DOI: 10.1002/adma.200701519
Controlling Morphology in Polymer–Fullerene Mixtures**
By Adam J. Moulé and Klaus Meerholz*
In the past several years, polymer–fullerene mixtures have
been intensely studied for use in organic solar cells because
they can be deposited from solution, are compatible with low-
cost roll-to-roll fabrication technology, and have shown high
power conversion efficiency (g), up to 4–5%.
[1–3]
The best de-
vices consist of a single bulk-heterojunction active layer, in
which the polymer (donor) and fullerene (acceptor) are de-
posited from a common solvent. As the solvent dries the do-
nor and acceptor components separate into domains. The final
efficiency of the solar cell has been shown to be extremely
sensitive to the size, composition, and crystallinity of the
formed domains.
[4,5]
Improvement of the morphology in de-
vices fabricated with a mixture of [6,6]-phenyl C
61
-butyric
acid methyl ester (PCBM) and regioregular poly(3-hexylthio-
phene) (P3HT) has been achieved by using heat-treatment
techniques
[2,6]
and long-time solvent curing,
[1]
with resulting
record efficiencies. More recently, a method for increasing the
crystallinity of the P3HT component has been introduced
which involves filtering preformed nanofibers of P3HT out of
solution and mixing the prepared nanofiber dispersion with
PCBM to increase the efficiency of as-cast devices.
[7]
Interest-
ingly, the best device performance was achieved by mixing
lower-molecular-weight (M
W
) amorphous P3HT back into the
solution to reduce the crystalline content of the active layer
and, thereby, to increase connection between crystalline do-
mains. Studies of the M
W
impact on P3HT/PCBM solar cells
have indicated that a large polydispersity and number-average
molecular weight (M
n
) over 19000 g mol
-1
leads to improved
efficiency.
[8,9]
Morphology studies of organic field-effect
transistor (OFET) devices indicate that the increased M
W
leads to better network formation between crystalline
domains.
[10,11]
The morphology of these improved devices has been stud-
ied using transmission electron microscopy (TEM),
[12]
graz-
ing-angle X-ray diffraction (XRD),
[13,14]
atomic force micros-
copy (AFM),
[10]
scanning electron microscopy (SEM),
[15]
NMR,
[16]
and a variety of other optical
[14]
and electrical tech-
niques.
[17]
The morphology studies give a picture of a device
in which the P3HT forms aligned/crystalline domains, be-
tween which are amorphous segments of P3HT and PCBM.
[14]
These domains form with larger size and crystallinity for high-
er heat-treatment temperatures
[18]
and longer solvent soaking
times.
[19,20]
Depending on the fabrication and measurement
techniques, the aligned domains of P3HT are depicted as fi-
bers
[12]
or as shapeless masses.
[14]
The majority of these studies do not, however, allow quanti-
fication of the percentage of the P3HT that is agglomerated/
crystalline in the final device. Only by making use of the
nanofiber filtration technique
[7]
have the authors been given
the ability to control the crystalline content of the P3HT in so-
lution and in the final device. The disadvantages of this tech-
nique are the necessity of more complicated preparation, and
filtered P3HT is restricted to a fibrous form that requires the
addition of amorphous P3HT to provide connections between
crystalline domains. We present here a simple method to de-
termine the agglomerated–amorphous ratio of the P3HT and
to control the degree of agglomeration/crystallinity of the
P3HT in the final device by using a solvent mixing method
and no further heat-treatment or prepreparation of the
polymer.
The most obvious change that heat-treatment and solvent
soaking yield on a P3HT:PCBM layers is the change in col-
or.
[6,21,22]
It has been widely reported that the P3HT absorp-
tion red-shifts and a series of vibronic peaks become visible at
k∼600 nm, 550 nm, and 510 nm.
[6,11]
This red-shift has been
assigned to increased planarization and stabilization of the
P3HT chains that accompanies self-stacking of the poly-
mer.
[10,14]
The crystal structure for these self-stacking domains
has been solved by using XRD and TEM, and shows a her-
ringbone interconnection of the alkyl chains and an a-dimen-
sion stacking distance of 1.6 nm.
[14]
The p–pchain stacking of
the P3HT chains in crystallites has been measured to be
0.38 nm.
[13]
The herringbone structure and planarization of
the P3HT with heating has been confirmed using heteronuc-
lear solid-state NMR measurements.
[16]
The red-shift in the
UV-vis spectrum occurs proportionally to the degree of ag-
glomeration of the P3HT.
[7,23]
The pure amorphous electronic spectrum of P3HT or a mix-
ture of P3HT and PCBM is simple to measure. A solution
UV-vis spectrum can also be measured in the liquid state. If
COMMUNICATION
240 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008,20, 240–245
–
[*] Prof. K. Meerholz, Dr. A. J. Moulé
[+]
Institute of Physical Chemistry
University of Cologne
Luxemburgerstr. 116, 50939 Köln (Germany)
E-mail: [email protected]
[+] Present address: University of California, Davis, Department of
Chemical Engineering & Materials Science, 1 Shields Av., Davis, CA
95616, USA.
[**] We would like to acknowledge the Alexander von Humboldt founda-
tion for the post-doctoral grant of A.M. We would also like the thank
German Ministry of Science and Education (BMBF) for funding
EKOS project (O3N2023D) and the ministry of Science and Innova-
tion of Northrhine-Westfalia (Elena-project). We thank Tanja Tege-
der for taking the SEM image. Supporting Information is available
online from Wiley InterScience or from the authors.
