Supporting Information for:
On the Control of Aggregate Formation in Poly(3-hexylthiophene) by Solvent,
Molecular Weight and Synthetic Method
Christina Scharsich1, Ruth Lohwasser2, Michael Sommer2, Udom Asawapirom3§,
Ullrich Scherf3, Mukundan Thelakkat2, Dieter Neher4, Anna Köhler1*
1
Organic Semiconductors, Experimental Physics II, Department of Physics, University of
Bayreuth, Bayreuth 95440, Germany.
2
Applied Functional Polymers, Macromolecular Chemistry I, Department of Chemistry,
University of Bayreuth, Bayreuth 95440, Germany
3
Macromolecular Chemistry, Bergische Universität Wuppertal, Wuppertal 42097, Germany
4
Soft Matter Physics, Institute of Physics and Astronomy, University of Potsdam, Potsdam
14476, Germany
e-mail: anna.koehler@uni-bayreuth.de
Absorption spectra of aggregated chains
This section describes how we derived the absorption spectra of chains in the
aggregated state. To induce poly(3-hexylthiophene) chains to aggregate, we
prepared solutions with different ratios of good:poor solvent (chloroform:ethyl
acetate). The measured absorption of these solutions is caused by coexisting coiled
and aggregated chains. The resulting isosbestic point (see Fig. 1 in the manuscript)
indicates that there are only these two species of chains, namely dissolved, and thus
coiled, polymer chains and planarized, aggregated chains. The latter show
structured and red-shifted absorption compared to the coiled chains.
Figure 1 illustrates exemplarily for each molecular weight compound how the
absorption spectra of the aggregated chains were extracted from the measured total
spectra. The shape of the absorption spectrum of well dissolved, coiled P3HT was
measured in 100% chloroform solution and scaled to fit the high energy shoulder of
the examined solution spectrum without changing its position. This scaled spectrum
of the coiled chains was then subtracted yielding the absorption spectrum due to
pure aggregates present in the according solution.
Figure 1
Measured absorption spectra for solutions with good:poor solvent (CHCl3:EtAc)
(black line) containing coexisting coiled and aggregated chains. The absorption of
well dissolved, coiled P3HT in 100% chloroform (red line) was scaled to fit the high
energy shoulder of the solution spectrum. The spectrum of the coiled chains was
then subtracted yielding the absorption due to aggregates (blue line).
Determination of relative oscillator strength
For our interpretation, we need to know the fraction of chains that are aggregated.
Above, we have derived which fraction of absorption is due to aggregates. However,
the fraction of aggregate absorption is not equal to the fraction of aggregates, since
the chain may have different oscillator strength in the coiled form and in the
aggregated form. The relative oscillator strength describes the change in oscillator
strength when going from coiled to aggregated P3HT chains. Knowledge of this
relative oscillator strength is therefore necessary to determine the fraction of
aggregates from the area of the aggregate absorption.
To determine the relative oscillator strength, we followed the procedure by Clark et
al.
1
Figure 2 illustrates the procedure. As outlined above, the total absorption
spectrum (blue solid line) can be deconvoluted into the absorption caused by
dissolved, coiled chains (black dashed line) and the absorption caused by
aggregated chains (blue dashed line). The change in absorption of the coiled chains,
Acoiled, corresponds to the difference between the spectrum from 100% coiled
chains (black solid line) and the fraction of absorption by the coiled chains in the
measured spectrum (black dashed line). In the left part of Figure 2, Acoiled is
indicated by the shaded area.
The corresponding increase in absorption of the aggregated chains, Aaggregate, is
given by the absorption of the aggregated chains formed (blue dashed line).
Aaggregate is shown as shaded area in the right part of Figure 2. Any difference
between the decrease of absorption by coiled chains and the increase in absorption
by aggregated chains must arise from a change in relative oscillator strength F as
follows.
F
ΔA aggregate
ΔA coiled
(1)
Figure 2
Exemplarily absorption spectra for P3HT solutions with solvent induced aggregate
absorption (blue solid line) indicating the change in oscillator strength between coiled
and aggregated chains (grey areas). The absorption of 100% well dissolved, coiled
chains is presented as solid black line. The dashed lines show the absorption due to
aggregated (dashed blue line) and coiled (dashed black line) chains, respectively.
Top figures show absorption of 30:70 (CHCl3:EtAc) solution of the defined 5 kD
sample and bottom figures show 80:20 (CHCl3:EtAc) solution of the 11 kD sample.
The resulting change in relative oscillator strength F for solutions with different ratios
of good:poor solvent (chloroform:ethyl acetate) for the defined and extracted poly(3hexylthiophene) compounds is summarized in Figure 3. Clark et al. obtained a
relative oscillator strength of (1.39 ± 0.10) for commercial P3HT in solution going
from 70° C to room temperature. This is in good agreement with the relative
oscillator strength we obtained for the solutions of the 19 kD samples with 25% of
poor solvent and higher.
Figure 3
The relative oscillator strength between coiled and aggregated P3HT chains as
function of poor solvent fraction for defined and extracted P3HT as indicted in the
figure. The molecular weight increases from top to bottom.
Franck-Condon analysis
The quantitative analysis of the P3HT aggregate absorption yields the excitonic
coupling J between the polymer chains within these aggregates. According to the
work of Spano2, the coupling is determined by the ratio of the first two absorption
peaks, A0-0/A0-1, as follows
𝑆𝜈
𝐽
𝐴0−0
𝐴0−1
=
𝑛0−0
𝑛0−1
2
(1−𝜔 𝑒 −𝑆 ∑𝜈>0𝜈!𝜈)
0
𝑆𝜈
𝐽
2
,
𝑆(1−𝜔 𝑒 −𝑆 ∑𝜈≠1𝜈!(𝜈−1))
0
where ω0 is the effective vibrational energy of the single emitter and S its Huang
Rhys parameter, n0-0 and n0-1 refer to the refractive index at the position of A0-0 and
A0-1. Thus, the correct determination of the excitonic coupling J requires the
knowledge of Huang Rhys parameter S and the effective vibrational energy ω 0 of the
single, i.e. non-aggregated, emitter. Both parameters can be extracted from a
Franck-Condon analysis of the photoluminescence of the single emitter (a nonaggregated chain). Therefore, we measured photoluminescence spectra of dilute
P3HT
solutions
in
100%
chloroform.
Figure
4
shows
the
experimental
photoluminescence data (blue symbols).
For the analysis of the excitonic coupling J, we need only a vibrational energy for an
effective oscillation. Thus, a single mode Franck-Condon analysis is sufficient. In a
Franck-Condon analysis, the photoluminescence spectrum is modeled as a sum of
Gaussian shaped transitions, Γ, from the first excited state S1(m=0) to the ground
state S0(m) with m=1,2,3,… being the vibration quantum number.
3
The fitting
procedure demands a normalization of the photoluminescence signal P(ћω) given by
P(ω)
Sm

Γδ[ω  (ω 0  m ω i )]

n 3 (ω)3 I 00
m m!
,
(2)
where I0-0 is the intensity of the emission from the 0-0 vibrational level of the first
excited state to the 0-0 vibrational level of the ground state. The integer m denotes
the excitation level of the vibration with energy ћωi and n is the refractive index of the
surrounding material at photon energy ћω. Here, the material surrounding the single
emitters was the solvent, chloroform. Its refractive index was calculated from the
Cauchy parameters published by Samoc.
4
Figure 4
Single mode Franck-Condon fits (red line) for the single emitter emission spectra in
dilute solution for defined P3HT. The molecular weight increases from top to bottom.
The experimental data (blue symbols) were fitted with Gaussian functions (grey
lines) at four vibration levels. The residue is presented as cyan line.
The Franck-Condon fits according to equation 2 were calculated using four
vibrational levels (m= 0, 1, 2, 3, 4) of the effective oscillator ħω0 and yield the fitting
parameters summarized in Table 1. The Franck-Condon fits (red line in Figure 4)
reproduce the experimental data with high accuracy. Thus, the resulting parameters
S and ω0 enable a precise calculation of the free excitonic coupling J.
P3HT
S
ω0
σ
5 kD
0.90
1390 cm-1
0.084 eV
11 kD
0.84
1390 cm-1
0.079 eV
19 kD
0.84
1390 cm-1
0.080 eV
Table 1
Fitting parameters of the Franck-Condon analyses for the poly(3-hexylthiophene)
(P3HT) single emitter photoluminescence spectra: Huang Rhys parameter S, energy
of effective oscillation ω0 and Gaussian standard deviation σ.
Absorption spectra of thin films
We want to investigate whether the results obtained for P3HT aggregates in solution
can be transferred to P3HT films that were spun from solution already containing
aggregates due to poor solvent fractions. Therefore, we studied the absorption of the
P3HT thin films, shown in Figure 5 normalized to the 0-1 absorption peak for the
defined P3HT samples. The absorption spectra of the films show both, a structured
absorption due to aggregated chains at low energies and absorption around 2.8 eV
and higher due to non-aggregated coiled chains in amorphous regions. We find that
films spun from solutions containing fractions of poor solvent have higher 0-0
absorption indicating aggregates with lower excitonic coupling and thus higher
conjugation length. Similar absorption spectra can be obtained for the extracted
P3HT samples as well.
Figure 5
Absorption spectra of thin films made from defined P3HT, normalized to the second
absorption peak. Films were spun from solutions with ratios of good:poor solvent
(CHCl3:EtAc) as indicted in the figure.
Assessment of thermal equilibrium state in solution
The absorption spectra of P3HT in a solvent mixture of chloroform and ethyl acetate
do not show any changes on a 24 hour timescale. We therefore wanted to know
whether the conformation obtained in such a solution corresponds to a thermal
equilibrium state. To probe this, P3HT was dissolved in a chloroform:ethyl acetate
50:50 mixture at a concentration of 0.25 mg/ml prepared in two different ways. In the
first approach, P3HT was fully dissolved in chloroform. Then, the necessary amount
of ethyl acetate was added at once. In the second approach, P3HT was again fully
dissolved in chloroform, but the necessary amount of ethyl acetate was added very
slowly (0.4 ml/h) in small amounts while stirring using an automated syringe. The
absorption spectra resulting from the two modes of preparation are different, with
larger aggregates falling out of solution in the second mode of preparation. From this
difference we infer that that the solutions prepared by the first mode are not in a
thermal equilibrium state.
References
1. Clark, J.; Chang, J. F.; Spano, F. C.; Friend, R. H.; Silva, C. Applied Physics
Letters 2009, 94, 3.
2. Spano, F. C. Journal of Chemical Physics 2005, 122, -.
3. Ho, P. K. H.; Kim, J. S.; Tessler, N.; Friend, R. H. Journal of Chemical Physics
2001, 115, 2709-2720.
4. Samoc, A. Journal of Applied Physics 2003, 94, 6167-6174.