DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
Investigation of Radical and Cationic Crosslinking in High Efficiency, Low Band Gap
Solar cell Polymers.
By Chin Pang Yau†, Sarah Wang†, Neil D. Treat, Zhuping Fei, Bertrand J. Tremolet de
Villers, Michael L. Chabinyc*, Martin Heeney*
Dr Chin Pang Yau, Dr Zhuping Fei and Prof. Martin Heeney
Department of Chemistry and Centre for Plastic Electronics
Imperial College London
Exhibition Road, South Kensington
London SW7 2AY
UK
E-mail: m.heeney@imperial.ac.uk
Sarah Wang, Dr Neil D. Treat, Dr Bertrand J. Tremolet de Villers and Prof Michael L.
Chabinyc
Materials Department
Elings Hall Room 3219
University of California
Santa Barbara CA 93106-5050
U. S. A.
E-mail: mchabinyc@engineering.ucsb.edu
†Both authors contributed equally.
Keywords: Dithienogermole, Organic Solar Cell, conjugated polymer, Crosslinking
Abstract
We report the synthesis and characterization of dithienogermole-co-thieno[3,4c]pyrroledione (DTG-TPD) polymers incorporating chemically crosslinkable sidechains and
compare their properties to a parent polymer with simple octyl sidechains. Two crosslinking
groups and mechanisms are investigated, UV promoted radical crosslinking of an alkyl
bromide crosslinker, and acid promoted cationic crosslinking of an oxetane crosslinker. We
find that random co-polymers with a 20% incorporation of the crosslinker demonstrate a
higher performance in bulk heterojunction solar cells than the parent polymer, whilst 100%
1
crosslinker incorporation results in deterioration in device efficiency. The use of 1,8diiodooctane (DIO) as a processing additive improved as-cast solar cell performance, but was
found to have a significant deleterious performance on solar cell performance after UV
exposure. The instability to UV could be overcome by the use of an alternative additive, 1chloronapthalene, which also promoted high device efficiency. Crosslinking of the polymer
was investigated in the presence and absence of fullerene highlighting significant differences
in behavior. Intractable films could not be obtained by radical crosslinking in the presence of
fullerene, whereas cationic crosslinking was successful.
2
1. Introduction
The surge of interest in organic photovoltaics over the last few years has resulted in a
steady increase in device efficiencies, with values now approaching thresholds for
commercialisation.1 Organic devices are typically comprised of a mixture of two components
in a bulk heterojunction (BHJ), with the device performance crucially dependent on the
complex nanostructure of this blend. This nanostructure is influenced by the miscibility of
individual components, as well as their tendency to crystallise and phase separate.2 Desirable
morphologies are thought to contain bicontinuous domains with sizes on the order of 10 nm
for effective charge generation and separation.2-3 This morphology has been typically
achieved by the manipulation of coating conditions, post processing thermal or solvent
annealing and by the choice of solvent and processing additives.2-3 In many cases the
nanostructure
of
the
blend
can
be
considered
kinetically trapped
rather
than
thermodynamically preferred.4 As such, the as-cast morphologies can change over time,
because the fullerene in polymer:fullerene BHJs is able to diffuse within the amorphous
regions of the polymer matrix.5 Therefore the lifetime of the device may be compromised by
the reorganisation of fullerene and polymer to a more thermodynamically stable state over
time. This change can be particularly relevant during device operation, in which relatively
high temperatures can be reached. One possible approach to overcome this issue is to ‘lockin’ the optimum kinetically trapped morphology by some form of chemical crosslinking after
the active layer has been formed.
Crosslinking of donor polymers has been shown to stabilise the nanostructure in
polymer:fullerene BHJs, principally by restricting the ability of PCBM, the most common
fullerene acceptor, to crystallise or aggregate into large domains in thin films.6 Crosslinkable
polymers also have other potential benefits, such as their use in bilayer devices where
3
deposition of the second layer can be facilitated by crosslinking of the first. For example some
element of structural control over phase connectivity and length scale has been recently
achieved by diffusion of fullerene derivatives into lightly cross-linked donor polymers during
solution deposition.7 In these examples swelling of the polymer network by the solvent
assisted the diffusion of fullerene into the film. Therefore, in general, polymer crosslinking
may offer more flexibility in methods to form BHJs, as well as providing methods to control
and stabilise the nanostructure in BHJs.
Crosslinking groups have been incorporated onto the lateral sidechains of conjugated
polymers to enable either thermal or photochemical crosslinking.6b Reactive groups such as
alkyl bromide, oxetane, azide and alkenyl have been reported as effective crosslinkers. Many
of these studies have used modified regioregular P3HT to demonstrate the proof of concept.
6a,8
However, there are relatively few examples in which low band gap donor-acceptor
polymers, which provide higher power conversion efficiency, have been investigated.6c,9
Within the small class of low band gap polymers reported thus far, device efficiencies have
been rather modest even before crosslinking. Furthermore there have been few studies
examining the differences in performance for radical and cationic crosslinking groups.
Two different reactive crosslinkers have previously demonstrated promising
performance in semiconducting polymers, alkyl bromide groups as a radical crosslinkers and
oxetanes as a cationic crosslinkers. Alkyl bromide functionalised sidechains have been
demonstrated as a promising route to stabilise blend morphology in both P3HT functionalised
polymers, and in low band gap benzodithiophene copolymers.6c,8c The process of crosslinking
is believed to be a radical based mechanism, whereby direct exposure to 254 nm UV light
causes homolytic cleavage of the carbon-bromine covalent bonds.10 Subsequent crosslinking
likely occurs by abstraction of a proton by the bromine radical from the sidechains of adjacent
4
polymers, followed by coupling of the alkyl radicals. The crosslinking of oxetane groups has
been widely investigated in OLED materials.11 Oxetane groups polymerise via a cationic ring
opening mechanism also known as CROP.12 In comparing oxetane to other cationic
crosslinking groups like epoxides, the oxetane functionality has the advantage of high stability
to a variety of chemical conditions, facilitating the synthesis of oxetane containing materials.
Furthermore unlike radical based crosslinking, which is sensitive to oxygen, cationic
polymerisation can be performed in ambient atmosphere. In addition the cationic reactive
center is generally less reactive and less prone to side reactions than highly reactive and nonselective radicals. The CROP mechanism typically requires small amounts of a photo-acid
generator (PAG) to initiate the reaction. Following initiation, heat is generally applied in the
form of a post-exposure bake in order to accelerate the crosslinking. Although several
conjugated polymers incorporating oxetane functionality have been successfully crosslinked
and photo-polymerised for OLED applications, at the outset of this work there were few
examples in OPV.8a,13
In this paper we report the synthesis and characterisation of random alternating donoracceptor co-polymers incorporating either oxetane or alkyl bromide crosslinkers, and compare
their optoelectronic properties to the parent alternating polymer. We focus on thieno[3,4c]pyrrole-4,6(5H)-dione (TPD) as a suitable readily functionalised electron poor co-monomer.
TPD has been utilised in various high performing donor-acceptor co-polymers, and generally
promotes high open circuit voltage and good photocurrent. 14 As an electron rich co-monomer,
we investigated dithienogermole (DTG) due to the promising performance of the PDTG-TPD
co-polymer previously reported, and our experience with this comonomer.15 We find that the
incorporation of small amounts (20%) of crosslinkable monomer has a beneficial impact on
overall device efficiency before crosslinking. The role of various processing additives is
5
investigated, and we find the that the use of 1,8-diiodoctane (DIO) causes unexpected stability
issues when the films are irradiated with UV radiation. The use of chloronapthalene (CN) as
an alternative additive circumvents these issues. Finally we compare and contrast radical and
cationic crosslinking in blends with fullerene derivatives and demonstrate that the presence of
fullerene in the blend reduces the efficacy of radical crosslinking.
2. Results and Discussion
2.1. Synthesis
4,4-Bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophene16 DTGSn2) and 1,3-dibromo-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione6c (TPD) were synthesised by
literature procedures. However, the purification of DTG-Sn2 was complicated due to its
chemical instability towards column chromatography over silica. Therefore, purification was
performed by preparative gel permeation chromatography (GPC) on a crosslinked polystyrene
size exclusion column using hexane as the eluent. The TPD oxetane, bromide and octyl
variants were synthesised according to a modification of literature procedure6c, as shown in
Scheme 1a. Hence TPD was deprotonated with sodium hydride in DMF, and the appropriate
alkylating agent was subsequently added. In the case of 1,6-dibromohexane, excess reagent
was added in order to prevent the reaction of two TPDs with one 1,6-dibromohexane.
Alternating co-polymers incorporating 100% of the crosslinking groups, as well as random
co-polymers incorporating 20% of the crosslinker and 80% of an octyl substituted TPD comonomer (TPD-C8) were prepared, along with the parent PDTG-TPD-C8 co-polymer.
Scheme 1b shows the nomenclature used to designate the percentage crosslinker in the
polymer backbone. Percentages refer to the statistical amounts of co-monomers added at the
beginning of the reaction, thus the 20% statistical co-polymers were synthesised using a 0.2
6
mol equivalence of the desired crosslinking TPD monomer (TPD-Ox or TPD-Br) with 0.8
mol equivalence of the TPD-C8. Five co-polymers, shown in Scheme 1c, were synthesised via
a microwave assisted Stille coupling method in chlorobenzene.17 After precipitation, they
were dissolved in chloroform then washed in sodium diethyldithiocarbamate trihydrate to
extract any residual palladium before being precipitated into methanol.18 Soxhlet extraction
was performed with methanol, acetone and hexane to remove catalyst residues and low
weight oligomers. Finally, the remaining polymer was extracted into chloroform and the
polymer was further fractionated by preparative GPC in hot chlorobenzene (80 °C). This
method was used to modulate the molecular weights and narrow the polydispersity (PDI) for
all five PDTG-TPD polymers.
All polymers demonstrated reasonable solubility in common organic solvents like chloroform
and chlorobenzene upon warming. The molecular weights were measured by GPC relative to
polystyrene standards, as shown in Table 1, which shows the number average molecular
weight (Mn), degree of polymerisation for Mn (DPn), weight average molecular weight (Mw),
degree of polymerisation for Mn (DPw) and polydispersity (PD) of all five polymers. The
molecular weight of PDTG-TPD-C8 was similar to that previously reported by Reynolds et
al.,15b and both of the random co-polymers showed comparable molecular weights, with
degrees of polymerisation around 60-70 for all polymers. For the PDTG-TPD-Ox100% and
PDTG-TPD-Br100% copolymers, slightly lower Mn‘s were observed, with degrees of
polymerisation around 35.
The composition of the random co-polymers was confirmed by 1H NMR in CDCl3
(see Supporting Information). Although the aromatic protons and methylene groups close to
the polymer backbone were rather broad and poorly resolved, likely due to restricted rotation,
the peaks attributable to the alkyl bromide or oxetane were sharp. Integration of the aromatic
7
protons (8.60-7.30 ppm) and the oxetane proton environment (4.51 and 4.35 ppm) gave a ratio
of 1:0.38, within the experimental error of the expected value of 1:0.4. Similarly, for PDTGTPD-Br20% the ratio was observed to be 1:1.1 (aromatic DTG proton environment (8.60-7.30
ppm):CH2 adjacent to the TPD nitrogen + CH2 adjacent to the bromine on TPD-Br (3.71-3.43
ppm)) which is in agreement with the theoretical reaction stoichiometry.
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were
performed under an inert atmosphere of nitrogen on all co-polymers. DSC measurements
showed no obvious transitions from -20 °C to 330 °C for all co-polymers. TGA of PDTGTPD-C8, PDTG-TPD-Ox100% and PDTG-TPD-Br20% shows 5% mass loss at 431 °C to 433 °C,
while PDTG-TPD-Ox20% shows a 5% mass loss at 450 °C and PDTG-TPD-Br100% at 385 °C
(Figure S6). The TGA shows that there are no low temperature decompositions, and since the
chemical composition of the backbone is similar for all polymers, it suggests that the 100%
alkyl bromide incorporation may be the reason for the earlier decomposition temperature of
PDTG-TPD-Br100%. The temperatures where decomposition occurs are much higher than any
annealing temperatures used in the fabrication process for BHJs making these polymers
highly stable for use in solar cells.
2.2. Optical and Electrochemical Properties
The UV-Vis spectra were measured in chlorobenzene solution at room temperature
and as thin films spin coated from a solution of chlorobenzene (5 mg/mL) (Figure 1). The
long wavelength onset for all polymers is 730 nm, corresponding to an optical band gap (Eg)
of 1.7 eV. All polymers exhibited similar shape spectra for both solution and film
measurements, with a pronounced series of peaks with the most intense around 680 nm and
620 nm (Table 1), similar to that previously reported.15b
This separation (~0.2 eV) is
consistent with a vibronic progression as observed in many ordered semiconducting polymers.
There was almost no change in the position of the absorption maxima (±2 nm shifts) upon
8
film formation suggesting aggregation in solution. Upon comparison of the five polymers, it is
clear that the parent alkylated polymer (TPD-C8) and both polymers with 20% crosslinker
groups show almost identical spectra, with very little difference in the relative heights of the
two absorption peaks, both in solution and thin film. However, for the 100% oxetane and
bromine containing polymers it can be seen there is a change in the relative peak amplitude of
the two maxima, with the long wavelength peak reducing in relative absorption versus the
shorter wavelength peak. Since the relative ratios of these two peaks are often related to
coupling of the transition dipoles in the solid state, this suggests that high amounts of the
relatively large crosslinking groups are influencing the intermolecular packing of the
conjugated backbones. We do not believe this is related to the lower degree of polymerisation
for both of the 100% polymers, because the MW should be sufficient to be above the effective
conjugation length in both cases. Additionally, PCPTD-TPD-C8 of much lower weight (Mn
16.3 KDa) has been reported to have an identical UV spectra to that of the higher weight
polymer.15e Therefore, we believe the origin is more likely related to the high percentage of
crosslinking groups changing the interactions of the sidechains.
The ionisation potential of polymer thin films was determined using photo-electron
spectroscopy in air (PESA) and is shown in Table 1. The reduction potential (LUMO) was
estimated by the addition of the optical Eg to the ionization potential. The ionization potentials
are all in the range of 5.2 to 5.3 eV, and fall within the standard error of 0.05 eV associated
with this particular measurement. These results support the fact that the 20% and non
crosslinkable polymer are optically and electronically very similar. For both of the 100%
crosslinkers (Br and Ox), a slightly lower HOMO of 5.3 eV was observed, which could be
due to the crosslinker functionality disrupting the packing and backbone planarisation.
9
2.3. Photovoltaic Properties
The optoelectronic properties of these polymers were measured in BHJs using a
glass|ITO|PEDOT:PSS|Polymer:Fullerene-Blend|LiF|Al device structure and their power
conversion efficiency was measured under 100 mW cm2 AM 1.5 illumination.
Polymer:fullerene BHJs were solution processed from chlorobenzene and due to the low band
gap absorption of the polymer, PC70BM was used to complement and capture the low
wavelength of the visible spectrum.19 All polymers were tested in 1:1, 1:2, 1:3 and 1:4 ratios
against PC70BM. As shown in Table 2, the best performances were obtained in a 1:2
polymer:PC70BM weight ratio for all polymers. Average PCEs from as cast devices through
the series are similar, around 1.6%, with the exception of PDTG-TPD-Ox100% which was
below 0.5% Following literature by Reynolds, So and co-workers, further optimisation
showed that the addition of 5 vol% 1,8-diiodooctane (DIO) increased the PCE of as-cast
devices in all cases.15c Interestingly, both of the 20% co-polymers performed significantly
better than the baseline polymer PDTG-TPD-C8, with the highest average efficiency of 7.35%
for PDTG-TPD-Br20% (Table 2). Both of the alkyl bromide crosslinkers gave higher
efficiencies compared to the analogous oxetanes. The 100% crosslinker polymers show a
decrease in overall PCE compared to equivalent 20% and non-crosslinkable polymers. The
limiting factor for both the PDTG-TPD-Ox100% and PDTG-TPD-Br100% is the fill factor, along
with a small drop in current in comparison to the 20% crosslinkable polymers. The
differences in performance may be due to changes in the blend microstructure from the
addition of different functional groups at the solubilising chain end. Similar results have been
seen from Bao and co-workers when altering the side groups with terminal siloxane groups.20
Our results further highlight the importance of solubilising chain effects on morphology and
device performance.
10
In order to investigate the possible influence of the end group on polymer morphology,
X-ray diffraction (XRD) using a lab source were obtained from drop cast films prepared from
solutions of polymer dissolved in chlorobenzene (5 mg/mL). The XRD traces are shown in
Figure S7. The polymers appear mostly disordered, with a lack of clearly resolved diffraction
peaks.
2.4. Crosslinking Studies
The different PDTG-TPD polymers allow an examination of the role of the cross-linking
mechanism on the ability to form intractable films and on the efficiency of photovoltaic
devices. The bromide and oxetane end groups crosslink by two different mechanisms, namely
free radical and cationic.12b,21 Here we investigated the crosslinking of PDTG-TPD-Br20% and
PDTG-TPD-Ox20% because they had the best performance in BHJs prior to crosslinking. To
gauge the necessary amount of curing required for cross-linking, we studied the ability of
crosslinked films to withstand solvent rinses using UV-Vis absorption spectra before and after
washing (in chloroform) of the crosslinked films.6c,8c,9a For PDTG-TPD-Br20%, 15 mg/mL
solutions were spin coated without any additional photoinitiator, and photo-crosslinking was
performed in a nitrogen glove box. Varying times of deep-UV exposure from a 15 W UV
lamp (254 nm) were used. As can be seen in Figure 2 the film without UV treatment shows
complete solubility after washing. However, 20 min of UV exposure afforded 80% film
retention and 30 min gave near 100% film retention.
For the oxetane crosslinkers, photoacid generators (PAGs) are generally required to
initiate the polymerisation.12 The use of both non-ionic and ionic PAGs was investigated,
utilising N-hydroxynaphthalimide triflate and tris(4-tert-butylphenyl)sulfonium triflate PAGs.
Preliminary attempts at crosslinking were investigated upon films prepared from 15 mg/mL
solutions of PDTG-TPD-Ox20% containing 1 wt% of PAG. Films were exposed to UV light
(365 nm) for 10 min, and followed by heating to 100 °C for 10 min. However, these initial
11
attempts proved unsuccessful, with no retention of the film after solvent rinsing. Increasing
the PAG to 5 wt% was also unsuccessful at 100 °C, but did afford semi-intractable films when
films were instead heated to 150 °C (note no cross-linking occurred in the absence of PAG
under these conditions). However, the high concentration of PAG necessary caused
significant issues with de-wetting of the polymer solution during spin-casting and in the
formation of smooth and homogenous films. The difficulties may have been due to the low
20% oxetane density, which may have reduced the probability of oxetane groups being in
close enough proximity to react, as well as reducing the ability of the catalytic species to
migrate from oxetane to oxetane in the film.
Because of difficulties with utilizing PAGs, we investigated the use of a volatile nonphotoactivated acid, trifluoroacetic acid (TFA), as a ring opening catalyst.
TFA has a
relatively non-nucleophilic counter ion, and its vapors can permeate through the film and
initiate the CROP mechanism. The volatility also meant that the excess TFA could
subsequently be removed by heating or vacuum treatment. Films of PDTG-TPD-Ox20% were
spun as described in the method above and then exposed to TFA vapour for 5 minutes.
Polymer thin films were placed in a glass petri dish containing a central shallow vial holding
0.5 mL of TFA. Warming to 100 °C on a hot plate created a sealed atmosphere containing
TFA. Films were subsequently cooled, and the UV-Vis absorption measured before and after
washing with chloroform. It can be seen from Figure 2, that 20 minutes at 100 °C resulted in
approximately 75% film retention, but longer heating offered little improvement. Heating the
films to higher temperatures did improve the retention, but was undesirable in terms of device
fabrication, so 100°C was chosen as a good compromise.
We believe the differences in film retention after solvent rinsing for the different
crosslinkers are due to the different mechanisms of crosslinking. In the case of cationic
12
crosslinking, the films are only able to crosslink when one oxetane ring is in close enough
proximity to another oxetane to react, so the difficulties may have been due to the relatively
low 20% crosslink density, which reduces the probability of such close interactions, as well as
reducing the ability of the catalytic species to migrate from oxetane to oxetane. In the case of
the radical crosslinking of the alkyl bromide, the cleavage of the C-Br is expected to generate
highly reactive alkyl and bromine radical species, which can react with a variety of species in
close proximity – for example extraction of a proton from a neighbouring alkyl chain by the
Br radical to generate HBr and another reactive alkyl radical. Dimerisation of these alkyl
radicals would then cause crosslinks. Undoubtedly, there are competing reactions such as
disproportionation, but the high reactivity and non-selective nature of the radicals generated
ensures sufficient crosslinks are formed to generate intractable films, even at low loadings of
the crosslinker.
There was little change observed in the UV/Vis spectra before and after washing of
the films in both cases, except for a slight decrease in the long wavelength peak of the spectra
relative to the shorter wavelength shoulder. This may be expected due to the reordering
necessary for the crosslinking process to occur, we note however this was a minor change and
the absorption maxima did not shift suggesting that the crosslinking does not cause a
significant perturbation of the chain packing.
2.5. Crosslinking of Solar Cell Devices prepared from 5 vol% DIO Additive
To study the impact of crosslinking on PCE, we fabricated crosslinked devices using
the optimal device parameters for uncrosslinked layers with 5 vol% DIO. PDTG-TPDBr20%:PC70BM films were spun cast in a nitrogen atmosphere, and crosslinked before vacuum
deposition of the LiF/Al contacts. Crosslinking was performed using the optimised conditions
for film retention by 30 min of exposure to 254 nm light. Upon irradiation the OPV
performance of the completed device suffered a very significant 99-100% drop in
13
performance (Figure 3), with very low Jsc and PCE. Surprisingly reducing the UV exposure
time to only 5 minutes still caused a very large degradation in device performance.
Our initial hypothesis was that the degradation upon UV exposure was caused by
residues from the crosslinking of the alkyl bromide in the polymer film (for example HBr or
bromine). To test this hypothesis, films of PDTG-TPD-Ox20%, cast from identical solvent and
DIO mixtures were also exposed to 254 nm UV (note we did not expect this to crosslink the
film). Interestingly this polymer which contains no alkyl bromide, showed an identical
degradation in performance (Figure 3), ruling out the possibility that the effect was due to
formation of HBr or molecular bromine. An alternative explanation may be that the polymer
backbone was degraded. However, no change to the film UV-Vis absorption was noted, and
in addition the literature has also shown many studies in which UV crosslinking is
demonstrated without backbone degradation.6c,8c Therefore we suspected that reaction of the
residual DIO in the films could be the cause, despite the high vacuum treatment (1×10-7-1×109
mbar) before electrode deposition which was expected to remove DIO traces.22 Other reports
have noted that DIO or high boiling additives can be retained in thin films after spin coating23,
and that its presence can cause films to crosslink when heated to 300ºC.24 In our case,
exposure of any residual DIO to UV could likely trigger the homolytic fission of the carboniodine bond (similar to the alkyl bromide in the polymer), and the resulting alkyl and iodine
radicals may subsequently react in the film, either with each other, the polymer or the
fullerene.25 Figure 3 shows the degradation effects to the J-V curves of a) PDTG-TPDBr20%:PC70BM and b) PDTG-TPD-Ox20%:PC70BM. A similar reduction in performance
occurred for both polymers, which may be attributed to the same volumetric qualities of DIO
(5 vol%) used in the solution before spinning the films. Carbon-iodine bonds are known to be
14
even more prone to homolytic fission of the bond than carbon-bromine because of the weaker
covalent bond,26 making it highly labile to deep-UV exposure.
We attempted to remove residual DIO by prolonged vacuum treatment (1×10-2 mbar)
for 2 or 24 hours, before subsequent UV exposure. Two hours was found to be insufficient
and resulted in similar effects to the freshly spun devices. However after 24 h, although a
reduction in Jsc and FF was still observed, this was similar to that seen for UV crosslinking
in the absence of DIO (see section 2.6) suggesting most of the DIO was removed.
Surprisingly we can find no reports on the effects of UV exposure to residual DIO in the
literature. It is likely that such effects have not been observed previously due to the minimal
UV exposure during routine testing of BHJs under simulated solar illumination. However we
believe this result is significant because with such labile carbon-iodine bonds, the use of DIO
may not be suitable for OPV applications where the long term exposure of sunlight may result
in similar degradation in situ.27 These results suggest that the use of DIO as an additive should
be avoided to achieve high PCE over long lifetimes, particularly since prolonged high vacuum
treatment is not viable for large scale printing.
2.6. Investigating Chloronaphthalene as an Alternative Additive
Due to the stability problems associated with the films cast with DIO, alternative
additives were investigated, which could still afford high efficiency devices but demonstrate
improved UV stability. Chloronaphthalene (CN) has been much investigated as a high boiling
point additive,28 and unlike the alkyl iodide, the aromatic carbon-chlorine bond is stronger and
stable towards UV light. Different concentrations of CN (3, 4, 5 and 6 vol%) were first
screened in the same device structure as reported earlier. The optimum performance was
achieved with 4 vol% of CN with both PDTG-TPD-Br20% and PDTG-TPD-Ox20%. As shown
in figure 4 and Table 3, the use of CN boosted FF, Jsc and overall PCE for all polymers by
approximately 1% compared to the use of DIO. CN has previously been suggested to promote
15
phase segregation of PCBM in P3HT:PCBM cells,29 affording higher degrees of polymer
crystallinity when compared to neat polymer film without PCBM, which was ascribed to the
slower evaporation rate of the high boiling additive. This gives rise to a super saturated
regime allowing more time for the alkyl chains to self-organise. In our case, similar effects
may help to explain the increase in Jsc and FF by improved thin film morphology and thus
better charge extraction in the active layer. The improvement of photocurrent and fill factor
suggest an improvement in exciton harvesting and charge extraction, which may indicate an
increase in donor-acceptor interfacial area and improved ordering of the polymer films upon
spin coating. The best performing device was again PDTG-TPD-Br20% with an average PCE
of 7.88%, followed by PDTG-TPD-Ox20% with an average PCE of 7.05%. Interestingly the
performance of both of the 100% polymers also improved as can be seen in table 4. It should
also be noted that these efficiencies, which have been measured in a normal device
architecture, are on par to those reported for the best inverted device structure by Reynolds
and co-workers.15b
2.7. Crosslinking of Solar Cell Devices prepared from 5 vol% CN Additive
Having substituted DIO for CN, further crosslinking studies with PDTG-TPD-Br20% were
performed using the same crosslink parameters. Gratifyingly, the results showed a significant
improvement to the previous study using DIO, although a deterioration in performance was
still observed upon crosslinking (Figure 5 and Table 4), mainly as a result of a reduction in
Jsc and FF. Exposing the film after crosslinking to further thermal annealing (120 °C and 150
°C for 5 minutes), or to high vacuum to remove possible breakdown residues did not improve
performance.
We also investigated the crosslinking of the PDTG-TPD-Ox20%/PC70BM blend with 4 vol%
CN. In this case, crosslinking was initiated by exposing the films to TFA vapour in a nitrogen
atmosphere for 30 min, followed by a thermal bake at 100 °C. Following the deposition of the
16
cathode, the device performance is shown in Figure 5 and summarised in Table 4.
Interestingly, we find that the photocurrent increased after crosslinking, from 10.5 to 12.7
mA/cm2, although the fill factor reduced from 0.66 to 0.46 resulting in an overall reduction of
PCE from 5.79 to 5.02% in this batch of devices. The increase in photocurrent may indicate
that a change in morphology has occurred during the crosslinking process.
2.8.
Differences
in
Crosslinking
behaviour
between
pristine
Polymer
and
Polymer:PC70BM Films.
The changes in device behaviour post crosslinking prompted us to investigate the
effectiveness of crosslinking in a blend film with fullerene present. Having established that
crosslinking can be performed without causing significant device degradation, further film
retention studies were performed using blend films on Glass|ITO|PEDOT:PSS in order to best
simulate device conditions and reaffirm the films solvent resistance. Many papers have used
the pure polymer films to exemplify crosslinking, but crosslinking studies of a blend with
fullerene have rarely been reported.6c,8c,21 Films were deposited using the same methodology
as for OPV fabrication with the 4 vol% CN additive. The films were then crosslinked with
varying amounts of time, either with UV (254 nm) exposure (for PDTG-TPD-Br20%) or heated
to 100 °C under a TFA vapour atmosphere (for PDTG-TPD-Ox20%). As before, film retention
was determined by a normalised loss in absorption at the UV maxima.
The results of PDTG-TPD-Br20%:PC70BM (1:2) demonstrated that even exposure to
254 nm UV for 60 minutes did not result in the formation of an intractable film. In all cases,
the film was removed completely upon insertion into a chloroform bath. Previous reports
have also reported a reduction in crosslinking efficacy upon the inclusion of fullerene to the
blend, which was rationalized on the basis that the fullerene acts as a UV screening layer,
absorbing most of the UV light and reducing crosslinking efficiency.21 However given the
long exposure time in the present case we believe that this is unlikely to be the only
17
explanation. Another possibility is that cleavage of the C-Br is occurring, but the resultant
radicals are preferentially reacting with fullerene, rather than cross-linking with another chain.
The well-known ability of fullerenes to act as radical scavengers suggests this may be a
possibility.30 However we were unsuccessful in attempts to directly detect any polymer with
bound fullerene by techniques such as MALDI or NMR (of the washed films). These results
suggest that, despite several reports of the efficacy of alkylbromides as useful crosslinkers,
caution must be exercised particularly in the presence of high loadings of fullerene.
In contrast, the cationic crosslinking of blends of 1:2 PDTG-TPD-Ox20%:PC70BM with
4 vol% CN in TFA vapour proved to be much more effective (Figure 6). Although longer
exposure times were needed than for the pristine polymer film, almost complete retention of
the polymer film could be obtained after 60 min of exposure to TFA at 100 °C. The longer
reaction times are probably a result of the dilution of the polymer by the fullerene in the thin
film. Interestingly examination of the UV spectra before and after washing (Figure 6), shows
that a 70% reduction in intensity is seen in the peak at 450 nm after washing in chloroform
whilst the polymer peak at 680 nm is barely affected. We attribute the drop in the short
wavelength to a reduction in PC70BM content after washing. One explanation for the loss of
PC70BM may have been due to the swelling of the crosslinked polymer in chloroform, thus
allowing for the release of PC70BM. This suggests that even though the polymer was
intractable, the fullerene which was blended into the film was not. Interestingly, with the
removal of PC70BM it is clear that the absorption spectra closely resembles that of the pure
polymer thin film, further suggesting that the polymer has not degraded during the
crosslinking process. Although this shows that PC70BM is able to freely move in a solvent
swollen film, crosslinking may still help to slow detrimental aging of the blend film
particularly as it has recently been shown that the formation of an electron blocking polymer
18
rich capping layer next to the cathode is one reason for reduced device efficiency upon
aging.31 In addition, it may be possible to pattern and crosslink the pure polymer film, before
subsequent solvent swelling in the presence of fullerenes to allow the laterally patterned blend
films. However, in order to form fully intractable blend films, it may be necessary that both
the polymer and fullerene contain crosslinkable functionalities.
We further investigated the stability of both sets of cross-linked devices, PDTG-TPDBr20%:PC70BM and PDTG-TPD-Ox20%:PC70BM (1:2) blends, with 4 vol% CN, under
accelerated aging conditions – namely thermal annealing. Devices were annealed at 120 °C
over a period of 5, 10 and 30 min. Although devices are not expected to reach this
temperature during actual operation, this temperature was chosen since any issues regarding
blend stability (for example detrimental large scale phase separation) were likely to happen
more rapidly at this temperature. Thus devices were fabricated as reported above, and then
annealed for the appropriate time and temperature within a glove box, before measurement at
room temperature. The results are shown in Table S1 and Figure 7.
In the case of PDTG-TPD-Ox20% using the optimal blend crosslinking procedure
resulted in a drop in overall efficiency, which we attribute to the extended baking time.
Exposure of the control non-crosslinked film to the same heating profile but without TFA
present also resulted in a similar reduction of efficiency. The two devices demonstrated
significant differences in the subsequent heating of the devices to 120 °C. The noncrosslinked devices exhibited a large initial drop in performance after just 5 min heating,
followed by a small improvement over time due to an increasing photocurrent. The
improvement of the control over time was surprising, but it may perhaps be related to
crosslinking of the film at 120 °C by acid present in the PEDOT:PSS.32 Previous reports have
shown that PEDOT:PSS can initate a surface initiated crosslinking upon heating. 12b
19
Morphology changes caused by such crosslinking could help explain the increased
photocurrent, similar to the increase in photocurrent observed for TFA exposed devices. In
contrast the cross-linked blend did not show the rapid reduction in performance upon
annealing at 120 °C, but rather a gradual decrease in efficiency over time, such that both sets
of material show similar performance after 30 min cumulative annealing. In conclusion it
seems that the crosslinking does not result in a clear improvement of device stability for the
oxetane polymer, although it does improve film intractability which may be beneficial for
multi-layer solution processing.
For PDTG-TPD-Br20% although UV crosslinking did not form intractable films, this
did appear to reduce device stability. On the contrary annealing at 120 °C resulted in a small
improvement in device efficiency, whereas the non-crosslinked polymer reduced in
performance. Subsequent thermal annealing of both films did not result in any further
significant changes of device efficiency.
In order to probe the origin of the differences in behavior for the oxetane polymers,
atomic force microscopy (AFM) topography was used to probe the surface of the 1:2 PDTGTPD-Ox20%:PC70BM film with (Figure S8) and without crosslinking (Figure S9). The images
of all films appear very similar, and no obvious change occurs to the morphology of either the
crosslinked or the non-crosslinked films during thermal annealing, despite the reduction in
solar cell performance. Certainly there appears to be no growth of large domains during the
annealing process, as has been seen in other reports. Indeed the surface morphology for all
films appears to show good intermixing of the two materials, with no large features observed,
in agreement with the high device efficiencies observed. RMS values shown in Table S2
demonstrate the smoothness of each film. The lack of any obvious difference in film
morphology over time and with post heating (at both 120 °C and 180 °C) with or without
20
crosslinking may indicate that the degradation in device performance is due to other factors
apart from morphology changes, or perhaps more simply that the effects are more subtle than
can be observed by AFM, which is only probing the surface morphology. Further
investigations into the thin film morphology and long term stability are required.
3. Conclusion
In conclusion, four novel crosslinkable polymers were synthesised based upon PDTGTPD with different ratios of crosslinker, and their optoelectronic properties and device
characteristics in OPVs were measured. Polymers with high degrees of crosslinking group
showed a reduction in solar cell performance compared to the parent polymer, whereas those
with 20% crosslinker demonstrated an improved overall PCE by increasing Jsc and FF.
Comparisons of two crosslinking methods, both radical and cationic, were performed for pure
polymer films as well as blends of polymer:fullerene, highlighting significant differences in
behaviour. We also demonstrated that the choice of processing additive is very important
when crosslinking devices with UV light. The use of diiodooctane resulted in a significant
reduction in device performance upon UV exposure suggesting that DIO may be a
problematic additive for BHJs with long lifetime under sunlight. We further demonstrated that
the oxetane containing PDTG-TPD-Ox20% could be readily crosslinked by exposure to a
saturated TFA vapour environment. Crosslinking by this route was successful for both pure
polymer as well as blends with fullerene. In contrast, although the pure alkyl bromide
containing PDTG-TPD-Br20% films could be crosslinked by exposure to 254 nm UV light, this
was unsuccessful for blend films containing fullerene. For blend devices, UV crosslinking of
the alkyl bromide containing polymers was shown to have a detrimental effect on
performance, whereas acid catalyzed crosslinking of the oxetane polymer resulted in a small
increase in device performance.
21
4. Experimental
All reactions were treated as air and light sensitive and performed under argon and in
the dark. All glassware used were washed using teepol surfactant, rinsing with excess water,
acetone and dichloromethane and dried in an oven at 120 °C. All solvents and reagents were
sourced commercially from Aldrich. 4,4-Bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4Hgermolo[3,2-b:4,5-b']dithiophene16 and 1,3-dibromo-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione6c
were synthesised as previously reported. 1H and 13C NMR spectra were recorded on a Bruker
AV-400 (400 MHz for 1H and 100 MHz for
13
C) unless otherwise stated, using the
residual solvent resonance of CDCl3 as an internal reference. Number-average (Mn) and
weight-average (Mw) molecular weight were determined by Agilent Technologies 1200
series GPC running in chlorobenzene at 80 °C, using two PL mixed columns in series and
calibrated
against
narrow
polydispersity
polystyrene
standards.
Electrospray mass
spectrometry was performed with a Thermo Electron Corporation DSQII mass spectrometer.
UV-Vis spectra were recorded on a UV-1601 Shimadzu UV-Vis spectrometer. Flash
chromatography was performed on silica gel (Merck Kieselgel 60 F254 230–400 mesh). A
custom build Shimadzu recSEC system was used to fractionate the polymers (chlorobenzene
at 80 °C) and purify the tin monomer (hexane at 40 °C). The system comprises of a DGU20A3 degasser, a LC-20A pump, a CTO-20A column oven, an Agilent PLgel 10 μm MIXEDD column and a SPD-20A UV detector. Photo electron spectroscopy in air (PESA)
measurements were recorded with a Riken Keiki AC-2 PESA spectrometer with a power
setting of 5 nW and a power number of 0.5. Samples for PESA were prepared on glass
substrates by spin coating. Data reported is the average of 2 separate thin film measurements
for pristine samples, and 2 films for blend samples. X-ray diffraction (XRD) measurements
were performed on the PANALYTICAL X′ PERT-PRO MRD diffractometer equipped with
22
nickel-filtered Cu K1 beam and X′ CELERATOR detector, using current I = 40 mA and
accelerating voltage V = 40 kV. Samples were prepared by drop casting.
The
conventional
devices
were
fabricated
using
the
structure
of
Glass/ITO/PEDOT:PSS (30-40 nm)/active layer (80-90 nm)/LiF (1 nm)/Al (100 nm). For
inverted device architecture it was Glass/ITO/ZnO (~30 nm)/active layer (80-90 nm)/MoO3
(10 nm)/Ag (100 nm). Indium tin oxide (ITO) coated glass was first cleaned using via
sequential sonication in detergent (Mucasol), water (DI), acetone, isopropanol and then dried
using nitrogen and finally plasma cleaned (228 mtorr 2 min). 30-40 nm of
poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P TP AI 4083,
Bayer AG) was then deposited via spin coating and dried at 150 ˚C for 20 min. Additives and
n-type material (PC70BM purchased from nano-c) were added to a 12 mg mL-1 polymer
solution and stirred overnight. The following blend solution was then spin coated to a
thickness of approximately 80-90 nm. LiF (1 nm) was then thermally deposited followed by
Al (120 nm) under high vacuum (1×10-6 mbar). The device area was 0.045 cm2 in both cases.
OPVs were characterized using a Xenon lamp at AM1.5 solar illumination (Oriel 300 W solar
simulator). Incident photon conversion efficiency (IPCE) measurements were made using a
100 W tungsten halogen lamp (Bentham IL1 with Bentham 605 stabilized current power
supply) coupled to a monochromator and a computer-controlled stepper motor (Bentham
M300, 300 mm focal length, slit width 3.7 nm, 1800 lines/m grating). The photon flux of light
incident to the samples was calibrated using a UV-enhanced silicon photodiode. A 590 nm
long-pass glass filter was inserted into the beam at illumination wavelengths longer than 620
nm to remove light from second-order diffraction. Photocurrent was measured using a
Keithley 2400 source meter; Measurement duration for a given wavelength was sufficient to
ensure the current had stabilized (up to around 5 s under low or zero bias light conditions). All
23
electrical measurements of OPVs were executed in the inert N2 purged devices chamber. For
crosslinking with TFA, films were placed on the base of a glass petri dish, and an open glass
vial containing 0.5 ml of TFA was placed adjacent to the film. The petri dish was covered,
and the whole ensemble was heated on a hot plate at 100C for the indicated time in a fume
cupboard. Films were then dried under vacuum before further fabrication.
1,3-Dibromo-5-(6-((3-methyloxetan-3-yl)methoxy)hexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)dione (TPD-Ox)
A solution of 1,3-dibromo-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (2 g, 6.43 mmol) in dry
DMF (12 mL) and added dropwise to NaH (60% mineral oil, 0.353 g, 8.36 mmol) at RT.
After which the reaction was left to stir for 1 hr at RT. The resulting solution was added
dropwise to a separate solution of 3-methyl-3-[(6-bromohexoxy)methyl]-oxetane (2.05 g, 7.72
mmol) in dry DMF (20 mL) and stirred overnight at RT. The reaction was diluted with diethyl
ether (~100 mL) and washed with water (3×50mL) and brine (3×50mL), dried (MgSO4),
filtered and concentrated under reduced pressure. The residue was purified by column
chromatography over silica (eluent: ethyl acetate:hexane, (1:1)) and recrystallised in aqueous
ethanol (ethanol:water, (4:1)) to afford light yellow crystals (1.34 g, 2.71 mmol, 42% yield).
Mp 92.3-92.8 °C; 1H NMR (400 MHz, CDCl3) δ 4.51 (d, J = 5.7, 2H), 4.36 (d, J = 5.7, 2H),
3.61 (t, J = 7.4, 2H), 3.52 – 3.41 (m, 4H), 1.70 – 1.55 (m, 4H), 1.48 – 1.34 (m, 4H), 1.32 (s,
3H);
13
C NMR (100 MHz, CDCl3) δ 160.38, 134.79, 112.99, 80.25, 76.08, 71.40, 39.91,
38.71, 29.34, 28.22, 26.61, 25.72, 21.39. HRMS (ESI) m/z: [M + H]+ calcd for
C17H22NO4SBr2, 493.9636; found, 493.9641.
24
1,3-Dibromo-5-(6-bromohexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD-Br)
A solution of 1,3-dibromo-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (2 g, 6.43 mmol) was
dissolved in dry DMF (12 mL) and added dropwise to NaH (60% mineral oil, 0.353 g, 8.36
mmol) at RT. After which the reaction was left to stir for 1 hr at RT. The resulting solution
was added dropwise to a separate solution of 1,6-dibromohexane (4.71 g, 19.3 mmol) in dry
DMF (20 mL) and stirred overnight at RT. The reaction was diluted with diethyl ether (~100
mL) and washed with water (3×50mL) and brine (3×50mL), dried (MgSO4), filtered and
concentrated under reduced pressure. The residue was purified by column chromatography
over silica (eluent: chloroform:hexane, (4:1)) and recrystallised in aqueous ethanol
(ethanol:water, (4:1)) to afford light yellow crystals (1.05 g, 2.215 mmol, 34% yield). Mp
104.4-105.0 °C; 1H NMR (400 MHz, CDCl3) δ 3.63 (t, J = 7.2, 2H), 3.42 (t, J = 6.8, 2H), 1.93
– 1.83 (m, 2H), 1.73 – 1.63 (m, 2H), 1.54 – 1.45 (m, 2H), 1.43 – 1.32 (m, 2H); 13C NMR (100
MHz, CDCl3) δ 160.39, 134.75, 113.09, 38.59, 33.68, 32.54, 28.08, 27.67, 25.96; HRMS
(ESI) m/z: [M + H]+ calcd for C12H12NO2SBr3, 470.8139; found, 470.8130.
Poly[(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophen-2-yl)-(5-octyl-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione)] (PDTG-TPD-C8)
4,4-Bis(2-ethylhexyl)-2,6-bis(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophene
(304.9
mg, 0.386 mmol) was weighed into a pre-dried microwave vial. 1,3-Dibromo-5-octyl-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione (164.0 mg, 0.386 mmol), Pd2dba3 (7.1 mg, 7.73 μmol) and
P(o-tol)3 (9.4 mg 0.031 mmol) were added. The microwave vial was capped and degassed
with argon and vacuum (×3). o-Xylene (2 mL) was added and the mixture further degassed
with stirring for 30 min. The resulting solution was then heated in the microwave reactor
(constant heating mode) at 100°C (2 min), 120°C (5 min), 140°C (5 min), 160°C (5 min) and
180°C (40 min). The reaction was cooled to 50 °C and then precipitated into methanol before
25
bring filtered into a cellulose thimble. Lower molecular weight fractions were removed via
Soxhlet extraction with methanol (1 day), acetone (1 day) and hexane (1 day). The remaining
polymer was then extracted in chloroform (3 hr), concentrated and dried under vacuum. The
polymer was then fractionated using preparative GPC (eluent: chlorobenzene). High
molecular weight fractions were combined, concentrated under reduced pressure and
precipitated into methanol to produce a black solid (21.3 mg, 8% yield). 1H NMR (500 MHz,
CDCl3, 40 °C) δ 8.59 – 7.31 (m, 2H), 3.70 (s, 2H), 2.16 – 0.68 (m, 49H); UV/Vis
(Chlorobenzene, 10-5 M) λmax (log ε) 618 (4.71), 679 (4.83) nm. GPC: (chlorobenzene, 80 °C)
Mn = 49,000 g/mol, Mw = 62,000 g/mol and PD = 1.27.
Poly[((4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophen-2-yl)-(5-octyl-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione))0.8-stat-((4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5b']dithiophen-2-yl)-(5-(6-((3-methyloxetan-3-yl)methoxy)hexyl)-4H-thieno[3,4-c]pyrrole4,6(5H)-dione))0.2] (PDTG-TPD-Ox20%)
Following a similar method reported for PDTG-TPD-C8, 4,4-bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophene (179.1 mg, 0.227 mmol), 1,3dibromo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (77.0 mg, 0.182 mmol), 1,3dibromo-5-(6-((3-methyloxetan-3-yl)methoxy)hexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione
(22.5 mg, 0.045 mmol), Pd2dba3 (4.2 mg, 4.54 μmol), P(o-tol)3 (5.5 mg, 0.018 mmol) and oxylene (2 mL) were reacted together. Following the workup and extraction the crude polymer
was then fractionated using preparative GPC (eluent: chlorobenzene), high molecular weight
fractions were combined, concentrated under reduced pressure and precipitated into methanol
to afford a black solid (76.3 mg, 45% yield). 1H NMR (500 MHz, CDCl3, 40 °C) δ 8.60 –
7.30 (m, 2H), 4.51 (s, 0.4H), 4.35 (s, 0.4H), 3.94 – 3.29 (m, 2.8H), 2.23 – 0.67 (m, 48.2H);
GPC: (chlorobenzene, 80 °C) Mn = 39,000 g/mol, Mw = 72,000 g/mol and PD = 1.86.
26
Poly[(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophen-2-yl)-(5-(6-((3methyloxetan-3-yl)methoxy)hexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)] (PDTG-TPDOx100%)
Following a similar method reported for PDTG-TPD-C8, 4,4-bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophene (209.6 mg, 0.266 mmol), 1,3dibromo-5-(6-((3-methyloxetan-3-yl)methoxy)hexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione
(132.0 mg, 0.266 mmol), Pd2dba3 (4.9 mg, 5.31 μmol) , P(o-tol)3 (6.5 mg, 0.021 mmol) and oxylene (2 mL) were reacted together. Following the workup and extraction the crude polymer
was then fractionated using preparative GPC (eluent: chlorobenzene), high molecular weight
fractions were combined, concentrated under reduced pressure and precipitated into methanol
to afford a black solid (28.6 mg, 13% yield). 1H NMR (500 MHz, CDCl3, 40 °C) δ 8.54 –
7.33 (m, 2H), 4.51 (s, 2H), 4.34 (s, 2H), 3.89 – 3.18 (m, 6H), 2.13 – 0.71 (m, 45H); GPC:
(chlorobenzene, 80 °C) Mn = 29,000 g/mol, Mw = 42,000 g/mol and PD = 1.46.
Poly[((4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophen-2-yl)-(5-octyl-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione))0.8-stat-((4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5b']dithiophen-2-yl)-(5-(6-bromohexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione))0.2] (PDTGTPD-Br20%)
Following a similar method reported for PDTG-TPD-C8, 4,4-bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophene (171.2 mg, 0.217 mmol), 1,3dibromo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (73.4 mg, 0.174 mmol), 1,3dibromo-5-(6-bromohexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (20.6 mg, 0.043 mmol),
Pd2dba3 (4.0 mg, 4.34 μmol), P(o-tol)3 (5.3 mg, 0.017 mmol) and o-xylene (2 mL) were
reacted together. Following the workup and extraction the crude polymer was then
fractionated using preparative GPC (eluent: chlorobenzene), high molecular weight fractions
27
were combined, concentrated under reduced pressure and precipitated into methanol to afford
a black solid (53.6 mg, 34% yield). 1H NMR (500 MHz, CDCl3, 40 °C) δ 8.62 – 7.33 (m, 2H),
3.92 – 3.27 (m, 2.2H), 2.14 – 0.60 (m, 47.8H); GPC: (chlorobenzene, 80 °C) Mn = 44,000
g/mol, Mw = 71,000 g/mol and PD = 1.63.
Poly[(4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophen-2-yl)-(5-(6-bromohexyl)-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione)] (PDTG-TPD-Br100%)
Following a similar method reported for PDTG-TPD-C8, 4,4-bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophene (199.0 mg, 0.252 mmol), 1,3dibromo-5-(6-bromohexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (120.0 mg, 0.252 mmol),
Pd2dba3 (4.6 mg 5.04 μmol), P(o-tol)3 (6.14 mg, 0.02 mmol) and o-xylene (2 mL) were
reacted together. Following the workup and extraction the crude polymer was then
fractionated using preparative GPC (eluent: chlorobenzene), high molecular weight fractions
were combined, concentrated under reduced pressure and precipitated into methanol to afford
a black solid (26.5 mg, 14% yield). 1H NMR (500 MHz, CDCl3, 40 °C) δ 8.61 – 7.33 (m, 2H),
3.90 – 3.28 (m, 4H), 2.20 – 0.62 (m, 42H); GPC: (chlorobenzene, 80 °C) Mn = 25,000 g/mol,
Mw = 35,000 g/mol and PD = 1.41.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This work was supported by EPSRC grants EP/G060738/1 and EP/I002936/1, and NSF grant
1026664. We would like to thank Dr Ester Buchaca-Domingo (Imperial College London) for
performing the WAXS experiments, and Scott E. Watkins, (CSIRO Melbourne) for the PESA
measurements.
28
References
(1) (a) Duan, C.; Huang, F.; Cao, Y. J. Mater. Chem. 2012, 22, 10416; (b) Son, H. J.; Carsten, B.; Jung, I. H.;
Yu, L. Energy Environ. Sci. 2012, 5, 8158; (c) Ameri, T.; Li, N.; Brabec, C. J. Energy Environ. Sci. 2013, 6,
2390.
(2) Wang, T.; Pearson, A. J.; Lidzey, D. G. J. Mater. Chem. C 2013, 1, 7266.
(3) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney, M. F. Chem. Rev. 2012, 112, 5488.
(4) Treat, N. D.; Chabinyc, M. L. Annual Review of Physical Chemistry 2014, 65, 59.
(5) (a) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Adv.
Energ. Mater. 2011, 1, 82; (b) Treat, N. D.; Mates, T. E.; Hawker, C. J.; Kramer, E. J.; Chabinyc, M. L.
Macromolecules 2013, 46, 1002; (c) Lee, K. H.; Zhang, Y.; Burn, P. L.; Gentle, I. R.; James, M.; Nelson, A.;
Meredith, P. J. Mater. Chem. C 2013, 1, 2593.
(6) (a) Miyanishi, S.; Tajima, K.; Hashimoto, K. Macromolecules 2009, 42, 1610; (b) Wantz, G.; Derue, L.;
Dautel, O.; Rivaton, A.; Hudhomme, P.; Dagron-Lartigua, C. Polym. Int. 2014, 63, 1346; (c) Griffini, G.;
Douglas, J. D.; Piliego, C.; Holcombe, T. W.; Turri, S.; Frechet, J. M. J.; Mynar, J. L. Adv. Mater. 2011, 23,
1660; (d) Png, R. Q.; Chia, P. J.; Tang, J. C.; Liu, B.; Sivaramakrishnan, S.; Zhou, M.; Khong, S. H.; Chan, H.
S. O.; Burroughes, J. H.; Chua, L. L.; Friend, R. H.; Ho, P. K. H. Nat. Mater. 2010, 9, 152.
(7) (a) Liu, B.; Png, R. Q.; Zhao, L. H.; Chua, L. L.; Friend, R. H.; Ho, P. K. H. Nat. Commun. 2012, 3, 1321;
(b) Tao, C.; Aljada, M.; Shaw, P. E.; Lee, K. H.; Cavaye, H.; Balfour, M. N.; Borthwick, R. J.; James, M.; Burn,
P. L.; Gentle, I. R.; Meredith, P. Advanced Energy Materials 2013, 3, 105.
(8) (a) Brotas, G.; Farinhas, J.; Ferreira, Q.; Rodrigues, R.; Martins, I. L.; Morgado, J.; Charas, A. J. Polym. Sci.
Part A Polym. Chem 2014, 52, 652; (b) Nam, C.-Y.; Qin, Y.; Park, Y. S.; Hlaing, H.; Lu, X.; Ocko, B. M.;
Black, C. T.; Grubbs, R. B. Macromolecules 2012, 45, 2338; (c) Kim, B. J.; Miyamoto, Y.; Ma, B.; Fréchet, J.
M. J. Adv. Funct. Mater. 2009, 19, 2273.
(9) (a) Qian, D.; Xu, Q.; Hou, X.; Wang, F.; Hou, J.; Tan, Z. a. J. Pol. Sci. A. Poly. Chem. 2013, 51, 3123; (b)
Chen, X.; Chen, L.; Chen, Y. J. Pol. Sci. A. Poly. Chem. 2013, 51, 4156.
(10) Tang, Y.; Ji, L.; Zhu, R.; Wei, Z.; Zhang, B. J. Phys. Chem. A 2005, 109, 11123.
(11) (a) Zuniga, C. A.; Barlow, S.; Marder, S. R. Chem. Mater. 2011, 23, 658; (b) Müller, D. C.; Falcou, A.;
Reckefuss, N.; Rojahn, M.; Wiederhirn, V.; Rudati, P.; Frohne, H.; Nuyken, O.; Becker, H.; Meerholz, K.
Nature 2003, 421, 829; (c) Zacharias, P.; Gather, M. C.; Köhnen, A.; Rehmann, N.; Meerholz, K. Angew. Chem.
Int. Ed. 2009, 48, 4038; (d) Jungermann, S.; Riegel, N.; Müller, D.; Meerholz, K.; Oskar, N. Macromolecules
2006, 39, 8911.
(12) (a) Feser, S.; Meerholz, K. Chem. Mater. 2011, 23, 5001; (b) Köhnen, A.; Brücher, M.; Reckmann, A.;
Klesper, H.; von Bohlen, A.; Wagner, R.; Herdt, A.; Lützenkirchen-Hecht, D.; Hergenröder, R.; Meerholz, K.
Macromolecules 2012, 45, 3487.
(13) (a) Farinhas, J.; Ferreira, Q.; Di Paolo, R. E.; Alcacer, L.; Morgado, J.; Charas, A. J. Mater. Chem. 2011,
21, 12511; (b) Brotas, G.; Farinhas, J.; Ferreira, Q.; Morgado, J.; Charas, A. Synth. Met. 2012, 162, 2052.
(14) (a) Chu, T.-Y.; Lu, J.; Beaupré, S.; Zhang, Y.; Pouliot, J.-R. m.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.;
Ding, J.; Tao, Y. J. Am. Chem. Soc. 2011, 133, 4250; (b) Guo, X.; Xin, H.; Kim, F. S.; Liyanage, A. D. T.;
Jenekhe, S. A.; Watson, M. D. Macromolecules 2010, 44, 269; (c) Li, Z.; Tsang, S.-W.; Du, X.; Scoles, L.;
Robertson, G.; Zhang, Y.; Toll, F.; Tao, Y.; Lu, J.; Ding, J. Adv. Funct. Mater, 2011, 21, 3331; (d) Piliego, C.;
Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. J. Am. Chem. Soc. 2010, 132,
29
7595; (e) Warnan, J.; Cabanetos, C.; Labban, A. E.; Hansen, M. R.; Tassone, C.; Toney, M. F.; Beaujuge, P. M.
Adv. Mater. 2014, 26, 4357; (f) Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ponce Ortiz, R.; Butler, M. R.;
Boudreault, P.-L. T.; Strzalka, J.; Morin, P.-O.; Leclerc, M.; López Navarrete, J. T.; Ratner, M. A.; Chen, L. X.;
Chang, R. P. H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2012, 134, 18427.
(15) (a) Kesava, S. V.; Fei, Z.; Rimshaw, A. D.; Wang, C.; Hexemer, A.; Asbury, J. B.; Heeney, M.; Gomez, E.
D. Advanced Energy Materials 2014, n/a; (b) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.;
So, F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062; (c) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.;
Tsang, S.-W.; Lai, T.-H.; Reynolds, J. R.; So, F. Nat. Photonics 2012, 6, 115; (d) Small, C. E.; Tsang, S.-W.;
Chen, S.; Baek, S.; Amb, C. M.; Subbiah, J.; Reynolds, J. R.; So, F. Adv. Energ. Mater. 2013, 3, 909; (e)
Gendron, D.; Morin, P.-O.; Berrouard, P.; Allard, N.; Aïch, B. R.; Garon, C. N.; Tao, Y.; Leclerc, M.
Macromolecules 2011, 44, 7188; (f) Fei, Z. P.; Shahid, M.; Yaacobi-Gross, N.; Rossbauer, S.; Zhong, H. L.;
Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Chem. Commun. 2012, 48, 11130; (g) Yau, C. P.; Fei, Z.;
Ashraf, R. S.; Shahid, M.; Watkins, S. E.; Pattanasattayavong, P.; Anthopoulos, T. D.; Gregoriou, V. G.;
Chochos, C. L.; Heeney, M. Adv. Funct. Mater. 2014, 24, 678.
(16) Fei, Z.; Kim, J. S.; Smith, J.; Domingo, E. B.; Anthopoulos, T. D.; Stingelin, N.; Watkins, S. E.; Kim, J.-S.;
Heeney, M. J. Mater. Chem. 2011, 21, 16257.
(17) Tierney, S.; Heeney, M.; McCulloch, I. Synth. Met. 2005, 148, 195.
(18) Nielsen, K. T.; Bechgaard, K.; Krebs, F. C. Macromolecules 2005, 38, 658.
(19) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J.
Angew. Chem. Int. Ed. 2003, 42, 3371.
(20) (a) Kim, D. H.; Ayzner, A. L.; Appleton, A. L.; Schmidt, K.; Mei, J.; Toney, M. F.; Bao, Z. Chem. Mater.
2013, 25, 431; (b) Mei, J.; Bao, Z. Chem. Mater. 2014, 26, 604.
(21) Carlé, J. E.; Andreasen, B.; Tromholt, T.; Madsen, M. V.; Norrman, K.; Jørgensen, M.; Krebs, F. C. J.
Mater. Chem. 2012, 22, 24417.
(22) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J.
J. Am. Chem. Soc. 2008, 130, 3619.
(23) (a) Ye, L.; Jing, Y.; Guo, X.; Sun, H.; Zhang, S.; Zhang, M.; Huo, L.; Hou, J. J. Phys. Chem. C 2013, 117,
14920; (b) Chang, L.; Lademann, H. W. A.; Bonekamp, J.-B.; Meerholz, K.; Moulé, A. J. Adv. Funct. Mater.
2011, 21, 1779.
(24) Chang, L.; Jacobs, I. E.; Augustine, M. P.; Moulé, A. J. Org. Elect. 2013, 14, 2431.
(25) Miranda, M. A.; Pérez-Prieto, J.; Font-Sanchis, E.; Scaiano, J. C. Prop. React. Kinect. Mech. 2001, 26, 139.
(26) Bernardes, C. E. S.; M. E. Minas da Piedade, M. E.; Amaral, L. M. P. F.; Ferreira, A. I. M. C. L.; Ribeiro
da Silva, M. A. V.; Diogo, H. P.; Costa Cabral, B. J. J. Phys. Chem. A 2007, 111, 1713.
(27) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Adv. Mater.
2012, 24, 580.
(28) (a) Chen, F.-C.; Tseng, H.-C.; Ko, C.-J. Appl. Phys. Lett. 2008, 92, 103316; (b) Guo, X.; Cui, C.; Zhang,
M.; Huo, L.; Huang, Y.; Hou, J.; Li, Y. Energy Environ. Sci. 2012, 5, 7943; (c) Moon, J. S.; Takacs, C. J.; Cho,
S.; Coffin, R. C.; Kim, H.; Bazan, G. C.; Heeger, A. J. Nano Letters 2010, 10, 4005.
(29) Shin, N.; Richter, L. J.; Herzing, A. A.; Kline, R. J.; DeLongchamp, D. M. Adv. Energ. Mater. 2013, 3,
938.
30
(30) Hoke, E. T.; Sachs-Quintana, I. T.; Lloyd, M. T.; Kauvar, I.; Mateker, W. R.; Nardes, A. M.; Peters, C. H.;
Nopidakis, N.; McGehee, M. D. Adv. Energ. Mater. 2012, 2, 1351.
(31) Sachs-Quintana, I. T.; Heumüller, T.; Mateker, W. R.; Orozco, D. E.; Cheacharoen, R.; Sweetnam, S.;
Brabec, C. J.; McGehee, M. D. Adv. Funct. Mater. 2014, 24, 3978.
(32) Köhnen, A.; Riegel, N.; Kremer, J. H.-W. M.; Lademann, H.; Müller, D. C.; Meerholz, K. Adv. Mater.
2009, 21, 879.
31
Scheme 1. a) Synthetic route for monomer, b) Nomenclature of polymers, c) general synthetic
route for polymer synthesis.
b)
Normalised Absorbance (a.u.)
Normalised Absorbance (a.u.)
a)
1.0 Solution
PDTG-TPD-C8
PDTG-TPD-Ox20%
PDTG-TPD-Ox100%
PDTG-TPD-Br20%
0.5
PDTG-TPD-Br100%
1.0 Film
PDTG-TPD-C8
PDTG-TPD-Ox20%
PDTG-TPD-Ox100%
PDTG-TPD-Br20%
0.5
PDTG-TPD-Br100%
0.0
0.0
400
600
400
800
600
800
Wavelength (nm)
Wavelength (nm)
Figure 1. UV-Vis spectra of all polymers: a) solution (CB) at room temperature and b) thin
films (spin coated from CB).
32
Figure 2. Percentage retention after chloroform washing against crosslinking time for: a)
PDTG-TPD-Br20% with 254 nm UV light and b) PDTG-TPD-Ox20% with TFA vapour.
Figure 3. Degradation of photovoltaic performance upon exposure to 254 nm UV light from
a) PDTG-TPD-Br20% and b) PDTG-TPD-Ox20% with 5 vol% DIO additive.
33
Figure 4. As cast polymer:PC70BM (1:2) devices with 4 vol% CN.
Figure 5. Initial non-crosslinked (solid) and crosslinked (dashed) devices of PDTG-TPDBr20% and PDTG-TPD-Ox20% with 4 vol% CN.
34
Figure 6. a) Retention of TFA vapour crosslinked films of PDTG-TPD-Ox20%:PC70BM (1:2)
blend with 4 vol% CN after solvent rinsing. b) UV-Vis spectra of crosslinked (in TFA
vapour/100 °C/60 min) glass|ITO|PEDOT|PDTG-TPD-Ox20%:PC70BM (1:2) with 4 vol% CN,
before and after washing.
Figure 7. Device performance of PDTG-TPD-Ox20% and PDTG-TPD-Br20%, crosslinked and
non crosslinked at 120 °C overtime.
35
Table 1. Molecular weight, degree of polymerisation and optoelectronic properties of all five
co-polymers.
Polymers
Mn
(kDa)
DPn
Mw
(kDa)
DPw
PD
λmax (sol)
(nm)
λmax (film)
(nm)
I.P.a
(eV)
Egb
(eV)
LUMOc
(eV)
PDTG-TPD-C8
49
68
62
86
1.27
679, 618
681, 620
5.2
1.7
-3.5
PDTG-TPD-Br20%
44
60
71
97
1.63
679, 618
681, 620
5.2
1.7
-3.5
PDTG-TPD-Br100%
25
32
35
45
1.41
675, 617
678, 619
5.3
1.7
-3.6
PDTG-TPD-Ox20%
39
53
72
98
1.86
679, 618
681, 620
5.2
1.7
-3.5
PDTG-TPD-Ox100%
29
36
42
56
1.46
675, 617
678, 619
5.3
1.7
-3.6
a)
Ionisation potential was measured using photo electron spectroscopy in air, b)Band gap extrapolated from the onset of optical
absorption, c)LUMO estimated from I.P. and Eg
Table 2. Table of photovoltaic properties of all five PDTG-TPD co-polymers in a
polymer:fullerene ratio of 1:2, averaged over 5 devices, as cast and with 5 vol% DIO additive.
As Cast
DIO 5 vol%
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
PDTG-TPD-C8
0.85
3.2
0.59
1.61
0.81
10.40
0.58
4.88
PDTG-TPD-Ox20%
0.86
3.8
0.49
1.63
0.79
12.2
0.62
6.00
PDTG-TPD-Ox100%
0.80
2.4
0.24
0.48
0.83
6.7
0.37
2.09
PDTG-TPD-Br20%
0.86
3.8
0.52
1.70
0.86
14.3
0.60
7.35
PDTG-TPD-Br100%
0.89
6.1
0.34
1.85
0.89
9.6
0.40
3.44
Table 3. OPV performance of polymer:PC70BM (1:2) blend (CB) with 4 vol% CN averaged
over five devices.
Polymer:PC70BM + CN 4
vol%
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
PDTG-TPD-C8
0.81
11.6
0.62
5.79
PDTG-TPD-Ox20%
0.84
13.2
0.63
7.05
PDTG-TPD-Ox100%
0.82
9.8
0.41
3.29
PDTG-TPD-Br20%
0.84
15.1
0.63
7.88
PDTG-TPD-Br100%
0.84
11.0
0.44
4.07
Table 4. OPV properties of crosslinked devices of PDTG-TPD-Br20% and PDTG-TPD-Ox20%
with 4 vol% CN.
Polymer:PC70BM + CN 4 vol%
Voc (V)
JSC (mA/cm2)
FF
PCE (%)
PDTG-TPD-Br20%
0.82
13.1
0.66
7.11
PDTG-TPD-Br20% crosslinked
0.84
10.7
0.52
4.65
PDTG-TPD-Ox20%
0.83
10.5
0.66
5.79
PDTG-TPD-Ox20% crosslinked
0.86
12.7
0.46
5.02
36