π-Conjugated gradient copolymers suppress phase separation and improve stability in bulk heterojunction solar cells

Article abstract of DOI:10.1039/c3tc32512a

Gradient sequence copolymers of 3-hexylthiophene (90 mol%) and 3-(6-bromohexyl)thiophene (10 mol%) were synthesized by catalyst transfer polycondensation. Post-polymerization conversion of the side-chain bromides into azides and subsequent Cu-catalyzed az

Full text of DOI:10.1039/c3tc32512a

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Materials Chemistry C
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p-Conjugated gradient copolymers suppress phase
separation and improve stability in bulk
heterojunction solar cells†
Cite this: J. Mater. Chem. C, 2014, 2,
3401
a
Edmund F. Palermo,a Seth B. Darlingbc and Anne J. McNeil
*
Gradient sequence copolymers of 3-hexylthiophene (90 mol%) and 3-(6-bromohexyl)thiophene (10 mol%)
were synthesized by catalyst transfer polycondensation. Post-polymerization conversion of the side-chain
bromides into azides and subsequent Cu-catalyzed azide–alkyne cycloaddition installed C60-functional
groups. Comparing blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester
(PCBM) with and without the gradient copolymer additive revealed that, when the gradient copolymer
was present, micron-scale phase separation was not observed even after prolonged thermal annealing
times. In addition, the PCBM was still able to quench the P3HT emission after thermal annealing,
indicating that the donor–acceptor interfacial area is maintained. Together, these data suggest that the
gradient copolymers are an effective compatibilizer for P3HT/PCBM physical blends. This stabilized film
morphology led to stable power conversion efficiencies (PCE) of the corresponding bulk heterojunction
solar cells even upon extended thermal annealing. Nevertheless, the short circuit current and fill factor
were reduced when the gradient copolymer was present, leading to a lower PCE. Overall, these gradient
copolymer additives represent a promising tool for inhibiting micron-scale phase separation and
producing robust polymer/fullerene-based solar cells.
Received 19th December 2013
Accepted 13th February 2014
DOI: 10.1039/c3tc32512a
www.rsc.org/MaterialsC
dissociation efficiency. As
a consequence, the long-term
Introduction
stability and performance of the solar cell is compromised.3,4
Stabilizing physical blends of organic materials has been
extensively studied and to some extent, solved for amorphous
homopolymers. Although chemically dissimilar homopolymers
naturally undergo phase separation in physical blends because
the enthalpic penalty for mixing outweighs the entropic gains,5
additives that localize at the interface can stabilize blends
against this phase separation. For example, Torkelson and co-
workers have demonstrated that gradient copolymers, which
exhibit a gradual change in composition along the normalized
chain length, localize at the interfaces in physical blends of the
two corresponding homopolymers.6,7 As a consequence, these
additives broaden the interfacial width, lower the interfacial
tension and suppress phase separation. Block copolymers have
also been used as additives to reduce phase separation in
polymer blends,8 but are ultimately limited by their tendency to
form micelles.6,7 Because gradient copolymers resist micelle
formation until reaching concentrations ten-fold greater than
their block copolymer counterparts,9 gradient copolymers may
be a promising approach for compatibilizing polymer/fullerene
blends.10,11
Due to a unique combination of semiconducting properties
with mechanical exibility, organic p-conjugated small mole-
cules and polymers are being utilized as the active layers in solar
cells.1 One of the most extensively studied systems is the
physical blend of poly(3-hexylthiophene) (P3HT, the electron
donor) and phenyl-C61-butyric acid methyl ester (PCBM, the
electron acceptor).2 In these bulk heterojunction (BHJ) devices,
the two materials initially undergo nanoscale phase separation
during solution processing, and this phase separation is further
evolved through solvent vapor or thermal annealing.3,4 This
nanoscale morphology provides extensive donor–acceptor
interfaces, which promote exciton dissociation. Upon extended
aging or thermal annealing, however, phase separation
continues, generating micron-scale domains, which dramati-
cally reduces the interfacial area and thus reduces charge
aDepartment of Chemistry and Macromolecular Science and Engineering Program,
University of Michigan, 930 North University Avenue, Ann Arbor, MI 48109-1055,
USA. E-mail: ajmcneil@umich.edu
bCenter for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue,
Using catalyst transfer polycondensation (CTP)12 we synthe-
sized a gradient copolymer containing P3HT and PCBM
wherein the side chain composition gradually varies from one
chain end to the other (Scheme 1).13 The side chains varied from
C6H13 to C6H12Br. We then converted the Br moieties to a PCBM
Argonne, IL 60439, USA
cInstitute for Molecular Engineering, The University of Chicago, Chicago, Illinois
60637, USA
† Electronic supplementary information (ESI) available: Materials and methods,
synthetic procedures, characterization data, and device fabrication. See DOI:
10.1039/c3tc32512a
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Scheme 1
derivative to access the targeted donor/acceptor gradient
copolymer architecture. The impact of these materials on the
phase separation, photoluminescence, and current–voltage
characteristics of P3HT/PCBM devices is demonstrated. The
gradient copolymer was found to compatibilize the P3HT/PCBM
blend and enhance the thermal stability. We conclude that
gradient copolymer additives represent an effective tool for
inhibiting micron-scale phase separation in physical blends of
polymers with fullerenes, which can lead to improved long-term
stability of the solar cell.
Experimental
Detailed experimental procedures and full characterization data
can be found in the ESI.†
Results and discussion
Polymer synthesis
Fig. 1 Plot of the cumulative mole fraction of 3BrHT in the copolymer
as a function of the normalized chain length.
The gradient copolymers described herein were prepared using
a living, chain-growth polymerization method known as catalyst copolymer (P1) exhibited a number-average molecular weight
transfer polycondensation.12 Depending on the (co)monomer (Mn ¼ 23 kDa) in agreement with the theoretical value (18 kDa),
and catalyst structures, the CTP method can afford materials narrow MW dispersity (Đ ¼ 1.17), and high regioregularity
with dened molecular weights (MW), low dispersities (Đ), high (>99%) (Fig. 2). The cumulative mole fraction of the 3BrHT
regioregularities (RR), and specic end groups. Herein, a comonomer was 0.10 at the end of the polymerization, which
gradient copolymer composed of 3-hexylthiophene (3HT, 90 matched the feed composition. Attempts at higher mol%
mol%) and 3-(1-bromohexylthiophene) (3BrHT, 10 mol%) was incorporation led to extensive cross-linking during the subse-
prepared according to our previously reported procedure quent functionalization reactions, as described in detail below.
(Scheme 1 and ESI†).13a The key to obtaining a gradient Nevertheless, 10 mol% of 3BrHT corresponds to 35 wt% PCBM
copolymer is the gradual, syringe pump addition of monomer 2 in the nal copolymer, which is within the range of typical
to a solution containing both monomer 1 and catalyst 3. 1H compositions of BHJ solar cells.2
NMR spectroscopic analysis of the aliquots withdrawn during
Two post-polymerization functionalization reactions were
the polymerization showed that the copolymer composition utilized to convert the P3HT/P3BrHT gradient copolymer (P1)
varied linearly with normalized chain length (as determined by into the target fullerene-functionalized copolymer (P3)
gel permeation chromatography (GPC)), conrming a gradient (Scheme 1). The side-chain bromides were rst converted to
sequence. (See Fig. 1 for a comparison of the gradient sequence azides using NaN3.14 1H NMR spectroscopic analysis conrmed
distribution relative to the analogous random and block quantitative conversion to the azides (Fig. 2). As expected, the
copolymer with the same composition.) The obtained gradient resulting copolymer (P2) showed little change in its GPC prole
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Fig. 3 Optical microscope images of a P3HT/PCBM blend after
annealing at 150 ꢁC for 1 h with (A) 0 wt%, (B) 1 wt% and (C) 10 wt%
gradient copolymer additive (P3). The scale bar represents 25 mm.
separation. Indeed, when the gradient copolymer was included
in the blend at 1 wt%, the size and density of PCBM needle-
shaped aggregates was markedly reduced (Fig. 3B). When
10 wt% gradient copolymer was included, no evidence of
micron-scale phase separation was observed (Fig. 3C). These
results are consistent with the notion that the gradient
copolymer serves as a compatibilizer for the blend. In principle,
this thermal stability should translate to increased longevity in
the bulk heterojunction-based solar cell (vide infra).
1
Fig. 2 (Left) Selected regions of the H NMR spectra for polymers P1
(pendant bromide), P2 (pendant azide), and P3 (pendant fullerene
derivative) showing complete conversion at each synthetic step.
(Right) Gel permeation chromatography data for polymers P1, P2, and
P3 after each synthetic transformation.
aer the post-polymerization reaction (Fig. 2). In a separate
step, the methyl ester of commercial PCBM was converted to the
corresponding alkynyl ester (4) via hydrolysis and esterication
using undec-10-yn-1-ol.
Photoluminescence quenching
To provide further evidence that the gradient copolymer addi-
tive is an effective tool to inhibit phase separation, we measured
the steady-state photoluminescence (PL) intensity of P3HT lms
and compared it to the P3HT/PCBM blends with and without
gradient copolymer additive. Contour maps were generated to
describe the thin lm PL intensity as a function of the excitation
and emission wavelengths (Fig. 4). As expected, the PL intensity
map for the pristine P3HT lm were similar aer annealing for
10 min versus 60 min at 150 ꢁC (Fig. 4A and B). In contrast, the
P3HT/PCBM blend initially showed signicant PL intensity
quenching due to efficient charge transfer from the donor P3HT
to the acceptor PCBM (Fig. 4C).18 This result suggests that these
two materials are intimately mixed at the nanoscale, which
enables efficient charge transfer at the interface. Upon pro-
longed thermal annealing of this blend, however, the relative PL
intensity increased (Fig. 4D). This decrease in quenching effi-
ciency by PCBM is presumably due to the micron-scale phase
separation that we observed in the optical microscope, which
will create regions of phase-pure P3HT that are larger than the
exciton diffusion limit. As a consequence, those excitons can be
emissive, leading to enhanced PL intensity.
Azide-functionalized gradient copolymer P2 was then reac-
ted with 4 using copper-catalyzed azide–alkyne cycloaddition
(CuAAC) to obtain P3. This reaction required some optimization
to avoid extensive polymer cross-linking. The recommendations
of Hashimoto and co-workers15 that were necessary included
keeping the reaction mixture strictly air-free, rigorously puri-
fying the solvent and ligand, and using an excess of 4. In
addition, we found that a longer linker between the alkyne and
fullerene lowered the prevalence of cross-linking and afforded
materials with higher solubility. Using these optimized condi-
tions, minimal broadening of the molecular weight distribution
1
was observed by GPC (Đ ¼ 1.24)16 and H NMR spectroscopic
analysis conrmed the complete conversion of the side chain
azides to triazoles (Fig. 2). As mentioned earlier, attempts to
functionalize copolymers with higher mol% 3BrHT were
unsuccessful due to polymer cross-linking (ESI†). With
successful preparation of gradient copolymer P3, we proceeded
to measure the impact of this additive on physical blends of
P3HT and PCBM.
The blend containing P3HT/PCBM and 10 wt% gradient
copolymer additive exhibited marked PL quenching even aer
prolonged thermal annealing (Fig. 4E and F). This result, in
combination with the optical characterization data, strongly
suggests that gradient copolymers are effective compatibilizers
for the P3HT/PCBM blend. Based on these data, we anticipated
that gradient copolymer additives would improve the thermal
stability of the corresponding bulk heterojunction solar cells.
Suppressed phase separation
One of the prevailing hypotheses about the decrease in power
conversion efficiencies upon prolonged thermal annealing for
P3HT/PCBM-based solar cells is that the materials undergo
micron-scale phase separation, which reduces both the inter-
facial area and the charge dissociation efficiency.3,4,17 We
conrmed the formation of needle-shaped PCBM aggregates
(ꢀ5–30 mm in length and ꢀ1 mm in width) in the conventional
P3HT/PCBM blend aer thermal annealing (150 ꢁC for 1 h,
Bulk heterojunction device performance
Fig. 3A). Based on literature precedent,6,7 we hypothesized that Photovoltaic devices were prepared with P3HT/PCBM with or
gradient copolymer additives might suppress this phase without gradient copolymer additive as the active layer. The
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Fig. 5 Current–voltage data for P3HT/PCBM blends with 0 wt% (black
solid line), 1 wt% (blue dash-dot line), or 10 wt% (red dashed line)
gradient copolymer additive (P3). The plots correspond to the devices
(A) as cast and after annealing at 150 ꢁC for (B) 10 min, (C) 30 min and
(D) 60 min. The active area was 0.049 cm3.
Fig. 4 Contour maps of PL intensity as a function of the emission and
excitation wavelengths for (A and B) P3HT; (C and D) P3HT/PCBM; (E
and F) P3HT/PCBM + 10 wt% gradient copolymer (P3). Each film was
annealed at 150 ꢁC for either 10 min (A, C and E) or 60 min (B, D and F).
recombination at the electrodes, blocking of the interfacial
buffer layers, accumulation of charge-trapped states, or imbal-
ance of the relative electron and hole mobilities in the active
The color represents the relative PL intensity with red as the maximum. layer.19 Although the gradient copolymers prevent micron-scale
phase separation, they may also inhibit the growth of pure
P3HT brils, which would restrict hole transport and, as a
device architecture was glass/ITO/PEDOT:PSS/active layer/Ca/Al. result, increase the likelihood of non-geminate recombina-
Detailed descriptions of the fabrication procedure and overall tion.20 Indeed, the P3HT/PCBM blend exhibited some modest
device conguration can be found in the ESI.† To obtain changes in the solid-state organization, as evidenced by powder
statistically signicant results, each thin lm composition was X-ray diffraction and differential scanning calorimetry, when
tested twice on different regions of the same sample (to account the gradient copolymer was added (ESI†). The origin of the J–V
for variation within a sample) and three unique samples of each curve distortion is outside the scope of this work, but will be a
composition were prepared and tested, giving a total of six focus of our future efforts. Excitingly, the blend of P3HT/PCBM
measurements.
with 1 wt% gradient copolymer additive exhibited an improved
The conventional devices comprised of a P3HT/PCBM active J–V curve shape relative to the sample with 10 wt% gradient, and
layer without any copolymer additive showed typical current– also maintained superior thermal stability relative to the
voltage characteristics that improved aer brief (10 min) conventional devices.
thermal annealing (Fig. 5 A and B). Annealing for longer time,
Plots of the PCE, ll factor (FF), open circuit voltage (Voc),
however, diminished the short circuit current density (Jsc) and and short circuit current density (Jsc) versus annealing time for
resulted in lower overall PCE (Fig. 5C and D). The most all samples are highlighted in Fig. 6. While the 10 wt% gradient
straightforward explanation is that the charge dissociation copolymer additive effectively suppressed phase separation in
efficiency has been reduced due to the changes in the donor/ the blends, it also signicantly reduced the device efficiency. On
acceptor interfaces during phase separation. This conclusion is the other hand, using 1 wt% gradient copolymer led to
supported by the observed PCBM crystallite formation with a reasonably improved PCEs relative to the devices containing 10
concomitant decrease in PL quenching.
wt% gradient, while still maintaining the enhanced thermal
In contrast, the blend of P3HT/PCBM with 10 wt% gradient stability. For comparison, the blends without gradient copoly-
copolymer additive showed dramatically different current– mer resulted in decreasing PCEs aer annealing for longer than
voltage data (Fig. 5). The observed “S-shaped” curves are oen 10 min. When the loading was further reduced to 0.1 wt%
ascribed to charge build-up in the device, which opposes the gradient copolymer, the devices performed similarly to the
ow of electrons from the anode to the cathode. There are blends without gradient copolymer (ESI†). Hence, it appears
several possible reasons for this behavior, including restricted that the lower limit for device stabilization is between 0.1 and 1
3404 | J. Mater. Chem. C, 2014, 2, 3401–3406
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improved thermal stability in the corresponding bulk hetero-
junction solar cells. Nevertheless, the power conversion effi-
ciency was reduced when the gradient copolymer was present,
and this result was attributed to a reduction in phase-pure
P3HT brils, which would hinder hole transport.
Future efforts will therefore focus on improving the device
performance. Specically, we will identify the ideal gradient
copolymer structure by varying the gradient strength, copoly-
mer composition, and molecular weight, and elucidating their
impact on blend morphology, thermal stability, and device
performance. We also plan to apply the gradient copolymer
strategy to other polymer/fullerene-based devices where the
performance is not as sensitive to the phase purity of the
polymer as P3HT. In addition, extending the principles gleaned
herein to donor–acceptor low band-gap copolymers with opti-
mized HOMO/LUMO levels will advance our understanding of
the complex interplay between mixing and de-mixing, with the
attendant effects on performance, in bulk heterojunction
devices.
Fig. 6 Plots of the (A) power conversion efficiency (PCE), (B) fill factor
(FF), (C) open circuit voltage (Voc), and (D) short circuit current density
(Jsc) as a function of annealing time for P3HT/PCBM blends with 0 (red
circles), 1 (black triangles), and 10 (blue squares) wt% gradient
copolymer additive (P3).
Acknowledgements
We thank the Army Research Office (ARO 58200-CH-PCS) for
support of this work. Use of the Center for Nanoscale Materials
at Argonne National Laboratory was supported by the U. S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. DE-AC02-06CH11357. A. J. M.
thanks the Camille and Henry Dreyfus Foundation for a
research fellowship. We also gratefully acknowledge Dr Chun-
Chih Ho, Dr Matthew Pelton and Dr David Gosztola for assis-
tance with device fabrication testing and PL measurements.
wt% gradient copolymer. This range is comparable to other
processing additives used in organic solar cells, such as diio-
dooctane and octanedithiol.21
Overall, these results provide strong evidence that gradient
copolymers can effectively stabilize the morphology of bulk
heterojunction active layers and enhance their thermal stability,
albeit with a reduced PCE. As such, these gradient copolymers
are comparable (and in some cases, better) than other copoly-
mer additives containing pendant fullerenes.22 For example,
both Jo and co-workers23a and Hadziioannou and co-workers23b
reported modest improvements in thermal stability with a
fullerene-containing copolymer additive, however the PCEs
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´
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Herein, a novel gradient sequence p-conjugated copolymer with
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PCBM blends. As a consequence, these materials afforded
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