Morphology Of Polymer-Fullerene Bulk Heterojunction Solar Cells, TYMCZASOWY, Incoming
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FEATURE ARTICLE
www.rsc.org/materials
| Journal of Materials Chemistry
Morphology of polymer/fullerene bulk heterojunction solar cells
Harald Hoppe{* and Niyazi Serdar Sariciftci
Received 25th July 2005, Accepted 2nd November 2005
First published as an Advance Article on the web 28th November 2005
DOI: 10.1039/b510618b
Within the different organic photovoltaic devices the conjugated polymer/fullerene bulk
heterojunction approach is one of the foci of today’s research interest. These devices are highly
dependent on the solid state nanoscale morphology of the two components (donor/acceptor) in
the photoactive layer. The need for finely phase separated polymer–fullerene blends is expressed
by the limited exciton diffusion length present in organic semiconductors. Typical distances that
these photo-excitations can travel within a pristine material are around 10–20 nm. In an efficient
bulk heterojunction the scale of phase separation is therefore closely related to the respective
exciton diffusion lengths of the two materials involved. Once the excitons reach the donor/
acceptor interface, the photoinduced charge transfer results in the charge separation. After the
charges have been separated they require percolated pathways to the respective charge extracting
electrodes in order to supply an external direct current. Thus also an effective charge transport
relies on the development of a suitable nanomorphology i.e. bicontinuous interpenetrating phase
structures within these blend films. The present feature article combines and summarizes the
experimental findings on this nanomorphology–efficiency relationship.
concept,
3–6
1. Introduction: history, materials and tools
bulk
heterojunction
the
quantum
efficiency
of
conjugated
polymer/fullerene
solar
cells
could
be
raised
The field of conjugated polymer/fullerene solar cells was born
with
considerably.
5
The first morphology study dedicated to the MEH-PPV :
C
60
bulk heterojunction system applying transmission electron
microscopy was reported by Yang and Heeger.
7
They
selectively dissolved the fullerene from thin blend films using
decahydronaphthalene, and observed both isolated and con-
nected regions with characteristic size of about 10 nm,
corresponding to the C
60
phase. For increasing C
60
content
the authors reported increasing percolation and bicontinuous
network formation of the C
60
phase within the MEH-PPV
network. Furthermore, the application of electron diffraction
the
discovery
of
the
photoinduced
electron
transfer
between
the
soluble
poly(para-phenylenevinylene)
(PPV)
Buckminsterfullerene.
1
derivative
MEH-PPV
and
the
C
60
First
bilayer
devices
delivered
the
proof
of
principle
for
conversion.
2
solar
energy
With
the
introduction
of
the
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry,
Johannes Kepler University Linz, Altenbergerstr. 69, A-4040 Linz,
Austria. E-mail: harald.hoppe@tu-ilmenau.de
{ New
address:
Institute
of
Physics,
TU
Ilmenau,
Germany.
E-mail: harald.hoppe@tu-ilmenau.de
Harald Hoppe reveived his
diploma degree in Physics at
the University of Konstanz,
Germany in 2000. He prepared
his diploma studies in polymer
physics under the mentorship of
Professor Jacob Klein at the
Department of Materials &
Interfaces, Weizmann Insitute
of Science, Israel. Thereafter
he received his doctorate degree
at the Linz Institute for
Organic Solar Cells (LIOS)
under the mentorship of
Professor Niyazi Serdar
Sariciftci at the Johannes
Kepler University of Linz, Austria, in 2004. In spring 2005 he
joined the group of Professor Gerhard Gobsch at the Technical
University of Ilmenau as a post-doctoral research assistant. His
main research activities are in the field of organic optoelectronics
with a special focus on plastic solar cells.
Niyazi Serdar Sariciftci received
his masters degree in Experi-
mental Physics and a doctorate
degree in Semiconductor Physics
under the mentorship of Professor
H. Kuzmany at the University of
Vienna in 1986 and 1989, respec-
tively. After a post doctoral period
at Stuttgart University, Germany,
with Professor M. Mehring, he
joined the Institute of Polymer and
Organic Solids at the University
of California, Santa Barbara, with
Professor Alan Heeger. He was
appointed Chair and Professor
in Physical Chemistry at the
Johannes Kepler University of Linz in 1996. He is the
Founding Director of the Christian Doppler Laboratory for
Plastic Solar Cells and the Linz Institute for Organic Solar Cells
(LIOS).
Harald Hoppe
Niyazi Serdar Sariciftci
His
main
research
activities
are
in
organic
semi-
conductor physics and chemistry.
This journal is
The Royal Society of Chemistry 2006
J.Mater.Chem., 2006, 16, 45–61 | 45
(PLEDs). It is even more intriguingly that among suitable
fullerenes only one representative warrants the best power
conversion efficiency for a decade, the soluble C
60
derivative
1-(3-methoxycarbonyl)propyl-1-phenyl [6,6]C
61
(PCBM).
37
Fig. 1 displays some representatives of the above-mentioned
groups of conjugated polymers used in bulk heterojunction
solar cell devices together with the most commonly used
fullerene PCBM. Table 1 summarizes some of the best power
conversion
efficiencies
reported
among
other
device
para-
meters for the same material systems.
Traditionally electron microscopy served as an exploration
tool for the bulk heterojunction morphology in thin films—
mainly in the form of transmission electron microscopy (TEM)
in connection with selected area electron diffraction (SAED).
7
Later scanning electron microscopy (SEM) proved its effec-
tiveness in resolving the fine structure of polymer–fullerene
composites.
38
Furthermore, atomic force microscopy (AFM)
especially in the non-contact (or tapping) mode was applied
to investigate the polymer–fullerene blend film topogra-
phy,
8,12,38,39
of which recently more sophisticated derivatives
like Kelvin probe force microscopy have been used.
40–42
Scanning near field optical microscopy (SNOM) detecting
photoluminescence
43
or photocurrent
44,45
has been applied as
well. Taking the results originating from these different
techniques together yields today a rather conclusive picture
of the underlying nanomorphology and its effect on power
conversion efficiency in solar cell devices.
Meanwhile also theoretical work on optimized morpho-
logies has been performed
46
and the general understanding
of today is converging towards some closely intermixed donor–
acceptor blend, where the individual phases have a certain
extend to allow for efficient transport of charges via percolated
pathways towards the respective electrodes. The challenge,
however, that remains to be resolved for each and every
materials
Fig. 1 Several representatives of suitable conjugated polymers are
shown together with the soluble C
60
fullerene derivative PCBM.
on MEH-PPV : C
60
blends with a weight ratio of approxi-
mately 1 : 4 revealed a crystalline organization of the C
60
. The
authors concluded the C
60
to be present in the blend in the
form of nanocrystallites having sizes of roughly 10 nm.
7
C
60
is
truly a spherical molecule and this property of spatial
symmetry allows for relatively high order in the fullerene
phase—even
within composites.
This
issue
will
be further
elucidated in the following sections.
The next major step in the development of polymer–
fullerene bulk heterojunction photovoltaics was achieved at
the turn of the millennium by Shaheen et al.,
8
demonstrating
for the first time power conversion efficiencies of 2.5% at AM
1.5 standard solar irradiation. This report also laid the basis
for many other investigations in the direction of studying the
link between morphology and efficiency in these polymer–
fullerene bulk heterojunctions.
Among organic solar cells, the polymer–fullerene blends
5,6,9
have demonstrated competitive power conversion efficiencies.
Interestingly, from the conjugated polymers only a few
material classes have been employed in polymer–fullerene
bulk heterojunctions: soluble poly(para-phenylenevinylene)
(PPV) derivatives,
5,8–20
polythiophenes (e.g. P3ATs),
21–32
polyfluorenes,
33,34
and poly(phenylene-ethynylene)-(para-
phenylenevinylenes) (PPE-PPVs).
35,36
These conjugated poly-
mers have already enjoyed development for many years,
especially for application in polymer light emitting diodes
combination
is
the
controlled
self-organization
during
and
after
the
film
formation
process
towards
the
desired optimal morphology.
2. Morphology determining parameters
The morphology of the photoactive polymer-fullerene blend
can be affected by controlling several production parameters
during the film formation or by treatments afterwards.
Experimentally the following parameters have been identified
as
most
significant
for
their
influence
on
the
nanoscale
morphology in these polymer–fullerene blends:
Table 1
Power conversion efficiencies and device parameters of several polymer–fullerene bulk heterojunction solar cells
Materials
Reference
Short circuit current
Open circuit voltage
Fill factor
Power conversion efficiency
5.25 mA cm
22
2.5%
a
MDMO-PPV : PCBM 1 : 4
8
@ 80 mW
820 mV
61%
5.0 mA cm
22
2.65%
a
MDMO-PPV : PCBM 1 : 4
19
@ 80 mW
800 mV
71%
7.6 mA cm
22
MDMO-PPV : C
70
-PCBM 1 : 4
17
@ 100 mW
770 mV
51%
3.0%
8.5 mA cm
22
P3HT : PCBM 1 : 2
25
@ 80 mW
550 mV
60%
3.5%
15.9 mA cm
22
P3HT : PCBM 1 : 2
26
@ 110 mW
560 mV
47%
3.8%
10.9 mA cm
22
P3HT : PCBM 1 : 1
30
@ 100 mW
630 mV
49%
3.4%
11.1 mA cm
22
P3HT : PCBM 1 : 0.8
31
@ 80 mW
640 mV
55%
4.9%
9.5 mA cm
22
P3HT : PCBM 1 : 0.8
32
@ 80 mW
630 mV
68%
5.1%
4.7 mA cm
22
PFDTBT : PCBM 1 : 4
33
@ 100 mW
1040 mV
46%
2.2%
4.3 mA cm
22
PPE-PPV : PCBM 1 : 2
35
@ 100 mW
810 mV
59%
2.0%
a
Corrected for spectral mismatch (factor 0.753) of the solar simulator.
46 | J.Mater.Chem., 2006, 16, 45–61
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a) the spin casting solvent,
b) the composition between polymer and fullerene,
c) the solution concentration,
d)
film nanomorphology, revealed by atomic force microscopy
(AFM).
8
The difference in photocurrents could not be related to an
increased photon absorption within the photoactive layers,
since the transmittances of toluene and chlorobenzene cast
films were nearly identical (see Fig. 3). Therefore the improved
photocurrent was assigned to a change in the underlying blend
nanomorphology.
In another study on the MEH-PPV : C
60
system, Liu et al.
correlated the solar cell device parameters with different
solvents (xylene, chlorobenzene, 1,2-dichlorobenzene, chloro-
form and tetrahydrofuran). They claimed that non-aromatic
solvents prevent intimate contact between the MEH-PPV
backbone and C
60
, thus reducing the charge transfer efficiency
and subsequently the photocurrent, but increasing the photo-
voltage. Also by AFM measurements they found tetrahydro-
furan (THF) based devices to exhibit a larger scale of phase
separation. Furthermore the authors used the phase image
of the non-contact AFM scans to determine the ratio of
C
60
and MEH-PPV exposed to the surface of the film and
correlated this to the observed open circuit voltages by a
simple linear combination of the corresponding magnitude for
the pristine devices.
More recently Rispens et al. have compared the surface
topography of MDMO-PPV : PCBM devices by varying the
solvent from xylene (XY) through chlorobenzene (CB) to 1,2-
dichlorobenzene (DCB). The authors found a decrease in phase
separation from XY through CB to DCB.
48
Furthermore they
proposed a certain crystal packing of the PCBM molecules,
with solvent molecules being introduced into the crystal lattice.
These data were based on crystals grown from solution.
48
Martens et al. have comparatively investigated the nano-
structure of MDMO-PPV : PCBM bulk heterojunctions by
applying transmission electron microscopy (TEM) on films
and
the
controlled
phase
separation
and
crystallization
induced by thermal annealing, and finally
e) the chemical structure of the materials.
The chemical structures of polymer and fullerene determine
to a large extent the solubility in common organic solvents
and the miscibility between these two compounds. The solvent
itself furthermore influences the drying time during film
formation, whereas thermal annealing enables the crystal-
lization and diffusion of one or both compounds in the blend
leading to a coarsening of the phase separation.
2.1 Solvent
Already in the first study on conjugated polymer/fullerene D/A
bulk heterojunction solar cells done by the Heeger group it
was reported that the limited solubility of pure C
60
in
organic solvents and its tendency to crystallize during film
formation prevents the use of high concentration blends.
5
This
limitation was overcome by the application of soluble C
60
derivatives, developed previously.
5,37
The soluble PPV deriva-
tive used was poly(2-methoxy-5(29-ethyl-hexyloxy)-1,4-phenyl-
enevinylene) (MEH-PPV).
47
On changing the solvent from
xylene to 1,2-dichlorobenzene ‘‘high-quality’’ spin cast MEH-
PPV : PCBM films with weight ratio compositions of up to 1 : 4
were achieved.
5
These devices outperformed pristine MEH-
PPV devices with a photocurrent increased by two orders of
magnitude at 430 nm 20 mW cm
22
monochromatic laser
illumination.
Shaheen et al. observed a dramatic power conversion
efficiency increase upon changing the solvent from which the
MDMO-PPV : PCBM (1 : 4 by weight) blend solution was
spun.
8
The authors reported a strong dependence of the
performance on the solvent used: whereas toluene cast devices
yielded power conversion efficiencies of only 0.9% the use of
chlorobenzene almost tripled the efficiency. This increase was
mainly due to an increase of the short circuit current (Fig. 2)
and the authors attributed this to finer grain sizes in the thin
cross-sections
of
films
spin
cast
from
toluene
and
chlorobenzene.
39,40,49
On increasing the PCBM concentration
Fig. 3 Optical transmission spectra of 100 nm thick MDMO-PPV :
PCBM (1 : 4 by wt.) films spin coated onto glass substrates from either
toluene (dashed line) or chlorobenzene (solid line) solutions (a).
Incident photon to collected electron (IPCE) spectra for photovoltaic
devices
Fig. 2 Effect on photocurrent of the solvent used for spin casting the
active layer, a blend of MDMO-PPV/PCBM 1 : 4 by weight. Reprinted
with permission from reference 8, Copyright 2001, American Institute
of Physics.
using
these
films
as
the
active
layer
(b).
Reprinted with
permission
from
reference
8,
Copyright
2001,
American
Institute
of Physics.
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J.Mater.Chem., 2006, 16, 45–61 | 47
Furthermore Martens has shown by AFM that the drying
time is an important parameter for the size of the phase-
separated structures. By introducing a hot air flow over a
drying film, the drying time could be decreased and
consequently the extent of phase separation was reduced.
40
Here the film thickness was kept constant by a first fast
thickness determining step, which was applied prior to the
drying. Then the films obtained with the same chlorobenzene
based solution were dried for different times.
Our recently published study on the nanoscale morphology
of MDMO-PPV : PCBM solar cells was also triggered by the
findings of Shaheen et al.
8
and aimed towards the decoding of
the different phases within these MDMO-PPV : PCBM blends
cast from both toluene and chlorobenzene.
38
Furthermore
the different power conversion efficiencies caused by these
morphologies needed a deeper understanding. In agreement
with prior studies, a large difference in the scale of phase
separation could be identified as major difference between
toluene and chlorobenzene cast blends (see Fig. 5).
For the first time we used high-resolution scanning electron
microscopy (HR-SEM) to image cross-sections of toluene and
chlorobenzene cast MDMO-PPV : PCBM blends (see Fig. 6).
Thereby much smaller ‘‘nanospheres’’ became observable and
have been assigned to the polymer MDMO-PPV in a coiled
conformation. It has been suggested previously that con-
jugated polymers are present as little particles and thus form
with the solvent rather dispersions than solutions.
50
The commonly observed larger scale of phase separation of
the toluene cast MDMO-PPV : PCBM blends has been
interpreted as the main reason for the reduced photocurrents
as compared to the chlorobenzene cast blends. Especially a
lower charge carrier generation efficiency could be understood
as a result of too small exciton diffusion lengths (10–20 nm) for
reaching the interface between the large fullerene clusters (200–
500 nm) and the polymer. Experimentally it has been identified
that indeed some unquenched photoexcitations give rise to
residual PCBM photoluminescence in toluene cast blends,
whereas in chlorobenzene cast blends the fullerene photo-
luminescence could not be detected any more (see Fig. 7).
38
Fig. 4 TEM cross-sectional view of 1 : 4 MDMO-PPV : PCBM films
spin cast from toluene (a) and chlorobenzene (b) on a PET substrate.
The darker regions were attributed to PCBM rich regions. Reprinted
from reference 39, Copyright 2003, with permission from Elsevier.
in the blends, the authors observed increasing dark clusters
and attributed these to the fullerene-rich phase.
Since for the ratio 1 : 1 (1 : 2) of MDMO-PPV : PCBM for
toluene (chlorobenzene) cast films there was no phase
separation visible, the authors concluded that a homogeneous
blend of PCBM and MDMO-PPV exists around the PCBM
clusters for the blends with a higher PCBM content.
Interestingly the chlorobenzene based blends were able to
incorporate more PCBM than the toluene based counterparts,
thus the compatibility between MDMO-PPV and PCBM
seems to be influenced by the choice of solvent. Systematically
the PCBM clusters in the toluene cast films were larger in size
(up to several 100 nm) as compared to chlorobenzene cast
films (less than 100 nm). In the TEM cross-sections of films
spin cast on PET the fullerene-rich clusters are visible as darker
regions (see Fig. 4).
Fig. 5 Tapping mode AFM topography scans of MDMO-PPV : PCBM 1 : 4 (by weight) blended films, spin cast from (a) chlorobenzene and (b)
toluene solution. The toluene cast film exhibits height variations that are one order of magnitude larger than those on chlorobenzene cast films.
Features of a few hundred nanometers in width are visible in (b), while features in (a) are around 50 nm. (Reproduced from reference 38 with
permission, Copyright 2004, Wiley VCH.)
48 | J.Mater.Chem., 2006, 16, 45–61
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Fig. 7 PL spectra for pure MDMO-PPV, pristine PCBM and
blends of the two are shown. It is clearly visible that the PCBM peak
at y735 nm is found for the toluene (Tol) cast blend, whereas for the
chlorobenzene (CB) cast blend no such peak occurs. (Reproduced from
reference 38 with permission, Copyright 2004, Wiley VCH.)
Fig. 6 SEM cross-sections of chlorobenzene (a, b) and toluene (c, d)
based MDMO-PPV : PCBM blends. Whereas chlorobenzene based
blends are rather homogeneous, toluene cast blends reveal large PCBM
clusters embedded in a polymer-rich matrix or skin-layer. Small
features—referred to as ‘‘nanospheres’’—are visible in all cases and can
be attributed to the polymer in a coiled conformation. The blending
ratio is depicted in the lower right corner. (Reproduced from reference
38 with permission, Copyright 2004, Wiley VCH.)
photocurrent is vital, it has to be concluded that indeed triplet
excitons take part to a large extent in the photocurrent
generation. This statement is supported by the large triplet
exciton diffusion lengths reported for C
60
-based bilayer
devices, where the photocurrent generation is indeed domi-
nated by those triplets.
52
This however leads to the conclusion
that this small photoluminescence signal of the singlet excitons
observed in toluene cast MDMO-PPV : PCBM blends can not
sufficiently explain the observed overall photocurrent loss.
Furthermore, if the large fullerene clusters or domains
were the reason for the decrease in photocurrent, the charge
generation due to the absorption of the polymer should not be
affected at all. This however is in contrast to the experimental
observation that indeed the spectral photocurrent is smaller
over the whole wavelength range detected.
8,38
In Fig. 8 experi-
mentally determined spectral photocurrents (incident photon
to collected electron, IPCE) for different MDMO-PPV :
PCBM mixing ratios and for the two solvents, chlorobenzene
and toluene, are depicted for comparison. The peak around
350 nm as well as the little kink slightly above 700 nm can be
clearly assigned to the absorption of PCBM, whereas the
However, the specific photoluminescence signal of even bare
PCBM films was very small (,1%) when compared to the
luminescence of the pristine polymer MDMO-PPV and
therefore it is not sufficient to explain the 2–3 fold difference
in the photocurrents observed earlier.
8
The smaller photo-
luminescence efficiency of PCBM is due to the symmetrically
forbidden LUMO–HOMO transition in fullerenes as well as
due to strong intersystem coupling to the dark triplet state.
In time-dependent photoluminescence measurements for
chlorobenzene based blends it has been shown that the life
time of the PCBM singlet state was indeed decreased by
addition of the polymer.
51
Furthermore it should be noted that
around the singlet exciton life time almost all of the excitons
are already in the triplet state due to the rapid intersystem
crossing (see reference 52). Since the total number of optically
observable photoexcitations in the fullerene phase is so
small, but the contribution of the fullerene to the spectral
Fig. 8 IPCE spectra for chlorobenzene (a) and toluene cast blends (b) are shown. The photocurrents of the toluene cast blends are lower over the
whole spectral range, but not due to missing PCBM absorption, as the PCBM absorption peak is clearly present at about 350 nm. Reproduced
from ref. 38 with permission, Copyright 2004, Wiley VCH)
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