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