Materials
Advances
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PAPER
Cite this: Mater. Adv., 2022,
3, 2515
View Journal | View Issue
Self-assembly of PBTTT–C14 thin films in
supercritical fluids†
Nastaran Yousefi,a Richard D. Pettipas,b Timothy L. Kelly
Loren G. Kaake *a
b
and
In order to develop more atom-economical deposition methods for polymer semiconductors, we
investigated physical supercritical fluid deposition (p-SFD) to form thin films of a popular bithiophene
semiconducting polymer (PBTTT). We deposited thin films of PBTTT–C14 on a substrate without the
need for in situ chemical reactions. Depositions were performed in n-pentane at several pressures above
the critical pressure of the pure liquid. The resulting films were studied with atomic force microscopy
(AFM) and grazing incidence wide angle X-ray scattering (GIWAXS). At lower pressures, nanowire
morphologies were observed and correlated with a more gradual decrease in the isobaric saturation
solubility with respect to temperature. The isotropic distribution of crystallite textures suggests that the
Received 13th September 2021,
Accepted 4th February 2022
wires are formed in solution prior to deposition. The addition of 0.5% mol ratio of toluene also had a
strong influence on thin film morphologies. Nanowires were observed at higher pressures in the
DOI: 10.1039/d1ma00847a
presence of toluene, which we correlated with increasing saturation solubility with respect to pressure.
Taken in sum, the results illustrate the profound influence of subtle changes in polymer solubility on the
rsc.li/materials-advances
self-assembly process.
1. Introduction
The combination of mechanical flexibility and electronic functionality offer the promise of flexible and conformal electronics
for a number of applications including organic field-effect transistors (OFETs),1–3 organic light emitting diodes (OLEDs),4,5
organic photovoltaics cells (OPVs),6,7 radio-frequency identification (RFID) tags,8 sensors,9 wearable technologies,10,11 and
medical devices.12 The ability of organic electronic materials to
be first synthesized and then deposited leverages both the benefits of synthetic organic chemistry and materials science. An
immense variety of polymers can potentially be synthesized, and
a suite of well-known tools allow them to be accurately characterized. This allows materials deposition techniques to be developed
and optimized independently.
Widespread commercial application of polymer semiconductors requires ecofriendly synthesis and thin film deposition
techniques.13,14 The introduction of direct heteroarylation polymerization has allowed a move away from Stille coupling and
its toxic metal reagents and waste products.15 In a related
a
Department of Chemistry, Simon Fraser University, 8888 University Dr, Burnaby,
BC V5A 1S6, Canada. E-mail: lkaake@sfu.ca
b
Department of Chemistry, University of Saskatchewan, 110 Science Place,
Saskatoon, SK S7N 5C9, Canada
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ma00847a
© 2022 The Author(s). Published by the Royal Society of Chemistry
direction, organic semiconductors that can be processed in
toluene, alcohols or even water are being developed in the
general move away from halogenated solvent processing.16,17
In comparison, small molecule organic semiconductors can
be deposited from the vapor phase, eliminating the need for
solvents altogether.18 However, this limits the chemical design
space of potential compounds, especially in terms of their
molecular weight. Indeed, polymers and high molecular weight
oligomers often cannot be sublimated without causing
chemical degradation. Many of the highest performing organic
semiconductors are high molecular weight materials that cannot take advantage of the scalable and reproducible technique
of vapor deposition. As such, there is an opportunity for the
development of a deposition technique that works with high
molecular weight materials while conferring the benefits of
physical vapor deposition.
Ideally, a deposition process provides films that are homogeneous over a large area both in terms of film thickness and
morphology. Examples of solution-based deposition techniques are coating (spin-coating,19 spray-coating,20,21 doctor
blade coating),22,23 drop-casting,24,25 Langmuir–Blodgett,26,27
and printing (inkjet,28 gravure,29 reverse-offset30). Printing
techniques are common due to their roll-to-roll compatibility
and ability to print on flexible substrates.31 However, each
technique offers trade-offs in terms of scalability, ease of ink
formulation, ultimate resolution, and use with substrates of
non-trivial curvature.
Mater. Adv., 2022, 3, 2515–2523 | 2515
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
We are developing a deposition method for high molecular
weight materials that relies on the unique solvation properties
of supercritical fluids, called physical supercritical fluid deposition (p-SFD).32 The technique is designed to maintain the
benefits of vapor phase processing and has the additional
benefit of not requiring line of sight access to the surface
where film formation takes place. When a solvent is at a
temperature well below its critical temperature, the saturation
concentration of a solid solute typically increases with temperature irrespective of the pressure. However, as a solvent is
heated towards its critical temperature, the solvent begins to
transition continuously from liquid-like to gas-like behavior
when the pressure exceeds the critical pressure. As this occurs,
the saturation solubility reaches a maximum, and then
decreases with increasing temperature, eventually approaching
zero in the limit of pure gas-like behavior. By holding the bulk
solvent at the solubility maximum and heating a substrate
already in the vessel, thin polymer films can be precipitated
from supercritical solvents. By controlling the local temperature on a substrate, the deposition process can be directed,
achieving linewidths of 5 microns, several times better than
aerosol jet printing methods.32
From an ecofriendly perspective, p-SFD has several benefits.
Firstly, the process is highly atom-economical. Following
deposition, the contents of the pressure vessel are exhausted
to a collection vessel, condensing the solvent, and collecting
unused deposition material. This process generates minimal
chemical waste. Secondly, the process opens the possibility of
using carbon dioxide as a solvent, which could be extracted
from the atmosphere. However, in order for this technique to
be useful in making organic optoelectronic devices, we must be
able to control the self-assembly processes that give rise to high
performance thin film morphologies.
Current strategies for controlling polymer semiconductor
thin film morphology include thermal annealing,33,34 solvent
engineering by addition of an antisolvent or cosolvent,35–42
solvent vapor annealing,43–45 and post formation alignment
via mechanical stretching or high-temperature rubbing.46,47
Amongst all these approaches, the use of solvent additives
has been widely explored for fabrication of high-performance
electronic devices.36,37,48,49
Supercritical fluids have also been used as a post processing
step for polymer morphology optimization. For example, supercritical fluids (SCFs) were used in the processing of OPVs50,51
OFETs,52 and hole-only diodes.53 In addition, they have been
used to modify the morphology of aliphatic polymer films,54–58
including polymer/polymer blends.59 In these studies, the general trend is that post-treatment with supercritical fluids promotes chain mobility, resulting in increased crystallinity,
dewetting, or vertical phase separation. This approach contrasts with ours in that supercritical fluids are used to modify
the morphology of films that were formed by some other
process, where we are forming films directly under supercritical
conditions. Previous studies from our lab on isotactic polypropylene suggests that pressure and solvent additive can exert a
profound influence on the thin film morphology.60,61 While we
2516 | Mater. Adv., 2022, 3, 2515–2523
Materials Advances
have formed semiconducting polymer films in our previous
work using p-SFD, this study represents the first detailed
examination of semiconducting polymer thin film morphologies formed using this process.
Here, we investigate the effect of solvent additives and
pressure on the morphology of PBTTT-C14 thin films processed
in supercritical n-pentane: toluene using atomic force microscopy (AFM), helium ion microscopy (HIM), and grazing incidence wide angle X-ray scattering (GIWAXS). The AFM results
demonstrate a stark contrast between films formed via p-SFD at
different pressures. Specifically, changing the pressure of the
fluid influences viscosity and the solubility behavior, both of
which can influence thin film morphology. We observed nanowire formation and a smoother thin film morphology with
more isotropic texturing in response to a steeper solubility
decline with respect to temperature. The addition of toluene
to the system resulted in nanowire formation at higher pressures due to higher solution viscosity. Based on our findings,
the morphology of the films processed by p-SFD technique
adopt smoother and more ordered films with increasing the
solution viscosity and are also influenced by subtle changes in
polymer solubility with temperature. Managing these variables
will allow films with good performance to be grown using this
atom-economical deposition technique.
2. Results and discussion
The pressure chamber used for investigating the polymer
solubility and thin film formation was a custom-made vessel,
fabricated from a solid block of beryllium copper (see Fig. S1,
ESI†). The chamber can be pressurized and heated by means of
a manual pressure generator and cartridge heaters. The front
and back of the chamber is fitted with sapphire windows to
allow for in situ transmission UV-vis measurements of the
solution.
Understanding the conditions required for thin film deposition requires an understanding of the saturation solubility
behavior of the solvent/polymer system. To investigate the
polymer solubility, a small glass crucible was filled with
poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]
(PBTTT–C14), capped with glass wool, and placed at the bottom
of the chamber. The chamber was then filled to the point of
overflowing with deoxygenated n-pentane or a n-pentane:
toluene (0.5% mol) solution. UV-vis spectra of the chamber
and its contents were collected approximately 15 minutes after
the chamber temperature had stabilized to ensure the solution
had come to its saturation concentration. The blank spectrum
used in the calculation of absorbance was collected at room
temperature. Blank spectra were collected at each pressure, as
subtle changes to cell transmission occur, presumably through
variations in the real index of refraction with respect to pressure. Fig. 1a shows absorbance spectra collected at 7.0 MPa as a
function of temperature for PBTTT–C14 in a pure n-pentane
solution. Because of the high absorbance values involved, the
resolution of the spectrometer was decreased, and the spectra
© 2022 The Author(s). Published by the Royal Society of Chemistry
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Materials Advances
Paper
Fig. 1 In situ UV-vis transmission results of PBTTT–C14 in (a) pure n-pentane and (b) n-pentane: toluene (0.5% mol) solution. UV-vis spectral
measurements for the chamber and its contents (solution) for several temperatures and a single pressure (left) and the integrated UV-vis absorbance as a
function of temperature for several pressures (right). Solid lines indicate the fit to the data using eqn (1).
were fitted with two Gaussian peaks to capture the behavior
accurately.
As shown in Fig. 1a, there is an increase in absorbance for
temperatures up to 125 1C. With further increases in temperature, the absorbance decreases. Similarly, there is an increase
in absorbance for temperatures up to 130 1C in the case of
n-pentane: toluene, Fig. 1b, and with further increase in temperature, the absorbance decreases. We interpret the absorbance as being reflective of the concentration of PBTTT–C14 in
solution. Non-monotonic solubility behavior as a function of
temperature was observed for both PBTTT–C14 and isotactic
polypropylene in pure n-pentane previously.32,61 Additionally,
the overall absorbance as a function of temperature for different pressures are displayed in Fig. 1 (right panel) using the total
integrated intensity obtained from a fit of the data. Based on
the observations, the overall absorbance does not change
significantly as a function of pressure in pure n-pentane.
However, there is a small but noticeable pressure dependence
in the solubility at the highest temperatures. When toluene is
added to the system the effect of pressure becomes more
pronounced. In the presence of toluene, the increase in the
solubility of PBTTT–C14 as a function of pressure reflects a
negative value for the volume change of mixing (DVm).
© 2022 The Author(s). Published by the Royal Society of Chemistry
As shown in Fig. 1b, the non-monotonic solubility behavior
as a function of temperature is observed for all the pressures
studied. To confirm this interpretation, gravimetric analysis
was carried out simultaneously to establish the PBTTT–C14
solubility as a function of temperature. Fig. S2 (ESI†) shows
both the results from the gravimetric analysis and in situ UV-vis
transmission of chamber and its contents (solution) for several
temperatures and a single pressure (7.0 MPa). The data gathered via gravimetric analysis are in good agreement with the
results from in situ UV-vis transmission results, confirming an
increase in the saturation solubility as a function of temperature below 130 1C and decreasing saturation solubility at
temperatures above that.
The change in saturation solubility as a function of temperature as extracted from the UV-vis data was fitted. The
expression used was developed previously56,57 and describes
the concentration of polymer (c) versus temperature:
c = b + a exp[–g(T – T0)2/RT].
(1)
pffiffiffiffiffiffiffiffiffiffiffiffi
In eqn (1) T0 DH=g describes the temperature of the
solubility maximum using the enthalpy of solvation (DH) and the
slope of solvent entropy with respect to temperature (DSsolvent p gT).
Mater. Adv., 2022, 3, 2515–2523 | 2517
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
The fitting results help to highlight the changes in the solubility behavior with respect to pressure. In the presence of
toluene, the solubility increases and T0 shifts towards higher
temperature with increasing pressure. The increase in T0 is
likely due to a decrease in g as toluene is a better solvent, with a
less positive DH in comparison with pentane. The increasing
solubility with increasing pressure is related to the volume
change of mixing (DVm). Negative values of DVm are typically
associated with increasing solubility with increasing pressure.
Microscopically, we attribute the negative value of DVm to the
formation of a dense solvent shell rich in toluene.
The nonmonotonic solubility behavior of PBTTT–C14 is the
vital to the process of p-SFD, allowing polymer films to be
grown in situ on a substrate without the need for chemical
reactions. To initiate the polymer deposition on a substrate, we
hold the temperature of the cell wall at the solubility maximum
(Twall E 130 1C) and resistively heat a substrate immersed in
the fluid (Tsub E 160 1C). Based on our previous studies, the
film thickness increases with respect to time in an approximately linear fashion, which allow us to control film thickness
in a relatively straightforward manner.32
To investigate the morphology of the PBTTT–C14 films, a
series of films were deposited in situ using p-SFD in pure
n-pentane and a mixture of n-pentane: toluene (0.5% mol) at
different pressures. AFM images shown in Fig. 2 demonstrate
the impact of pressure and solvent additive on film morphology. In pure n-pentane (Fig. 2a), at lowest pressure (3.5 MPa) we
observe a fibrillar morphology reminiscent of p-stacked nanowires of poly(3-hexylthiophene).62 As the pressure increases to
7.0 MPa, we observe a more sweeping fibrillar morphology with
fibers spanning almost the entire image that resembles the
isotropic nanoribbons observed in casted films of PBTTT in
previous reports.63,64 As pressure increases further, the length
of the fibers decreases, and the morphology becomes more
featureless. In the presence of toluene (Fig. 2b), the fibers are
more arranged in a broad sweeping manner at lower pressures
Fig. 2
Materials Advances
and exhibit nanowires at the highest pressures. We interpret
nanowires as demonstrating crystalline ordering of individual
chains through p-stacking with the backbone axis perpendicular to the fiber axis (that is stacking direction). We interpret
the broader, sweeping morphologies to indicate an alternative
stacking motif, one with the polymer axis being parallel to the
fiber axis. In addition, PBTTT–C14 films deposited via p-SFD are
markedly different than spin-coated films that exhibit semicrystalline but otherwise relatively featureless surfaces, similar
to the previous reports for spin-coated and unheated PBTTT
films (Fig. S5, ESI†).64,65
In order to better understand the film formation process,
GIWAXS measurements were carried out. Fig. 3(a) and (b)
shows the results, which exhibit d-spacings of 0.29 Å 1 which
is comparable to previous measurements of PBTTT–C14
films.66,67 Comparing the GIWAXS measurements of PBTTT–
C14 deposited films via p-SFD with those of a spin-coated
PBTTT–C14 film (Fig. 3 and Fig. S6, ESI†) does not suggest that
the films are of markedly different crystallinity. UV-vis spectra
of the spin-coated and p-SFD grown PBTTT–C14 films collected
ex situ (Fig. S7, ESI†) suggests that the local ordering is somewhat lower in films processed in supercritical fluids, presumably a reflection of shorter chain correlation lengths as opposed
to lower crystallinity.
As shown in Fig. 3a, the PBTTT–C14 film at 3.5 MPa pure npentane is still highly crystalline, however, they show a more
isotropic texture relative to spin-coated films (Fig. S6, ESI†). At
higher pressures, the film appears to be more textured, showing a greater population of molecules with edge-on orientation.
This is especially true with films deposited at 7.0 MPa, however,
the behavior is somewhat subtle and lacks a clear monotonic
dependence, making it difficult to reach further conclusions.
Fig. 3b displays the GIWAXS results of PBTTT–C14 films grown
n-pentane: toluene at different pressures. In a manner similar to
pure n-pentane, the film at 3.5 MPa is highly crystalline, and has a
more isotropic texture than the PBTTT–C14 spin-coated film.
Atomic force microscopy images of PBTTT–C14 films formed in supercritical (a) pure n-pentane and (b) n-pentane: toluene at different pressures.
2518 | Mater. Adv., 2022, 3, 2515–2523
© 2022 The Author(s). Published by the Royal Society of Chemistry
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Materials Advances
Paper
Fig. 3 GIWAXS patterns of PBTTT–C14 films grown in (a) n-pentane and (b) n-pentane: toluene at different pressures. (c) The molecular structure of
PBTTT–C14 and the drawing illustrating the different molecular orientation of PBTTT–C14 (edge-on and face-on) in thin layers with respect to the
substrate surface.
Texture analysis (Fig. S8, ESI†) confirms what can estimated from
the radial scattering patterns; among films formed at different
pressures, the films at 7.0 MPa are the most strongly edge-on
oriented, with a somewhat lower crystallinity. This observation is
also irrespective of the solvent used. Fig. 3c contains the chemical
structure of PBTTT–C14 and a drawing illustrating the difference
between edge-on and face-on orientations with respect to the
substrate.
In order to understand the self-assembly process of PBTTT–
C14 in supercritical solvents, we made a comparison with
previous studies on isotactic polypropylene, where we hypothesized that deposition occurs in two steps. The first process is a
pre-aggregation step that occurs in solution and is triggered by
a drop in polymer solubility near the heated substrate surface.
Self-assembly processes occurring in this step depend upon the
solution properties, in particular the Rayleigh number, which
describes the amount of turbulence near the substrate surface.
It was found out that increasing solution turbulence is correlated with decreases in polymer crystallinity. In the second step,
deposition material contacts the substrate, where it can then
move in a process reminiscent of surface diffusion in physical
vapor deposition. We discovered that the presence of high
boiling point solvent additives can promote ordering in
this step.
In Fig. 1a (right panel), we observe very similar saturation
concentrations with respect to pressure at the solubility maximum, and so, are forced to conclude that changes in the
Rayleigh number with respect to pressure do not drive the
changes in sample morphology. However, it was observed that
© 2022 The Author(s). Published by the Royal Society of Chemistry
the decrease in solubility with respect to temperature is pressure dependent above the solubility maximum. This suggests
that the amount of material precipitating from solution near
the substrate is greater at low pressures and at a less at higher
pressures. As a result, the amount of material available to form
large-scale crystallites in solution is smaller at higher pressures.
This hypothesis explains the appearance of nanowires at low
pressures where solution phase self-assembly has sufficient
material to create nanowire aggregates that are then deposited
onto the substrate. Presumably, the larger aggregates would be
less capable to diffusing on the substrate, leading to a more
uniform large scale thin film morphology characterized by
nanowires of isotropic orientation. Isotropic, highly crystalline
nanowire formation is supported by our GIWAXS observations.
Film uniformity at longer length scales is supported by helium
ion microscope images (Fig. S9, ESI†). At higher pressures, the
amount of material that can pre-aggregate is limited, inhibiting
nanowire growth. These smaller structures can then diffuse on
the substrate surface, leading to rougher with more textured
crystallinity. The GIWAXS data supports greater texturing at
high pressures and helium ion microscope images demonstrate
that films exhibit dewetting and enhanced roughness.
In the case of n-pentane: toluene, polymer solubility is
strongly pressure dependent. This should accompany a change
in Rayleigh number with more viscous flows occurring at
higher pressure. The more viscous flow decreases the amount
of turbulence, leading to the formation of nanowires at
higher pressures. At lower pressures, the formation of large
wires is inhibited, presumably because of a shallow decrease in
Mater. Adv., 2022, 3, 2515–2523 | 2519
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
solubility with increasing temperature which results in a lower
nucleation density and smaller crystallites. This conclusion is
supported by a broadening of the higher order diffraction arcs
in n-pentane: toluene relative to pure n-pentane at 3.5 MPa.
However, the presence of solvent additives would allow for the
motion of polymer chains on the surface once deposited,
explaining generally improved film uniformity with the toluene
solvent additive.
Taken in sum, this study illustrates that self-assembly in
supercritical fluids is a complicated phenomenon with a number of subtle effects that can lead to dramatic changes in
sample morphology. In particular, this study highlights the
importance of solution viscosity and the magnitude of the
decrease in polymer solubility with respect to temperature
during the deposition process. Gaining mastery over these
effects is paramount to leveraging the full potential of p-SFD
as a green processing technique.
3. Conclusion
The development of physical supercritical fluid deposition
(p-SFD) as an environmentally friendly means of thin film
deposition requires a deep understanding of the relationship
between processing and film structure. We studied the selfassembled film structure of PBTTT-C14 grown from n-pentane
and n-pentane: toluene solutions. We found that thin film
morphologies differ dramatically from spin-coated films and
that subtle changes in the isobaric solubility behavior of the
polymer exert important influences on the self-assembly process. In particular, we found that nanowire formation in
solution is promoted by a steeper decrease in solubility with
respect to temperature. This is accompanied by a more isotropic texturing and smoother thin films. In the presence of
toluene, nanowire formation is preferred at higher pressures as
a result of higher solution viscosity. More broadly, we conclude
that smooth, ordered films are more likely to be deposited from
viscous solutions with a small change in solubility with respect
to temperature.
4. Experimental
4.1
Supercritical fluid chamber description
A custom pressure vessel was constructed from a block of
beryllium copper (BeCu) with 6 ports surrounding vessel interior. The bottom port serves as a fluid inlet and the top port
serves as an outlet. One of the side ports is used to introduce a
substrate for thin film deposition. The chamber also possesses
a pair of sapphire windows which allow the internal chamber
contents to be monitored in situ. The system is pressurized by
using a manual pressure pump generator (HiP 62-6-10), and the
pressure is monitored with a transducer (Swagelok PTI series).
All the constituent parts of the system are rated to greater than
53.0 MPa and are regularly pressure tested to 24.0 MPa. In order
to ensure the integrity of the system, there is a 38.0 MPa rupture
disc to prevent over-pressurization. The volume of the chamber
2520 | Mater. Adv., 2022, 3, 2515–2523
Materials Advances
was kept as small as feasible (E27 mL) to ensure safety in an
academic laboratory setting. Additionally, oxygen was rigorously removed from solvent and the system by purging our
solvent with nitrogen and overfilling the vessel. The chamber is
also placed in Lexan safety enclosure that is actively ventilated
to remove any inadvertently generated solvent vapors.
4.2
Solubility measurements of PBTTT–C14
Transmission UV-vis spectroscopic measurements were carried
out by placing PBTTT–C14 into a crucible at the bottom of the
chamber before the vessel was sealed and filled with solvent.
The chamber exterior was heated by an Omega benchtop PID
controller (CSi32 Series 0.04 1C temperature stability) used to
drive four cartridge heaters connected in parallel and placed
symmetrically about the edges of the chamber. After the
temperature stabilized, the solution was pressurized to 3.5,
7.0, 10.3, and 17.2 MPa by using the manual pressure generator. The system was given B15 min to reach equilibrium
before solution absorbance measurements were collected.
Transmission UV-vis spectroscopic measurements of the pressure vessel were performed using an Ocean Optics USB4000
spectrometer, which covers 200–1100 nm ranges with a Toshiba
TCD1304AP (3648-element linear silicon CCD array) detector.
The short-arc xenon discharge lamp (USHIO UXL-75W with
300–1100 nm wavelength range) was used as the light source. A
solution of malachite green in water (10 6–10 4 M) was used as
a light absorber and placed on the light path before the light
reaches the chamber to decrease the light source intensity. The
blank absorbance used for the measurements was simply the
absorbance of malachite green solution plus the chamber
solution at room temperature. A different blank was collected
for each pressure to take into account the slight transmission
increase with pressure. The cut-off pressure for solubility
measurements was 10.3 MPa due to UV-vis detector saturation.
The resolution of the spectra was decreased to 8 nm to achieve
higher signal-to-noise ratio and obtain accurate readings at
high absorbance values. Data were fitted with two Gaussian
peaks using IGOR Pro (Wavemetrics).
Gravimetric analysis of PBTTT–C14 saturated solutions was
measured by exhausting the chamber contents through the
chamber outlet into a polypropylene vessel. The vessel had an
inlet port to accept the chamber contents and a wide outlet port
to prevent from pressurizing vessel. As a matter of safety, it is
very important to provide active ventilation of the vessel to
rapidly remove any flammable solvent vapors and prevent
pressure buildup inside the collection vessel. The rapid expansion of the chamber contents cooled the solution below its
boiling point, allowing liquid solvent and (precipitated)
PBTTT–C14 to be collected. Solvent was removed from the slurry
under reduced pressure, and the dried PBTTT–C14 material was
weighed.
4.3
Deposition technique and conditions
The substrates used for deposition were ITO-coated glass slides
purchased from Colorado Concept Coatings LLC. The thickness
of ITO coating was 40 nm with average resistance of 60 O sq 1.
© 2022 The Author(s). Published by the Royal Society of Chemistry
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Materials Advances
ITO glass slides were suitable substrates due to their optical
transparency, electrical conductivity, and ease of use. A thin
layer of gold (E50 nm) was deposited near the edges of the
substrates by using physical vapor deposition to facilitate a
uniform current through the ITO film. The ITO glass slides
were cleaned with acetone and 2-propanol before mounting
them on the sample holder and placing the assembly into the
chamber.
The deposition of PBTTT–C14 in pressurized solvent was
performed by first increasing the temperature of the solution to
130 1C. When pressure and temperature had stabilized, the
temperature of the ITO glass slide was increased to 160 1C
by resistively heating the ITO glass substrates. Increasing
the substrate temperature had a subtle effect on the
chamber pressure which was adjusted to maintain the desired
pressure.
4.4
Film characterization
Ex situ UV-vis spectra of the deposited films, including those
prepared by spin-coating, were collected with an Agilent 8453
UV-vis spectrophotometer. Spin-coated films were prepared by
using PBTTT–C14 in o-dichlorobenzene (10 mg mL 1), and the
resulting film thickness was B30 nm as measured by a Bruker
Dimension Icon atomic force microscope.
Atomic force microscopy measurements were carried out at
the 4D LABS facility at Simon Fraser University using a Bruker
Atomic Force Microscopy System (Dimension Icon model) via
ScanAsyst mode. ScanAsyst imaging mode is based on the
general-purpose imaging mode, Peak Force Tappingt. The
ScanAsyst-Air probe has a triangular cantilever shape with
70 kHz resonant frequency, 0.4 N m 1 spring constant, and
2 nm tip radius.
Helium ion microscopy images were captured via ORION
NanoFab Helium Ion Microscope (HIM) from Zeiss company at
the 4D LABS facility at Simon Fraser University. The helium ion
beam selected for imaging the PBTTT–C14 thin films were in
the range of 10–35 kV, beam current 0.1–100 pA.
GIWAXS measurements were performed at the Hard X-ray
MicroAnalysis (HXMA) beamline of the Canadian Light Source.
An energy of 12.688 keV was selected using a Si(111) monochromator. The beam size was defined by slits having a 0.2 mm
vertical gap and a 0.3 mm horizontal gap, and the angle of
incidence was set to 0.11. The diffraction patterns were collected on a Rayonix SX165 CCD camera (80 mm pixel size;
16.3 cm diameter) using an acquisition time of 30 s. The
sample-to-detector distance (224 mm) was calibrated using a
silver behenate powder standard. The GIWAXS data were
processed using the GIXSGUI software package in MATLAB;68
the patterns were calibrated, solid angle and polarization
corrections applied, and the data was reshaped to account for
the missing wedge along qz.
Conflicts of interest
There are no conflicts to declare.
© 2022 The Author(s). Published by the Royal Society of Chemistry
Paper
Acknowledgements
L. G. K. acknowledges funding from the Natural Science and
Engineering Research Council of Canada (NSERC,RGPIN-201505981) under the Discovery Grants program. The Natural
Sciences and Engineering Research Council of Canada (NSERC,
RGPIN-2017-03732) and the University of Saskatchewan are
acknowledged for financial support. T. L. K. is a Canada
Research Chair in Photovoltaics. The research was undertaken,
in part, thanks to funding from the Canada Research Chair
program. The Canadian Light Source (CLS) is supported by CFI,
NSERC, the University of Saskatchewan, the Government of
Saskatchewan, Western Economic Diversification Canada, the
National Research Council Canada, and the Canadian Institutes of Health Research. Portions of this research were also
carried out in the 4D LABS facility at Simon Fraser University.
We also thank Dr Marc Patrick Courte for help in collecting
AFM image of the spin-coated PBTTT–C14. Technical support
from HXMA beamline scientist Dr Chang-Yong Kim is gratefully acknowledged.
References
1 M. Shin, J. H. Song, G. H. Lim, B. Lim, J. J. Park and
U. Jeong, Adv. Mater., 2014, 26, 3706–3711.
2 A. Chortos, J. Lim, J. W. F. To, M. Vosgueritchian,
T. J. Dusseault, T. H. Kim, S. Hwang and Z. Bao, Adv. Mater.,
2014, 26, 4253–4259.
3 K. Fukuda, Y. Takeda, M. Mizukami, D. Kumaki and
S. Tokito, Sci. Rep., 2014, 4, 3947.
4 L. X. Xiao, Z. J. Chen, B. Qu, J. X. Luo, S. Kong, Q. H. Gong
and J. J. Kido, Adv. Mater., 2011, 23, 926–952.
5 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,
R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. D. Santos,
J. L. Brédas, M. Lögdlund and W. R. Salaneck, Nature, 1999,
397, 121.
6 P. Cheng, G. Li, X. Zhan and Y. Yang, Nat. Photonics, 2018,
12, 131–142.
7 R. A. J. Janssen and J. Nelson, Adv. Mater., 2013, 25,
1847–1858.
8 S. Soeren, M. Kris, A. Vladimir, D. Carsten, V. Stijn De,
G. Jan and H. Paul, Nat. Mater., 2005, 4, 597.
9 L. Wang, D. Fine, D. Sharma, L. Torsi and A. Dodabalapur,
Anal. Bioanal. Chem., 2006, 384, 310.
10 J. Kang, D. Son, G. J. N. Wang, Y. Liu, J. Lopez, Y. Kim,
J. Y. Oh, T. Katsumata, J. Mun, Y. Lee, L. Jin, J. B. H. Tok and
Z. Bao, Adv. Mater., 2018, 30, 1706846.
11 C. Zhu, H.-C. Wu, G. Nyikayaramba, Z. Bao and B. Murmann,
IEEE Electron Device Lett., 2019, 40(10), 1630–1633.
12 K. Dae-Hyeong, V. Jonathan, J. A. Jason, X. Jianliang, V. Leif,
K. Yun-Soung, A. B. Justin, P. Bruce, S. F. Eric, C. Diego,
L. K. David, G. O. Fiorenzo, H. Yonggang, H. Keh-Chih,
R. Z. Mitchell, L. Brian and A. R. John, Nat. Mater., 2010,
9, 511.
13 M. Irimia-Vladu, E. D. Głowacki, G. Voss, S. Bauer and
N. S. Sariciftci, Mater. Today, 2012, 15, 340–346.
Mater. Adv., 2022, 3, 2515–2523 | 2521
View Article Online
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
14 M. Irimia-Vladu, Chem. Soc. Rev., 2014, 43, 588–610.
15 J. R. Pouliot, F. Grenier, J. T. Blaskovits, S. Beaupre and
M. Leclerc, Chem. Rev., 2016, 116, 14225–14274.
16 S. Q. Zhang, L. Ye, H. Zhang and J. H. Hou, Mater. Today,
2016, 19, 533–543.
17 C. R. Harding, J. Cann, A. Laventure, M. Sadeghianlemraski,
M. Abd-Ellah, K. R. Rao, B. S. Gelfand, H. Aziz, L. Kaake,
C. Risko and G. C. Welch, Mater. Horiz., 2020, 7, 2959–2969.
18 S. R. Forrest, Nature, 2004, 428, 911–918.
19 Y. Yuan, G. Giri, A. L. Ayzner, A. P. Zoombelt, S. C. B. Mannsfeld,
J. Chen, D. Nordlund, M. F. Toney, J. Huang and Z. Bao, Nat.
Commun., 2014, 5, 3005.
20 S. F. Tedde, J. Kern, T. Sterzl, J. Fürst, P. Lugli and
O. Hayden, Nano Lett., 2009, 9, 980–983.
21 A. Abdellah, B. Fabel, P. Lugli and G. Scarpa, Org. Electron.,
2010, 11, 1031–1038.
22 R. Z. Rogowski, A. Dzwilewski, M. Kemerink and A. A.
Darhuber, J. Phys. Chem. C, 2011, 115, 11758–11762.
23 S. R. Tseng, H. F. Meng, K. C. Lee and S. F. Horng, Appl.
Phys. Lett., 2008, 93, 153303.
24 D. H. Kim, J. T. Han, Y. D. Park, Y. Jang, J. H. Cho,
M. Hwang and K. Cho, Adv. Mater., 2006, 18, 719–723.
25 C. S. Kim, S. Lee, E. D. Gomez, J. E. Anthony and Y. L. Loo,
Appl. Phys. Lett., 2008, 93, 103302.
26 J. Matsui, S. Yoshida, T. Mikayama, A. Aoki and
T. Miyashita, Langmuir, 2005, 21, 5343–5348.
27 H. Xu, Y. Wang, G. Yu, W. Xu, Y. Song, D. Zhang, Y. Liu and
D. Zhu, Chem. Phys. Lett., 2005, 414, 369–373.
28 H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M.
Inbasekaran, W. Wu and E. P. Woo, Science, 2000, 290,
2123–2126.
29 G. Grau and V. Subramanian, Adv. Electron. Mater., 2016,
2, 1500328.
30 K. Fukuda, Y. Yoshimura, T. Okamoto, Y. Takeda,
D. Kumaki, Y. Katayama and S. Tokito, Adv. Electron. Mater.,
2015, 1, 1500145.
31 R. R. Sondergaard, M. Hosel and F. C. Krebs, J. Polym. Sci.,
Part B: Polym. Phys., 2013, 51, 16–34.
32 N. Yousefi, J. J. Maala, M. Louie, J. Storback and L. G. Kaake,
ACS Appl. Mater. Interfaces, 2020, 12, 17949–17956.
33 S. Cho, K. Lee, J. Yuen, G. M. Wang, D. Moses, A. J.
Heeger, M. Surin and R. Lazzaroni, J. Appl. Phys., 2006,
100, 6.
34 H. Kim, W. W. So and S. J. Moon, J. Korean Phys. Soc., 2006,
48, 441–445.
35 Y. F. Zheng, G. Wang, D. Huang, J. Kong, T. Goh, W. Huang,
J. S. Yu and A. D. Taylor, Sol. RRL, 2018, 2, 8.
36 J. Y. Na, M. Kim and Y. D. Park, J. Phys. Chem. C, 2017, 121,
13930–13937.
37 J. K. Keum, K. Xiao, I. N. Ivanov, K. L. Hong, J. F. Browning,
G. S. Smith, M. Shao, K. C. Littrell, A. J. Rondinone,
E. A. Payzant, J. H. Chen and D. K. Hensley, CrystEngComm,
2013, 15, 1114–1124.
38 J. Gao, L. Y. Duan, G. H. Yang, Q. Zhang, M. B. Yang and
Q. Fu, Appl. Surf. Sci., 2012, 261, 528–535.
39 V. Vittoria and F. Riva, Macromolecules, 1986, 19, 1975–1979.
2522 | Mater. Adv., 2022, 3, 2515–2523
Materials Advances
40 H. Cornelis, R. G. Kander and J. P. Martin, Polymer, 1996,
37, 4573–4578.
41 J. Wang, A. T. Dibenedetto, J. F. Johnson, S. J. Huang and
J. L. Cercena, Polymer, 1989, 30, 718–721.
42 S. B. Lin and J. L. Koenig, J. Polym. Sci., Part B: Polym. Phys.,
1983, 21, 1539–1558.
43 Y. Wang, H. Cui, M. Zhu, F. Qiu, J. Peng and Z. Lin,
Macromolecules, 2017, 50, 9674–9682.
44 D. J. Mascaro, M. E. Thompson, H. I. Smith and V. Bulović,
Org. Electron., 2005, 6, 211–220.
45 C. Liu, T. Minari, X. Lu, A. Kumatani, K. Takimiya and
K. Tsukagoshi, Adv. Mater., 2011, 23, 523.
46 L. Biniek, N. Leclerc, T. Heiser, R. Bechara and
M. Brinkmann, Macromolecules, 2013, 46, 4014–4023.
47 K. Tremel, F. S. U. Fischer, N. Kayunkid, R. Di Pietro,
R. Tkachov, A. Kiriy, D. Neher, S. Ludwigs and
M. Brinkmann, Adv. Energy Mater., 2014, 4, 13.
48 O. Bubnova, Z. U. Khan, H. Wang, S. Braun, D. R. Evans,
M. Fabretto, P. Hojati-Talemi, D. Dagnelund, J. B. Arlin,
Y. H. Geerts, S. Desbief, D. W. Breiby, J. W. Andreasen,
R. Lazzaroni, W. M. M. Chen, I. Zozoulenko, M. Fahlman,
P. J. Murphy, M. Berggren and X. Crispin, Nat. Mater., 2014,
13, 190–194.
49 K. Zhao, H. U. Khan, R. P. Li, Y. S. Su and A. Amassian, Adv.
Funct. Mater., 2013, 23, 6024–6035.
50 J. A. Amonoo, E. Glynos, X. C. Chen and P. F. Green, J. Phys.
Chem. C, 2012, 116, 20708–20716.
51 P. V. Ambuken, H. A. Stretz, M. Dadmun and S. Michael
Kilbey, Sol. Energy Mater. Sol. Cells, 2015, 140, 101–107.
52 B. X. Dong, J. A. Amonoo, G. E. Purdum, Y. L. Loo and
P. F. Green, ACS Appl. Mater. Interfaces, 2016, 8, 31144.
53 N. S. Jiang, L. Sendogdular, M. Sen, M. K. Endoh, T. Koga,
M. Fukuto, B. Akgun, S. K. Satija and C. Y. Nam, Langmuir,
2016, 32, 10851–10860.
54 M. Asada, N. S. Jiang, L. Sendogdular, J. Sokolov, M. K. Endoh,
T. Koga, M. Fukuto, L. Yang, B. Akgun, M. Dimitrioug and
S. Satija, Soft Matter, 2014, 10, 6392–6403.
55 Y. T. Shieh and H. S. Yang, J. Supercrit. Fluids, 2005, 33,
183–192.
56 E. Kiran, K. Liu and K. Ramsdell, Polymer, 2008, 49,
1853–1859.
57 E. Kiran, J. Supercrit. Fluids, 2009, 47, 466–483.
58 Q. Lan, J. Yu, J. Zhang and J. He, Macromolecules, 2011, 44,
5743–5749.
59 H. Zhou, H. Fang, J. C. Yang and X. M. Xie, J. Supercrit.
Fluids, 2003, 26, 137–145.
60 N. Yousefi, B. Saeedi Saghez, R. D. Pettipas, T. L. Kelly and
L. G. Kaake, New J. Chem., 2021, 45, 11786–11796.
61 N. Yousefi, B. Saeedi Saghez, R. D. Pettipas, T. L. Kelly and
L. G. Kaake, Mater. Chem. Front., 2021, 5, 1428–1437.
62 J. A. Merlo and C. D. Frisbie, J. Phys. Chem. B, 2004, 108,
19169–19179.
63 T. Schuettfort, B. Watts, L. Thomsen, M. Lee, H. Sirringhaus
and C. R. McNeill, ACS Nano, 2012, 6, 1849–1864.
64 D. M. DeLongchamp, R. J. Kline, Y. Jung, D. S.
Germack, E. K. Lin, A. J. Moad, L. J. Richter, M. F.
© 2022 The Author(s). Published by the Royal Society of Chemistry
View Article Online
Materials Advances
66 M. L. Chabinyc, M. F. Toney, R. J. Kline, I. McCulloch and
M. Heeney, J. Am. Chem. Soc., 2007, 129, 3226–3237.
67 P. Boufflet, Y. Han, Z. Fei, N. D. Treat, R. Li, D. M. Smilgies,
N. Stingelin, T. D. Anthopoulos and M. Heeney, Adv. Funct.
Mater., 2015, 25, 7038–7048.
68 Z. Jiang, J. Appl. Crystallogr., 2015, 48, 917–926.
Open Access Article. Published on 07 February 2022. Downloaded on 11/18/2023 4:05:21 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Toney, M. Heeney and I. McCulloch, ACS Nano, 2009, 3,
780–787.
65 I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I.
Macdonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner,
W. M. Zhang, M. L. Chabinyc, R. J. Kline, M. D. McGehee and
M. F. Toney, Nat. Mater., 2006, 5, 328–333.
Paper
© 2022 The Author(s). Published by the Royal Society of Chemistry
Mater. Adv., 2022, 3, 2515–2523 | 2523