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

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Page 1 of 13 Nanoscale

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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

[email protected]

Page 2 of 13Nanoscale

mailto:[email protected]

Nanoscale

Cite this: DOI: 10.1039/c0xx00000x

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2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

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 dynamic 10

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 correlation 20

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 emitting 25

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 The 30

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 variation 35

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 increase 40

structural order, ,21,22 leading to a diversity of polymer

morphologies including nanofibers23, , nanosheets24, branched

nanostructures25, discoids16

, rectangular parallelpipeds11

, and

nanoribbons21

.

Changes in the electronic absorption and luminescence 45

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 spectrum 50

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 interactions17,28. 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 chains28

.

Analysis of absorption, excitation, luminescence 65

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 coupling22,32-35. Here we apply

Multivariable Component Analysis (MCA) to study dynamic 70

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 for 75

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 Density 80

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 in 5

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, PDI 10

= 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 in 15

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 system 20

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 dilute 25

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 were 30

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 a 35

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 a 40

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 by 45

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 SAXSess 50

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 images 55

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 for 60

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 “The 65

Unscrambler X” software (CAMO Software AS) and non-

negative concentration, non-negative spectra, closure and

unimodality constrains.

Results and Discussion 70

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 to 75

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 and 85

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 transitions 90

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 to 5

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 the 10

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 of 15

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 during 20

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. MCA 25

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 and 30

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 intensity 40

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. 32from 50

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 than 60

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 pronounced 65

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

146.01

48.01

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 even 10

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 in 15

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 marginal 20

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 the 25

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 the 30

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 optical 35

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 80/20 40

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 saturation 45

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 1m. This reveals the additive aggregation

of P3BT chains with the incorporation of a marginal solvent.

Normalized absorption spectra of the NF seeded thin films are 50

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 the 60

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, as 65

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. A 70

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 binary 5

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 incorporated 10

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 aggregate 25

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 the 30

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 following 35

equation:

Eq (2)

where λ is the Huang-Rhys factor assumed here to be 1. Hence it 40

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/I0-1 45

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

No

rm.

Inte

nsit

y

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 the 5

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 the 10

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 a 15

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 to 20

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 amorphous 25

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. The 30

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 emission 35

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 μm 40

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 molecular 45

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 through 50

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 and 55

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 in 60

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 =𝐾𝐹𝑊𝐻𝑀𝜆

𝐵𝐹𝑊𝐻𝑀𝑐𝑜𝑠𝜃100 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 the 75

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 acetone 5

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 for 10

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 with 15

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-π 20

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 within 25

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 the 30

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 between 35

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 binary 40

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 successfully 45

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 17 50

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 intensity 55

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 is 60

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 on 70

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 of 75

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.

Acknowledgements 80

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 Center 85

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:[email protected]

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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Controlling Poly(3-butylthiophene) Assembly in Solution Manuscript

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