Controlling Poly(3-butylthiophene) Assembly in Solution Manuscript

Controlling Poly(3-butylthiophene) Assembly in Solution
and Thin Films by tuning H-bonding in Chloroform-Acetone
Binary Solvent: Towards Improved Structure and Optical
Response of Thin Films
Journal: Nanoscale
Manuscript ID: NR-ART-01-2015-000360
Article Type: Paper
Date Submitted by the Author: 17-Jan-2015
Complete List of Authors: Stanford, Michael; University of Tennessee,
Hu, Davis; University of Tennessee,
Keum, Jong; Oak Ridge National Laboratory, Spallation Neutron Source
Zhu, Jiahua; Oak Ridge National Laboratory, Center for Nanophase
Materials Science
Hong, Kunlun; Oak Ridge National Laboratory, Center for Nanophase
Materials Science
Hu, Bin; The University of Tennessee, Department of Materials Science and
Engineering
Sumpter, Bobby; Oak Ridge National Laboratory, Center for Nanophase
Materials Science
Smith, Sean; Oak Ridge National Laboratory, Centre for Nanophase
Materials Sciences
Ivanov, Ilia; Oak Ridge National Laboratory, Center for Nanophase
materials Science
Nanoscale

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Page 1 of 13 Nanoscale

Dear Prof. Dirk Guldi,
We would be pleased to submit our manuscript " Controlling Poly(3-butylthiophene) Assembly in Solution and Thin
Films by tuning H-bonding in Chloroform-Acetone Binary Solvent: Towards Improved Structure and Optical
Response of Thin Films" for consideration of publication as an article in Nanoscale.
The manuscript addresses and important topic on control of optical properties and structure of polythiophenes via tunable
aggregates in solutions and films cast from a binary solvent mixtures. The manuscript combines results of optical diagnostic of
polymer aggregation with modeling of optical properties of aggregated using Large-scale Atomic/Molecular Massively Parallel
Simulator (LAMMPS) for Raman spectra and Density Functional Theory Tight Binding (DFTTB) calculations to electronic
spectra of different polymer aggregates. Application of multivariable Component Analysis (MCA) to study dynamic changes in
the electronic spectral features of P3BT as a function of solvent quality allowed deconvolution of the dynamic changes in
concentrations of amorphous and aggregate components during aggregation.
Excitonic coupling and bandwidth calculations from electronic absorption reveal comparable inter- and intra-chain order for
aggregate nanofibers in the solution phase and their resultant thin films. X-ray diffraction, employed to determine structural
properties, reveal a crystallite size of 90 Å in films cast from nanofibers thus revealing similar nanofiber diameter although
nanofiber concentration was revealed to be dependent upon marginal solvent concentration. Combined spectral features and
diffraction patterns help provide fundamental insight into the correlation of structural and optical signatures of P3BT nanofibers
in binary solvents and thin films.
The results reported in the manuscript present scientific and practical importance, especially in application for
controllable assembly of polymers for flexible electronics and photovoltaics.
Thank you for your consideration and time.
Best regards
Ilia Ivanov
Center for Nanophase Materials Sciences
Oak Ridge National Laboratory
1 Bethel Valley Rd, bld. 8610, M166
Oak Ridge TN 37831-6488
Tel. (865)7716213
ivanovin@ornl.gov
Page 2 of 13Nanoscale

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Controlling Poly(3-butylthiophene) Assembly in Solution and Thin
Films by tuning H-bonding in Chloroform-Acetone Binary Solvent:
Towards Improved Structure and Optical Response of Thin Films
Michael G. Stanforda,b
, Davis Hub
, Jong K. Keuma
, Jiahua Zhua
, Kunlun Honga
, Bin Hub
, Bobby G.
Sumptera
, Sean Smitha
and Ilia N. Ivanova,
*5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Optical properties and structure of poly (3-butyl-thiophene) (P3BT) aggregates were studied in solutions and films cast from a binary
solvent. In this study, chloroform and acetone (marginal solvent) were utilized as the binary solvent to incrementally tailor solvent
polarity, thus influencing solution phase P3BT aggregation. Multivariable Component Analysis (MCA) was employed to study dynamic10
changes in the electronic spectral features of P3BT as a function of solvent quality and thereby to deconvolute the pure spectra of
amorphous and aggregate components. From MCA deconvolution, we observed two distinct regions of aggregate formation as a
function of marginal solvent. For P3BT solutions of 0-17 vol% acetone, a nanofiber growth region (Region I) was exhibited where
increasing acetone concentration drove the onset of nanofiber formation. Exceeding 20 vol. % acetone, P3BT nanofiber saturation
occurred due to the critical solvent-solute interaction energy which drove incorporation of free chains into π-stacked nanofibers.15
Comparable spectral features were exhibited in films cast as a function of marginal solvent concentration. Excitonic coupling and
bandwidth calculations from electronic absorption reveal comparable inter- and intra-chain order for aggregate nanofibers in the solution
phase and their resultant thin films. X-ray diffraction, employed to determine structural properties, reveal a crystallite size of 90 Å in
films cast from nanofibers thus revealing similar nanofiber diameter although nanofiber concentration was revealed to be dependent upon
marginal solvent concentration. Combined spectral features and diffraction patterns help provide fundamental insight into the correlation20
of structural and optical signatures of P3BT nanofibers in binary solvents and thin films.
Introduction
The past two decades have seen the rise of the organic
electronics including field-effect transistors, light emitting25
devices, and organic photovoltaic cells with performance slowly
approaching that of silicon counterparts1-3
. A common component
of these devices is poly(3-alkyl thiophene) (P3AT), which
enables light absorption and transport of holes through the
network of amorphous and crystalline domains.4-9
The30
morphology of polycrystalline P3AT films is dependent on
multiple parameters including the length of the alkyl substituent,
regioregularity, polydispersity, and molecular weight of P3AT as
well as processing conditions such as solvent boiling point and
substrate passivation.10,11,12,13,14
This results in a broad variation35
of device performances15-17
. Thermal and solvent annealing
further increase the size of the crystalline domains and decrease
the fraction of amorphous polymer5,18
. Slow growth of polymer
aggregates in binary solvents allows further control of the
polymer aggregation to enhance charge transport19,20
and increase40
structural order, ,21,22
leading to a diversity of polymer
morphologies including nanofibers23,
, nanosheets24
, branched
nanostructures25
, discoids
16
, rectangular parallelpipeds
11
, and
nanoribbons
21
.
Changes in the electronic absorption and luminescence45
spectra associated with the formation of P3AT aggregates in the
solutions are usually described in terms of polaronic Frenkel
excitons and the H- and J-aggregate model developed for small
molecules by M. Kasha and applied to P3AT by F. Spano26,27
.
The vibronic progression observed in the luminescence spectrum50
of aggregates is associated with excitation in symmetric C=C
stretching mode, which is coupled with the electronic excitation.
The difference in the selection rules for H-and J-aggregates
produces signature features in the absorption and emission
spectra indicative of inter and intra-molecular interactions
17,28
.55
Long-range intra-chain order (and aggregate planarity) which
suppresses inter-chain exciton coupling in J- aggregates of P3HT
can be eased to induce H-type aggregate structure by lowering
temperature or applying pressure29
. Within an isolated aggregate,
a combination of H- and J-like emission was reported by Grey30
60
and Barns31
which can be described by a model of J/H
aggregation based on HOMO-LUMO overlap of adjacent
polymer chains, which arises as a result of charge transfer
interactions between chains
28
.
Analysis of absorption, excitation, luminescence65
spectra or Raman spectra of aggregates usually involves peak
fitting or direct measurement of the ratio of the vibronic peak
intensities used to identify the type of aggregate and the extent of
excitonic bandwidth coupling
22,32-35
. Here we apply
Multivariable Component Analysis (MCA) to study dynamic70
changes in the spectral features of poly-3 butyl thiophene (P3BT)
with an increasing fraction of marginal solvent (acetone). MCA
of electronic absorption, luminescence and Raman spectra
enables deconvolution of pure spectral features of aggregates and
their concentration-profiles as a function of marginal solvent for75
solutions and thin films. Measurements of excitonic coupling and
excitonic bandwidth using the deconvoluted spectra of aggregates
allows characterization of the aggregates free of the influence of
contributions from non-aggregated forms. The electronic spectra
of P3BT aggregates were also derived from the Density80
Functional Theory Tight Binding (DFTTB) calculations.
Molecular dynamics simulation of the polymer chain structure as
a function of binary solvent composition were carried out with

This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3
Large-scale Atomic/Molecular Massively Parallel Simulator
(LAMMPS). The geometries used to obtain the derived Raman
and electronic absorption spectra are defined as unimers, dimers,
and trimers in order to study the effects of interchain interactions.
The structural order and crystallite size of P3BT aggregates in5
thin films as a function of marginal solvent concentration was
obtained from TEM and x-ray diffraction measurements.
Experimental
Materials and Sample Preparation. Regio-regular poly(3-
butylthiophene) (P3BT) was synthesized (Mw = 14.9kg/mol, PDI10
= 1.09, RR > 95%) and dissolved into chloroform. All chemical
grade solvents used in this study were purchased from Sigma-
Aldrich and used as received without further purification.
The P3BT was first fully dissolved in chloroform at
ambient temperature in order to confirm that aggregation seen in15
this study was resultant of interactions between poor solvent and
polymer. Absorption spectroscopy was used as a non-destructive
bulk method to confirm complete dissolution. Acetone (poor
solvent) was then incrementally added to the well dissolved
solutions. For simplification, we adopted an abbreviation system20
for binary solvents used as follows: Volume % chloroform /
volume % acetone. The following solvent volume % ratios were
used: 100/0, 92/8, 88/12, 83/17, 80/20, 75/25, 71/29, 67/33.
Dilute solutions of 0.01 wt% P3BT were used to study the optical
signatures of aggregation due to high transparency. The dilute25
nature of the solutions also isolate the solvent effects on
aggregation, hence we do not demonstrate concentration driven
aggregation which would be present in more concentrated
solutions. In order to characterize thin films, each solution was
drop cast on clean Si/SiO2 and quartz substrates. Substrates were30
sectioned and rigorously cleaned by sonication in chloroform,
acetone, and isopropyl alcohol prior to film deposition.
Instrumentation. Optical absorption spectra were recorded
using a Cary 5000 UV-vis-NIR spectrometer in spectral range
between 350 to 800nm. Photoluminescence was measured using a35
Jobin Yvon Horiba Fluorolog Flourometer with an integration
time of 1.0s with a slit size of 3 nm on excitation and emission
monochromators. The emission measurements were recorded
using a right-angle configuration for solutions and front-face
configuration for thin films. Raman spectra were recorded using a40
Renishaw micro Raman Microscope 1000. A 633 nm HeNe laser
was used to excite the samples. Raman mapping acquisitions
were recorded in static mode using a 1s acquisition time and 100x
objective with 0.6 μm step size.
The molecular order of P3BT films were examined by45
x-ray diffraction. One-dimensional scans were performed using a
PANalytical Powder Diffractometer with x-rays generated at
45kV/40mA. Grazing incidence x-ray diffraction (GIXD) was
utilized on the P3BT films in order to determine film orientation
with respect to the substrate. Specifically an Anton Paar SAXSess50
mc2
system equipped with a multipurpose VarioStage was
utilized. X-rays were generated at 45kV/40mA with a wavelength
of λ = 1.541 Å. The angle of incidence was set to 0.2o
in order to
record 2D scattering patterns which revealed long range
aggregation and molecular order of the thin films. The 2D images55
consist of in-plane (qxy) and out-of-plane (qz) components. An
image plate (Multisensitive Storage Phosphor) recorded the 2D
diffraction pattern and was read using a Perkin Elmer Cyclone®
Plus Storage Phosphor System. Transmission electron
microscope (TEM) images recorded aggregate dimensionality for60
solutions cast from 88/12 and 80/20 binary solvent P3BT
solutions. Specifically, a Libra 120 PLUS TEM from Carl Zeiss
was used to record images.
Multivariable Component Analysis (MCA) of normalized spectra
(absorption, luminescence and Raman) was done using “The65
Unscrambler X” software (CAMO Software AS) and non-
negative concentration, non-negative spectra, closure and
unimodality constrains.
Results and Discussion70
The electronic π-π* transition of conjugated polymers are
intimately related to the electronic structure of the HOMO and
LUMO and coupling with vibrational transitions. The Franck-
Condon progression of P3BT is significantly altered with
molecular aggregation, thus providing selection rules unique to75
aggregates. These selection rules give rise to unique spectral
vibronic progression signatures which may be revealed by
absorption, emission, and Raman spectroscopy for solutions and
films alike16,31 ,32
.
Density Functional Theory Tight Binding (DFTTB)80
calculations were used to model the effects of interchain
interactions on the electronic absorption spectra of P3BT. The
derived spectra, which account for the complex structural and
electronic interactions between chains, are shown in Fig. 1. An
evident redshift in the absorption spectrum of the dimer and85
trimer reveals the onset of low energy states. The rise in peak
intensities at 560 and 610 nm exhibited in the dimer and trimer
components correspond with 0-1 and 0-2 vibronic progressions34
.
The trimer is characterized by the same low energy absorption
peak as the dimer, and by two additional high energy transitions90
suggesting H- character of the trimer. Thus, already at the stage
of trimer aggregation we observe evidence of mixed H/J
Figure 1: The absorption spectra of P3BT unimer, dimer, and
trimer determined from Density Functional Theory Tight Binding
(DFTTB) calculations.

aggregation of P3BT.
Figure 2a reports changes in an aged P3BT solution absorption
spectrum, observed experimentally as the fraction of marginal
solvent is increased. Freshly aggregated P3AT nanoparticles in
binary solvents are kinetically unstable, and their “aging” leads to5
the formation of more stable nanofibers.36,37
The spectrum of the
P3BT unimer is characterized by a broad peak with maximum at
410 nm corresponding to π-π* transition. As the fraction of
marginal solvent (acetone) increases the π-π* peak red-shifts to
485 nm and new peaks at 560 and 610 nm, consistent with the10
interpretation that 0-1 and 0-0 peaks of vibronic progressions34
are being formed. The quantized vibronic progression (0-0 and 0-
1) of absorption bands with aggregation is in agreement with the
results of the DFTTB simulations. A 0.17 eV energy difference
between 560 and 610 nm vibronic peaks coincides with energy of15
the C=C symmetric vinyl stretch of the thiophene ring,
confirming interaction between thiophene -systems within the
aggregate.
Multivariable Component Analysis (MCA) of the absorption
spectra reveals two pure components in P3BT solutions during20
acetone-induced aggregation, Fig. 2b. The first component with a
broad absorption peak at 407 nm corresponds to the random
P3BT coils. The second component absorption spectrum has a
broad peak at 495 nm and two shoulders at 560 and 606 nm
consistent with the spectral features of P3BT aggregates38
. MCA25
allows quantification of the fractional contribution of each
component to the absorption spectra as a function of acetone
fraction, Figure 2c, clearly indicating two distinct regions (I and
II). Region I (0-17 vol. % acetone) reveals the onset of
aggregation directly proportional to acetone concentration and30
Region II shows an absorption spectrum dominated by P3BT
aggregates (17-30 % vol. acetone). The threshold of 17 % vol.
acetone to aggregate all P3BT in chloroform is about 5% higher
than previously reported solubility parameters of the
P3AT/chloroform/acetone system39
.35
The order of aggregates on the molecular level can be
characterized in terms of excitonic coupling, the coupling of
electronic excitation to molecular elongation, which largely
depends upon molecular order. The excitonic coupling constant,
j0, is estimated from the relative 0-0/0-1 absorption intensity40
ratios in eq. 1,40
Eq (1)
45
where εp is the energy of the vibronic transition coupled to the
electronic excitation, which corresponds to the energy difference
between 0-0 and 0-1 vibronic peaks, 0.17 eV. Excitonic coupling
and bandwidth were calculated in accordance with Ref. 32from50
the absorption spectra of P3BT aggregates which were
deconvoluted using MRC, and found to be 47.3 meV and 133
meV respectively. The positive value of the excitonic coupling
indicates the contribution of weak inter-chain coupling which
attenuates the 0-0 vibronic transition in the solution of P3BT,55
according to Spano’s H-J aggregation model. The values of
excitonic coupling and bandwidth of P3BT aggregates in the
solution will be used to reference molecular order in P3BT thin
films. However, it is well known that absorption spectra are less
sensitive for determining inter/intra-chain coupling than60
emission. We will explore this later.
Figure 3a shows an interesting progression in the
photoluminescence of P3BT solutions with the incorporation of
marginal solvent. The spectrum of the well dissolved P3BT is
characterized by a broad peak at 595 nm with a pronounced65
shoulder at 670 nm. PL peak position exhibits a blue-shift to 565
nm as aggregate-inducing marginal solvent is incorporated into
2
0
0
10
00
146.0
1
48.0
1

















p
p
j
j
I
I


Figure 3: (a) Emission spectra for P3BT solutions in different
binary solvent combinations. (b) Deconvoluted emission spectra
of components within films calculated from a multi-component
regression of the emission spectra shown in (a). (c) Relative
contribution of P3BT unimer and perturbed species to the
combined emission spectra.
Figure 2: (a) Normalized absorption spectra for P3BT
solutions of differing chloroform/acetone volume ratios
(denoted in figure legend). (b) Deconvoluted spectra for
random coil and aggregates species formed within the binary
solvent. (c) Contribution of random coil and aggregate species
to the combined absorption spectra as a function of acetone vol.
%.

the solution. The blue-shift with acetone addition may seem
counterintuitive since aggregation normally exhibits a
characteristic red-shift and broadening of spectral line widths,
however this PL behavior can be explained by considering the
emissive characteristics of weakly bound aggregates. H-5
aggregation results in some quenching of the radiative emission
process due to selection rules. Also, as acetone is introduced, H-
aggregates begin to form and settle out of the solution with little
contribution to the photoluminescence spectra. Some P3BT
unimers remain well-disperse and suspended in the solution even10
after the introduction of marginal solvent. Hence, after the
introduction of marginal solvent, a narrow linewidth PL spectra is
exhibited which is consistent with that of an isolated P3AT
unimer41
. When marginal solvent concentration is sufficiently
high (>20 vol% acetone), unimer concentration left suspended in15
the solution decreases and results in a decrease in PL intensity.
MCA of the spectra reveal two components (Fig. 3b) in the
P3BT solutions during acetone-induced aggregation. A broad
spectra with a peak at 595 nm is exhibited in low marginal20
solvent concentration solutions. Broadening occurs due to weak
interactions between neighbouring unimers suspended within the
solution. The narrow spectra with a peak at 565 nm corresponds
with the well-dispersed P3BT unimers suspended within the
solution which is prevalent once H-aggregates settled out of the25
solution. There is no significant PL contribution from the
aggregates since they settled out.
Chloroform + acetone binary solvent is known for its negative
deviation from the Raoult’s law, which is explained in terms
strong C

-H…O=C hydrogen bonding,42
which changes the30
structure of the binary solvent and also exerts influence on the
structure of solute, including large protein molecules.43
A free
energy of mixing of chloroform and acetone exhibits a broad
minimum of around ~ 20 vol. % of acetone, suggesting direct
influence of C

-H…O=C hydrogen bond on the optical35
properties of fresh and aged P3BT through -system
planarization and intrachain interaction. The C

-H…O=C
interaction can be further increased in the presence of solute
through cooperative phenomena.44
The P3BT aggregate films cast from 88/12 and 80/2040
chloroform/acetone vol. % solutions were examined using
transmission electron microscopy (TEM), Figure 4. These
samples represent aggregates formed in the growth region (I), and
in the saturated region (II). Average NF size in the region (I) is
400 nm long with a diameter of 6.5+0.5 nm. In the saturation45
region (> 17 vol. % acetone), nanofibers are formed with similar
diameters but the aggregates cluster with a diameter 12.6+0.5 nm
and length exceeding 1m. This reveals the additive aggregation
of P3BT chains with the incorporation of a marginal solvent.
Normalized absorption spectra of the NF seeded thin films are50
shown in Figure 5a. The high energy π-π* transition is discerned
in solutions formed from negligible concentrations of marginal
solvent. Contribution of the 0-1 and 0-0 vibronic transitions at
560 and 610 nm respectively are prevalent in all films cast from
binary solution with > 8 vol. % of acetone.55
MCA deconvolution of P3BT film absorption spectra reveals
two components corresponding to amorphous and aggregate
species, Figure 5b. The P3BT aggregation completes at 12 vol. %
acetone as compared to 17 vol. % for aggregation in solutions. It
is worth noting that P3BT NFs may act as seeds for the60
nucleation of weakly ordered aggregates upon solvent
evaporation when drop cast. This may be caused by NF- seeded
aggregation during film spin-casting.
The P3BT aggregates in binary solvent and film have an
excitonic coupling of 47.3 meV and 45.8 meV, respectively, as65
determined from the deconvoluted spectra using Equation (1).
These values suggest inter-molecular nature of P3BT aggregate
of roughly the same molecular order present in the solutions and
films. Therefore, evaporation of binary solvent has minimal
effects on the order of aggregates formed in the solution phase. A70
positive excitonic coupling suggests that inter-molecular coupling
is dominant with consideration of the more comprehensive HJ
aggregate model. Nearly-alike values of the excitonic bandwidth
(W) of the aggregates within the solutions and films, estimated to
Figure 5: (a) Absorption spectra for films cast from varying
chloroform/acetone ratios. A rise in vibronic progression (560
and 610nm) correspond with films cast from high volume %
acetone. (b)Deconvoluted absorption spectra of amorphous and
aggregate species within the film. (c) Contribution of each
component to the combined absorption spectra as a function of
acetone vol. %.
Figure 4: TEM images showing P3BT aggregate growth
formed in (a) 88/12 and (b) 80/20 chloroform/acetone
solutions. Nanofibers are formed with the addition of
acetone. Images represent (a) nanofiber growth region I and
(b) nanofiber growth termination region II.

be 133 and 129 meV, suggests similar conjugation length and
intra-chain order, further confirming successful transfer of
structural aggregates from solution to the P3BT film.
The peaks at 606, 660, and 715 nm in photoluminescence
spectra for P3BT films drop cast from the series of binary5
solvents and shown in Fig. 6a were assigned to 0-0, 0-1, and 0-2
transitions in vibronic progression. MCA deconvolution of PL
spectra reveals two components with characteristic features of
amorphous and aggregated P3BT, Figure 6b. The amorphous
species corresponds with unimers which were not incorporated10
into aggregates and have a PL peak which corresponds with that
of the unimer species in the solution. The concentration of
aggregates level off in films cast from binary solvent with ~17-20
vol % acetone, as indicated in Fig. 6c. This is in strong agreement
with aggregate saturation levels exhibited in the solution phase.15
However, this differs from the saturation indicated from film
absorption because the emission process is drastically different in
highly ordered NFs as opposed to weakly coupled evaporation
driven aggregates. High NF molecular order results in chain
planarization which readily permits radiative emission. However,20
torsional rotation between thiophene rings in weakly ordered
aggregates tends to attenuate radiative processes. Therefore, when
species contribution to the absorption (Fig. 5c) and
photoluminescence (Fig. 6c) spectra are compared, information
may be extracted about molecular order of multiple aggregate25
species present. Specifically, films cast from at least 17-20 vol %
acetone result in morphology consisting predominately of NFs
that were formed in the solution state. Films cast from 12-17
vol% acetone consist of NFs and weakly ordered aggregates (as
made evident by strong aggregate features in this region of the30
absorption spectra).
The emission spectrum of the aggregate species reported in Fig
6b was further analysed to determine the interplay of intra and
inter-chain order. The H-J aggregate model28
presents a method
to determine intra/inter-chain contributions by the following35
equation:
Eq (2)
where λ is the Huang-Rhys factor assumed here to be 1. Hence it40
was determined that I0-0/I0-1 scales with |Jintra|/Jinter. For the
aggregate species, |Jintra|/Jinter ~ 2.11 which is considerably larger
than unity. Therefore the aggregate species has significant
contribution of intra-chain order which is consistent with the
formation of ordered nanofibers. The large difference in I0-0/I0-145
between absorption (Figure 5) and emission (Figure 6)
characteristics must indicate that inter-chain coupling dominates
Figure 6: (a) Emission spectra of P3BT films cast from the binary
solvents. (b) Deconvoluted emission spectra of amorphous and
aggregated species within films calculated from a multi-
component regression of the emission spectra shown in Figure 3.
(c) Contribution of amorphous and aggregated P3BT species as a
function of acetone vol. % as determined by the contribution of
each species in the combined emission spectra
1000 1200 1400 1600
1350 1400 1450 1500
Norm.Intensity
Raman shift (cm-1
)
Raman shift (cm
-1
)
Figure 7: DFT Raman spectra of P3BT unimer (black), dimer
(blue), and trimer (red) to simulate the effects of aggregation
on Raman spectra. Inset images reveals the shift in the C=C
stretching mode.
Figure 8: Deconvoluted Raman spectra for (a) P3BT film cast
from pure chloroform and (b) P3BT film cast from 67/33 binary
solvent. Contributions of the amorphous and aggregated species
(dotted lines) were located at 1455cm-1 and 1440cm-1,
respectively. (c) Contribution of each species to the integrated
area of the C=C Raman peak.
er
ra
PL
PL
J
J
I
I
int
int
2
0
10
00
35.1




absorption characteristics whereas emission is more sensitive to
determining intra-chain coupling. This further supports the
findings of the H-J aggregate model.
First principles theoretical calculations were employed to
determine the effects of aggregation and π-π stacking on the5
Raman spectra of P3BT. Changes in the Raman spectra upon
aggregation were modelled using Quantum Tight Binding
approach. Specifically, the calculated Raman spectra for a single
chain, dimer, and trimer are reported in Fig. 7. The inset figure
reveals the P3BT C=C symmetric stretching mode from the10
theoretical calculation. Fig. 7 reports a single sharp C=C
symmetric stretching mode peak for a P3BT unimer. Lack of
interchain interactions, such as π-π stacking, permits the presence
of a single stretching mode due to chain isolation. Interchain
interactions associated with dimer and trimer formation result in a15
multicomponent C=C symmetric stretching mode Raman peak.
These multicomponent peaks reveal that π-π stacking alters the
symmetric stretching mode frequency thus creating a
distinguishable difference between aggregate and amorphous
peak position. This allows experimental Raman spectra to20
provide insight on the extent of aggregation exhibited within thin
films.
Raman spectroscopy was used to characterize the P3BT in
films (Fig 8) using difference in inelastic light scattering
properties of C=C stretching modes of aggregated and amorphous25
polythiophenes45
. Figure 8 (a,b) reports Raman spectra of C=C
stretching region of P3BT films cast from pure chloroform and
from binary solvent with 20 vol. % acetone. Raman frequencies
C=C symmetric stretching for nanofibers and amorphous species
are found to be 1440 cm-1
and 1455 cm-1
respectively. The30
contribution of each species to the C=C Raman peak was
determined from the deconvoluted spectra and reported in Fig.
8c. There are 2 distinct growth regions: from 0 – 20 vol. %
acetone, with saturation at 20 vol. % acetone. MCA
deconvolution of Raman signatures coincides with film emission35
characteristics.
The relative intensity ratio of the polythiophene C=C
stretching mode of aggregated and amorphous (IC=C,agg/IC=C,am)
P3BT were calculated for each spectra and used to create a
spatially-resolved 400 μm2
map of the film with 0.6 μm40
resolution. Fig. 9a and 9b reveals the spatial aggregation of P3BT
in films cast from pure chloroform and binary solvent with 20
vol. % acetone in terms of the IC=C,agg/IC=C,am ratio. Percolations
of fibers provide continuous network formation with minimal
regions void of significant aggregation and hence molecular45
order. The morphology and thus bulk physical properties may be
expected to be primarily isotropic throughout the thin film when a
drop casting technique is utilized.
Verification of structural order and crystallite size (related to
NF thickness) of the P3BT films were extracted through50
utilization of 1-D x-ray diffraction. Figure 10 compares the
diffraction pattern of films cast from the chloroform/acetone
solutions. The lack of well-defined diffraction peaks for the film
cast from pure chloroform indicates that this is an amorphous
film with no preferred orientation. However, as acetone and55
hence NF concentration increases, a sharp peak at 2θ=6.88o
(12.8
A) is exhibited, which corresponds to the (100) reflection. This
reveals an enhanced molecular order and the formation of the
P3BT unit cell. This finding validates that spectroscopically
revealed solution-driven aggregation indeed results in60
significantly enhanced molecular order. Molecular organization
appears to saturate in films cast from at least 17-20 vol% acetone
as indicated by (100) peak intensity. This indicates that these
films are saturated with highly ordered nanofibers that were
formed in the solution phase.65
The effect of solvent quality on the crystalline
morphology of cast thin film was further investigated by
comparing the stacking height of (100) crystal plane, i.e.,
crystallite size (L100). From the measured XRD profiles, L100 was
calculated by Scherrer’s formula which is given by46
70
𝐿100 =
𝐾 𝐹𝑊𝐻𝑀 𝜆
𝐵 𝐹𝑊𝐻𝑀 𝑐𝑜𝑠𝜃100
Eq.(3)
with KFWHM and  being the shape factor (0.9) and wavelength of
X-ray beam (1.5406 Å). BFWHM and 100 are the full-width at half
maximum of (100) crystal reflection after correcting for the75
instrumental broadening and half of the diffraction angle of (100)
crystal plane. In the Figure 10b, a sharp change in crystallite size
is observed at around the acetone volume of 8%. The sharp
changes in L100 strongly suggest that the quality of solvent greatly
Figure 9: Spatially resolved Raman spectra indicating the C=C
aggregated/C=C unaggregated Raman peak intensity ratio. (a)
P3BT film cast from pure chloroform solvent
Figure 10: Out-of-plane x-ray diffraction pattern for P3BT films
cast from varying chloroform/acetone solutions. Peak intensity
saturates for films cast above 17 vol% acetone. (b) Crystallite
size, L100 calculated from the FWHM of (100) reflection profiles
of each film.

affects the crystalline morphology of cast thin film. In particular,
the increase in crystallite size suggests the onset of nanofiber
formation at this acetone concentration. Saturation of the
crystallite size indicates minimal changes in nanofibers diameter
as a function of acetone concentration. Instead, as acetone5
concentration increases, random coils are getting incorporated
into additional fibers which increase NF concentration.
Grazing-incidence x-ray diffraction (GIXD) was employed to
determine film orientation with respect to the substrate as well as
aggregate dimensionality. Figure 11 shows the GIXD patterns for10
films cast from pure chloroform (Fig. 11a) and 20 volume%
acetone (Fig.11b). The amorphous halo seen in the diffraction
pattern of the film cast from pure chloroform in Fig. 11a indicates
the lack of any preferred orientation. The lack of ordered seeds
provides no precursors for the assembly of domains with15
molecular order. Figure 11b reveals the diffraction pattern for the
film cast from 20 volume% acetone. Diffractions peaks
associated with the (100) (qz= 4.89 nm-1
) and (010) (qxy= 13.9
nm-1
) planes are clear, thus indicating the preferred orientation of
the film. The (010) plane in the P3BT unit cell is indicative of π-π20
stacking. Clearly, aggregate NFs formed within the solution
phase were transferred to and extend throughout the film. The
(100) plane results in out-of-plane scattering whereas the (010)
plane exhibits in-plane scattering. Fig. 11c illustrates the
proposed orientation of the extended aggregate domains within25
the film as determined from the 2-D diffraction patterns. The
aggregate domains assume an “edge-on” orientation in which
main-chain and π-π stacking directions are oriented parallel to the
substrate. This is consistent with the nanofibers which lay on the
substrate. This orientation provides a plane, parallel to the30
substrate, which exhibits high charge mobility. Hence, the initial
solution-phase NFs assume edge-on orientation during casting.
This provides precursors for the nucleation of an extended
aggregate network with edge-on orientation. Through this
determination, a greater understanding of the correlation between35
structure and optical signatures of P3BT nanofibers was realized.
This work provides a clear correlation of the effects of
marginal solvent incorporation on P3BT aggregation and the
subsequent film formation. MCA deconvolution of optical spectra
reveals the saturation of P3BT aggregate growth in binary40
solution with 17 vol. % acetone. Previous work suggested that
acetone/chloroform hydrogen bonding drives the aggregation of
P3ATs, thus resulting in aggregate saturation when critical
solubility parameters are met37
. MCA deconvolution of films cast
from the aggregate solutions reveal that NFs were successfully45
transferred from the solution phase. This is evident by the rise in
vibronic progressions (absorption and emission) and alteration of
the C=C symmetric stretching mode (Raman) realized in
spectroscopic signatures. It was found that aggregate saturation in
films also resulted when cast from solutions of greater than 1750
vol% acetone. Similarities in excitonic coupling and bandwidth
suggest that film deposition results in marginal alteration of intra-
and inter-chain order of solution phase aggregates. Structural
parameters extracted from diffraction patterns reveal a correlation
with that of spectroscopic signatures. The (100) peak intensity55
increases with the incorporation of acetone but the peak largely
saturates in films cast from greater than 17 vol% acetone.
Scherrer’s formula indicates that crystallite (NF) formation is
induced with the incorporation of 8 vol% acetone and saturates
with over 12 vol% acetone. This reveals that NF diameter is60
independent of marginal solvent concentration when cast from
greater than 12 vol% acetone. The spectroscopic and diffraction
analysis of thin films demonstrates that NF were successfully
transferred to thin polymer films.
65
Conclusions
We demonstrated that molecular order of polymer films cast
from solutions of P3BT can be controlled by controlled
aggregation of polymer in binary solvent of chloroform.
Two distinct nanofiber formation regions were found based on70
analysis of optical spectra. The Region I (0-17 vol. % acetone) is
the nanofiber growth region in which acetone addition results in
the seeding of nanofibers. The Region II ( 17-20 vol% acetone)
is characterized by nanofiber growth termination to drive the
majority of dissolved P3BT chains into π-stacked nanofibers of75
high molecular order. Films cast from Region II solutions result
in high molecular order and nanofiber saturation. Films cast from
12-17 vol% acetone consist of nanofibers along with weakly
coupled aggregates.
Acknowledgements80
We acknowledge support from ORNL Laboratory Directed
Research and Development for synthesis of P3BT, modelling and
simulation efforts. DH and MS were supported through U.S.
Department of Energy Science Undergraduate Laboratory
Internships program. This research was conducted at the Center85
for Nanophase Materials Sciences, which is sponsored at Oak
Ridge National Laboratory by the Scientific User Facilities
Division, Office of Basic Energy Sciences, U.S. Department of
Energy.
90
Notes and references
a
The Center for Nanophase Materials Sciences, ORNL, Oak Ridge, TN
37831-6496, USA; E-mail:ivanovin@ornl.gov
Figure 11: 2-D GIXD patterns for films cast from P3BT solutions
(5mg/mL) with (a) 100/0 and (b) 80/20 chloroform/acetone
binary solvents. (c) Schematic of P3BT chain orientation on
substrate as determined from 2-D GIXD patterns

b
Department of Materials Science and Engineering, Knoxville, TN
37996-2100,USA;
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/5
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
10
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