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In-Situ Observations of the Growth Mode of Vacuum Deposited α-Sexithiophene


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The real-time morphological evolution of vacuum deposited α-sexithiophene (α-6T) on a weakly interacting (glass) substrate at ambient temperature is reported. In-situ grazing incidence small angle X-ray scattering (GISAXS) enabled the observation of nanoscale aggregates while in-situ grazing incidence wide angle scattering (GIWAXS) allowed the study of the molecular-scale morphology. The in-situ GISAXS measurements revealed that the α-6T growth proceeds via a Stranski-Krastanov mode, whereby 2-4 complete monolayers are deposited followed by subsequent layers formed via island growth. In-situ GIWAXS also showed the evolution of the polymorph composition during the thin film growth. Initially the disordered β-phase and the low-temperature (LT) phase are deposited in nearly equal proportion until a thickness of 8 nm whereby the LT-phase begins to dominate until a final α-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the β-phase. The change in polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain relief as the thin film moves from surface to bulk mediated growth. The GISAXS findings were confirmed through direct imaging using ex-situ atomic force microscopy (AFM) at various thicknesses revealing the existence of both initial monolayers and intermediate and final island morphologies. The findings reveal the real-time morphological evolution of α-6T across both the molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of thin film microstructure in device performance, it is expected that these results will aid in the development of the structure-property relationships necessary to realise the full potential of organic electronics.
In Situ Observations of the Growth Mode of Vacuum-Deposited
T. L. Derrien,*A. E. Lauritzen, P. Kaienburg, J. F. M. Hardigree, C. Nicklin, and M. Riede*
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ABSTRACT: The real-time morphological evolution of vacuum-deposited α-sexithiophene (α-6T) on a weakly interacting (glass)
substrate at ambient temperature is reported. In situ grazing-incidence small-angle X-ray scattering (GISAXS) enabled the
observation of nanoscale aggregates, while in situ grazing-incidence wide-angle scattering (GIWAXS) allowed the study of the
molecular-scale morphology. The in situ GISAXS measurements revealed that the α-6T growth proceeds via a StranskiKrastanov
mode, whereby 24 complete monolayers are deposited, followed by subsequent layers formed via island growth. In situ GIWAXS
also showed the evolution of the polymorph composition during the thin-lm growth. Initially, the disordered β-phase and the low-
temperature (LT)-phase are deposited in nearly equal proportion until a thickness of 8 nm, whereby the LT-phase begins to
dominate until a nal α-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the β-
phase. The change in the polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain
relief as the thin lm moves from surface to bulk-mediated growth. The GISAXS ndings were conrmed through direct imaging
using ex situ atomic force microscopy (AFM) at various thicknesses, revealing the existence of both initial the initial and intermediate
monolayers and nal island morphologies. The ndings reveal the real-time morphological evolution of α-6T across both the
molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of the thin-lm
microstructure in device performance, it is expected that these results will aid in the development of structureproperty relationships
necessary to realize the full potential of organic electronics.
Small-molecule organic semiconductors (OSCs) have emerged
as interesting materials for a variety of applications including
organic eld-eect transistors (OFETs),
organic light-
emitting diodes (OLEDs),
and organic photovoltaics
In addition to low-cost and environmentally-
friendly processing techniques, the diverse range of molecules
available allow tailored structures to be created that can
optimize a variety of physical parameters.
However, OPV
and OFET devices are still outperformed by their inorganic
counterparts, in part due to the microstructural complexity of
OSC thin lms. Such thin-lm morphologies are typically
amorphous or polycrystalline with several crystalline poly-
morph components aecting numerous factors, such as
crystalline orientation, domain size, and purity, that inuence
the optical and electronic properties.
This complicates
theoretical predictions of performance and necessitates de-
tailed studies across multiple length scales, targeting both bulk
and interfaces to understand and engineer the structures that
inuence the properties and realize the potential of OSCs.
α-Sexithiophene (α-6T) is an OSC material that has been
extensively studied for use in OLEDs,
due to its combination of high carrier mobility
and appropriate optical gap. Several polymorphs of α-6T have
been shown to exist in thin lms.
The selection of the
appropriate polymorph or combination of polymorphs is
crucial to device performance, as each polymorph displays
distinct properties. For example, the high-temperature (HT)
and low-temperature (LT) phases of α-6T show diering
absorption strengths (under normal incidence).
While the
HT and the LT phases exist both in bulk and thin lms,
another α-6T polymorph is the kinetically favored and
disordered β-phase, which is unique to thin lms.
polymorph control in thin lms has been shown to be
dependent on several parameters such as illumination
substrate temperature,
deposition rate,
deposition annealing,
or vacuum incubation.
Here, we used several methods to characterize the real-time
growth of α-6T on a commercially relevant glass substrate to
characterize the growth mode across several length scales from
several angstroms to 200 nm. In situ grazing-incidence small-
angle X-ray scattering (GISAXS) was used to characterize the
nanoscale structure of the growing lms, while in situ grazing-
incidence wide-angle X-ray scattering (GIWAXS) was used to
characterize the molecular- scale microstructure. The mor-
phologies observed via X-ray scattering were conrmed
through direct surface topography imaging by atomic force
microscopy (AFM), revealing a StanskiKrastanov growth
mode. These observations of both the nal and intermediate α-
Received: January 16, 2020
Revised: May 9, 2020
Published: May 12, 2020
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6T growth stages provide important insight into the structural
control of OSC thin lms.
Grazing-Incidence Small-Angle X-ray Scattering (GI-
SAXS). GISAXS measurements used the Diamond Light
Source surface diraction beamline (I07). In situ experiments
were performed using the purpose-built MINERVA sample
that consists of dual low-temperature thermal
evaporation sources whose deposition rates are monitored by
quartz crystal microbalances (QCMs). Prior to α-6T
evaporation, the Corning Eagle XG substrates (manufactured
with an root mean square (RMS) of 5 Å, low roughness
conrmed by the lack of discernible diuse scattering) were
cleaned in an ultrasonic bath for 10 min using 2.5% Hellmanex
(non-etching) solution, deionized water, acetone, and nally
isopropanol. The cleaned substrate was mounted, and the
chamber was pumped down until a base pressure of 107mbar
was achieved. α-6T was deposited onto the substrate at a rate
of 0.30.45 Å/s, as monitored by the QCMs, which were
calibrated using spectroscopic ellipsometry using lm thick-
nesses of 5, 10, and 50 nm. In situ GISAXS measurements were
recorded every 10 s using an exposure time of 5 s with an X-ray
beam energy of 10.0 keV and an incidence angle of 0.1°(below
the critical angle of the glass substrate αc0.14°). No beam
damage was detected for thick lms, as veried by comparing
the images of the X-ray illuminated section with an o-beam
section after the deposition. Images were collected using a
Pilatus 2M detector at a sample to detector distance of 3.02 m
that was calibrated using a silver behenate standard (AgBeh).
Data reduction was performed in DAWN.
Grazing-Incidence Wide-Angle X-ray Scattering (GI-
WAXS). In situ GIWAXS measurements were recorded using
the previously described MINERVA chamber and deposition
conditions on glass substrates with a beam energy of 20.0 keV
and an incidence angle of 0.05°(αc0.07°). Images with a 1 s
exposure time were collected every 10 s using a Pilatus 2M
detector at a sample to detector distance of 41.8 cm that had
been calibrated using AgBeh.
Atomic Force Microscopy (AFM). The AFM images were
collected using a Veeco Multimode V in tapping mode
mounted with Bruker SCANASYST-AIR-HR tips. A scanning
rate of 0.5 Hz was used, and the images were analyzed using
The samples were prepared in the same manner
as the in situ samples, but the sample shutter was closed at
QCM readings corresponding to 2.5, 3.5, 4.5, and 5.5 nm.
To obtain a complete understanding of the growth mode of α-
6T, we employed several characterization methods capable of
probing the thin-lm microstructure across broad length scales.
In situ GISAXS was used to track the real-time morphology of
nanoscale aggregates, and these ndings were conrmed via
AFM imaging. The evolution of the molecular structure of the
α-6T lm growth was then monitored in real-time via in situ
GIWAXS, which enabled the tracking of the polymorph
composition during growth.
Selected detector images from the in situ GISAXS are shown
in Figure 1A. The images are taken from time points
corresponding to multiples of approximately 0.7-1.3 mono-
layers (MLs). The number of MLs deposited was calculated
using the recorded QCM thicknesses and assuming an LT-
phase structure (2.2 nm).
This assumption leads to only
approximate ML multiples, due to variation in the α-6T
polymorph composition, but is convenient for discussing the
resulting small-angle scattering. From these detector images, it
can be seen that o-specular scattering is strongest at
incomplete ML multiples (Figure 1A; 0.7, 1.5, and 2.6 MLs)
and weakest at complete or near-complete MLs (Figure 1A;
1.1 and 1.9 MLs). This is a result of strong scattering from
island-like morphologies for unlled MLs and weak scattering
from the smooth, complete, ML surface
(Figure 1B). Such
strong oscillations in the scattering intensity only occurred
during these early stages of the α-6T deposition. This can be
seen in the real-time intensity variation of the scattering in the
in-plane direction shown in Figure 2 and in a video of the
detector images of the full 50 nm thin-lm deposition
(available in the electronic Supporting Information). The
data in Figure 2, composed of stacked linecuts at q= 0.095
Å1, are indicative of the changing morphology of the growing
α-6T lm. The appearance and disappearance of the o-
specular scattering can be clearly seen in the early time points
corresponding to thicknesses of up to 2.6 MLs. The absolute
peak intensity was calculated by tting a Lorentizian shape on
a Gaussian background in the linecut data and was plotted
against lm thickness, as shown in Figure 3A. The rst two
minima indicated by the dashed lines correspond to the nearly
complete monolayer coverage shown in Figure 1, at 1.1 and 1.9
ML coverage. Upon further α-6T deposition, or after 2 MLs
have been deposited, the degree of oscillation in the o-
specular scattering intensity is reduced and completely absent
after 4 MLs (8.4 nm) corresponding to a loss of the layerwise
growth and the presence of α-6T islands.
In addition to the intensity variations, Figure 2 shows the
narrowing and broadening of the peaks occurring during these
oscillations. Shifts in the peak positions (dashed lines in Figure
2A) are due to the varying correlation length resulting from the
changing separation of the growing islands. This disparity can
be clearly seen in the sample linecuts and ts for the peaks
corresponding to 0.7 and 1.5 MLs shown in Figure 2B, which
highlight the variations seen from the rst two scattering peaks
seen in the time series. The peak positions from the ts were
used to calculate the characteristic length scale of the average
Figure 1. (A) Two-dimensional (2D) in situ GISAXS images obtained
at various α-6T ML coverages showing the variation in scattering
during the course of the deposition. At 0.7 ML (1.7 nm), 1.5 ML (3.3
nm), and 2.6 ML (5.7 nm) coverages, strong scattering peaks are
observed, while at 1.1 ML (2.4 nm) and 1.9 ML (4.1 nm) coverages,
the peaks disappear. (B) Schematic of the growth mode elucidated
from the detector images.
The Journal of Physical Chemistry C Article
J. Phys. Chem. C 2020, 124, 1186311869
separation of the islands, or the correlation length (Lc), via Lc=
2π/q, where qis the peak position in q-space (as conrmed
via simulation using BornAgain,
Figure S1). The correlation
length of the growing islands is plotted in Figure 3B, which
indicates that the islands formed on top of the rst monolayer
grow with a larger correlation length than those grown on the
glass substrate. The correlation length is signicantly increased
near complete ML coverage. This can be attributed to the fact
that near complete ML coverage, the scattering signal is the
result of the gaps in the lm, not the island separation. The
diuse scattering close to the specular beam (at q=0Å
calculated from a 5 ×5 (0.0015 Å1×0.015 Å1) pixel region
centered at q= 0.11 Å1, was also used to monitor the
progression of the lm growth (Figure 3C), revealing a period
of layerwise oscillation. The oscillation in the peak intensity,
correlation length, and diuse scattering near the specular
beam were all reduced after 2 MLs were deposited and absent
after 4 MLs, indicating that after this point, the growth of the
α-6T lm proceeds via the formation of adsorbate islands. The
decreased correlation length observed indicates that in the
latter stages of the lm growth the islands formed were more
closely spaced together. Collectively, the GISAXS data show
that α-6T growth by vacuum deposition on glass proceeds via a
StranskiKrastanov (SK)
growth mode where beyond a
critical wetting layer thickness of two monolayers, α-6T
deposition progresses via island growth (Figure 1B). The
observation of SK growth, which depends on lattice
interactions (strain) between the surface and the adsorbate,
demonstrates the templating potential of the thin-lm
substrate. Though SK growth modes for α-6T have been
observed in situ on metal substrates using photoelectron
emission microscopy (PEEM) and reectance dierence
spectroscopy (RDS),
they have never been expressly
captured on weakly interacting, device-relevant substrates such
as the glass substrates used here. Furthermore, the in situ
GISAXS employed here enabled the calculation of morpho-
logical parameters, with nm resolution, of the growing lm not
accessible with other techniques.
Direct imaging of the various growth stages via AFM of ex
situ-prepared samples was consistent with the analysis of the
GISAXS data. We were able to image the early stages of α-6T
growth by stopping the deposition at thicknesses of 2.5 nm
(Figure 4A), 3.5 nm (Figure 4B), 4.5 nm (Figure 4C), and 5.5
nm (Figure 4D), as measured by QCM. Additionally, Figure
4E shows the surface structure of the 50 nm α-6T lm, where
the island morphology is clearly visible. Watershed analysis of
the islands of the 50 nm-thick lm indicated a mean island size
of 53 ±15 nm. The size of the islands was also estimated using
the o-specular GISAXS peaks according to the Scherrer
Dhkl =2πK/FWHM, where Dhkl is the average grain
size, taken here as the size of the islands, and Kis a shape
constant, 0.9 for spherical grains. Due to instrumental
broadening and detector resolution considerations, Scherrer
analysis can be used to calculate a lower limit grain size.
the 50 nm-thick lm, Scherrer analysis returned a lower limit
grain size of 41 nm, which is within the standard deviation of
progression from near monolayer coverage (Figure 4A), to
island formation (Figure 4B), growth (Figure 4C), coalescence
(Figure 4D), and repeated island formation (Figure 4E) of the
α-6T lm. This was also observed through the increase of RMS
roughness, as calculated from the AFM images (Table 1),
where at thicknesses below 2 MLs (2.5, 3.5, and 4.5 nm), the
Figure 2. Real-time GISAXS data of α-6T deposited on glass at room
temperature. (A) Surface plot of real-time α-6T GISAXS scans
measured up to 20 nm thickness showing variation in o-specular
scattering intensity and peak positions. The surface plot is a
composite of linecuts taken at q= 0.095 Å1. The peaks indicated
with dashed lines occur between the deposition of the rst (0.7 ML)
and second monolayers (1.5 ML), as shown in Figure 1A. The
linecuts taken from the detector images for these peaks are shown in
(B), along with the corresponding ts.
Figure 3. Evolution of the (A) peak scattering intensity, (B)
correlation length, and (C) intensity at q=0Å
1(calculated from a
5×5 pixel region centered at q= 0.11 Å1) of the α-6T lm during
growth as measured by GISAXS. Dashed lines correspond to the
deposition of one and two monolayers.
The Journal of Physical Chemistry C Article
J. Phys. Chem. C 2020, 124, 1186311869
RMS roughness remains approximately constant, after which it
quickly increases as a result of island formation (5.5 and 50
nm). The ex situ AFM observations further conrm the SK
growth mode observed in situ above. Additional image analysis
was conducted to extract an approximate correlation length of
the islands (Figure S2). The results shown in Table 1
correspond with the correlation lengths extracted from the
GISAXS data.
The crystalline structure of the growing α-6T lm was
monitored by real-time in situ GIWAXS measurements. By
performing these time-resolved measurements under the same
conditions as the above GISAXS experiments, we were able to
track the evolution of the dierent α-6T polymorphs
throughout the deposition process. Figure 5A, composed of a
series of linecuts taken from the in-plane direction, shows the
evolution of the β- and the LT-phase peaks during the lm
growth. The heatmap clearly shows the inverse relation
between the intensity of the βand LT peaks at the beginning
and the end of the evaporation. At the beginning of the thin-
lm deposition, the βpeak at q= 1.38 Å1intensity was
stronger in intensity than the (020) LT peak (q1.6 Å1).
After the full 50 nm lm growth, the intensity of the βpeak
was notably less intense than its maximum, while the (020) LT
peak intensity was strongest. Additionally, a weak LT (011)
peak at q= 1.32 Å1was detected, which was not present in
the early stages of the α-6T deposition. A timelapse of the full
scattering images is available in the electronic Supporting
Information. The peaks were tted as described above to
calculate the scattering intensity and the full width half-
maximum (FWHM). The size of the islands was estimated
according to the Scherrer equation. Using this equation, the
grain size, i.e., the diameter of the growing polymorph grains,
can be plotted against the lm thickness. The results, plotted in
Figure 5B, show that after an initial increase in the grain size
during the early stages of the lm growth, the domains of both
the β- and LT-phases contracted in size until a lm thickness of
19 nm where the LT-phase reached an equilibrium grain size
value of 15 nm and the β-phase continued to shrink to a nal
grain size of 10 nm until the deposition was stopped. Finally,
the integrated scattering intensities (ISI) of the βpeak and the
LT (020) peak were normalized to the maximum value (LT
ISI at the end of the lm growth). The normalized ISI can then
be used to compare the scattering intensity of the α-6T-phases
throughout the lm growth (Figure 5C).
From the ISI data, three regions can be distinguished: region
(1), where the β- and LT-phase ISI increase quickly, and the β-
phase ISI dominates; (2), where the rate of ISI increase slows
for both the β- and the LT-phases; and (3), where the LT-
phase ISI again increases while the β-phase ISI remains
relatively constant. The rst region of rapid ISI increase occurs
up to about 4.8 nm (2 ML), roughly corresponding to the
region of monolayer growth. The ISI then slows at the
beginning of region 2, which occurs at the onset of island
growth up to a thickness of approximately 8 nm. Region 3 then
begins at a lm thickness of 8 nm, and it is at this point that
the ISI of the LT-phase continues to increase while the β-phase
ISI stagnates until the deposition is stopped at 50 nm and the
ISI of the LT-phase is more than twice that of the β-phase.
Analyses of the β- and LT-phase peak positions (Figures S2
and 6, respectively) revealed that the start of region 3
corresponded with a sudden increase in the LT-phase (020)
peak spacing, as seen in Figure 6, near a thickness of 8 nm.
This abrupt shift in d-spacing is indicative of strain relief, which
accompanied the transition from monolayer to island growth
associated with the SK growth mode. However, the GISAXS
data indicated that the island growth occurred from a thickness
of approximately 4 nm, which corresponds to the start of
region 2. This nding suggests that although the island growth
found in the GISAXS and AFM data occurs at smaller
thicknesses, the full extent of the lattice strain induced by the
glass surface is not relieved until a thickness of 8 nm. Indeed,
this is the thickness in which the o-specular peak intensities
completely vanish. In the intermediate growth region (2), the
growth of two polymorphs stagnates until the lattice strain is
relaxed, at which point the LT-phase grows notably quicker
than the β-phase. In contrast to the relaxing of the α-6T LT-
phase, the d-spacing of the β-phase peak did not undergo any
substantial shifts following the growth mode (Figure S3) shift.
The in situ GIWAXS results obtained here closely mirror
those reported by Lorch et al.,
who reported the growth of
competing α-6T crystal phases on SiO2substrates at both 308
and 373 K. However, our ambient temperature measurements
on glass more closely resemble the growth they reported for
higher substrate temperatures (373 K), where the β-phase
dominated in the early stages of growth, after which the LT-
phase began to dominate. They reported that at high
temperature, at thicknesses above 7 nm, the LT-phase takes
over the β-phase, nearly equal to the 8 nm thickness where we
observed the β- to LT-phase transition. Conversely, on SiO2at
308 K, a temperature similar to our ambient measurements on
glass substrates, their results showed that the β-phase
dominates throughout the deposition process. Furthermore,
though the measurements by Lorch et al. show a larger LT-
phase d-spacing for the lms grown at high temperature, the
Figure 4. AFM images of vacuum-deposited α-6T lms at (A) 2.5 (1.1 ML), (B) 3.5 (1.6 ML), (C) 4.5 (2.0 ML), (D) 5.5 (2.5 ML), and
(E) 50 nm thickness.
Table 1. RMS Roughness and Correlation Length
Calculated from Figure 4
sample thickness (nm) RMS roughness (nm) correlation length (nm)
2.5 1.2 157
3.5 1.2 132
4.5 1.5 142
5.5 3.2 150
50 8.2 98
The Journal of Physical Chemistry C Article
J. Phys. Chem. C 2020, 124, 1186311869
real-time GIWAXS results at both high and low temperatures
do not indicate surface strain such as we have observed on
glass at ambient temperature. This disparity highlights the
strain-mediated templating potential of the substrate on which
α-6T is deposited, a key consideration in device fabrication,
given the distinct physical properties, such as absorption
displayed by each α-6T polymorph.
Through the use of real-time in situ X-ray scattering
experiments, we were able to directly probe the growth
mode of α-6T on glass at room temperature from the
molecular scale to 200 nm. We employed in situ GISAXS to
probe the formation and morphology of nanoscale aggregates,
and in situ GIWAXS to probe the evolution of the crystalline
structure of the growing lms, both in real time. The GISAXS
data revealed that α-6T lms grow according to a Stranski
Krastanov growth mode where two-dimensional (2D) layer-
by-layer growth transitioned to three-dimensional (3D) island
growth. The shift from 2D to 3D island growth was found to
occur after two monolayers were deposited. The presence of
both monolayer and island morphologies was conrmed
through direct imaging via AFM. In situ GIWAXS measure-
ments revealed the changing crystalline structure of the thin
lm, where initially more of the α-6T β-phase formed on the
glass surface, but when the lm thickness exceeded 8 nm, the
LT-phase began to dominate. Taken together, the results
present a multi length scale characterization of the evolution of
vacuum-deposited α-6T lms on weakly interacting substrates,
revealing a growth mode where both the molecular scale and
the nanoscale morphology undergo a transition from surface to
bulk-mediated growth. In addition to emphasizing the
capability of the combination of in situ GIWAXS and GISAXS
to characterize the thin-lm growth, the ndings highlight the
importance of the surface in controlling the morphology and
crystalline structure of vacuum-deposited lms. Further in situ
studies on additional OSC-relevant substrates and the
characterization of device parameters will aid in the develop-
ment of structureproperty relationships necessary to improve
sıSupporting Information
The Supporting Information is available free of charge at
Simulation model and parameters; comparison of
experimental and simulated ospecular scattering
peaks; real time evolution of the α-6T β-phase peak
Figure 5. Real-time GIWAXS data of α-6T deposited on glass at
room temperature. (A) Real-time GIWAXS scans of vacuum-
deposited α-6T showing the evolution of the LT- and β-phases
during the lm growth. (B) Real-time evolution of the α-6T β-phase
and LT-phase (020) grain sizes. (C) Normalized integrated scattering
intensity, with dashed lines indicating the regions of rapid β-phase
growth (1), slowed growth (2), and LT-phase dominated growth (3).
Figure 6. Real-time evolution of the α-6T (020)-LT peak position
(black) and d-spacing during lm deposition.
The Journal of Physical Chemistry C Article
J. Phys. Chem. C 2020, 124, 1186311869
position and d-spacing during lm deposition; AFM
Real-time GISAXS timelapse (AVI)
Real-time GIWAXS timelapse (AVI)
Corresponding Authors
T. L. Derrien Diamond Light Source, Didcot, Oxfordshire
OX11 0DE, United Kingdom;
2278; Email:
M. Riede Clarendon Laboratory, Department of Physics,
University of Oxford, Oxford, Oxfordshire OX1 3PU, United
A. E. Lauritzen Clarendon Laboratory, Department of Physics,
University of Oxford, Oxford, Oxfordshire OX1 3PU, United
P. Kaienburg Clarendon Laboratory, Department of Physics,
University of Oxford, Oxford, Oxfordshire OX1 3PU, United
J. F. M. Hardigree Clarendon Laboratory, Department of
Physics, University of Oxford, Oxford, Oxfordshire OX1 3PU,
United Kingdom
C. Nicklin Diamond Light Source, Didcot, Oxfordshire OX11
0DE, United Kingdom
Complete contact information is available at:
The authors declare no competing nancial interest.
T.L.D. prepared the original manuscript of the draft with all
authors contributing to the reviewing and editing process.
T.L.D., A.E.L., P.K., and J.F.M. performed the experiments and
T.L.D. analyzed the data. The project was conceived by T.L.D.,
A.E.L., C.N., and M.R., with C.N. and M.R. providing
supervision. T.L.D., P.K., C.N., and M.R. acknowledge funding
for this work from the UKRI-GCRF grant Synchrotron
Techniques for African Research and Technology (START)
ST/R002754/1. A.E.L. thanks EPSRC for funding through the
Doctoral Training Partnership (EP/N509711/1) as well as
STFC and the ISIS Neutron and Muon facility and project
(1948713). Access to Diamond beamtime at I07 was provided
under experiment Nos. SI20426-1 and NT24871-1. The
authors thank A. Warne, J. Rawle, H. Hussain, and F. Carla
(Diamond Light Source) for their assistance with beamline
instrumentation. They are grateful to J. Naylor, D. Wicks, and
A. Dorman of K. J. Lesker Ltd. for generously providing
deposition control and evaporation sources along with
technical support for MINERVA, which was the result of an
STFC CLASP project (ST/L006294/1).
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... A key component of device engineering is the ability to tune the thickness of each layer, which requires an understanding of growth mechanisms. This is particularly relevant for organic crystals where it is rather common for the roughness to evolve via more three-dimensional Stranski-Krastanov or Volmer-Weber island growth, [12][13][14] rather than the desirable, two-dimensional, layer-by-layer outcome (Frank-van-der-Merwe growth). Rough interfaces should be avoided as they can lead to poor electrical properties in transistors 15 and layer-inhomogeneities that can cause electrical shorts. ...
... Many of the molecules considered here have multiple single bonds within their conjugated core, resulting in non-planar conformations. This is in contrast to the numerous roughness evolution studies of planar 24,29-31 and rod-like 12,19,[32][33][34][35][36] conjugated molecules. Prior to this work, most roughness evolution research into more three-dimensional (3D) molecules with the ability to crystallize was limited to materials like C 60 20,37-39 and rubrene. ...
Exploiting the capabilities of organic semiconductors for applications ranging from light-emitting diodes to photovoltaics to lasers relies on the creation of ordered, smooth layers for optimal charge carrier mobilities and exciton diffusion. This, in turn, creates a demand for organic small molecules that can form smooth thin film crystals via homoepitaxy. We have studied a set of small-molecule organic semiconductors that serve as templates for homoepitaxy. The surface roughness of these materials is measured as a function of adlayer film thickness from which the growth exponent (β) is extracted. Notably, we find that three-dimensional molecules that have low molecular aspect ratios (AR) tend to remain smooth as thickness increases (small β). This is in contrast to planar or rod-like molecules with high AR that quickly roughen (large β). Molecular dynamics simulations find that the Ehrlich-Schwöbel barrier (EES) alone is unable to fully explain this trend. We further investigated the mobility of ad-molecules on the crystalline surface to categorize their diffusion behaviors and the effects of aggregation to account for the different degrees of roughness that we observed. Our results suggest that low AR molecules have low molecular mobility and moderate EES which creates a downward funneling effect leading to smooth crystal growth.
... The devicerelevant thin-film morphology and microstructure differ from bulk and factors such as molecular and macroscopic ori-entation [11,12], polymorphism including surface-induced phases (SIPs) [13][14][15][16], nonequilibrium growth conditions, and interfacial interactions all play important roles. Several methods can be used to investigate the structural properties of OSC films, including x-ray scattering and near edge x-ray absorption fine-structure techniques [17][18][19], in situ variations thereof [20][21][22][23], fluorescence spectroscopy and microscopy [24], Raman spectroscopy [25], optical spectroscopy [26], and electron microscopy [27][28][29], to name a few. ...
... SK growth is often observed in other systems of rod-like small-molecules, including other molecules from the family of oligothiophenes such as α-sexithiophene 053402-9 (α-6T) [23], which has been shown to form a 2-ML-thick substrate-wetting layer on glass substrates before 3D growth sets in. In comparison, DIP on silicon dioxide exhibits layer-by-layer growth for seven monolayers before 3D growth sets in [20]. ...
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We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, 5,5′-bis(naphth-2-yl)-2,2′-bi- and 5,5′′-bis(naphth-2-yl)-2,2′:5′,2′′-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented MoS2 layers, while the NaT3 molecules form tall, isolated islands on vertically oriented MoS2 layers. The molecules are standing on silicon dioxide and on vertically oriented MoS2, but lying flat on horizontally oriented MoS2. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes.
... While OLEDs have met commercial success due to their high efficiencies, sufficient lifetimes, and the scalability of vacuum deposition, the performance of OFETs and OPVs still lags behind those of inorganic devices. This is partially due to the complex mechanisms by which thin film OSCs develop, where elaborate microstructures occur across several length scales and can evolve as the thin film grows [7][8][9][10][11]. Device microstructure is critical for OPV and OFET performance and controlling the microstructure has even been found to improve OLED properties [12,13]. ...
Full-text available
We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the γ-ZnPc polymorph thereafter (thickness ≈1.0 nm). As the growth of the bulk γ-ZnPc progresses, a substantial out-of-plane lattice strain (≈15% relative to γ-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of ≈13 nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.
... The microstructure of the evaporated thin-films for OSC is usually amorphous or polycrystalline with up to several crystalline polymorph components. Numerous factors, such as crystalline orientation, domain size and purity influence the optical and electronic properties [6][7][8][9][10][11][12][13][14]. ...
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Organic solar cells (OSCs), also known as organic photovoltaics (OPVs), are an emerging solar cell technology composed of carbon-based, organic molecules, which convert energy from the sun into electricity. Key for their performance is the microstructure of the light-absorbing organic bulk heterojunction. To study this, organic solar films composed of both fullerene C60 as electron acceptor and different mole percentages of di-[4-(N,N-di-p-tolyl-amino)-phenyl]-cyclohexane (TAPC) as electron donor were evaporated in vacuum in different mixing ratios (5, 50 and 95 mol%) on an ITO-coated glass substrate held at room temperature and at 110 °C. The microstructure of the C60: TAPC heterojunction was studied by grazing incidence wide angle X-ray scattering to understand the effect of substrate heating. By increasing the substrate temperature from ambient to 110 °C, it was found that no significant change was observed in the crystal size for the C60: TAPC concentrations investigated in this study. In addition to the variation done in the substrate temperature, the variation of the mole percent of the donor (TAPC) was studied to conclude the effect of both the substrate temperature and the donor concentration on the microstructure of the OSC films. Bragg peaks were attributed to C60 in the pure C60 sample and in the blend with low donor mole percentage (5%), but the C60 peaks became nondiscernible when the donor mole percentage was increased to 50% and above, showing that TAPC interrupted the formation of C60 crystals.
A high-quality ultrathin dielectric film is important in the field of microelectronics. We designed a composite structure composed of Al2O3/HfO2 with different Al2O3/HfO2 cycles prepared by atomic layer deposition (ALD) to obtain high-quality ultrathin (1-12 nm) dielectric films. Al2O3 protected HfO2 from interacting with the Si substrate and inhibited the crystallization of the HfO2 film. High permittivity material of HfO2 was adopted to guarantee the good insulating property of the composite film. We investigated the physical properties as well as the growth mode of the composite film and found that the film exhibited a layer growth mode. The water contact angle and grazing-incidence small-angle X-ray scattering analyses revealed that the film was formed physically at 3 nm, while the thickness of the electrically stable film was 10 nm from grazing-incidence wide-angle X-ray scattering and dielectric constant analyses. The composite film was applied as a dielectric layer in thin-film transistors (TFTs). The threshold voltage was decreased to 0.27 V compared to the organic field-effect transistor with the single HfO2 dielectric, and the subthreshold swing was as small as 0.05 V/dec with a carrier mobility of 49.2 cm2/V s. The off-current was as low as 10-11 A, and the on/off ratio was as high as 5.5 × 106. This ALD-prepared composite strategy provides a simple and practical way to obtain the high-quality dielectric film, which shows the potential application in the field of microelectronics.
The formation mechanism of metal oxide films is very important to nanomanufacturing and microelectronic devices. We have prepared the aluminum oxide (Al2O3) films with the thicknesses ranging from 1 to 30 nm by using atomic layer deposition technology. By investigating their morphologies and optoelectrical properties, we found that the Al2O3 films grow on substrate via a layer growth mode by atomic force microscope measurement. Grazing incidence small-angle x-ray scattering further proved that a layer structure parallel to the substrate at the thickness of 5 nm. The water contact angle revealed that 5 nm is critical thickness of physical-film formation. The analysis of dielectric characteristics and the performance of thin film transistors revealed that 30 nm is critical thickness of dielectric-film formation. The understanding on structure-function relationships is necessary to realize the potential application of metal oxide films.
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BornAgain is a free and open-source multi-platform software framework for simulating and fitting X-ray and neutron reflectometry, off-specular scattering, and grazing-incidence small-angle scattering (GISAS). This paper concentrates on GISAS. Support for reflectometry and off-specular scattering has been added more recently, is still under intense development and will be described in a later publication. BornAgain supports neutron polarization and magnetic scattering. Users can define sample and instrument models through Python scripting. A large subset of the functionality is also available through a graphical user interface. This paper describes the software in terms of the realized non-functional and functional requirements. The web site provides further documentation.
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We report on a method for fabricating balanced hole and electron transport in ambipolar organic field-effect transistors (OFETs) based on the co-evaporation of zinc-phthalocyanine (ZnPc) and its fluorinated derivative (F 8 ZnPc). The semiconducting behaviour of the OFET can be tuned continuously from unipolar p-type, with a hole mobility in the range of (1.7 ± 0.1) × 10 ⁻⁴ cm ² /Vs, to unipolar n-type, with an electron mobility of (1.0 ± 0.1) × 10 ⁻⁴ cm ² /Vs. Devices of the pristine ZnPc and F 8 ZnPc show a current on/off ratio of 10 ⁵ . By co-evaporating the p-type ZnPc with the n-type F 8 ZnPc, we fabricate ambipolar transistors and complementary-like voltage inverters. For the ambipolar devices, the optimum balance between the hole and electron mobilities is found for the blend of 1:1.5 weight ratio with hole and electron mobilities of (8.3 ± 0.2) × 10 ⁻⁷ cm ² /Vs and (5.5 ± 0.1) × 10 ⁻⁷ cm ² /Vs, respectively. Finally we demonstrate application of the ambipolar devices in a complementary-like voltage inverter circuit with the performance comparable to an inverter based on separate ZnPc and F 8 ZnPc OFETs.
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A sample environment to enable real-time X-ray scattering measurements to be recorded during the growth of materials by thermal evaporation in vacuum is presented. The in situ capabilities include studying microstructure development with time or during exposure to different environmental conditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop, to reduce the background scattering from the X-rays passing through the entrance and exit windows, together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used in organic solar cells and organic light emitting diodes, including the development of the microstructure with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room temperature under vacuum, highlighting the importance of using real time measurements to understand the as-deposited pristine film structure and its development with time. More generally, this sample environment is versatile and can be used for investigation of structure-property relationships in a wide range of vacuum deposited materials and their applications in, for example, optoelectronic devices and energy storage.
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A software package for the calibration and processing of powder X-ray diffraction and small-angle X-ray scattering data is presented. It provides a multitude of data processing and visualization tools as well as a command-line scripting interface for on-the-fly processing and the incorporation of complex data treatment tasks. Customizable processing chains permit the execution of many data processing steps to convert a single image or a batch of raw two-dimensional data into meaningful data and one-dimensional diffractograms. The processed data files contain the full data provenance of each process applied to the data. The calibration routines can run automatically even for high energies and also for large detector tilt angles. Some of the functionalities are highlighted by specific use cases.
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Thin-film growth is important for novel functional materials and new generations of devices. The non-equilibrium growth physics involved is very challenging, because the energy landscape for atomic scale processes is determined by many parameters, such as the diffusion and Ehrlich-Schwoebel barriers. We review the in situ real-time techniques of x-ray diffraction (XRD), x-ray growth oscillations and diffuse x-ray scattering (GISAXS) for the determination of structure and morphology on length scales from Å to µm. We give examples of time resolved growth experiments mainly from molecular thin film growth, but also highlight growth of inorganic materials using molecular beam epitaxy (MBE) and electrochemical deposition from liquids. We discuss how scaling parameters of rate equation models and fundamental energy barriers in kinetic Monte Carlo methods can be determined from fits of the real-time x-ray data.
In this work two novel donor:acceptor (D:A) complexes, namely diindenoperylene(DIP):1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane(F6TCNNQ) and alpha-sexithiophene(6T):F6TCNNQ, are studied. The D:A complexes segregate in form of π-π stacked D:A co-crystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin film crystalline texture and composition of the D:A mixtures significantly. In equimolar mixtures, for DIP:F6TCNNQ the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T:F6TCNNQ a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D:A complex, respectively, are observed. Using optical absorption spectroscopy we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D:A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D:A complex.
The development of organic semiconductors for photovoltaic devices, over the last three decades, has led to unexpected performance for an alternative choice of materials to convert sunlight to electricity. New materials and developed concepts have improved the photovoltage in organic photovoltaic devices, where records are now found above 13% power conversion efficiency in sunlight. The author has stayed with the topic of organic materials for energy conversion and energy storage during these three decades, and makes use of the Hall of Fame now built by Advanced Materials, to present his view of the path travelled over this time, including motivations, personalities, and ambitions.
2017 saw the publication of several new material systems that challenge the long‐held notion that a driving force is necessary for efficient exciton dissociation in organic photovoltaics (OPVs) and that a loss of ≈0.6 eV between the energy of the charge transfer state Ect and the energy corresponding to open circuit is general. In light of these developments, the authors combine insights from device physics and spectroscopy to review the two key tradeoffs limiting OPV performances. These are the tradeoff between the charge carrier generation efficiency and the achievable open circuit voltage (Voc) and the tradeoff between device thickness (light absorption) and fill factor. The emergence of several competitive nonfullerene acceptors (NFAs) is exciting for both of these. The authors analyze what makes these materials compare favorably to fullerenes, including the potential role of molecular vibrations, and discuss both design criteria for new molecules and the achievable power conversion efficiencies. Organic photovoltaics have made astounding progress both in fundamental understanding and device performance. In particular, nonfullerene acceptors bring into question previous empirical estimates of the achievable power conversion efficiencies. This report aims to provide a perspective on the physics underlying the conventional and new systems and their performance limits.
Herein, we report an efficient approach to control the crystalline polymorph of evaporated α-sexi-thiophene (6T) thin-films by keeping them overnight (12 h) under vacuum. Further, we investigated the effects on the performance of organic photovol-taic devices of controlling the 6T polymorph via this vacuum technique so that the films take on low-temperature (LT) pol-ymorph (in which the backbones of the 6T molecules lie flatter on the substrate—the so-called 'lying-down' orientation). Our results revealed that when the organic layer was deposited directly onto cupper iodide (CuI) interlayer, the angle between the organic backbone and the substrate was reduced in the LT polymorph compared with the high-temperature (HT) poly-morph. The power conversion efficiency of solar cells could thus be enhanced from 0.58 to 1.77% via a change in the crys-tal polymorph of the 6T layer from HT to LT by simply keeping the films in vacuum for 12 h.
Adding a twist for enhanced performance The efficiency of organic light-emitting diodes (OLEDs) is fundamentally governed by the ratio of emissive singlet to dark triplet excitons that are formed from spin-polarized electron and hole currents within the material. Typically, this has set an upper limit of 25% internal quantum efficiency for OLEDs. Di et al. manipulated the ratio of spin states through a modification of process chemistry. They introduced a rotation of the molecular structure, which inverted the spin-state energetics and enhanced OLED performance. Science , this issue p. 159