Supporting Information
The phase behavior of a polymer-fullerene bulk heterojunction system that contains bimolecular crystals
Nichole Cates Miller,1 Roman Gysel,1 Chad E. Miller,1,2 Eric Verploegen,2,3 Zach Beiley,1 Martin Heeney,4 Iain
McCulloch,4 Zhenan Bao,3 Michael F. Toney,2 Michael D. McGehee1
1
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
Stanford Synchrotron Radiation Laboratory, Menlo Park, California 94025
3
Department of Chemical Engineering, Stanford University, Stanford, California 94305
4
Department of Chemistry, Imperial College, London SW7 2AZ, U.K
2
Calculated and Measured PC71BM Melting Enthalpies
We compared the PC71BM melting enthalpies in the hypereutectic mixtures to enthalpies calculated by scaling
the melting enthalpy of pure PC71BM by the weight fraction of the pure PC71BM phase at the PC71BM melting
temperature. Note that the weight fraction of the pure PC71BM phase, which was calculated using the lever
rule, is not equivalent to the overall PC71BM weight fraction since some PC71BM is in the liquid phase. Figure
S1 shows a plot of the calculated enthalpies versus the measured enthalpies as well as a line indicating where
the calculated and measured values are equal. The good agreement between the calculated and measured values
verifies the pBTTT:PC71BM phase diagram.
Figure S1: The calculated PC71BM melting enthalpies for the hypoeutectic blends versus the measured values.
Zoomed 80:20 pBTTT:PC71BM GIXS Patterns
A
B
Figure S2: Zoomed GIXS images showing two sets of lamellar peaks in 80:20 pBTTT:PC71BM at 25 °C (A)
and 180 °C (B). The horizontal and vertical axes show -0.35 Å-1 ≤ qxy ≤ 0.35 Å-1 and 0 Å-1 ≤ qz ≤ 1.35 Å-1,
respectively.
1
Supporting Information
Rapid Thermal Annealing (RTA)
50:50 pBTTT:PC71BM blends were heated using RTA. The RTA procedures that kept the samples above the
eutectic temperature (~225 °C) for ~11 seconds and ~19 seconds produced intercalated pBTTT:PC71BM blends
as indicated by the lamellar spacing of ~21 Å for these blends (Figure S3). The blend heated above 225 °C for
~34 seconds shows peaks from both pure pBTTT and the bimolecular crystal, indicating that RTA can be used
to suppress interaction. Unfortunately, micron-scale phase separation, which is detrimental to solar-cell
performance, exists in the sample that was kept above 225 °C for ~34 seconds, which is the only sample for
which RTA suppressed the intercalation (Figure S4). Auger elemental analysis was used to show that the xshaped crystals are PC71BM.
~11 seconds
~19 seconds
~34 seconds
Intensity (arb. un.)
1000
pBTTT
BMC
d200
100
10
BMC
d100
1
0.1
d200
0.2
pBTTT
d100
0.3
0.4
0.5
q (1/Å)
Figure S3: Ex-situ specular X-ray diffraction measurements of 50:50 pBTTT:PC71BM blends annealed using
RTA procedures that heated the samples above the eutectic temperature of 225 °C for ~11 seconds (black), ~19
seconds (red), and ~34 seconds (blue).
Figure S4: Optical microscopy images of 50:50 pBTTT:PC71BM blends after RTA heated the samples above
225 °C for ~11 (A), ~19 (B), and ~34 seconds (C).
2
Supporting Information
Differential Scanning Calorimetry Measurements
When pBTTT:PC71BM blends are heated above ~225 °C, the bimolecular crystals disappear, giving way to a
liquid phase and either pure pBTTT or pure PC71BM depending on the blend ratio. It appears that there is not
enough time for the bimolecular crystals to completely reform upon cooling at a rate of 10 °C/minute. Figure
S5A shows three heating runs for 40:60 pBTTT:PC71BM (pure bimolecular crystal) in the case where the first
run heats the samples above the eutectic temperature (~225 °C). The BMC  L + PC71BM peak (~225 °C) is
significantly larger for the first scan than for the second two scans. We believe that the larger enthalpy
associated with the peak at 225 °C in the first scan is caused by interactions in the bimolecular crystal. This
peak is much smaller in the second and third scans due to incomplete intercalation. In addition, the absence of a
side-chain melting peak (~137 °C) in the first scan and its appearance in the second and third scans indicate that
there is no pure pBTTT in the first scan, but that there is pure pBTTT in the second and third scans.
A
B
Figure S5: The first (blue), second (red) and third (green) DSC heating curves for a 40:60 pBTTT:PC71BM
blends with heating and cooling rates of 10 °C/min. The first scan heats the samples above 225 °C, the eutectic
temperature, in (A), whereas the first scan only heats the blend to 170 °C in (B).
Figure S5B shows the 40:60 pBTTT:PC71BM heating curves in the case where the first run only heats the
samples to 170 °C. In this scan, the second run has a large BMC  L + PC71BM peak, which gets smaller on
the third run. There is also no side-chain melting peak in the second scan, but this peak does occur in the third
scan. These results provide further evidence that the bimolecular crystals do not completely reform upon
cooling at a rate of 10 °C/minute.
However, we know that the intercalated phase is stable, since bilayer pBTTT:PC71BM films, which are not
intercalated prior to annealing, become intercalated upon annealing at temperatures of 125, 150 and 180 °C.1
References
1.
Gysel, R.; Cho, E.; Miller, N. C.; Risko, C.; Kim, D.; Miller, C. E.; Richter, L.; Kline, R. J.; Heeney, M.;
McCulloch, I.; Toney, M. F.; Brédas, J.-L.; McGehee, M. D. under review.
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