Efficiency enhancement of P3HT/PCBM bulk heterojunction solar cells by attaching zinc phthalocyanine to the chain-end of P3HT

Article abstract of DOI:10.1039/c1jm11563d

A new solution processable zinc phthalocyanine dye (ZnPc), as an interface modifier between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C ₆₁-butyric acid methyl ester (PCBM) in bulk heterojunction solar cells, was successively synthesized an

Full text of DOI:10.1039/c1jm11563d

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PAPER
Efficiency enhancement of P3HT/PCBM bulk heterojunction solar cells by
attaching zinc phthalocyanine to the chain-end of P3HT†
a
b
b
b
Jea Uk Lee,‡ Young Do Kim,‡ Jea Woong Jo, Jae Pil Kim and Won Ho Jo
b
*
Received 12th April 2011, Accepted 19th August 2011
DOI: 10.1039/c1jm11563d
A new solution processable zinc phthalocyanine dye (ZnPc), as an interface modifier between poly(3-
hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in bulk heterojunction
solar cells, was successively synthesized and linked to the chain-end of P3HT through the formation of
a coordination complex. ZnPc dye molecules do not aggregate but preferentially locate at the interface
between P3HT and PCBM, and thus contribute to the photocurrent generation by both direct photo-
excitation and enhancement of charge transfer between P3HT and PCBM. To localize the zinc
phthalocyanine dyes at the donor–acceptor interface more effectively, another new organic dye
molecule, fullerene-functionalized zinc phthalocyanine (ZnPc-C60) was also synthesized and linked to
the chain-end of the P3HT, where ZnPc-C60 contributes not only to the photocurrent generation by
direct photo-excitation, but also lowers the interfacial tension, resulting in the reduction of the domain
size and the suppression of the macrophase separation of the P3HT/PCBM blend for prolonged
thermal annealing. This leads to higher device efficiency with 20% enhancement of the short circuit
current and to enhancement of long-term thermal stability of device performance as compared to that
of the reference P3HT/PCBM device.
circuit voltage (VOC) of each subcell, there have been few
Introduction
successful reports on tandem solar cells with high efficiency
because the overall current is limited by the subcell that delivers
the smaller photocurrent.11 Another avenue to achieve high
efficiency is to develop donor–acceptor type low-bandgap poly-
mers which have broader absorption range and consequently
harvest solar light more efficiently than P3HT.12 However, it is
not always easy to synthesize low-bandgap conjugated polymers
with high charge carrier mobility.13
Conjugated polymer-based solar cells have widely been studied
as low-cost alternatives for renewable energy sources. Particu-
larly, the polymer solar cell with bulk heterojunction (BHJ)
structure based on poly(3-hexylthiophene) (P3HT) and [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) has intensively
been investigated and almost reached the optimum device
performance: power conversion efficiencies (PCEs) of 4–5% and
an incident photon-to-current conversion efficiency (IPCE) of
over 70% under AM 1.5 (AM ¼ air mass) illumination.1–5
In recent years, new device structures6,7 and novel active layer
materials based on low-bandgap polymers8–10 have been devel-
oped to further enhance the performance of polymer solar cells.
Although tandem architectures, in which two subcells with
complementary absorption bands are connected in series
through a recombination zone, lead to summation of the open-
A recent approach for improving the device performance of
polymer solar cells is to employ the dye sensitization method,
modifying the interface between the conjugated polymer and
electron acceptor material in BHJ solar cells. A previous work
has demonstrated that the addition of a small molecule chro-
mophore with an energy level intermediate between P3HT and
fullerene extends the absorption of P3HT to longer wavelength,
and as a result improves the photocurrent of P3HT/PCBM solar
cells.14–16 Very recently, other research groups have also reported
that dye molecules can improve the photocurrent significantly by
promoting charge separation and strongly suppressing back
recombination at the donor–acceptor interface.17–19 However,
the PCEs have not been enhanced. For the enhancement of the
PCE, the following prerequisites should be taken into account
for successful application of dye molecules as the interface
modifier in BHJ solar cells. First, to effectively mediate charge
transfer between the donor and acceptor, the energy level of the
dye molecule must be located at the intermediate between those
aComposite Materials Research Group, Korea Institute of Materials
Science, Changwon, Gyeongnam, 642-831, Korea
bWCU Hybrid Materials Program and Department of Materials Science
and Engineering, Seoul National University, Seoul, 151-742, Korea.
E-mail: whjpoly@snu.ac.kr; Fax: +82-2-876-6086; Tel: +82-2-880-7192
† Electronic supplementary information (ESI) available: Cyclic
voltammetry data, 1H NMR spectrum, XRD patterns, average
photovoltaic parameters and deviations, and TEM image. See DOI:
10.1039/c1jm11563d
‡ These authors contributed equally to this work.
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of the donor and acceptor materials: the lowest unoccupied
molecular orbital (LUMO) level of the organic dye should be
lower than the donor LUMO level and higher than the acceptor
LUMO level; the HOMO level of the dye should be lower than
the HOMO level of the donor.20 Second, dye molecules should
not aggregate in the blend film. It has been generally accepted
that dye molecules with a planar structure easily aggregate and
consequently deteriorate the device performance by reducing the
absorption efficiency and the charge mobility of the active layer,
when blended as the third component of the active layer.18 Third,
dye molecules should be located at the donor–acceptor interface.
In this study, for the ideal interface modifier to be realized,
a novel material, tetra-tert-butyl zinc(II) phthalocyanine-end
capped P3HT (P3HT-ZnPc) has been synthesized and used as an
interface modifier in BHJ solar cells. Solution processable tetra-
tert-butyl zinc(II) phthalocyanine (ZnPc), which has widely been
used for dye sensitization, was linked to the chain-end of the
pyridyl-terminated P3HT (P3HT-Py) through coordination of
the zinc atom of phthalocyanine to the pyridyl group of P3HT-
Py. Since some experimental and theoretical studies have
demonstrated that the end group of the polymer chain prefer-
entially migrates to the polymer–polymer interface in immiscible
polymer blends,21 it is easily expected that the phthalocyanine
dyes bound to the P3HT chain-end are located at the interface
between P3HT and PCBM. Furthermore, to more effectively
localize the zinc phthalocyanine dyes at the donor–acceptor
interface, another new organic dye molecule, fullerene-func-
tionalized ZnPc, (ZnPc-C60) has been synthesized and also linked
to the chain-end of the P3HT-Py. Since the ZnPc-C60 molecule
has better compatibility with PCBM than ZnPc, it is fully
expected that the P3HT-ZnPc-C60 complex is more effective for
modification of the interface between P3HT and PCBM. In this
paper, we report the synthesis of dye-end capped P3HTs,
together with an extensive investigation on their optical prop-
erties, crystal structures, and their application to BHJ solar cells.
should be noted here that an amide bond formed between
fullerene and ZnPc breaks the conjugation between ZnPc and
fullerene, and thus the absorption and the energy levels of ZnPc
and fullerene remain intact after coupling.
The chemical structure of ZnPc-C60 is identified by comparing
FT-IR spectra of ZnPc, ZnPc-NH2, and ZnPc-C60, as shown in
Fig. 1. The characteristic absorption bands of ZnPc-NH2 are
observed at 3366 cmꢀ1 and 3221 cmꢀ1 (N–H stretch of primary
amine), 3072 cmꢀ1 (aromatic C–H stretch), 2950–2850 cmꢀ1
(aliphatic C–H stretch), and 1612 cmꢀ1 (N–H bend), while the
FT-IR spectrum of ZnPc-C60 shows the appearance of a single
N–H stretching vibration and C]O stretching vibration of
amide bond at 3376 cmꢀ1 and 1661 cmꢀ1, respectively. The
appearance of a sharp peak at 528 cmꢀ1, which is characteristic
for fullerene, also confirms that PCBA is linked covalently to
ZnPc.
As another evidence for successful synthesis of phthalocya-
nines, matrix-assisted laser desorption ionization time of flight
(MALDI-TOF) mass spectroscopy and elementary analysis were
carried out. Fig. 2 reveals that the mass spectra of all phthalo-
cyanines match well with the calculated molecular weights.
A new pyridyl-terminated P3HT was synthesized through the
controlled Grignard metathesis polymerization of 2,5-dibromo-
3-hexylthiophene and subsequent post-polymerization func-
tionalization (Scheme 2). Hydroxypropyl-terminated P3HT
(P3HT-OH) was first synthesized according to the procedure
reported in the literature,24–26 and then combined with iso-
nicotinic acid by using the carbodiimide-activated Steglich
esterification method to connect the pyridine unit to the chain-
end of the P3HT. The Steglich esterification is one of the most
convenient and effective methods for end-group modification of
polymer chains, because the reaction condition is mild never-
theless the conversion is very high due to the favorable catalytic
action of dimethylaminopyridine.27,28 The pyridyl-terminated
P3HT was then reacted with ZnPc or ZnPc-C60 dye in refluxing
anhydrous THF, affording the corresponding complexes in
which the pyridyl end group forms a ligand with Zn in the axial
direction of phthalocyanine dyes (Scheme 2). Since the solubility
of phthalocyanine dyes with tert-butyl groups is much better
than P3HT, the final complex materials are readily purified by
filtering and washing the unreacted dye molecules with MeOH
and dichloromethane after the reaction. The final products,
P3HT-ZnPc and P3HT-ZnPc-C60, are soluble in various organic
solvents such as THF, toluene, chloroform, and 1,2-
dichlorobenzene.
Results and discussion
The synthetic routes for preparation of ZnPc and ZnPc-C60 are
shown in Scheme 1. Metal phthalocyanine (Pc) dyes such as
CuPc, ZnPc, FePc, CoPc, and NiPc have well been recognized as
electron donor materials (or p-type semiconductors) in organic
solar cells. The central metals in these phthalocyanines are
transition metals, with different electrochemical properties
derived from the 3d orbitals. Among these metal phthalocya-
nines, ZnPc was selected in this study because of its broader
absorption range and higher open circuit voltage than other
metal phthalocyanines when it is used as an electron donor
material in BHJ solar cells.22,23 Furthermore, the energy levels of
the solution processable ZnPc (LUMO ¼ ꢀ3.8 eV, HOMO ¼
ꢀ4.9 eV) are located at the intermediate between those of P3HT
(LUMO ¼ ꢀ3.2 eV, HOMO ¼ ꢀ4.7 eV) and PCBM (LUMO ¼
ꢀ4.0 eV, HOMO ¼ ꢀ6.0 eV) (see ESI†). For the purpose of
further enhancing the probability for ZnPc dyes to locate at the
interface between the P3HT and PCBM domain, fullerene-
modified ZnPc was also synthesized, as represented in Scheme 1,
where tri-tert-butyl-amino zinc(II) phthalocyanine (ZnPc-NH2)
was first synthesized and then coupled with carboxylate-func-
tionalized fullerene, [6,6]-phenyl-C61-butyric acid (PCBA). It
The formation of pyridyl-terminated P3HT and polymer-dye
complexes was identified by 1H NMR. The down-field shift of the
methylene proton signal adjacent to the hydroxyl group of
P3HT-OH from 3.75 to 4.2 ppm and the appearance of two
aromatic proton signals at 7.8 and 8.8 ppm due to the pyridine
group provide a solid evidence that P3HT-Py was successfully
synthesized through the esterification reaction between P3HT-
OH and isonicotinic acid (see ESI†). When P3HT-Py is reacted
with ZnPc, the 1H NMR spectrum of the product unambiguously
exhibits further down-field shift of the two proton signals of the
pyridine group in P3HT-Py to 8.3 and 9.5 ppm, respectively,
confirming the formation of a coordination complex between
P3HT-Py and ZnPc (Fig. 3). The 1H NMR spectrum of the
P3HT-ZnPc-C60 complex also shows both the down-field shift of
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Scheme 1 Synthesis of ZnPc and ZnPc-C60 dyes.
Fig. 2 MALDI-TOF MS spectra of ZnPc (black line), ZnPc-NH2 (red
line), and ZnPc-C60 (blue line).
Fig. 1 FT-IR spectra of ZnPc, ZnPc-NH2, and ZnPc-C60
.
P3HT-Py has a very small shoulder at shorter elution times. This
is due to the chain coupling that may occur during the end-group
modification of P3HT, because the thiophene-bromine bond at
the opposite chain-end is active in various side reactions.29
When we measured the molecular weights of chain-end
modified P3HTs using MALDI-TOF MS, it showed that the
average number of the molecular weights of P3HT-Py, P3HT-
ZnPc, and P3HT-ZnPc-C60 were 17 250, 18 020, and 18 890 g
molꢀ1, respectively, as shown in Fig. 5. The molecular weight
differences between these polymers (770 and 870 g molꢀ1) are well
matched with the molecular weight of ZnPc and PCBA, clearly
indicating that most of the P3HT chain-ends are functionalized
by the ZnPc and ZnPc-C60 dyes.
the pyridine signals and the appearance of new proton signals at
d ¼ 7.5–8 ppm due to the phenyl hydrogens of PCBA.
The formation of P3HT-dye complexes was further verified by
using gel permeation chromatography (GPC) (Fig. 4). P3HT
synthesized in this study has a reasonable molecular weight for
the application of an electron donor in BHJ solar cells, with
a narrow molecular weight distribution (Mn ¼ 24 000, Mw/Mn ¼
1.15). After P3HT was reacted with ZnPc and ZnPc-C60, the
GPC traces slightly shifted to higher molecular weights (Mn ¼
25 000 for P3HT-ZnPc and Mn ¼ 27 000 for P3HT-ZnPc-C60),
indicating that the ZnPc and ZnPc-C60 dyes were linked to the
P3HT chain. It should be mentioned here that the GPC trace of
Fig. 6 shows the UV-Vis absorption spectra of the dye mole-
cules, P3HT-ZnPc, and P3HT-ZnPc-C60 in THF solution. The
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Scheme 2 Synthesis of P3HT-Py, P3HT-ZnPc, and P3HT-ZnPc-C60. DCC: 1,3-dicylohexylcarbodi-imide; DMAP: dimethylaminopyridine.
polymer-dye complexes absorb at 300–550 nm and 600–700 nm,
corresponding to the typical absorption range of P3HT and
phthalocyanine, respectively. A small red-shift of the maximum
absorption wavelength (lmax) of the dye molecules was observed,
as the fullerene derivative was covalently bonded to a side arm of
the phthalocyanine dye. When the amount of dye molecules
bound at the polymer chain-end is estimated by comparing the
absorbance of P3HT with that of the dye molecule, it reveals that
92% and 86% of the chain-ends of P3HT are modified by ZnPc
and ZnPc-C60, respectively. However, when the same amount of
ZnPc dye as that in P3HT-ZnPc is simply mixed with P3HT, the
simple mixture (P3HT + ZnPc) does not absorb at the ZnPc
absorption range, probably because the dye molecules are
aggregated in the mixture.
modified by phthalocyanine dyes. However, when the ZnPc dyes
are simply mixed with P3HT/PCBM, the blend film exhibits
considerably lower crystallinity than the others, indicating that
ZnPc dye molecules in the simple mixture of P3HT + ZnPc/
PCBM disturb the crystallization of P3HT, presumably due to
the formation of dye aggregation in the blend film.
Table 1 shows the average photovoltaic parameters and their
deviations of BHJ devices fabricated from four different blends,
where the average power conversion efficiency of each blend
sample was evaluated from the values of 12 devices. The repre-
sentative current density-voltage (J-V) curves were also
compared in Fig. 8. The reference device of P3HT/PCBM has
VOC ¼ 0.56 V, JSC ¼ 9.23 mA cmꢀ2, and FF ¼ 0.61, yielding the
PCE of 3.15%, which is slightly lower than the literature value.
This lower PCE value can be explained as follows: the high
crystallinity of P3HT synthesized in our laboratory may raise the
HOMO level (ꢀ4.7 eV) due to increased inter-chain orbital
delocalization (see ESI†),30 and therefore the rise of the HOMO
To investigate the effect of ZnPc dye attached at the chain-end
of P3HT on the film morphology and the device performance,
three blend films of P3HT/PCBM, P3HT-ZnPc/PCBM, and
P3HT-ZnPc-C60/PCBM were prepared. For comparison,
another film was also prepared by simple mixing of P3HT/PCBM
with the same amount of ZnPc dyes as that in P3HT-ZnPc
(hereafter denoted by P3HT + ZnPc/PCBM). When the X-ray
diffraction (XRD) patterns of four blend films are compared
after thermal annealing at 150 ꢁC for 5 min, as shown in Fig. 7,
the P3HT/PCBM blend film shows a very sharp and intense (100)
reflection peak at 2q ¼ 5.4ꢁ corresponding to the chain-chain
interlayer distance, and weak (200) and (300) reflection peaks at
2q ¼ 10.7 and 15.9ꢁ, respectively. Although the (100) peak
intensities are somewhat weaker than that of the P3HT/PCBM
film, P3HT-ZnPc/PCBM and P3HT-ZnPc-C60/PCBM blend
films still show a strong (100) reflection peak, implying that the
P3HT chains crystallize even after the chain-end of P3HT is
of P3HT results in a decrease of VOC
.
After introducing ZnPc and ZnPc-C60 to the chain-end of
P3HT, the JSC of both devices was substantially increased, while
the fill factor was nearly unchanged, leading to PCEs of 3.28 and
3.56% for the P3HT-ZnPc/PCBM and P3HT-ZnPc-C60/PCBM
devices, respectively. It should be noted here that the JSC of the
P3HT-ZnPc-C60/PCBM device was increased by ca. 20% in
comparison with the reference device of P3HT/PCBM. On the
other hand, the device fabricated by simple mixing with ZnPc
exhibited a much lower PCE of 2.25%.
The IPCEs of the P3HT-dye/PCBM devices were measured and
compared with that of the P3HT/PCBM device, as shown in
Fig. 9. Both the P3HT-ZnPc/PCBM and P3HT-ZnPc-C60/PCBM
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Fig. 4 GPC traces of P3HT-Py (black line), P3HT-ZnPc (red line), and
P3HT-ZnPc-C60 (blue line).
Fig. 3 1H NMR spectra of (a) P3HT-ZnPc and (b) P3HT-ZnPc-C60
complexes.
devices exhibit an additional photocurrent at the ZnPc absorption
range (650–800 nm), indicating that dye molecules attached at the
polymer chain-end effectively absorb the incident light and
contribute to the photocurrent generation. Furthermore, the
IPCE value at 520 nm corresponding to the P3HT absorption also
increases up to 66% and 74% for the P3HT-ZnPc/PCBM and
P3HT-ZnPc-C60/PCBM device, respectively, from 62% for the
reference P3HT/PCBM device, suggesting that the phthalocya-
nine dye promotes charge generation from excitons in P3HT,31
whereas the P3HT + ZnPc/PCBM device shows a low IPCE at the
P3HT absorption range and very small signal at the ZnPc
absorption range, because the ZnPc dyes are easily aggregated in
the simple blend of P3HT/PCBM.
Fig. 5 MALDI-TOF MS spectra of P3HT-Py, P3HT-ZnPc, and P3HT-
ZnPc-C60
.
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Fig. 8 Typical J-V curves of P3HT/PCBM (black line), P3HT + ZnPc/
Fig. 6 UV-Visible absorption spectra of ZnPc (red dashed line), ZnPc-
60 (blue dashed line), P3HT-ZnPc (red line), P3HT-ZnPc-C60 (blue line),
PCBM (green line), P3HT-ZnPc/PCBM (red line), and P3HT-ZnPc-C60
/
C
PCBM (blue line) BHJ devices (100 mW cmꢀ2, AM 1.5 G). All the devices
were thermally annealed at 150 ꢁC for 5 min.
and P3HT + ZnPc (green line) in THF solution.
Fig. 9 IPCE spectra of P3HT/PCBM (black circle), P3HT + ZnPc/
PCBM (green circle), P3HT-ZnPc/PCBM (red circle), and P3HT-ZnPc-
60/PCBM (blue circle) BHJ devices (100 mW cmꢀ2, AM 1.5 G). All the
devices were thermally annealed at 150 ꢁC for 5 min.
Fig. 7 XRD patterns of P3HT/PCBM (black line), P3HT-ZnPc/PCBM
(red line), P3HT-ZnPc-C60/PCBM (blue line), and P3HT + ZnPc/PCBM
(green line) blend films after thermal annealing at 150 ꢁC for 5 min.
C
When the nanoscale morphology of the active layer from each
device after 5 min annealing is examined by TEM, as shown in
Fig. 10, the TEM image of the P3HT/PCBM film shows the
nanoscale phase morphology with network structure where well
developed P3HT nanocrystals are clearly observed in the bright
P3HT domain (Fig. 10a), whereas the thin film of P3HT + ZnPc/
PCBM shows leaf-like aggregation of dye molecules after
thermal annealing at 150 ꢁC for 5 min (Fig. 10b). To more clearly
identify the aggregates formed from ZnPc dyes, we prepared
separately another P3HT/PCBM sample blended with a larger
amount of ZnPc and examined the thin film morphology. As the
weight fraction of ZnPc is increased, the size of the aggregate
becomes larger in the blend film (see ESI†). This observation
leads us to conclude that ZnPc dyes, when blended as the third
component of the active layer, form large aggregates after
thermal annealing and this aggregation may suppress the
Table 1 The photovoltaic parameters of P3HT/PCBM, P3HT + ZnPc/PCBM, P3HT-ZnPc/PCBM, and P3HT-ZnPc-C60/PCBM BHJ devices
(100 mW cmꢀ2, AM 1.5 G). All the devices were thermally annealed at 150 ꢁC for 5 min
Device
VOC (V)
JSC (mA cmꢀ2
)
FF (%)
PCE (%)
P3HT/PCBM
0.56 ꢂ 0.02
0.54 ꢂ 0.03
0.56 ꢂ 0.02
0.56 ꢂ 0.02
9.22 ꢂ 0.7
8.52 ꢂ 0.8
9.91 ꢂ 0.5
10.96 ꢂ 0.4
61 ꢂ 1.0
49 ꢂ 3.0
59 ꢂ 1.0
58 ꢂ 1.5
3.15 ꢂ 0.15
2.25 ꢂ 0.25
3.27 ꢂ 0.20
3.56 ꢂ 0.18
P3HT + ZnPc/PCBM
P3HT-ZnPc/PCBM
P3HT-ZnPc-C60/PCBM
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Fig. 11 TRPL spectra of P3HT (gray line), P3HT/PCBM (black line),
P3HT + ZnPc/PCBM (green line), P3HT-ZnPc/PCBM (red line), and
P3HT-ZnPc-C60/PCBM (blue line) blend films after thermal annealing at
150 ꢁC for 5 min.
Fig. 10 TEM images of (a) P3HT/PCBM, (b) P3HT + ZnPc/PCBM, (c)
P3HT-ZnPc/PCBM, and (d) P3HT-ZnPc-C60/PCBM blend films after
thermal annealing at 150 ꢁC for 5 min.
450 ps, which is nearly equal to that of P3HT/PCBM, both the
P3HT-ZnPc/PCBM and P3HT-ZnPc-C60/PCBM films have
shorter charge separation time (140–160 ps) than the P3HT/
PCBM film. This is because excitons in P3HT-dye/PCBM are
more efficiently dissociated into charge carriers while ZnPc
molecules in the simple mixture of P3HT + ZnPc/PCBM are
aggregated and thus do not contribute to the charge separation.
In short, the ZnPc-C60 dye molecules linked to the chain-end of
P3HT do not aggregate but preferentially locate at the interface
between P3HT and PCBM, and thereby contribute to the
enhancement of JSC via three different mechanisms: First, dye
molecules absorb the incident light and contribute to the
photocurrent generation; second, the dye molecule at the donor–
acceptor interface enhances the charge transfer from P3HT to
PCBM by acting as an ‘‘electronic tunnel’’; third, P3HT-ZnPc-
C60 acts as a ‘‘compatibilizer’’ to control the donor/acceptor
morphology.35–37 Among these contributions, the first contribu-
tion to the enhancement of JSC must be small because the
photocurrent at the ZnPc absorption range is not so large in
ICPE spectra, as shown in Fig. 9. On the other hand, more
important contributions would be both charge transfer and
morphology control by ZnPc-C60, because differences in the
TRPL decay time (20 ps) and JSC (1.0 eV) between P3HT-ZnPc-
C60/PCBM and P3HT-ZnPc/PCBM are observed.
crystallization of P3HT, resulting in inefficient charge transport.
On the other hand, the dye aggregation was not observed in
P3HT-ZnPc/PCBM and P3HT-ZnPc-C60/PCBM after thermal
annealing, as shown in Fig. 10c and 10d, suggesting that dye
molecules linked to the polymer chain-end do not aggregate
upon thermal annealing and thus do not prohibit the crystalli-
zation of P3HT. It should be noted that the pyridine–Zn bond is
not dissociated upon thermal annealing because the dissociation
energy of pyridine–Zn metal (248 KJ/mol) is much larger than
the thermal energy at 150 ꢁC (RTz3.5 kJ molꢀ1).32 Furthermore,
it is notable that the domain size of P3HT-ZnPc-C60/PCBM is
ꢃ50 nm, which is much smaller than the domain size of P3HT/
PCBM (ꢃ200 nm). It has been well known that the addition of
block copolymer compatibilizer to the corresponding homopol-
ymer blend leads to reduction of interfacial tension and thus
suppresses macrophase separation due to the attractive interac-
tion between the block copolymer and the corresponding blend
component at the interface.33,34 Hence, it is expected that the
ZnPc-C60 part in P3HT-ZnPc-C60 interacts effectively with
PCBM, and thus reduces the interfacial tension between P3HT
and PCBM, resulting in the reduction of domain size. The high
JSC of the P3HT-ZnPc-C60/PCBM device is attributed to the
relatively small domain size.
Another important role of the interface modifier is the
improvement of the long-term stability of the nano-scaled
morphology against macrophase separation. To examine the
The time-resolved photoluminescence spectroscopy (TRPL)
was measured under different film conditions, as shown in
Fig. 11, from which the fluorescence kinetic parameters of the
blend films were evaluated and summarized in Table 2. The
fluorescence at 650 nm of P3HT has the mono-exponential decay
time of 700 ps whereas that of P3HT/PCBM has bi-exponential
decay times of 190 ps and 450 ps at room temperature. The fast
decay time (190 ps) is identical with the time of charge (exciton)
separation at the interface between P3HT and PCBM, while the
slow decay time (450 ps) is ascribed to the exciton-hole recom-
bination time of P3HT. Whilst the P3HT + ZnPc/PCBM sample
also exhibits bi-exponential decay with a lifetime of 180 and
Table 2 Fluorescence kinetic parameters of P3HT/PCBM, P3HT +
ZnPc/PCBM, P3HT-ZnPc/PCBM, and P3HT-ZnPc-C60/PCBM blend
films after thermal annealing at 150 ꢁC for 5 min
Sample
lex (nm)
lem (nm)
Decay time (ps)
P3HT
P3HT/PCBM
P3HT + ZnPc/PCBM
P3HT-ZnPc/PCBM
P3HT-ZnPc-C60/PCBM
532
650
700
190/450
180/450
160/470
140/430
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effect of the ZnPc-C60 interface modifier on the long-term
stability of the device performance, the PCEs of the P3HT-ZnPc-
C60/PCBM devices were measured as a function of annealing
time at 150 ꢁC and compared with those of devices without ZnPc-
C60 interface modifier. When the PCEs are plotted against the
annealing time, as shown in Fig. 12, it is obvious that the PCE of
the P3HT-ZnPc-C60/PCBM device decreases much more slowly
than the other devices with increasing the annealing time. This is
probably because the ZnPc-C60 dyes located at the interface
between the P3HT and PCBM phases effectively act as a com-
patibilizer to suppress the macrophase separation.
Tetrahydrofuran (THF) was dried over sodium/benzophenone
under nitrogen and freshly distilled before use. Buckminsterful-
lerene derivatives, PCBA (Materials Technologies Research) and
PCBM (Nano-C) were used as received. Poly(3,4-ethyl-
enedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS)
(Baytron P VP AI 4083) was purchased from H. C. Stark and
passed through a 0.45 mm PES syringe filter before spin-coating.
All other reagents were purchased from Sigma-Aldrich and used
as received.
Synthesis of tetra-tert-butyl zinc(II) phthalocyanine (ZnPc)
A mixture of 4-tert-butylphthalonitrile (737 mg, 4 mmol) and
ZnCl2 (136 mg, 1 mmol) were refluxed in dimethylaminoethanol
(DMAE, 1.5 mL) under argon for 15 h. After the reaction
mixture was concentrated under reduced pressure, the blue solid
was extracted with dichloromethane and then washed with
water. Yield 85.5%; FT-IR (KBr, cmꢀ1): 2956 (C–H), 1613 (N–
H); elementary analysis calcd (%) for C48H48N8Zn (802.36): C
70.14, H 6.34, N 12.58, found: C 70.53, H 6.79, N 12.12;
MALDI-TOF MS: m/z 800.3 (100%, [M + 2H]+).
Conclusions
We have synthesized two kinds of zinc phthalocyanine dyes and
linked them to the chain-end of P3HT. It was found that the dye
molecules linked to the chain-end of P3HT do not aggregate in
the blend film and preferentially locate at the P3HT/PCBM
interface. Hence, the dye molecules linked to P3HT do not
disturb the crystallization of P3HT, and rather lower the inter-
facial tension, affording the reduction of the domain size of the
P3HT/PCBM blend. More importantly, the ZnPc-C60 dye
located at the interface enhances the photocurrent generation by
both direct photo-excitation and effective charge separation. The
P3HT-ZnPc-C60/PCBM devices achieved the PCE of 3.56% with
20% enhancement of JSC compared to that of the reference
P3HT/PCBM device. Furthermore, the P3HT-ZnPc-C60/PCBM
device exhibited remarkable enhancement of the long-term
thermal stability of the device performance, because the ZnPc-
Synthesis of tri-tert-butyl-amino zinc(II) phthalocyanine (ZnPc-
NH2)
A mixture of 4-tert-butylphthalonitrile (553 mg, 3 mmol) and
4-aminophthalonitrile (143 mg, 1 mmol) was refluxed in DMAE
(1.5 mL) under argon for 15 h in the presence of ZnCl2 (136 mg,
1 mmol). After the reaction mixture was concentrated under
reduced pressure, the greenish blue solid was extracted with
dichloromethane and then washed with water. The correspond-
ing tri-tert-butyl-amino zinc(II) phthalocyanine was separated
from tetra-tert-butyl zinc(II) phthalocyanine, di-tert-butyl-di-
amino zinc(II) phthalocyanine, and tert-butyl-tri-amino zinc(II)
phthalocyanine by column chromatography using a mixed
solvent of petroleum ether and dioxane (4 : 1/v:v) as an eluting
solvent. Yield 18.5%; FT-IR (KBr, cmꢀ1): 3366, 3221, 1612 (N–
H), 2955 (C–H); elementary analysis calcd (%) for C44H41N9Zn
(761.27): C 67.88, H 5.81, N 14.84, found: C 66.16, H 6.24, N
13.55; MALDI-TOF MS: m/z 760.0 (100%, [M + H]+).
C60 interface modifier suppresses large-scale phase separation of
P3HT/PCBM blend even after prolonged annealing times. It is
expected that our concept to attach dye molecules to the polymer
chain-end can be applied to other low bandgap conjugated
polymers to enhance the device performance of polymer solar
cells.
Experimental section
Materials
4-tert-Butylphthalonitrile and 4-aminophthalonitrile were
purchased from TCI and used without further purification.
Synthesis of tri-tert-butyl zinc(II) phthalocyanine-C60 (ZnPc-
C60)
PCBA (150 mg, 0.17 mmol) was dissolved in 50 mL of dry CS2
under argon atmosphere. Thionyl chloride (10 mLꢁ) was added
using a syringe and the mixture was heated at 80 C for 18 h.
After the reaction mixture was cooled down to room tempera-
ture, the volatile compounds were removed under reduced
pressure. The remaining solid ([6,6]-phenyl-C61-acyl chloride)
was washed with anhydrous THF three times and dried under
vacuum. The tri-tert-butyl-amino zinc(II) phthalocyanine (155
mg, 0.18 mmol) and triethylamine (4.5 mL) were dissolved in dry
CS2 (60 mL), and the solution was dropwise added to [6,6]-
phenyl-C61-acyl chloride using a syringe. After the reaction
solution was refluxed for 24 h, the volatile CS2 was removed with
a rotary evaporator. The crude product was suspended in diethyl
ether using ultrasonication. The suspension was poured into
500 mL of water, and the precipitate was filtered and dried at
Fig. 12 The power conversion efficiencies of P3HT/PCBM (black line),
P3HT + ZnPc/PCBM (green line), P3HT-ZnPc/PCBM (red line), and
P3HT-ZnPc-C60/PCBM (blue line) BHJ devices as
annealing time at 150 ꢁC.
a function of
ꢁ
50 C under vacuum. After the product was suspended in ethyl
17216 | J. Mater. Chem., 2011, 21, 17209–17218
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acetate using ultrasonication, the suspension was centrifuged at
3500 rpm, and the supernatant was decanted and dried at 50 ꢁC
under vacuum. Yield 30.5%; FT-IR (KBr, cmꢀ1): 3376, 1554 (N–
H), 2925 (C–H), 1661 (C]O), 528 (C60); elementary analysis
calcd (%) for C115H51N9OZn (1640.1): C 84.22, H 3.13, N 7.69,
found: C 83.93, H 3.06, N 7.95; MALDI-TOF MS: m/z 1638.8
(100%, [M + H]+).
4 wt%. For comparison, the solution of P3HT/PCBM blended
with the same amount of ZnPc dyes as that in P3HT-ZnPc was
also prepared. These solutions were stirred for at least 24 h at
room temperature and then were passed through a 0.2 mm PES
syringe filter before spin coating. These solutions were coated on
the top of PEDOT:PSS layer by spin-coating at 700 rpm for 60 s.
Al (100 nm in thickness) was thermally evaporated on the top of
the active layer under high vacuum lower than 10ꢀ6 Torr. The
ꢁ
devices were then annealed at 150 C for 5 min on a digital hot
Synthesis of pyridyl-terminated P3HT (P3HT-Py)
plate under nitrogen atmosphere inside a glove box.
Hydroxypropyl-terminated P3HT was synthesized by following
the procedure reported in the literature,25 before pyridyl-termi-
nated P3HT was synthesized by the Steglich esterification
between hydroxypropyl-terminated P3HT and isonicotinic acid.
In a 100 mL round-bottom flask, 0.14 g of hydroxypropyl-
terminated P3HT (Mn ¼ 24 000, Mw/Mn ¼ 1.15 by GPC) and
isonicotinic acid (0.04 g, 0.32 mmol) were added, and the flask
was degassed under vacuum and backfilled with argon gas.
40 mL of CHCl3 and 0.01 mg of dimethylaminopyridine (0.1
mmol) were added to the reaction mixture. Then, 0.07 mg of 1,3-
dicyclohexylcarbodiimide (0.32 mmol) dissolved in 10 mL of
CHCl3 was dropwise added using a syringe, and the solution was
stirred at 30 ꢁC for 2 days. The polymer was purified by soxhlet
extraction consecutively with methanol, hexane, ethyl acetate,
and toluene. The polymer was isolated from toluene solution,
and dried under vacuum to afford 0.125 g (90% yield).
Characterization and measurement
1
The chemical structure was identified by a 500 MHz H NMR
spectrometer (Avance 500, Bruker). FT-IR spectra of the
synthesized materials were obtained on an IR spectrometer (FT/
IR-660 plus, Jasco). The molecular weight and its distribution for
all synthesized polymers was measured by GPC (Knauer K-501
pump with a K-2301 refractive index detector) using THF as an
eluent. The molecular weights were reported relative to poly-
styrene standards. MALDI-TOF mass spectra were collected
(Voyager-DE STR Biospectrometry Workstation, Applied Bio-
systems Inc.) with a-cyano-4-hydroxy-cynamic acid (CHCA) as
a matrix. Elementary analyses of all synthesized dyes were per-
formed by an elemental analyzer (Flash EA 1112, Thermo
Electron Co.).
The UV-visible absorption and fluorescence spectra were
obtained on a Perkin-Elmer Lambda 25 spectrometer and
a Photon Technology International QM-4 fluorescence spec-
trometer, respectively. The morphology of the active layer films
was observed by TEM (JEM-1010, JEOL) with an accelerating
voltage of 120 kV. The crystallinity of the active layer was
investigated by X-ray diffractometer (M18XHF-SRA, Mac
Science Co.) using Cu-Ka (l ¼ 0.154 nm) radiation. The cyclic
voltammetry (CV) measurements were carried out using Pt
working and counter electrodes and a Ag/AgCl (3 M NaCl)
reference electrode. The photovoltaic performance was measured
under nitrogen atmosphere inside a glove box. The current
density-voltage (J-V) characteristics were measured with
Synthesis of zinc(II) phthalocyanine-terminated P3HT (P3HT-
ZnPc)
A mixture of tetra-tert-butyl zinc(II) phthalocyanine (0.1 g,
0.125 mmol) and pyridyl-terminated P3HT (0.1 g, 0.004 mmol)
were refluxed in anhydrous THF (10 mL) under argon for 12 h.
The polymer solution was precipitated into MeOH and filtered,
and the precipitation and filtration step was repeated 3 times to
remove the unreacted zinc(II) phthalocyanine. The precipitated
polymer was dried under vacuum to afford 0.09 g (87% yield).
Synthesis of zinc(II) phthalocyanine-C60-terminated P3HT
(P3HT-ZnPc-C60)
a Keithley 4200 source-meter under AM 1.5 G (100 mW cmꢀ2
)
simulated by a Newport-Oriel solar simulator. The light intensity
was calibrated using a NREL certified photodiode and light
source meter prior to each measurement. The active area was
determined at 0.04 cm2 by attaching a shadow mask onto the
solar cell device. The IPCE was measured using a lock-in
amplifier with a current preamplifier (K3100, Mac Science Co.)
under short circuit current state with illumination of mono-
chromatic light.
A mixture of tri-tert-butyl zinc(II) phthalocyanine-C60 (0.08 g,
0.05 mmol) and pyridyl-terminated P3HT (0.1 g, 0.004 mmol)
were refluxed in anhydrous THF (10 mL) under argon for 24 h.
The polymer solution was precipitated into dichloromethane and
filtered, and this precipitation and filtration step was repeated
three times to remove the unreacted zinc(II) phthalocyanine-C60.
The precipitated polymer was dried under vacuum to give 0.08 g
(76% yield).
Photovoltaic cells fabrication
TRPL measurement
ITO-coated glass (15 U sqꢀ1) was cleaned with acetone and iso-
propyl alcohol, and then dried at 200 ꢁC for 30 min. After
complete drying, the ITO-coated glass was treated with UV-
ozone for 15 min. PEDOT:PSS was spin coated onto the ITO
with 40 nm in thickness, and the PEDOT:PSS film was annealed
at 120 ꢁC for 30 min in a N2-filled glove box. The mixture of
P3HT/PCBM (P3HT-ZnPc/PCBM, or P3HT-ZnPc-C60/PCBM)
was dissolved in 1,2-dichlorobenzene at a concentration of
A mode-locked Nd:YAG laser (Pizzicato, Quantel) with the
pulse duration of 25 ps was employed for excitation, and a streak
camera of 10 ps (C2830, Hamamatsu) attached to a CCD
detector (RTE128H, Princeton Instruments) was used for
detection. Samples were excited with the second harmonic pulses
(532 nm) of the laser. Emission wavelengths were collected at
near magic angle for the fluorescence kinetic measurements and
selected by combining band-pass filters and cut-off filters.
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19 S. Honda, H. Ohkita, H. Benten and S. Ito, Chem. Commun., 2010,
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