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In-situ observation of stacking fault evolution in vacuum-deposited C 60


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We report an in-situ study of stacking fault evolution in C60 thin films using grazing-incidence x-ray scattering. A Williamson-Hall analysis of the main scattering features during growth of a 15 nm film on glass indicates lattice strain as high as 6% in the first 5 nm of the film, with a decrease to 2% beyond 8 nm thickness. Deformation stacking faults along the {220} plane are found to occur with 68% probability and closely linked to the formation of a nanocrystalline powder-like film. Our findings, which capture monolayer-resolution growth, are consistent with previous work on crystalline and powder C60 films, and provide a crystallographic context for the real-time study of organic semiconductor thin films.
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In-situ observation of stacking fault evolution in vacuum-deposited C60
J. F. M. Hardigree, I. R. Ramirez, G. Mazzotta, C. Nicklin, and M. Riede
Citation: Appl. Phys. Lett. 111, 233305 (2017);
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Published by the American Institute of Physics
In-situ observation of stacking fault evolution in vacuum-deposited C
J. F. M. Hardigree,
I. R. Ramirez,
G. Mazzotta,
C. Nicklin,
and M. Riede
Clarendon Laboratory, Department of Physics, University of Oxford, Oxfordshire OX1 3PU, United Kingdom
Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom
(Received 12 July 2017; accepted 13 November 2017; published online 8 December 2017)
We report an in-situ study of stacking fault evolution in C
thin films using grazing-incidence x-ray
scattering. A Williamson-Hall analysis of the main scattering features during growth of a 15 nm film
on glass indicates lattice strain as high as 6% in the first 5 nm of the film, with a decrease to 2%
beyond 8 nm thickness. Deformation stacking faults along the {220} plane are found to occur with
68% probability and closely linked to the formation of a nanocrystalline powder-like film. Our find-
ings, which capture monolayer-resolution growth, are consistent with previous work on crystalline
and powder C
films, and provide a crystallographic context for the real-time study of organic semi-
conductor thin films. V
C2017 Author(s). All article content, except where otherwise noted, is
licensed under a Creative Commons Attribution (CC BY) license (
The structure of small-molecule organic semiconductors
in thin film electronic devices has been the subject of numer-
ous studies, stimulated by the interest in developing low-cost
electronic devices for health monitors,
and digital logic
from simple organic building
blocks. Thin films of small-molecule organic semiconductors
can exhibit a wide range of microstructural motifs, which are
intimately linked to properties as diverse as absorption,
photogenerated exciton diffusion,
charge carrier mobility,
and analyte gas diffusivity.
In-situ techniques that enable real-time monitoring of
vacuum-deposited thin films can provide highly granular
spatio-temporal information for probing the underlying phys-
ics of organic film formation
and help elucidate the gen-
eralisable processing-structure-property relationships sought
for advanced device fabrication.
The molecular and thin film structure of C
fullerene, a
ubiquitous organic semiconductor, has been investigated in
detail since its successful synthesis nearly 30 years ago.
Among the many mesoscale features of C
solids are a high
density of twins and stacking faults along the h111iclose-
packing direction, attributed to the weak van der Waals inter-
actions between fullerenes.
Stacking faults are a feature
of materials with face-centered cubic (FCC) structures, in
which several sets of tetrahedral positions are available in a
given h111iplane. While FCC stacking consists of planes
following a 3-plane sequence of alternating tetrahedral posi-
tions (A-B-C-A-B-C), a stacking fault occurs when atoms in
the third plane in this sequence occupy tetrahedral positions
corresponding to those of the A planes. A particular type of
stacking fault known as crystal twinning occurs when the
fourth plane takes the position of a B plane instead (e.g.,
A-B-C-B-A). Such faults disrupt translational symmetry
along the close packed h111idirection, impacting macro-
scopic properties such as charge transport through the forma-
tion of gap states and interfacial energy barriers in inorganic
semiconductor thin films.
Although studies of bulk crystal-
and vacuum-deposited thin film
have identi-
fied the presence of stacking faults and twins, there has been
much less work aimed at quantifying these and probing how
they evolve during deposition, especially under industrially
relevant processing conditions.
To investigate the evolution of C
, we employed a
recently developed multi-source deposition chamber at the
Diamond Light Source that enables grazing-incidence X-ray
scattering (GIXS) measurements of growing molecular thin
films during thermal deposition.
Capturing the growth
dynamics of organic molecular films at industrially relevant
deposition rates (0.1–1 A
presents various technical chal-
lenges. The low scattering density of organic materials neces-
sitates longer exposure times than that of films of a similar
thickness of high-Z atomic species to achieve similar con-
It is also established that organic thin films—in partic-
ular samples less than 10 nm thick—are susceptible to x-ray
beam damage.
Consequently, tracking the scattering of
the growing film requires balancing the deposition rate and x-
ray exposure time, such that each image provides a suffi-
ciently discrete snapshot of the film’s current state without
subjecting it to detrimental beam damage that could influence
subsequent layers. To balance these competing needs, we set
a fullerene evaporation rate of 0.1 A
˚/s (1 monolayer/min)
and used a 10 s exposure time on a 2D detector (Pilatus 2M).
To limit the beam exposure, we spaced measurements by 65 s
[Fig. S1(a) in the supplementary material]. Because the angle
of incidence during in-situ measurements was set to 0.072,
the minimum x-ray penetration depth is z
¼7.5 nm in C
As a result, the images
obtained during the first 7.5 nm capture the accumulating
growing film on the substrate, while images acquired at
greater thickness capture the upper 7.5nm of the film. Images
were calibrated using silver behenate (AgBeh), and data
reduction was performed using DAWN;
further details on
sample preparation and alignment procedures can be found in
the supplementary material and in a separate report with the
technical details of the deposition chamber.
Author to whom correspondence should be addressed: josue.martinez-
0003-6951/2017/111(23)/233305/5 V
CAuthor(s) 2017.111, 233305-1
APPLIED PHYSICS LETTERS 111, 233305 (2017)
During the initial few nm of growth, we observe the nucle-
ation of C
crystallites on the glass surface, as revealed by the
rapid changes in the diffuse scatter captured in the images of
Figs. 1(a)–1(d). The convergence of the broad scattering near
¼0.15 A
towards q
¼0.138 A
in the first 300 s or 3 nm
of growth is consistent with the complete coverage of the sur-
face with 4 monolayers of C
. By monitoring the parallel
scattering q
over the first 300 s in Fig. 1(e),anestimateofthe
distance between island centers
can be extracted from the
peak position of the diffuse scatter, with the correlation length
. Features appear near q
¼0.02 A
depositing just under 1 nm of C
(1 monolayer), consistent
with an island-island spacing of 29.1 nm. The tapering of
to a constant value of 45 nm after 8 nm thickness
reflects that the film has reached steady-state growth and coin-
cides with the expected penetration depth of x-rays at this
energy and angle of incidence. This value is consistent with
47.69 nm found from the FFT analysis of atomic force micros-
copy (AFM) of a 15 nm C
film (Fig. S4 in the supplementary
Simple reflectivity models with constant roughness
¼0.2 nm for the C
layer and glass roughness
¼1.0 nm (Fig. S2 in the supplementary material)
are only able to match the oscillation period obtained at
¼0.138 A
, but a full description of the measured off-
specular reflectivity using an adjustment of the model by
Woll et al.
is required to fully capture the growth behav-
ior of the film. As a consequence, the reported film thickness
is based on the response of a calibrated quartz crystal monitor
(QCM) within the deposition chamber
and does not account
for changes in thickness which may arise from differences in
the sticking coefficient of C
at room temperature within the
first few layers of the film. For a comprehensive investigation
of the sticking coefficient and thermally assisted dewetting of
and its implications on observed film growth, the reader
is referred to a recent study that makes use of in-situ x-ray
reflectivity to quantify dewetting and upward mass transport
by monitoring the specular signal from film deposition up to
60 min post-deposition.
Complementary in-plane scattering along the substrate
horizon (q
¼0.022 A
) affords insights into the crystallo-
graphic evolution of the upper 7.5 nm of the growing film.
Reflections from background-subtracted images (details in sup-
plementary material) were fitted to Gaussian peaks using open-
source software tools.
reflection (q
¼0.8 A
) appears after the first nanometer of
growth, indicating that the surface is populated by hexagonally
close packed islands of C
. Initially, the features near q
¼1.3 A
and 1.4 A
are indistinguishable from a broad scat-
tering area, but within the first 4 nm, these features quickly
become more readily distinguishable from one another, and
their peak widths begin to narrow consistent with domain
growth. During the first 5 nm of growth, we observe a marked
transition from near the {310}
plane to a value between the
plane and the {021}
plane. Simultaneously, the
feature near 1.3 A
initially appears near the {221}
but with increasing thickness converges towards a qvalue
between that of the {220}
and {111}
planes. These
shifts of the scattering vector for both features to intermediate
FIG. 1. Grazing-incidence x-ray scattering (GIXS) images during nucleation
and growth on glass. (a)–(d) Detector images in the small angle region near
the beamstop. The black band in the images corresponds to intermodular gap
on the Pilatus detector. (e) Diffuse scatter as a function of film thickness mea-
sured in the region denoted by the white box (q
¼0.138 A
) indicated in (a).
FIG. 2. Comparison of scattering features observed in the GIXS measure-
ments at q
¼0.022 A
. (a) Williamson-Hall (WH) plot of Dqvs qfor
thickness starting at 5 nm. Note: For each qvalue, large icons indicate val-
ues for the 15 nm film from the post-growth scan. The arrow indicates the
direction of the increasing thickness. (b) Evolution of the peak position for
each scattering feature with the increasing film thickness. Solid lines: FCC-
indexed planes; dashed lines: HCP-indexed planes; and broad dashed lines:
qvalues where both FCC and HCP planes can be indexed. The feature at
0.7 A
is the Pilatus intermodular gap, and the step at 1.1 A
is a shadow
from the substrate shutter on the Be window. The arrow indicates the direc-
tion of the increasing thickness. (c) Extracted grain size (left) and strain
(right, orange) from individual peak fitting and WH analysis. (d) Stacking
fault probability calculated from Dqfor each plane; green symbols are cal-
culated relative to the feature at 1.3 A
and red symbols relative to the
feature at 1.48 A
233305-2 Hardigree et al. Appl. Phys. Lett. 111, 233305 (2017)
values are consistent with the accumulation of several % stack-
ing faults in FCC systems.
According to Warren’s selection rules,
stacking faults
in FCC materials modify reflections in planes for which
hþkþl3m. In the frame of an FCC lattice, it has been
shown that shifts in the peak position and increased breadth
(FWHM) of {220} peaks mark the onset of stacking faults
along the close-packing direction.
In the case of powder-
like samples, a Williamson-Hall (WH) plot can be a useful
tool to decouple the influence of grain size and defects on
shifts in scattering vector and line broadening. In a WH plot,
Dqvs qacross all peaks is fit to a linear or a quadratic func-
tion of the scattering vector using the Williamson-Hall (WH)
Dq¼2p=Dþ2eq, where the domain size Dand
lattice strain ecan be extracted from the intercept and slope
of the plot, respectively. Figure 2(a) shows the WH plot for
the film thickness starting at 5 nm, beyond which all three
planes could be accurately fit to Gaussians. Despite only fit-
ting three peaks [20 thickness values, linear least squares fit-
ting with mean (l) and standard deviation (r) for the squared
residuals R
:l¼0.84 and r¼0.18], linear fits consistently
yield a non-zero positive intercept, indicating that part of the
line broadening arises from small (10 nm) grain scattering
in the film [Fig. 2(c)]. The larger grain sizes extracted from
the analysis below 8 nm thickness may reflect a greater
extent of in-plane grain connectivity in the film, consistent
with the larger q (and smaller island-island distance D
at low thickness seen in Fig. 1(e). However, it is important to
note that these grain sizes may also include contributions
from changes in the peak breadth Dqand position q,as
both strain and grain size [Fig. 2(c)] are seen to exhibit simi-
lar changes below 8 nm film thickness where individual
peaks only begin to emerge in the diffraction [Fig. 2(b)].
Additionally, fits of the data for films above 7 nm thickness
converge towards a common slope, indicating a smaller role
of microstrain on the observed q-dependence of the line
broadening away from the substrate.
As shown in Fig. 2(c),
this strain decreases from as much as 5% when probing C
within 7.5 nm of the substrate interface, to just under 2%
in the upper 7.5 nm of the film away from the substrate.
Previously reported strain values near the substrate interface
using in-situ reflection high energy electron diffraction
(RHEED) measurements of laser-deposited C
on mica
yielded a value of 3.2%, based on double lattice constant
estimates. Given that amorphous glass is not expected to pro-
vide long-range templating for the C
lattice, this extracted
strain value is qualitatively in line with the powder-like
growth observed herein (Fig. S5 in the supplementary
Because the in-situ measurements at this angle of inci-
dence are most sensitive to the upper 7.5 nm of the film, in-
plane scattering effectively captures both 1D line defects
such as dislocations at grain boundaries between nanocrys-
tals and 2D planar defects such as stacking faults within the
7.5 nm (10 monolayer) cross-section of the nanocrystals.
To compare the surface and bulk properties of the thin film,
immediately after deposition, the substrate shutter was
closed and the sample was probed between x¼0and 0.2
(Fig. S4 in the supplementary material). As observed during
the in-situ measurements of the upper 7.5 nm layer, the
peak exhibits a larger width and hence a smaller
coherence length Dthan the other planes in the film. When the
full film is measured along the horizon at a sample tilt
¼8.6 nm, D
¼4.9 nm,
and D
¼4.7 nm, all consistent with those derived from the
individual peak fitting of the in-situ data. The good fit (R
¼0.999) and negligible intercept suggest that a high degree of
defects is the main cause for the q-dependent peak broadening.
The probability that the 4th plane in an FCC 111 plane
sequence of the form A-B-C-A-B-C incorporates a planar fault
of the form A-B-C-B-[…] is the stacking fault probability a,
and the average number of planes between faults can be esti-
mated as 1/a. The probability of deformation stacking faults a
in a powder sample can be estimated using the Warren and
Warekois formula,
which is given as
jtan hpeak cos2u270 ffiffi
where D(2h) is the peak offset in degrees, h
jhþkþlj,jis the fraction of faulted planes for the family of
planes {hkl}, h
is the scattering angle of the reference
peak, and cosuis the average angle between the [111] plane
and faulting planes within {hkl}. A closer examination of
Fig. 2(b) (and Fig. S3 in the supplementary material) indi-
cates that all three main peaks are shifted relative to their
crystallographic references, and so, it is useful to compare
relative shifts between two separate peaks, using the relation
D2hhkl ¼Gav jtan ha;(2)
where hG
, and h
is the reciprocal
vector averaged over all faulting hkl planes given by
. To estimate the stacking fault probability,
the two diffraction features at 1.3 A
and 1.48 A
were ref-
erenced to the {220} and {311} planes, respectively. As
shown in Fig. 2(d), the probability of deformation stacking
faults in this film reaches a value of a¼68% relative to the
{220} planes and a¼4.2% for the {311} planes as measured
in the full 15 nm film. These values correspond to an average
number of {111} planes between stacking faults relative to
these references of 1.5 and 24 planes, respectively. The large
calculated probabilities for the {220} planes below 7 nm are
attributable to the significant uncertainty in the position of
the scattering vector within the broad signal [cf. Fig. 2(b)].
As the film increases beyond z
¼7.5 nm, the decreasing
values of aare averaged-out with less influence from the
substrate, with the plateau near 12 nm, marking the mea-
surement of just the upper “bulk” of the thin film. Although
a full averaging out would be expected at 2z
¼15 nm, this
difference may simply reflect the outsize influence of the
2 nm nearest the glass interface, where nucleation is taking
place on the substrate. Although high for FCC metals, these
values for aare consistent with simulations of stacking faults
in vacuum-deposited C
by Vaughan et al.
in which thin
film samples were best described by models with a 50%
probability of HCP stacking on FCC C
Lastly, based on the in-situ and post-growth coherence
lengths for the different reflections, it appears that once
the C
film transitions to steady state island growth, the
233305-3 Hardigree et al. Appl. Phys. Lett. 111, 233305 (2017)
crystallites are limited to a size of roughly 10 nm along the
FCC h111iand FCC h22 Li/h300idirections. However, the
WH analysis for the full 15 nm film yields a larger grain size
of 174 nm [square symbol, Fig. 2(c)], a factor 20 greater than
the individual peaks and roughly 8the 19.5 nm size calcu-
lated from AFM images (supplementary material, Table S1).
This larger value arises from the fact that when probing the
full film, the diffracted volume is a superposition of the
microstructure through the vertical composition of the film.
As seen in Fig. 2(a), both the measured peak position and
width for the full film measurement are closer to the average
values than the those of the upper half of the film. Moreover,
comparison of the sizes obtained from integral breadth and
WH analysis for the in-situ measurements and the full film
[Fig. 2(c)] indicate close agreement between the two meth-
ods for all but the full film measurement, suggesting that the
differences arise from the WH fitting of the full film data.
Although the higher deposition rate, and short time interval
between in-situ and post-deposition scans as compared to
other studies suggest that dewetting might not be a main con-
sideration for the observed grain size differences, combining
this analysis with in-situ and post-deposition XRR as done
by Bommel et al.
would facilitate a quantitative compari-
son between grain size distribution and film homogeneity
normal to the surface, enabling a further link between the
kinetics of film dewetting and as-grown film microstructure.
Additionally, capturing a greater region of qspace, e.g., by
probing at higher incident x-ray energy, would improve grain
size estimates by using a greater number of diffraction orders
in the analysis. It is well-documented that WH analyses can
overestimate grain sizes by more than an order of magni-
in our specific case we can trace this inconsistency to
the vertical inhomogeneity of the film, which is only
resolved when comparing the analysis of the post-deposition
and in-situ GIXS measurements.
In summary, our results comprise in-situ monitoring of
stacking fault evolution in C
fullerene thin films while
employing deposition rates consistent with scalable, state-of-
the-art organic electronic device manufacturing. Although ini-
tial room-temperature growth on glass is marked by low
roughness and nearly 6% in-plane microstrain, C
incorporates stacking faults, with a
¼4.2% and a
The observation that these material parameters vary with the
distance from the substrate underscore the need for in-situ
characterisation to understand the coupling of interfacial and
bulk material properties in organic thin films. Our findings
highlight the potential for in-situ GIXS as a powerful multi-
length-scale probe for elucidating the structural and morpho-
logical evolution of vacuum-deposited molecular thin films for
next-generation organic electronic devices.
See supplementary material for details on GIXS image
reduction, in-situ lattice constant evolution, and post-
deposition AFM analysis of the deposited thin film.
This work was supported by a Science and Technology
Facilities Council (STFC) Challenge-Led Applied Systems
Programme (CLASP, ST/L003309/1) focused on advancing
the commercialization of organic solar cells. J.F.M.H. thanks
Wolfson College, Oxford, for Junior Research Fellowship
support. G.M. was supported by the Energy and Physical
Sciences Research Council (EPSRC) Centre for Doctoral
Training in New and Sustainable Photovoltaics and by the
University College Oxford through the Oxford-Radcliffe
scholarship. The authors thank G. Christoforo, P. R. Warren,
S. V. Kesava, and H. Ye (Univ. of Oxford) and A. Warne
and J. Rawle (Diamond Light Source) for their assistance
with beamline instrumentation.
J.F.M.H. and M.R. developed the thin film research
program. J.F.M.H. wrote the manuscript with contributions
from all authors and directed the thin film characterisation
and data analysis. I.R.R., G.M., and C.N. assisted with GIXS
measurements and data analysis.
The authors declare no competing financial interests.
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... It gives a measure for the plane distance variation in relation to the plane distance itself ( d d Δ ) and is caused by defects. 28,29 A decrease in slope with increasing temperature is observed: 0.012 at 30°C, 0.008 at 70°C, 0.006 at 90°C, and 0.005 at 110°C. ...
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The highly luminescent dicyanodistyrylbenzene-based charge-transfer (CT) cocrystal based on isometric donor and acceptor molecules with a mixing ratio of 2:1 is characterized in the thin film regime. Physical vapor deposited films prepared at different substrate temperatures are analyzed in terms of their thin film structure and transistor performance. The thin film morphologies and crystallographic properties including microstrain and mosaic spread strongly dependent on the substrate temperature. Enhanced crystal growth with rising temperatures leads to a better transistor performance reaching its maximum at 90 °C with a hole and electron mobility of 1.6 × 10–3 and 2.3 × 10–5 cm² V–1 s–1, respectively. At higher temperatures performance decreases limited by percolation pathways between the enlarged crystals.
... Using the Scherrer equation, this peak value belongs to the pure C60 sample evaporated at RT and leads to an estimated crystallite size of 10.1 nm. This value is consistent with data mentioned in a previous study [25], in which the investigators obtained a C60 crystallite size of 10 nm. ...
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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.
... 16 This method has successfully been applied to thin films of organic semiconductors. 17,18 The molecule 2-decyl-7-phenyl- [1]benzothieno [3,2-b][1]benzothiophene (Ph-BTBT-10) is a recently developed organic semiconductor that exhibits excellent performance in thin-film transistors. 19,20 The asymmetric nature of the molecule, there is a phenyl ring at one terminal end of the aromatic benzothieno−benzothiophene core and a decyl chain at the other terminal end, causes a specific phase behavior as a function of temperature. ...
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The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) is an organic semiconductor with outstanding performance in thin-film transistors. The asymmetric shape of the molecule causes an unusual phase behavior, which is a result of a distinct difference in the molecular arrangement between the head-to-head stacking of the molecules versus head-to-tail stacking. Thin films are prepared at elevated temperatures by crystallization from melt under controlled cooling rates, thermal-gradient crystallization, and bar coating at elevated temperatures. The films are investigated using X-ray diffraction techniques. Unusual peak-broadening effects are found, which cannot be explained using standard models. The modeling of the diffraction patterns with a statistic variation of the molecules reveal that a specific type of molecular disorder is responsible for the observed peak-broadening phenomena: the known head-to-head stacking within the crystalline phase is disturbed by the statistic integration of reversed (or flipped) molecules. It is found that 7–15% of the molecules are integrated in a reversed way, and these fractions are correlated with cooling rates during the sample preparation procedure. Temperature-dependent in situ experiments reveal that the defects can be healed by approaching the transition from the crystalline state to the smectic E state at a temperature of 145 °C. This work identifies and quantifies a specific crystalline defect type within thin films of an asymmetric rodlike conjugated molecule, which is caused by the crystallization kinetics.
... In order to quanti fy the dislocation densities and degrees of distortion out of the total coherent size of the crystallinity, more precise analyses using e.g. 2D-GIXD measurements in a higher resolution are needed [53]. Nevertheless, since such structural imperfections are known to diminish the charge carrier mobility in general molecular semiconductors [46,[54][55][56][57], the present results clarifying factors for pursuit of well-ordered molecular heterojunctions will lead to development of organic optoelectronic devices of improved operation efficiencies taking advantage of the high charge carrier mobility [58,59] and In-plane mean crystallite size of C 60 on RubSC depending on the growth temperature derived from the HR-GIXD spot profiles of the C 60 {22 0} spots. ...
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Uniform and well-defined interfaces are required for clarification of fundamental processes at internal interfaces between donor and acceptor molecules constituting organic optoelectronic devices. In this study, evolution of a well-ordered molecular interface, epitaxially grown C60 on the single crystal rubrene (C42H28) surface, was accurately investigated by grazing incidence X-ray diffraction (GIXD) techniques. Contrasting to the case of C60 on the single crystal pentacene forming uniquely aligned epitaxial interfaces, coexistence of two inequivalent crystalline domains of C60 was identified on the single crystal rubrene. Nevertheless, crystallinity of C60/rubrene exhibited even more remarkable improvement to extend its in-plane average crystallite size up to 250 nm as raising the growth temperature. Probable leading factors determining the structures and crystallinity of the well-defined molecular interfaces are discussed based on close comparison of the present results with the C60/pentacene interfaces. The techniques presented herein for enhancement of the crystallinity in epitaxial molecular interfaces are potentially applicable to development in the photoelectric power conversion efficiency of organic photovoltaics via improved charge carrier mobility in donor-acceptor interfaces.
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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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In this paper, we demonstrate microwave flexible thin-film transistors (TFTs) on biodegradable substrates towards potential green portable devices. The combination of cellulose nanofibrillated fiber (CNF) substrate, which is a biobased and biodegradable platform, with transferrable single crystalline Si nanomembrane (Si NM), enables the realization of truly biodegradable, flexible, and high performance devices. Double-gate flexible Si NM TFTs built on a CNF substrate have shown an electron mobility of 160 cm2/V·s and f T and f max of 4.9 GHz and 10.6 GHz, respectively. This demonstration proves the microwave frequency capability and, considering today's wide spread use of wireless devices, thus indicates the much wider utility of CNF substrates than that has been demonstrated before. The demonstration may also pave the way toward portable green devices that would generate less persistent waste and save more valuable resources.
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The purpose of this review is to provide a basic physical description of the exciton diffusion in organic semiconductors. Furthermore, experimental methods that are used to measure the key parameters of this process as well as strategies to manipulate the exciton diffusion length are summarized. Special attention is devoted to the temperature dependence of exciton diffusion and its relationship to Förster energy transfer rates. An extensive table of more than a hundred measurements of the exciton diffusion length in various organic semiconductors is presented. Finally, an outlook of remaining challenges for future research is provided.
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
Anisotropic charge transfer (CT) state absorption originating from the natural transition dipole alignment at planar donor–acceptor interfaces can be exploited to reveal the full CT state absorption spectrum in organic solar cells. Application of this method reveals the existence of previously unobserved, higher-lying CT absorption buried beneath the excitonic background.
The control on the charge transport properties of ternary organic photovoltaic P3HT:PCBM:QBT devices is enabled by modulating the distribution of P3HT polymorphs in the device photoactive layers. Negligible amounts of QBT induce striking modifications in the P3HT lamellar stacking direction, forming both densely packed and non-densely packed P3HT chains. The former reduce the charge carrier recombination rate, enabling an increased fill factor and short-circuit device photocurrent.
Understanding and controlling the growth and stability of molecular thin films on solid surfaces is necessary to develop nanomaterials with well‐defined physical properties. As a prominent model system in organic electronics, we investigate the post‐growth dewetting kinetics of the fullerene C 60 on mica with real‐time and in situ X‐ray scattering. After layer‐by‐layer growth of C 60 , we find a thermally‐activated post‐growth dewetting, where the smooth C 60 ‐layer breaks up into islands. This clearly shows that growth is kinetically limited before the system moves over an activation barrier into an energetically favored configuration. From the temperature‐dependent dewetting kinetics we find an effective activation barrier of 0.33 eV, which describes both the temperature‐dependent macroscopic changes in the surface morphology and the microscopic processes of inter‐ and intralayer diffusion during dewetting. magnified image (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)
Though charge transport is sensitive to subtle changes in the packing motifs of molecular semiconductors, research addressing how intermolecular packing influences electrical properties has largely been carried out on single-crystals, as opposed to the more technologically relevant thin-film transistors (TFTs). Here, independent and reversible access to the monoclinic and triclinic crystal structures of a core-chlorinated naphthalene tetracarboxylic diimide (NTCDI-1) is demonstrated in polycrystalline thin films via post-deposition annealing. Time-resolved measurements of these transitions via UV–visible spectroscopy and grazing-incidence X-ray diffraction indicate that the polymorphic transformations follow second-order Avrami kinetics, suggestive of 2D growth after initial nucleation. Thin-film transistors comprising triclinic NTCDI-1 consistently outperform those comprising its monoclinic counterpart. This behavior contrasts that of single-crystal transistors in which devices comprising monoclinic crystals are consistently superior to devices with triclinic crystals. This difference is attributed to more uniform in-plane charge transport in polycrystalline thin films having the triclinic compared to the monoclinic polymorph. As the mobility of TFTs is a reflection of ensemble-average charge transport across crystalline grains having different molecular orientations, this study suggests that among different polymorphs of a particular molecular semiconductor, those with smaller in-plane anisotropy are more beneficial for efficient lateral charge transport in polycrystalline devices.