Influence of the annealing temperature on the photovoltaic performance and film morphology applying novel thermocleavable materials

Article abstract of DOI:10.1021/cm1019537

Di-2-thienyl-2,1,3-benzothiadiazole (DTBT) bearing thermally cleavable ester groups in different positions were prepared and copolymerized with alkylsubstituted cyclopentadithiophene (CPDT). The polymers were found to have band gaps in the range of 1.66-2.03 eV and were explored in polymer photovoltaic devices as mixtures with soluble methanofullerenes. The positioning of the ester groups proved to be very significant despite the identical conjugated backbone of 2-methyl-2-hexyl 5-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4- b′]dithiophen-2-yl)-2-(7-(3-(((2-methylhexan-2-yl)oxy)-carbonyl) thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T1) and 2-methyl-2-hexyl 2-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b′] dithiophen-2-yl)-5-(7-(4-(((2-methylhexan-2-yl)oxy)carbonyl)thiophen-2-yl) benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T2). Power conversion efficiencies of up to 1.92% were observed for polymers bearing ester groups on the 4-positions of the thienyl groups (T2), but shifting them to the 3-positions (T1) reduced the efficiency significantly to 0.18%. The thermal behavior of the polymers was studied with thermogravimetric analysis (TGA) that showed a weight loss around 200 °C corresponding to elimination of the ester side chains followed by a second weight loss around 300 °C corresponding to loss of CO₂ via decarboxylation. The temperature of thermocleavage of the active layer films was optimized to 265 °C whereby the T2:PCBM solar cells maintained a significant performance giving efficiencies up to 1.49%.

Full text of DOI:10.1021/cm1019537

Chem. Mater. 2010, 22, 5617–5624 5617
DOI:10.1021/cm1019537
Influence of the Annealing Temperature on the Photovoltaic Performance
and Film Morphology Applying Novel Thermocleavable Materials
Martin Helgesen,*,† Morten Bjerring,‡ Niels Chr. Nielsen,‡ and Frederik C. Krebs†
†Risø National Laboratory for Sustainable Energy, Technical University of Denmark,
Frederiksborgvej 399, DK-4000 Roskilde, Denmark, and ‡Center for Insoluble Protein Structures (inSPIN),
Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University,
DK-8000 Aarhus C, Denmark
Received July 14, 2010. Revised Manuscript Received August 25, 2010
Di-2-thienyl-2,1,3-benzothiadiazole (DTBT) bearing thermally cleavable ester groups in different
positions were prepared and copolymerized with alkylsubstituted cyclopentadithiophene (CPDT). The
polymers were found to have band gaps in the range of 1.66-2.03 eV and were explored in polymer
photovoltaic devices as mixtures with soluble methanofullerenes. The positioning of the ester groups
proved to be very significant despite the identical conjugated backbone of 2-methyl-2-hexyl 5-(4,4-
bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b0]dithiophen-2-yl)-2-(7-(3-(((2-methylhexan-2-yl)oxy)-car-
bonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T1) and 2-methyl-2-hexyl
2-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b0]dithiophen-2-yl)-5-(7-(4-(((2-methylhexan-2-yl)oxy)-
carbonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T2). Power conversion
efficiencies of up to 1.92% were observed for polymers bearing ester groups on the 4-positions of the
thienyl groups (T2), but shifting them to the 3-positions (T1) reduced the efficiency significantly to 0.18%.
The thermal behavior of the polymers was studied with thermogravimetric analysis (TGA) that showed a
weight loss around 200 °C corresponding to elimination of the ester side chains followed by a second
weight loss around 300 °C corresponding to loss of CO2 via decarboxylation. The temperature of
thermocleavage of the active layer films was optimized to 265 °C whereby the T2:PCBM solar cells
maintained a significant performance giving efficiencies up to 1.49%.
Introduction
material which offers significant advantages, both in terms of
conjugated polymer synthesis,7-11 device processing,3,12,13
and operational stability.14-17 To overcome the fact that
the softness/photochemistry provided by solubilizing chains
has been linked to the instability of polymer solar cells,18-25
the thermocleavable materials exploit a thermally labile
bond in the molecule that functions as the linker between
From a materials point of view the state-of-the-art in the
field of organic photovoltaics is currently represented by
bulk heterojunction solar cells based on a polymer/copolymer
and a methanofullerene ([60]PCBM and [70]PCBM) where
power conversion efficiencies are approaching encourag-
ing 8% for small area devices.1 The steady increase in
performance during the past 10 years reveals a great poten-
tial for polymer solar cells as a low-cost renewable energy
source,2-6 but aside from the efficiency, processing and
stability are two other important aspects that have to be
addressed with equal intensity to fully realize that potential.
To combine all three parameters into a useful material and
device further research in device science and new materials is
needed. A recent approach is to utilize a thermocleavable
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*To whom correspondence should be addressed. E-mail: manp@risoe.
dtu.dk.
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2010 American Chemical Society
Published on Web 09/08/2010
pubs.acs.org/cm

5618 Chem. Mater., Vol. 22, No. 19, 2010
Helgesen et al.
high chromophore density. The positioning of the ester
groups together with the annealing temperature has a
pronounced effect on the photovoltaic performance and
film morphology of the polymer:PCBM solar cells which
is presented.
Experimental Section
Synthetic procedures for synthesis of monomers and poly-
mers according to Schemes 1 and 2, and their characterization
data (incl. 1H and 13C liquid-state -NMR) are described in detail
in the Supporting Information together with general experi-
mental details.
Polymer Solar Cell Fabrication and Analysis. Photovoltaic
devices were made by spin coating PEDOT:PSS (Aldrich,
1.3 wt % aqueous solution) onto precleaned, patterned indium
tin oxide (ITO) substrates (9-15 Ωper square) (LumTec) followed
by annealing at 140 °C for 5 min. The active layer was deposited, in
a glovebox, by spin coating a blend of the polymer and PCBM
dissolved in o-dichlorobenzene (40 mg/mL). After a thermal
treatment (see Table 2) the counter electrode of aluminum was
deposited by vacuum evaporation at 2-3 ꢀ 10-6 mbar. The active
area of the cells was 0.5 cm2. I-V characteristics were measured
under AM1.5G corresponding to 100 mW/cm2 white light from a
multiwavelength high-power LED array using a Keithley 2400
source meter. IPCE spectra were recorded on the same solar test
platform28 with the LED based illumination system.
Figure 1. Polymers based on DTBT bearing thermocleavable ester
groups and CPDT.
the solubilizing group and the photoactive material. When
the polymer is heated this bond breaks whereby the solubi-
lizing group can be removed in a post-processing step form-
ing a harder insoluble material. At this point, thermoclea-
vage of bulk heterojunction polymer:PCBM solar cells has
generally lead to a drop in the photovoltaic performance
which has been linked to extensive phase segregation of the
polymer and PCBM upon annealing.26 However, there
are a few examples where similar or better performance
has been obtained after thermocleavage of the film15,27
and these examples possibly represent cases where the
morphology does not change before thermocleavage.
Herein we report our efforts in advancing thermocleav-
able materials in bulk heterojunction solar cells through
the synthesis of two new polymers with reduced band gap.
The polymers T1 and T2 (Figure 1) are copolymers based
on di-2-thienyl-2,1,3-benzothiadiazole (DTBT) and 4,4-
bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b0]dithiophene
(CPDT). For thermocleavability and improved solubil-
ity, ester side chains are attached to the thiophene units
through a thermally labile ester bond. The ester groups
can be removed, at temperatures in the range 200-
300 °C, via elimination and decarboxylation in a post-
processing step leaving a harder polymer material with
Results and Discussion
Synthesis. The synthetic steps involved in the prepara-
tion of the monomers 7a and 7b are outlined in Scheme 1.
The esterification of thiophene-3-carboxylic acid (1) em-
ploys an equivalent amount of 4-dimethylaminopyridine
(DMAP) in combination with N,N0-diisopropylcarbodii-
mide (DIPC) giving 2-methyl-2-hexyl thiophene-3-carboxy-
late (2) in high yield. 2 can then be deprotonated in the
2-position using lithium diisopropylamine (LDA) followed
by treatment with trimethyltin chloride. This affords the
stannylated thiophene 3 which can undergo Stille coupling
with 4,7-dibromobenzo[c][1,2,5]thiadiazole giving the
monomer 6a. 6a was finally functionalized by NBS bromi-
nation. Activation of 1 in the 5-position is done by dropwise
addition of bromine to a solution of 1 in glacial acetic acid
which affords 5-bromothiophene-3-carboxylic acid (4) in
reasonable yield. Using the same procedure as for 2, ester-
ification of 4 gives the tertiary ester 5 in good yield. Suzuki
cross-coupling of 5 with the boronic ester 4,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c][1,2,5]thia-
diazole affords 6b which is NBS brominated to give mono-
mer 7b. Copolymerization of 7a and 7b via Stille coupling
with the distannylated CPDT unit gives polymer T1 in 48%
yield as a dark red solid (Mw = 57700 g/mol, PDI = 1.9)
and T2 in 89% yield as a dark purple solid (Mw = 41600 g/
mol, PDI = 2.7). The solubility of T1 in organic solvents
such as chloroform and toluene is higher than that of T2
which is probably caused by a more twisted backbone.
Thermal Behavior. The thermal behavior of the thermo-
cleavable polymers was investigated by thermogravimetric
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697–712.
(19) Lira-Cantu, M.; Norrman, K.; Andreasen, J. W.; Krebs, F. C.
Chem. Mater. 2006, 18(24), 5684–5690.
(20) Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2006, 90
(2), 213–227.
(21) Norrman, K.; Larsen, N. B.; Krebs, F. C. Sol. Energy Mater. Sol.
Cells 2006, 90(17), 2793–2814.
(22) Chambon, S.; Rivaton, A.; Gardette, J. L.; Firon, M. Sol. Energy
Mater. Sol. Cells 2008, 92(7), 785–792.
(23) Chambon, S.; Manceau, M.; Firon, M.; Cros, S.; Rivaton, A.;
Gardette, J. L. Polymer 2008, 49(15), 3288–3294.
(24) Manceau, M.; Rivaton, A.; Gardette, J. L. Macromol. Rapid
Commun. 2008, 29(22), 1823–1827.
(25) Manceau, M.; Rivaton, A.; Gardette, J. L.; Guillerez, S.; Lemaitre,
N. Polym. Degrad. Stab. 2009, 94(6), 898–907.
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1260.
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Chem. Mater., Vol. 22, No. 19, 2010 5619
Figure 2. TGA of (a) T1 and (b) T2 in the temperature range 50-500 °C. A derivative weight loss curve has been included to tell the point at which weight
loss is most apparent.
Scheme 1. Synthetic Steps Involved in the Preparation of the Monomers 7a and 7b
Scheme 2. Synthesis of the Polymers T1 and T2
analysis (TGA). The sample holders were carefully weighed,
and the samples introduced. TGA was then carried out using
heating rate of 10 °C min-1. TGA of T1 and T2 in the
temperature range 50-500 °C indicates that the ester bond
starts to break around 200 °C (Figure 2) as expected from
our earlier studies.27,29 A second loss peak is clearly detected
for T2 at ∼300 °C which corresponds to loss of CO2.30
According to Figure 2, the decarboxylation of T2 starts to
happen around 265 °C whereas the exact transition of T1 is
not as clear as for T2. At temperatures over 400 °C decom-
position of the polymers sets in, and the TGA data of T1 and
T2 does exhibit the same overall weight loss.
Solid-State NMR. The thermocleavage of the polymers
was also followed by solid-state NMR spectroscopy.
Here, 13C cross-polarization (CP) magic-angle spinning
(MAS) spectra of T1 and T2 were acquired before and
after heating the materials at 200 and 300 °C for 15 min in
a nitrogen atmosphere. Figure 3 shows the 13C CP/MAS
spectra of T1 and T2 before (a,b), after heating at 200 °C
(c,d), and after heating at 300 °C (e,f ). It is seen that a large
fraction of the aliphatic signals (0-60 ppm) are removed
by the heating, indicating that the ester side chains have
been cleaved. Also, the signals from the tertiary carbons
at the ester side chains at 83 ppm are seen to be totally
removed, being consistent with high cleavage efficiency.
When heated at 200 °C the ester is converted to a carboxylic
acid, for which a characteristic signal at 170 ppm is seen most
clearly for T2 (Figure 3d), and it is evident that this signal
(29) Petersen, M. H.; Gevorgyan, S. A.; Krebs, F. C. Macromolecules
2008, 41(23), 8986–8994.
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molecules 2007, 40(16), 6012–6013.

5620 Chem. Mater., Vol. 22, No. 19, 2010
Helgesen et al.
Figure 3. 13C CP/MAS NMR spectra of T1 (a,c,e) and T2 (b,d,f) before heating (a,b) and after heating at 200 °C (c,d), 300 °C (e,f) for 15 min. Each spectrum
were recorded using 22528 scans in 19 h. The dashed lines at the tertiary carbon and carboxylic acid signals at 83.0 and 169.5 ppm are guides to the eye.
Figure 4. Possible chemical transitions of T1 and T2. The same polymer material is possibly reached after heating at 300 °C.
disappears when the sample is heated at 300 °C (Figure3f) as
a result of the decarboxylation. From this point of view, the
same precursor film prepared by standard solution proces-
sing of either T1 or T2 can thus in principle give access to the
same polymer material when heated at 300 °C as shown in
Figure 4. The 13C CP/MAS NMR data of T1 and T2 do not
become identical when heating to 300 °C thus implying that
the chemical environment in the solid state differs for the
materials obtained after heating. The different chemical
environments could arise because of the geometrical con-
straints imposed by the position of the thermocleavable
groups as illustrated in Figure 7 (vide supra).
Optical Properties. The absorption spectra of the poly-
mers in chloroform solution and in thin film are shown in
Figures 5 and 6 where a significant effect is observed on
the optical properties depending upon where the ester
groups are attached on the DTBT unit. In solution the
optical band gaps, defined by the onset of absorption, of
the polymers range from 1.79 to 2.07 eV (Table 1). The
lowest band gap is observed for T2 suggesting that
incorporation of the ester groups on the 4-positions of
the thienyl groups introduces minimum steric hindrance
to the conjugated backbone thereby improving electron
delocalization compared to T1. The absorption maxima
(λmax) of T1 is blue-shifted over 100 nm compared to T2
which also indicate that incorporation of the ester groups
on the 3-positions of the thienyl groups will lead to a more
twisted conjugated backbone consequently reducing elec-
tron delocalization. In thin films, mainly T2 shows a
tendency to π-stack in the solid state where absorption
is red-shifted >50 nm reducing the band gap to 1.66 eV. A
minor red shift is observed for T1 in the solid state giving
it a band gap of 2.03 eV. Upon thermocleavage, by
annealing at high temperatures for 1 min, the films

Article
Chem. Mater., Vol. 22, No. 19, 2010 5621
maintained the optical quality but showed lower absorp-
tion intensity (Figure 6) which might be associated with
the change in film thickness and the dielectric constant of
the materials. The absorption spectra for T2 show no
significant shifts when the films are annealed but T1
shows a significant shift toward lower energies after
annealing the films at 300 °C. Thermocleavage of the
bulky ester groups in the 3-positions of the thienyl groups
most likely improve the planarity of the conjugated back-
bone thereby improving electron delocalization which
lowers the band gap. However, the absorption spectra
of T1 and T2 do not match after thermocleavage at 300 °C
(Figure 6) although it should lead to the same material T3
(Figure 4). This indicates that there are conformational
differences in the conjugated backbone of T1 and T2
(Figure 7) that get locked through thermocleavage of the
ester groups. Consequently, T1:PCBM and T2:PCBM
composites upon heating give T3:PCBM composites with
the same chemistry, but it is the path to T3:PCBM
composites that determines the optical properties, photo-
voltaic performance, and morphology of the final film.
Photovoltaic Performance. Bulk heterojunction solar
cells with an active area of 0.5 cm2 were prepared on an
indium tin oxide (ITO) covered glass substrate, using
conventional device architecture. A thin layer of poly-
(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate)
(PEDOT-PSS) was spin coated on top of the ITO coating
followed by spin coating of the active layer. The active
layer contained a blend of the respective polymer and
PCBM. After spin coating of the active layer the devices
were either processed directly into a solar cell by evapora-
tion of aluminum as back electrode or subjected to a
thermal treatment at the temperature of thermocleavage
immediately before evaporation of the back electrode.
The most efficient devices comprised a polymer/PCBM
ratio of 1:4 spin-coated from o-dichlorobenzene with a
polymer concentration of 8 mg/mL. The obtained current-
voltage curves of the polymer:[60]PCBM solar cells are
presented in Figure 8. As discussed above, incorpora-
tion of the ester groups on the 3-positions of the thienyl
groups in the DTBT unit will introduce significant steric
hindrance to the conjugated backbone thereby reducing
electron delocalization. The twisted backbone can also
weaken intermolecular interactions between the polymer
chains in the solid state which could explain the low
efficiencies observed for the T1:PCBM solar cells (0.15-
0.18%). Without thermal treatment of T1:PCBM devices
a typical Voc of 0.60 V was obtained. Upon heating the
device to 225 °C the Voc drops to 0.49 V and is reduced
further to 0.42 V when annealing at 265 °C. The current
density is enhanced from 1.06 mA/cm2 to 1.35 mA/cm2
after thermocleavage at the optimum temperature 265 °C.
The effect of thermocleavage is also observed in the inci-
dent photon to current efficiency (Figure 9a). For the
unannealed T1:[60]PCBM solar cells, IPCE is relatively
Figure 5. UV-vis absorption spectra of T1 and T2 in chloroform
solution.
Figure 6. (a) UV-vis absorption spectra of T1 (native and cleaved) and
(b) T2 (native and cleaved) in thin film before and after thermocleavage
for 1 min.
Table 1. GPC and Spectroscopic Data for Polymers T1 and T2
solution
max (nm)
film
polymer
Mw (g/mol)
PDI
R(λmax) (L/g cm)
λ
λonset (nm)
Eg (eV)
λmax (nm)
λ
onset (nm)
Eg (eV)
3
480
610
2.03
490a
456b
608
675a
715b
746
1.84a
1.73b
1.66
T1
T2
57700
41600
1.9
2.7
21
27
490
593
600
691
2.07
1.79
a Annealed at 300 °C for 1 min. b Annealed at 300 °C for 10 min.

5622 Chem. Mater., Vol. 22, No. 19, 2010
Helgesen et al.
Figure 7. Possible structure of one repeating unit of T1 (left) and T2 (right) with minimized energy. MM2 calculation with the software ChemBio3D Ultra
(Minimized energy to minimum rms Gradient of 0.100).
Figure 8. J-V characteristics of (a) T1:[60]PCBM solar cells and (b) T2:[60]PCBM solar cells measured under 100 mW/cm2 white light before and after a
thermal treatment.
Figure 9. IPCE of (a) T1:[60]PCBM solar cells and (b) T2:[60]PCBM solar cells before and after a thermal treatment.
Table 2. Photovoltaic Performance of Devices Based on Blends of Polymer
and PCBM
low with quantum efficiencies of about 5-10% in the
range 390-620 nm but after thermocleavage at 225-
265 °C the IPCE increases by up to 4%. The enhanced IPCE
after thermocleavage can be explained by partial recovery of
the conjugated backbone planarity (i.e., improved charge
carrier mobility).
Shifting the ester groups to the 4-position (T2) has a major
impact on the photovoltaic performance. Without thermal
treatment of the T2:[60]PCBM solar cells a typical Jsc of 5.68
mA/cm2 was obtained (Table 2). Upon heating the device to
225 °CtheJsc drops to 3.68 mA/cm2 and then increases again
to 4.58 mA/cm2 when annealing at 265 °C. The open-circuit
voltage and fill factor experience the same development
where you first observe a drop upon annealing at 225 °C
followed by an increase when annealing at 265 °C. This
results in a nearly retained power conversion efficiency (η =
1.24%) when T2 is thermocleaved at 265 °C compared to the
unannealed device based on the soluble precursor polymer
(η= 1.36%). The same effect of annealing is clearly visible in
thermal
treatmenta (°C)
Jsc
(mA/cm2)
polymer
Voc (V)
FF
η (%)
T1
T1
T1
T2
0.60
0.49
0.42
0.69
0.59
0.70
0.81
0.79
0.76
1.06
1.27
1.35
5.68
3.68
4.58
6.79
4.55
5.61
0.29
0.28
0.27
0.35
0.33
0.38
0.35
0.41
0.35
0.18
0.17
0.15
1.37
0.72
1.22
1.92
1.47
1.49
225
265
T2
225
265
T2
T2b
T2b
T2b
225
265
a Heated for 20-30 s. b Devices prepared with [70]PCBM.
the incident photon to current efficiency (Figure 9b). IPCE
lies over 25% (unannealed device) in the wavelength range
400-650 nm reaching a maximum of 36%. After annealing
at 225 °C the IPCE drops up to 15% but the initial IPCE is
nearly retained when annealing at 265 °C.

Article
Chem. Mater., Vol. 22, No. 19, 2010 5623
Figure 10. J-V characteristics of (a) T2:[70]PCBM solar cells measured under 100 mW/cm2 white light (b) IPCE of T2:[70]PCBM solar cells before and
after a thermal treatment.
Figure 11. AFM topography images (2 μm ꢀ 2 μm) of solar cells based on blends of [60]PCBM and (a) T1 unannealed, Sq = 0.3 nm, (b) T1 annealed at
225 °C, Sq = 0.3 nm, (c) T1 annealed at 265 °C, Sq = 0.3 nm, (d) T2 unannealed, Sq = 0.3 nm, (e) T2 annealed at 225 °C, Sq = 0.5 nm, (f) T2 annealed at
265 °C, Sq = 0.3 nm.
In an attempt to increase the photocurrent, T2 was also
tested in solar cells with [70]PCBM and the obtained J-V
curves and IPCE are depicted in Figure 10. [70]PCBM
and [60]PCBM has similar electrochemical properties,
but [70]PCBM absorbs more light because of its lower
symmetry that allow low energy transitions.31 The incor-
poration of [70]PCBM improved the performance which
is clearly reflected in the IPCE for the unannealed and
thermocleaved (265 °C) device which is now found to be
higher than 30% in the wavelength range 400-650 nm
reaching a maximum of 47% (Figure 10b). A relatively
low fill factor limits the power conversion efficiency to
1.49% (thermocleaved at 265 °C) and 1.92% (unannealed).
As for T1, T2 can be regarded as thermocleaved to the free
carboxylic acid when the device films are annealed at
200-230 °C. However, according to TGA (Figure 2b),
the decarboxylation of T2 start to happen around 265 °C
which is also the optimized temperature for thermocleavage
of the T2:PCBM solar cells. It is likely that thermocleavage
under the optimized conditions (265 °C) only leads to partial
transformation, where the final film presents chemistry
corresponding to both the free carboxylic acid and the
decarboxylated material (Figure 4) to varying degrees.
Annealing at higher temperatures around 300 °C involved
a drastic drop in the Voc and Jsc thereby reducing the
performance.
Morphology. The performance of bulk heterojunction
solar cells based on a mixture of donor and acceptor
material is known to be very dependent on the morphol-
ogy of the active layer.32-34 Different factors have been
shown to have an influence on the morphology including
the donor-acceptor ratio, solvent, and the annealing
(32) van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma,
C. W. T.; Sieval, A. B.; Hummelen, J. C.; Janssen, R. A. J. Adv.
Funct. Mater. 2004, 14(5), 425–434.
(33) Yang, X.; Loos, J. Macromolecules 2007, 40(5), 1353–1362.
(34) Barrau, S.; Andersson, V.; Zhang, F. L.; Masich, S.; Bijleveld, J.;
Andersson, M. R.; Inganas, O. Macromolecules 2009, 42(13), 4646–
4650.
(31) 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(29), 3371–3375.

5624 Chem. Mater., Vol. 22, No. 19, 2010
Helgesen et al.
temperature.26,35-37 Changes in the surface topography
of T1:PCBM and T2:PCBM device films annealed at
different temperatures, as measured by atomic force
microscopy (AFM), are shown in Figure 11. All films
are very smooth with an average surface roughness rang-
ing from 0.3 to 0.5 nm. Comparing the films before and
after thermocleavage reveals that only the T2:PCBM
films shows clear changes in the surface morphology.
The domain sizes increase to larger features when the film
is annealed at 225 °C (Figure 11e) that indicate extensive
phase segregation of the polymer and PCBM which has
also been observed in our earlier work.26 The extensive
phase segregation can possibly limit charge carrier gen-
eration and transport to the electrodes which might
explain the reduced current densities of the T2:PCBM
devices annealed at 225 °C. The unannealed film and the
film annealed at 265 °C (Figure 11d and 11f ) reveals a
rather uniform phase separation with relatively small
domain sizes indicating that thermal cleavage of the
tertiary esters at 265 °C takes place before any undesirable
change in morphology. Ideally, thermocleavage of the
ester groups forms a material with a high glass transition
temperature that can limit the possible migration and
segregation of the PCBM molecules leading to high
thermal stability of the photovoltaic characteristics.38
unit are thermocleavable alkyl ester groups which allow
processing of three chemically different thin films from the
same soluble precursor polymer through elimination and
decarboxylation. A significant effect is observed on the
optical properties and photovoltaic performance depend-
ing upon where the ester groups are attached on the DTBT
unit. Inchloroformsolutionthe polymershadopticalband
gaps ranging from 1.79 to 2.07 eV that are lowered to
1.66-2.03 eV in thin film (Table 1). The absorption
maximum (λmax) of T1 is blue-shifted over 100 nm com-
pared to that of T2 which indicates that incorporation of
the ester groups on the 3-positions of the thienyl groups
will lead to a more twisted conjugated backbone conse-
quently reducing electron delocalization. Furthermore, T1
shows a significant shift toward lower energies after an-
nealing the films at 300 °C which indicate that thermo-
cleavage of the bulky ester groups in the 3-positions of the
thienyl groups most likely improve the planarity of the
conjugated backbone.
The best performing polymer in a solar cell was T2
giving efficiencies up to 1.92% (unannealed) when mixed
with [70]PCBM, significantly higher than that of T1 (η =
0.18%). The greater performance of T2 suggests that
incorporation of the ester groups on the 4-positions of
the thienyl groups provide minimum steric hindrance
thereby improving electron delocalization and possibly
the charge carrier mobility. The temperature of thermo-
cleavage of the active layer films was optimized to 265 °C
where the T2:PCBM solar cells maintained a significant
performance giving efficiencies up to 1.49%.
Conclusion
Two new thermocleavable polymers based on di-2-
thienyl-2,1,3-benzothiadiazole (DTBT), bearing solubiliz-
ing chains on the thienyl groups, and 4,4-bis(2-ethylhexyl)-
4H-cyclopenta[1,2-b:5,4-b0]dithiophene (CPDT) have
been synthesized. The solubilizing chains on the DTBT
Acknowledgment. This work was supported by the Danish
Strategic Research Council (DSF 2104-05-0052 and 2104-
07-0022), the Danish National Research Foundation
(through support of inSPIN), and the Danish Biotechnolog-
ical Instrument Center (DABIC).
(35) Li, G.; Shrotriya, V.; Yao, Y.; Yang, Y. J. Appl. Phys. 2005, 98(4),
043704.
(36) Nilsson, S.; Bernasik, A.; Budkowski, A.; Moons, E. Macromole-
cules 2007, 40, 8291–8301.
(37) Martens, T.; D’Haen, J.; Munters, T.; Beelen, Z.; Goris, L.; Manca,
J.; D’Olieslaeger, M.; Vanderzande, D.; De Schepper, L.; Andriessen,
R. Synth. Met. 2003, 138(1-2), 243–247.
(38) Bertho, S.; Janssen, G.; Cleij, T. J.; Conings, B.; Moons, W.;
Gadisa, A.; D’Haen, J.; Goovaerts, E.; Lutsen, L.; Manca, J.;
Vanderzande, D. Sol. Energy Mater. Sol. Cells 2008, 92(7), 753–
760.
Supporting Information Available: General procedures and
characterization data including NMR spectra and FTIR spectra of
the polymers. Experimental procedures for the synthesis of the
monomers and polymers according to Schemes 1 and 2. This mate-
rial is available free of charge via the Internet at http://pubs.acs.org.