This document analyzes the morphology of thin films made of poly(methyl-methacrylate) (PMMA) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at different ratios using wide-angle X-ray scattering (WAXS). It finds that as the concentration of PCBM increases, its diffraction peaks increase in amplitude and decrease in width, while the PMMA peaks remain constant. It identifies four peaks as belonging to PCBM and four as belonging to PMMA. The locations and amplitudes of the PCBM peaks change with concentration in accordance with past studies of PCBM in amorphous polymers.

1. Morphology of PMMA:PCBM thin films
Edward Burt Driscoll∗
Department of Physics, North Carolina State University, Raleigh, North Carolina 27607, USA
(Dated May 7, 2015)
Abstract
Poly(methyl-methacrylate) (PMMA) blended with [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) in different ratios is analyzed using wide-angle
X-ray scattering (WAXS) for morphological characteristics. Like past mea-
surements of PCBM, the diffraction peaks increase in amplitude and de-
crease in width with an increase in concentration, while PMMA peaks re-
main constant.
Introduction
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is an
essential ingredient in organic photovoltaic devices, where it
acts as an electron acceptor[1]. In devices it is paired with
an electron donor such as poly(3-hexylthiophene,2,2-diyl)
(P3HT) in a donor-acceptor bulk heterojunction system[2].
Poly(methyl-methacrylate) is an amorphous polymer
that acts as a base for the PCBM. This heterogeneous mix-
ture allows for coagulation of the PCBM when processed in
spin-coating.
When mixed, PMMA and PCBM molecules are not dis-
tributed evenly, but PCBM instead exists in a PMMA ma-
trix. The miscibility is a measurement of how much the ma-
terials are mixed, as opposed to how much is separated[3].
This can be altered through material ratios and annealing
time and temperature.
Carrier mobility is an important factor in the perfor-
mance of devices, and comparing miscibility and crys-
tallinity to carrier mobility has yielded optimization of
those factors, which is beneficial to the overall performance
of OPVs.
Experimental Methods
The samples were prepared via spin-coating. First, glass
substrates were cut into approximately 0.5 in by 1 in sec-
tions, small enough to fit inside dishes used for transporta-
tion, and to minimize the need for large amounts of solu-
tion. These substrates were then cleaned ultrasonically in
soap and de-ionized (DI) water, then DI water, and finally
acetone, in that order. After each 10-minute cleaning step,
the substrates were dried using pressurized nitrogen glass,
and after the final cleaning placed in a UV-ozone cleaner for
10 minutes.
The solutions were prepared with the proportions of
PMMA and PCBM being added to a small vial, transported
into a glovebox, where the solvent was added alongside a
magnetic stirrer, and spun until dissolved. The substrates
were then placed on a spin-coater where the solutions were
dropped onto the surface and immediately spun for a minute
to insure even coverage. The samples were transported to a
hot plate to anneal.
These samples were sent to the Advanced Light Source
(ALS) synchrotron at University of California Berkeley,
where they were subject to wide-angle X-ray scattering
(WAXS) measurements at Beamline 7.3.3, shown in Fig-
ure 1. Each sample, ranging from pure PMMA to 2:8
PMMA:PCBM, was tested for a multitude of angles: 0.100o
,
0.110o
, 0.120o
, 0.130o
, 0.140o
, and 0.200o
. This data was
then input into Igor Pro programs which converts the data
from 2-D to 1-D (SAS 2D Nika) and then allows for multi-
peak fitting. The 0.130o
data was chosen to be analyzed. A
sample of the raw 2-D data is shown in Figure 2.
Figure 1[4]: Beamline layout at the ALS for WAXS measure-
ments.
The peaks, which correspond to q-values, are noted
for their location, amplitude, and FWHM (full width half
maximum). These peaks are then compared throughout
the different concentrations to identify them as PMMA or
PCBM peaks. These can be used for comparing the amount
of crystallinity in the sample for corresponding q-vectors
lengths.
∗e-mail: ebdrisco@ncsu.edu
1

2. Figure 2: Sample 2D WAXS data taken from the 6:4 ratio of
PMMA:PCBM at 0.130o. A black mask is added to the image
to remove mechanical blind spots.
Results
An example of the data and the fit for a sample is shown
in Figure 3. The data, the red trace in the middle, is com-
pared with the peaks, red on the bottom, and the baseline,
shown in green in the middle, and from this the residual,
the red line above, is found. This was done for each of the
concentrations, and each peak’s location, amplitude, area,
and FWHM are found and compared. An example for peak
6 is shown in Figure 4. Unfortunately, due to the lack of
normalization features, patterns in the amplitudes between
samples cannot be directly compared without a large possi-
bility of error.
Figure 3: The corresponding 1-D form of Figure 2, for 6:4
PMMA:PCBM at 0.130o. Eight peaks are visible through all
samples in the given range of q, with three peaks and two double
peaks visible in this example. Peak 2, a PMMA peak, is only
visible in pure PMMA and 2:8 PMMA:PCBM.
In total, eight peaks were found in the data, four of
which corresponded to PMMA and four of which corre-
sponded PCBM. From Figure 3, peaks 1,3,6, and 7 are at-
tributed to PCBM and peaks 2,4,5, and 8 are attributed
to PMMA. The PCBM peaks are not visible in the pure
PMMA sample and increase in amplitude compared with
the PMMA peaks as the PCBM concentration is increased.
The PMMA peaks though, on the most part, stay constant
and are visible through all the sample concentrations.
Figure 4: The location, amplitude, and FWHM for the PCBM
peak identified as peak 6 in Figure 3. Positive trends in the
location and amplitude are visible, along with a slight downward
trend in the FWHM, with respect to concentration of PCBM.
The four PCBM peaks appear at around 0.35 ˚A−1
, 0.70
˚A−1
, 1.28 ˚A−1
, and 1.92 ˚A−1
. The size of these peaks
display a strong crystalline structure. These peaks can be
characterized as [100], [200], amorphous, and [010] respec-
tively. The large width of the peak at 1.28 ˚A−1
is character
of an amorphous peak. The four PMMA peaks are visible
at 0.65 ˚A−1
, 0.90 ˚A−1
, 1.15 ˚A−1
, and 2.10 ˚A−1
.
Discussion
Unfortunately, normalization of the peak areas and mis-
cibility measurements were not able to be finished. This
would entail peak fitting the 0.200o
data and using peak ar-
eas from this fit to normalize the amplitudes from the 0.130o
fit. Using a visual light microscope (VLM), patterns in the
transmission and reflection of light will be recorded via a
camera and analyzed for how much the PCBM coagulates
in the PMMA matrix. Then for miscibility, a separate mi-
croscope will be directed through the PMMA parts of the
samples, analyzing the amount of PCBM still mixed in.
The results from this part of the experiment, however,
is in accordance with past measurements of PCBM in an
amorphous polymer, where PCBM peaks increase in am-
plitude and decrease in width with an increase in concen-
tration, while polymer peaks stay more or less constant[5].
The data compiled by this project can be led into more
experimentation with PMMA:PCBM and other amorphous
polymer-electron acceptor systems, and eventually in perfor-
mance of PMMA:PCBM in heterojunction with an electron
donor in a OPV or diode.
References
[1] C.M. Proctor, J.A. Love and T.-Q. Nguyen, Adv. Mater.,
26 (2014) 5957-5961.
[2] M.A. Brady, G.M. Su and M.L. Chabinye, Soft Matt., 7
(2011) 11065-11077.
[3] M.H. Lee, J.H. Jung, J.H. Shim and T.W. Kim Org.
Elect., 12(8) (2011) 1341-1345.
[4] A. Hexemer, et al., J. Phys., 247 (2010) 12007.
[5] S.V. Kesava, et al., Adv. Energy Mater., 4 (2014)
1400116.
2

3. Peak-by-Peak Analysis
3

4. 4