1 : 4.
recorded for photovoltaic devices fabricated from PTAT2BT-8
.
Hole-only devices were fabricated using the structure
glass/ITO/PEDOT:PSS/polymer:PC70BM/Au. The dark J-V curves
of hole-only devices are presented in Figure 6. Following the
space charge limited current (SCLC) theory, the hole mobilities
of the two polymer:PC70BM films was calculated.46 The hole
mobility of PTAT2BT-8:PC70BM blends (9.7 x 10-5 cm2 V-1 s-1) is
higher than PTATBT-8:PC70BM blends (1.6 x 10-5 cm2 V-1 s-1).
PTAT2BT-8 displays a higher hole mobility relative to PTATBT-8
despite it possessing a lower molecular weight. We speculate
this is a consequence of the incorporation of additional
thiophene spacer units in PTAT2BT-8. These promote planarity
of the polymer backbone which facilitates charge
transportation. Taking both the EQE and hole mobility
characterisation into consideration, the improved photovoltaic
performance of PTAT2BT-8:PC70BM based devices can be
attributed to improved charge carrier separation and
subsequent charge extraction.
Figure 6.Dark J-V curves of hole only devices fabricated from
PTATBT-8 and PTAT2BT-8
.
A detailed device-fabrication process is described in the
Experimental section. As mentioned previously, the limited
solubility of PTATffBT meant that photovoltaic devices could
not be fabricated using this polymer. Photovoltaic
measurements revealed that devices with the active layer cast
from 1:3 polymer:PC70BM (w/w ratio) had optimum
performance. The influence of casting solvent and additives
were also investigated (see Table 2). A chlorobenzene casting
solvent yielded the best results for PTATBT-8 whilst
PPATBT-8 and PPAT2BT-8 synthesised by Iraqi et al displayed
PCEs of 3.92 and 4.17 %, respectively.33 These are higher than
the TIPS-functionalised polymers synthesised. This occurs as a
result of higher Jsc and FF values for PPATBT-8 and PPAT2BT-8
to their comparable TIPS-functionalised analogues.
Surprisingly, the smaller optical and electrochemical band-gaps
of PTATBT-8 and PTAT2BT-8 did not translate into a larger Jsc
values.
chloroform was found to be the best solvent for PTAT2BT-8
.
(a)
(b)
The addition of 3.0% diiodooctane (DIO) as an additive did not
improve the photovoltaic properties of the polymer but had a
detrimental effect on the photovoltaic performance. The J-V
characteristic curves for the best performing devices are
shown in Figure 5a.
Both PTATBT-8 and PTAT2BT-8 exhibit modest performances
with PCEs of 2.36 and 3.15 %, respectively. Optimised
photovoltaic devices fabricated from PTATBT-8 demonstrated
an open-circuit voltage (Voc) of 0.90 V, a short-circuit current
(Jsc) of -6.88 mA cm-2 and a fill-factor (FF) of 37.8 %. In
contrast, PTAT2BT-8 displayed a Voc of 0.74 V, a Jsc of -8.37 mA
(c)
(d)
cm-2 and a FF of 50.0 %. The higher Jsc and FF of PTAT2BT-8
,
relative to PTATBT-8, presumably arise as a result of improved
packing of polymer chains in polymer:PC70BM blends as a
result of improving charge mobility and thus extraction of
photo-generated charge carriers. It is well documented that Figure 7. Tapping-mode atomic force microscopy topography
the Voc of photovoltaic devices is closely related to the energy (a and b) and phase images (c and d) of: PTATBT-8 : PC70BM
difference between the HOMO of the donor and the LUMO (1:3 w/w) (a and c); and PTAT2BT-8 : PC70BM (1:3 w/w) (b and
level of the acceptor.45 The lower HOMO level of PTAT2BT-8
,
d).
relative to PTATBT-8 is thus translated into a higher Voc value.
External quantum efficiencies (EQE) for corresponding devices
were measured (Figure 5b). PTAT2BT-8 demonstrated higher
6 | J. Name., 2012, 00, 1-3
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Atomic Force Microscopy (AFM) Images
Experimental Section
Materials
Atomic force microscopy (AFM) in tapping mode was used to
image the surface morphologies of PTATBT-8:PC70BM (1:3) and
PTAT2BT-8:PC70BM (1:3) blend films spin-coated from
chlorobenzene and chloroform solutions respectively, as
shown in Figure 7. It should be noted that there is no
significant difference in the surface morphologies of the
PTATBT-8 and PTAT2BT-8 based blends. Both exhibit smooth,
amorphous looking surfaces with no indication of
unfavourable large-scale phase separation. Such morphologies
should result in efficient excitation and photocharge
generation. As AFM reveal no significant difference in the
surface morphologies of the two types of blend film we
tentatively speculate that the improved Jsc associated with
devices fabricated from PTAT2BT-8 are a result of difference in
improved charge carrier separation and transportation
properties, as evidenced by the EQE spectra. This is also
reflected by the improved FF associated with devices
fabricated from PTAT2BT-8; a property that results from
improved charge-carrier mobility.
4.7-Bis(5-bromothiophen-2-yl)-5,6-
difluorobenzo[c][1,2,5]thiadiazole(M2),47,48 4,7-bis-(5-bromo-
thiophen-2-yl)-5,6-bis-octyloxy-benzo[c][1,2,5]thiadiazole
49
(
M3
)
and 4,7-bis-(5’-bromo-[2,2’]bithiophenyl-5-yl)-5,6-bis-
50
octyloxy-benzo-[1,2,5]thiadiazole
(
M4
)
were prepared
according to previous established literature procedures.
PC70BM (95 %) was purchased from Ossila Ltd. All chemicals
and solvents, with the exception of those stated below, were
of reagent grade quality, purchased commercially and used
without further purification unless otherwise stated. Toluene
was dried and distilled over sodium under an inert argon
atmosphere.
Acetonitrile
(high
performance
liquid
chromatography (HPLC) grade) was dried and distilled over
phosphorous pentoxide under an inert argon atmosphere,
then stored over molecular sieves, 3 A.
Measurements
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded on either Bruker AV 250 (250 MHz), Bruker AV 400
(400 MHz) or Bruker Avance III HD 500 (500 MHz)
spectrometers at room temperature, using chloroform-d
(CDCl3) or at 100oC using 1,2-dideutrotetrachloroethane as the
solvent. NMR spectra were recorded in parts per million (ppm)
relative to tetramethysilane (δH0.00). Coupling constants are
given in hertz (Hz). Carbon, nitrogen and sulphur elemental
analysis was performed on a Perkin Elmer 2400 series 11
CHNS/O analyser. Analysis of halides was undertaken using the
Schöniger flask combustion method. GPC analysis was
conducted on polymer solutions in 1,2,4-trichlorobenzene at
140oC. Polymer samples were spiked with toluene as a
reference. GPC curves were obtained using a Viscotek GPCmax
VE 2001 GPC solvent/ sample module and a Waters 410
Differential Refractometer, which was calibrated using a series
of narrow polystyrene standards (Polymer Laboratories). TGA
Conclusions
Three triisopropylsilylacetylene-functionalised anthracene
(TIPSAnt) based polymers were synthesised by copolymerising
TIPSAnt
[1,2,5]thiadiazole, dithienyl-benzo[c]-[1,2,5]thiadiazole or
dibithiophenyl-benzo[c]-[1,2,5]thiadiazole to yield PTATffBT
PTATBT-8 and PTAT2BT-8 respectively. Whilst chemical
with
either
dithienyl-5,6-difluoro-benzo[c]-
,
,
characterisation of PTATffBT was possible, the limited
solubility of the conjugated polymer in common organic
solvents at elevated temperatures meant that fabrication of
BHJ photovoltaic devices via solution processing was not
possible. Both PTATBT-8 and PTAT2BT-8 displayed excellent
solubility in common organic solvents; a consequence of the
incorporation of octyloxy-chains onto the 5,6-positions of the
benzothiadiazole acceptor-moiety. UV-visible spectroscopy
revealed that PTAT2BT-8 had the lowest optical band gap of all
polymers synthesised with a value of 1.81 eV. We find that
traces were obtained using
a
Perkin Elmer TGA-1
Thermogravimetric Analyser at a scan rate of 10 °C min-1 under
an inert nitrogen atmosphere. Powder XRD samples were
recorded on a Bruker D8 Avance diffractometer with a CuKα
radiation source (1.5418 Å, rated as 1.6 kW). The scanning
angle was conducted over the range 2-40o. UV-visible
absorption spectra were recorded at ambient temperature
PTATffBT had the deepest HOMO level (-5.48 eV);
a
consequence of utilising a fluorinated-benzothiadiazole
acceptor monomer. PTATBT-8 and PTAT2BT-8had HOMO
levels of -5.46 and -5.31 eV. The addition of an additional
thiophene spacer unit in PTAT2BT-8 increases intramolecular
charge transfer along the polymer backbone resulting in a
shallower HOMO level relative to PTATBT-8. BHJ photovoltaic
devices fabricated from PTATBT-8:PC70BM and PTAT2BT-
using
a
Hitachi U-2010 Double Beam UV/Visible
Spectrophotometer. Polymer solutions were made using
chloroform (spectrophotometric grade) and measured using
quartz cuvettes (path length = 1 x10-2 m). Thin films used for
absorption spectra were prepared by drop casting solutions
onto quartz plates using 1 mg cm-3polymer solutions that were
8:PC70BM had efficiencies of 2.36 and 3.15 %, respectively. The
improved PCE values of PTAT2BT-8 occur primarily as a result
of increased Jsc and FF. Taking both the EQE and hole mobility
characterisation into consideration the improved photovoltaic
performance of PTAT2BT-8:PC70BM based devices can be
attribute to improved charge carrier separation and charge
transportation.
prepared
voltammograms were recorded using a Princeton Applied
Research Model 263A Potentiostat/Galvanostat. three
using
chloroform
(HPLC
grade).
Cyclic
A
electrode system was employed comprising a Pt disc (area =
3.14 x 10-2 cm2), platinum wire and Ag/Ag+ as the working
electrode, counter electrode and reference electrode,
respectively, in a tetrabutylammonium perchlorate acetonitrile
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Journal Name
solution (0.1 mol dm-3). Measurements were conducted on 130.61, 126.91, 105.22, 103.33, 83.93, 25.00, 18.94, 11.58.
polymer films that were prepared by drop casting 1.0 mm3 of [M]+ calculated forC48H72B2O4Si2, 790.515529; found,
polymer solution (1 mg cm-2 in chloroform (HPLC grade)). In 790.513046. Elemental Analysis (%) calculated for
accordance with IUPAC’s recommendations, ferrocene was C48H72B2O4Si2: C, 72.90; H, 9.18. Found: C, 73.22; H, 7.72.
employed as a reference redox system.51
Poly[9,10-bis(triisopropylsilylacetylene)-anthracene-2,6-diyl-alt-
Fabrication and Testing of BHJ polymer solar cells
(4,7-dithiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole-5-5-
diyl] (PTATffBT):4.7-Bis(5-bromothiophen-2-yl)-5,6-
The polymers and PC70BM were dissolved separately in either
CF, CB or CB with 3 % DIO, and were then put on a hot plate
held at 70oC overnight with stirring to allow dissolution. The
solutions were then mixed at different polymer:fullerene
blend ratios before spin casting. Photovoltaic devices were
fabricated onto pre-patterned ITO glass substrates (20 Ohms
per square) that were supplied by Ossila Limited. The ITO/glass
substrates were first cleaned by sonication in dilute NaOH
followed by IPA. A 30 nm thick PEDOT:PSS layer was spin-
coated onto the ITO/glass substrates. These were then
transferred to a hot-plate held at 120oC for 10 minutes before
being transferred to a nitrogen glove-box. All active layers
were spin cast onto the glass/ITO/PEDOT:PSS substrate. The
devices were then transferred into a thermal evaporator for
deposition of a cathode (5 nm of calcium followed by a 100 nm
of aluminium evaporated at a base pressure of ~ 10-7 mbar).
The cathode was deposited through a shadow-mask producing
a series of independent pixels. Devices were encapsulated
using a glass slide and epoxy glue before testing. PCEs were
determined using a Newport 92251A-1000 AM 1.5 solar
simulator. An NREL calibrated silicon cell was used to calibrate
the power output to 100 mW cm-2 at 25oC. An aperture mask
having an area of 2.06 mm2 was placed over devices to define
the test area. At least two devices were prepared for each
sample to give 12 pixels. Each reporting an independent J-V
curve. EQE values were determined over the wavelength range
of interest by comparing the photocurrent of the OPV cell to a
difluorobenzo[c][1,2,5]thiadiazole (0.181 g, 0.367 mmol) and
2,6-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,10-
bis(triisopropylsilylacetylene) anthracene (0.290 g, 0.367
mmol) were added to a flask and placed under an inert argon
atmosphere using standard schlenk line techniques. Dry
toluene (9 cm3) and tetraethyl ammonium hydroxide (20 % wt,
2 cm3, degassed) were added and the reaction was degassed
again. Pd(OAc)2 (6.5 mg, 28.9 μmol) and tri(o-tolyl)phosphine
(17.3 mg, 56.8 μmol) was added and the reaction mixture was
degassed again. The mixture was heated to 90oC for 30
minutes. Upon completion, the mixture was cooled to room
temperature and bromobenzene (0.1 cm3, 0.94 mmol) was
added. The mixture was degassed and heated at 90oC for 1
hour. Upon completion, the reaction mixture was cooled to
room temperature and phenyl boronic acid (150 mg, 1.23
mmol) was added. The mixture was degassed and heated at
90oC for a further hour. The mixture was cooled to room
temperature and precipitated into methanol (500 cm3,
degassed) and stirred overnight. The resulting mixture was
filtered through a membrane filter and the polymer was
obtained as a deep purple powder. The polymer was cleaned
using Soxhlet extraction with solvents in the following order:
methanol, acetone, hexane and toluene. The toluene fraction
was concentrated (~ 50 cm3) and then poured into degassed
methanol (500 cm3). The resulting mixture was stirred
overnight and the solid was collected by filtration (92.4 mg, 29
%). GPC analysis: Mn = 2,100 Da, Mw = 2,500 Da, PDI = 1.19.
1H-NMR (500 MHz, C2D2Cl4, 100oC) (δH/ppm) 8.96 (m, 1H),
8.77-8.52 (m, 2H), 8.38-8.20 (m, 2H), 7.97 (m, 1H), 7.67 (m,
2H), 7.49-7.27 (m, 2H).19F-NMR (500 MHz, C2D2Cl4, 100oC)
(δF/ppm) -127.22, -127.70.Anal.Calcd.for C50H52F2N2S3Si2: C,
68.92; H 6.02; N, 3.22; S, 11.04. Found: C, 67.85; H, 5.93; N,
2.78; S, 9.68.
reference silicon photodiode having
response.
a known spectral
2,6-Bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,10-
bis(triisopropylsilylacetylene) anthracene (M1)
A single neck round bottom containing 2,6-dibromo-9,10-
bis(triisopropylsilylacetylene)anthracene (3.00 g, 4.3 mmol),
bis(pinacolato)diboron (3.83 g, 15.01 mmol), potassium
acetate (2.52 g, 24.97 mmol) and Pd(dppf)Cl2 (0.2 g, 0.25
mmol) was placed under an argon atmosphere using standard
schlenk line techniques. To this mixture, DMF (50 cm3) was
added, degassed and heated at 90oC for 48 hours. The reaction
mixture was cooled to room temperature and poured into
water (50 cm3) and extracted with diethyl ether (3 x 100 cm3).
The organic phases were combined, washed with H20 (2 x
100cm3) and dried (MgSO4). The crude product was purified by
dissolving in the minimum amount of hot ether and
precipitated in methanol which had been run through a basic
Poly[9,10-bis(triisopropylsilylacetylene)-anthracene-2,6-diyl-
alt-(5,6-bis(octyloxy)-4,7-di(thiophene-2-
yl)benzo[c]thiadiazole-5,5-diyl] (PTATBT-8):PTATBT-8 was
synthesized according to the polymerisation method outlined
for PTATffBT
:
4,7-bis(5-bromo-2-thienyl)-5,6-bis(octyloxy)-
2,1,3-benzothiadiazole (0.262 g, 0.367 mmol) and 2,6-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,10-
bis(triisopropylsilylacetylene) anthracene (0.290 g, 0.367
mmol) were added to a flask and placed under an inert argon
atmosphere using standard schlenk line techniques. Dry
toluene (10 cm3) and tetraethyl ammonium hydroxide (20 %
wt, 2.1 cm3, degassed) were added and the reaction was
degassed again. Pd(OAc)2 (6.0 mg, 26.7 μmol) and tri(o-
tolyl)phosphine (16.3 mg, 53.6 μmol) was added and the
1
column (1.41 g, 1.8 mmol, 41 %). H NMR (250 MHz, CDCl3):
(δH/ppm) 9.30 (s, 2H) 8.62 (d, J = 8.48 Hz, 2H), 7.94 (dd, J =
0.98 Hz and J = 8.48 Hz, 2H), 1.41 (s, 24H), 1.32 (m, 42H). 13C
NMR (250 MHz, CDCl3) ∂ (ppm): 136.42, 133.44, 132.37,
8 | J. Name., 2012, 00, 1-3
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4.
5.
6.
7.
8.
Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H.
Ade and H. Yan, Nat. Commun., 2014, 5, 1–8.
reaction mixture was degassed again. The mixture was heated
to 90oC. The polymerisation was deemed complete after 3
hours. The polymer was obtained as a purple solid (230 mg, 57
%). GPC analysis: Mn = 13,600 Da, Mw = 38,700 Da, PDI = 2.84.
1H-NMR (500 MHz, C2D2Cl4, 100oC) (δH/ppm) 8.89 (br, 1H),
8.82-8.32 (m, 4H), 7.99 (br, 1H), 7.67 (br, 2H), 7.49-7.22 (m,
2H), 4.21 (br, 4H), 1.94 (br, 4H), 1.55-1.01 (m, 62H), 0.80 (br,
6H).Anal.Calcd.for C66H86N2O2S3Si2: C, 72.61; H 7.94; N, 2.57; S,
8.81. Found: C, 70.16; H, 6.55; N, 3.35; S, 10.35.
M. C. Scharber and N. S. Sariciftci, Prog. Polym. Sci., 2013,
38, 1929–1940.
J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li and
J.-X. Tang, Adv. Mater., 2015, 27, 1035–1041.
S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev.,
2007, 107, 1324–1338.
Poly(9,10-bis(triisopropylsilylacetylene)-anthracene-2,6-diyl-
alt-(5,6-bis(octyloxy)-4,7-di(2,2’-bithiophen-5-
M. M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C.
Hummelen, P. A. Van Hal and R. A. J. Janssen, Angew.
Chemie - Int. Ed., 2003, 42, 3371–3375.
yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] (PTAT2BT-8):PTAT2BT-
8was synthesized according to the polymerisation method
outlined for PTATffBT: 4,7-bis(5’-bromo-2,2’-bithien-5-yl)-5,6-
bis(octyloxy)-2,1,3-benzothiadiazole (0.323 g, 0.367 mmol) and
2,6-bis-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,10-
bis(triisopropylsilylacetylene) anthracene (0.290 g, 0.367
mmol) were added to a flask and placed under an inert argon
atmosphere using standard schlenk line techniques. Dry
toluene (10 cm3) and tetraethyl ammonium hydroxide (20 %
wt, 2.1 cm3, degassed) were added and the reaction was
degassed again. Pd(OAc)2 (6.1 mg, 27.2 μmol) and tri(o-
tolyl)phosphine (16.1 mg, 53.0 μmol) was added and the
reaction mixture was degassed again. The mixture was heated
to 90oC. The polymerisation was deemed complete after 1
hour. The polymer was obtained as a deep purple solid. (76.7
mg, 17 %). GPC analysis: Mn = 6,600 Da, Mw = 13,200 Da, PDI =
2.00. 1H-NMR (500 MHz, C2D2Cl4, 100oC) (δH/ppm) 8.87 (s, 1H),
8.62 (d, 2H, J = 8.50 Hz), 8.44 (m, 2H), 7.88 (d, 2H, J = 8.50 Hz),
7.67-6.95 (m, 9H), 4.20 (br, 4H), 1.97 (br, 4H), 1.56-1.09 (m,
62H), 0.86 (br, 6H). Anal.Calcd.for C74H90N2O2S5Si2: C, 70.67; H
7.22; N, 2.23; S, 12.76. Found: C, 71.01; H, 7.01; N, 2.92; S,
14.98.
9.
C. J. Brabec, G. Zerza, G. Cerullo, S. De Silvestri, S. Luzzati, J.
C. Hummelen and S. Sariciftci, Chem. Phys. Lett., 2001, 340,
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R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884–
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Acknowledgements
We would like to thank EPSRC for financial support of this
work via research grants EP/J017361/1 and EP/I028641/1. YZ
Y. Li, T.-H. Kim, Q. Zhao, E.-K. Kim, S.-H. Han, Y.-H. Kim, J. I.
N. Jang and S.-K. Kwon, J. Polym. Sci. Part A Polym. Chem.,
2008, 46, 5115–5122.
thanks the University of Sheffield for the award of
a
scholarship. TW acknowledges financial support from the
Recruitment Program of Global Experts and the Natural
Science Foundation of China (Grant No. 21504065).
17.
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DOI: 10.1039/C5RA20927G
Graphical contents entry for article:
Triisopropylsilylacetylene-Functionalised
Benzothiadiazole Copolymers for Application in Bulk
Heterojunction Solar Cells
Anthracene-alt-
a
a
Luke Cartwright,a Lois. J. Taylor, Hunan Yi, Ahmed Iraqi,*a Yiwei Zhang,b Nicholas. W.
Scarratt,b Tao Wang,c David. G. Lidzey*b
Si
S
N
N
n
S
S
F
F
Si
PTATffBT
Si
S
N
N
n
S
S
C8H17
O
OC8H17
Si
Si
PTATBT-8
S
S
N
N
S
S
C8H17
O
S
OC8H17
PTAT2BT-8
n
Si
Three triisopropylsilylacetylene-functionalised anthracene (TIPSAnt) based polymers were
synthesised by copolymerising TIPSAnt with either dithienyl-5,6-difluoro-benzo[c]-
[1,2,5]thiadiazole,
dithienyl-benzo[c]-[1,2,5]thiadiazole
or
dibithiophenyl-benzo[c]-
[1,2,5]thiadiazole to yield PTATffBT, PTATBT-8 and PTAT2BT-8, respectively. The optical,
electrochemical, thermal, structural and photovoltaic properties in bulk heterojunction solar
cells are investigated.