World Journal Of Engineering
Relations between phase diagram, kinetics of thermal annealing process, and
morphological stability in polymer:fullerene blends for bulk heterojunction solar cells
Guy Van Assche1, F. Demir1, B. Van Mele1, J. Manca2, D. Vanderzande2
1
Physical Chemistry and Polymer Science, Vrije Universiteit Brussel - VUB, Pleinlaan 2, B-1050 Brussels, Belgium.
2
Institute for Materials for Research, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium.
Introduction
For the efficiency of bulk heterojunction solar cells
based on blends of conjugated polymers and fullerenes,
the formation of nanostructured morphology is crucial.
As the diffusion distance of the excitons, the holeelectron pairs generated upon photon absorption is
limited to a few nanometer, at least one of the phase
dimensions should not exceed 10 nm [1-3]. To permit
an efficient charge transport of the charges generated,
a co-continuous phase morphology is needed.
To improve the understanding of the forces driving the
morphology formation and its stability, we
investigated the phase behavior of several polymerfullerene blends. For blends of poly(3-hexyl
thiophene) (P3HT) and [6,6]–phenyl C61 – butyric
acid methyl ester (PCBM), the crystallization of the
components drives the phase formation [4]. As,
increasing the crystallinity of the polymer-fullerene
blend components favors the charge mobility and
improves the efficiency (as long as the dimensions of
the crystals remains sufficiently small), a welldesigned thermal annealing of the spin-cast blends is
needed for optimal performance.
We will report on a study of the phase behavior and
the crystallization kinetics, and their relation with the
annealing behavior and the long-term morphological
stability in bulk heterojunction solar cells. Blends of
amorphous polymers, such as poly(2-methoxy-5(3’,7’-dimethyloctyloxy)-1,4-phenylene
vinylene)
(MDMO-PPV) and a high Tg-PPV, and semicrystalline polymers, such as P3HT, with PCBM will
be discussed (Fig. 1).
After drying (25 °C, N2, 50 hr), the solid films were
recovered in powder form.
Techniques
Differential scanning calorimetry (DSC) in both
standard and modulated modes were performed on a
TA Instruments Q1000 Tzero™ DSC equipped with
an RCS cooling accessory. DSC measurements were
made at 10 K.min-1. For modulated temperature DSC
(MTDSC) measurements, the modulation amplitude
was 0.5 K with a period of 60 s. After melting the
samples at 310 °C, they were rapidly cooled inside the
DSC and reheated to 310 °C at 2.5 K min-1 with
modulation to observe the glass transition (Tg).
Rapid-heat-cool calorimetry (RHC) measurements
were made on a TA Instruments RHC prototype
equipped with a liquid nitrogen cooling accessory.
Samples of 250-500 µg were placed in aluminum RHC
crucibles with a lid. For Tg measurements, samples are
quench-cooled (max. rate ca. 1500 K.min-1) and
subsequently reheated to measure the Tg behavior. For
crystallization rate measurements, samples are
first molten and then isothermally crystallized
from the melt state or the glassy state for various
times. The crystallinity is determined from the
subsequent melting peak. For more details see [5].
Atomic force microscopy images were recorded for
samples spin-coated on silicon wafers with an Asylum
Research MFP-3D AFM operating in AC-mode.
O
P3HT
S
MDMO-PPV
n
n
O
Experimental
O
OC10
Materials
Weighted amounts of P3HT (Merck, 35 kg mol-1),
MDMO-PPV (Merck, 1 771 kg mol-1), or High TgPPV (Merck) and PCBM (Solenne) were dissolved in
chlorobenzene at a concentration of 1 wt% and were
stirred overnight at 50 °C. Films with a thickness of 1
µm were deposited by drop-casting under nitrogen.
RO
OR
O
PCBM
OC10
x
OR'
y
OMe
z
n
High Tg-PPV
Fig. 1. Conducting polymers and fullerene derivative
for organic solar cells.
1149
World Journal Of Engineering
Results and Discussion
long-term
stability at application
temperature
from
MTDSC: 2 (heating
after quench),
Figure 17a
K minelevated
, +/- 0.5 K temperatures.
per 60 s
accelerated ageing tests at2.5
more
nd
Conclusion
A thorough study of the thermal transitions and
isothermal annealing of polymer:fullerene blends
revealed several key aspects for the usage of these
materials for solar cell applications. The glass
transition of the polymer:fullerene blends is crucial for
the long-term stability of the required nanostructured
morphology needed for optimal performance.
The temperature dependence of the crystallization rate
can be used to control the thermal annealing on the one
hand, but might also prove useful for predicting the
o
150
P3HT/PCBM:
MDMO-PPV/PCBM:
HTg-PPV/PCBM:
single
lower,
lower,
upper
upper
100
50
0
0
20
40
60
80
100
Weight fraction of PCBM, fw (wt %)
Fig. 2. Tg versus weight fraction PCBM for blends of
P3HT, MDMO-PPV, and high-Tg PPV with
PCBM (RHC).
1E-1
Crystallization rate (1/s)
For all polymers, the thermal transitions for blends
with PCBM contents ranging from 0 to 100 wt% were
studied by DSC, MTDSC, and RHC. At sufficiently
high PCBM contents, the crystallization of PCBM is
detected when cooling at 10 K.min-1, however, the
crystallization temperature decreases as the PCBM
content is lowered. For blends with the semicrystalline P3HT, the crystallization of P3HT is
observed up to 50 wt% PCBM, and the crystallization
and melting temperature gradually decrease with
increasing PCBM content.
For the amorphous polymers, a single Tg, indicating a
homogeneous blend, is observed in most cases. For
MDMO-PPV, a double Tg, indicating phase separation
in the amorphous phase, is observed at 70-90 wt%
PCBM. This is especially clear for samples rapidly
cooled by RHC, as in this case crystallization could be
avoided, even for pure PCBM, permitting the analysis
of the materials in their amorphous state (Error!
Reference source not found.).
For the reorganization of the material during annealing
or during application, the Tg is crucial: if the Tg of the
amorphous phase is sufficiently high, all cooperative
mobility is frozen and the phase morphology should be
stable. As the Tg’s of P3HT (~20°C) and MDMO-PPV
(~60°C) are quite low, the Tg’s of their blends with
PCBM at relevant PCBM contents (50-80 wt%) do not
exceed 80°C, indicating that a long-term stability of
the nanomorphology can not be expected during
application. For high-Tg PPV, the Tg is sufficiently
high to expect an improved long-term stability.
The rate of crystallization during the annealing of
P3HT:PCBM blends at temperatures ranging from
60°C to 160°C goes through a maximum around
125°C, and is generally faster for samples crystallized
after quenching to the glassy state (Fig. 3).
Glass transition temperature, Tg ( C)
-1
1E-2
1E-3
from the Melt
from the Glass
1E-4
50
90
130
170
Temperature (°C)
Fig. 3. Crystallization rate versus temperature for a
P3HT:PCBM 50:50 blend (RHC).
References
1. Yang, X. N., Loos, J., Veenstra, S. C., Verhees, W. J. H.,
Wienk, M. M., Kroon, J. M., Michels, M. A. J., and Janssen, R.
A. J. Nanoscale morphology of high-performance polymer
solar cells. Nano Letters, 5(4) (2005) 579-583.
2. Ma, W. L., Yang, C. Y., and Heeger, A. J. Spatial Fouriertransform analysis of the morphology of bulk heterojunction
materials used in "plastic" solar cells. Adv. Mater., 19(10)
(2007) 1387-+.
3. Thompson, B. C. and Frechet, J. M. J. Polymer-fullerene
composite solar cells. Angew. Chemie-Int. Ed., 47(1) (2008)
58-77.
4. Zhao, J., Swinnen, A., Van Assche, G., Manca, J., Vanderzande,
D., and Van Mele, B. Phase Diagram of P3HT/PCBM Blends
and Its Implication for the Stability of Morphology. J. Phys.
Chem. B, 113(6) (2009) 1587-1591.
5. Demir, F., Van den Brande, N., Van Mele, B., Berth, S.,
Vanderzande, D., Manca, J., Van Assche, G. Isothermal
1150
World Journal Of Engineering
crystallization of P3HT:PCBM blends studied by RHC. J.
Therm. Anal. Calorim., published oinline (2011)..
1151