Article
pubs.acs.org/Macromolecules
Crystallization of Sensitizers Controls Morphology and Performance
in Si-/C-PCPDTBT-Sensitized P3HT:ICBA Ternary Blends
Xiaoyan Du,† Xuechen Jiao,∥ Stefanie Rechberger,‡ José Darío Perea,§ Markus Meyer,† Negar Kazerouni,§
Erdmann Spiecker,‡ Harald Ade,∥ Christoph J. Brabec,§,⊥ Rainer H. Fink,*,†,‡ and Tayebeh Ameri*,§
†
Department of Chemistry and Pharmacy, ‡Institute of Micro- and Nanostructure Research, & Center for Nanoanalysis and Electron
Microscopy (CENEM), and §Institute of Materials for Electronics and Energy Technology (I-MEET), FAU Erlangen-Nürnberg,
91058 Erlangen, Germany
∥
Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-7548, United States
⊥
Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstr. 2a, 91058 Erlangen, Germany
S Supporting Information
*
ABSTRACT: Organic solar cells based on multinary
components are promising to further boost the device
performance. The complex interplay of the morphology and
functionality needs further investigations. Here, we report on a
systematic study on the morphology evolution of prototype
ternary systems upon adding sensitizers featuring similar
chemical structures but dramatically different crystallinity,
namely poly(3-hexylthiophene) (P3HT) and indene-C60-bisadduct (ICBA) blends with poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)2,1,3-benzothiadi-azole)-5,5′-diyl] (Si-PCPDTBT) and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT), employing energy-filtered transmission electron microscopy (EFTEM) and resonant soft X-ray scattering (RSoXS). In addition, a
combined density functional theory (DFT) and artificial neuronal network (ANN) computational approach has been utilized to
calculate the solubility parameters and Flory−Huggins intermolecular parameters to evaluate the influence of miscibility on the
final morphology. Our experiments reveal that the domain spacing and purity of ICBA-rich domains are retained in SiPCPDTBT-based systems but are strongly reduced in C-PCPDTBT-based ternary systems. The P3HT fiber structure are
retained at low sensitizer content but dramatically reduced at high sensitizer content. The theoretical calculations reveal very
similar miscibility/compatibility between the two sensitizers and ICBA as well as P3HT. Thus, we conclude that mainly the
crystallization of Si-PCPDTBT drives the nanostructure evolution in the ternary systems, while this driving force is absent in CPCPDTBT-based ternary blends.
■
transport are much more complex in ternary solar cells than in
binary systems. So far, several models have been proposed for
the ternary solar cells, namely charge transfer,16−19 energy
transfer,20,21 alloy formation,22 and two CT states or parallel
like model,23 which are very system dependent. In all cases, the
morphology change upon adding a third component in the host
matrix must be well investigated in order to fully understand
the working mechanism.
In previous publications,16−18 the low band gap heteroanalogues poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadi-azole)5,5′-diyl] (Si-PCPDTBT)24 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-
INTRODUCTION
Organic solar cells (OSCs) have improved considerably in
power conversion efficiency (PCE) over the past two decades
owing to the effort of new material development, advanced
device engineering, and further understanding of device
working mechanism.1−6 The most intensively studied polymer:fullerene binary single junction devices based on newly
developed polymers still face the problem of having a narrow
absorption range over the whole solar spectrum. To increase
the absorption range, tandem and/or ternary solar cells are
considered to be the most promising strategies. Tandem solar
cells have achieved great success by device engineering and
materials development.7−12 Although ternary solar cells are
much more promising in terms of device fabrication, there are
still only a few successful cases.13−22 Understanding the
mechanism of success/failure of ternary solar cells is critical
for further optimization of such devices. Charge generation and
© XXXX American Chemical Society
Received: December 15, 2016
Revised: February 8, 2017
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benzothiadiazole)] (C-PCPDTBT)25 were reported as nearinfrared (NIR) sensitizers in poly(3-hexylthiophene) (P3HT)
and indene-C60-bis-adduct (ICBA) blends. It has been
demonstrated that there is ultrafast hole transfer from Si-/CPCPDTBT to P3HT after photoexcitation.17 Although the
optical properties of Si-PCPDTBT and C-PCPDTBT are quite
similar, the former leads to an increased PCE in ternary cells
while the latter leads to a severe PCE decrease, which mainly
comes from low short-circuit current densities (JSC) and fill
factor (FF) (see Table S1). Based on the similarities in energy
levels and optical properties of the two sensitizers, the different
functionality mainly arises from their influence on the
morphology of the host matrix, which in turn has a large
influence on charge generation and transport properties.
Considering the surface energy derived from contact angle
measurements, photoluminescence (PL) spectra, and charge
carrier mobility measured by space charge limited current
(SCLC) method, it is proposed that the strong recombination
by adding C-PCPDTBT are due to the fact that the CPCPDTBT tends to stay in the amorphous mixed P3HT:ICBA
and ICBA-rich domains, and thus both the electron transport
and hole transport are disturbed, while Si-PCPDTBT interacts
more with P3HT-rich domains rather than ICBA-rich domains,
which benefits both hole and electron transport.26 However,
these predictions based on surface energy might not fully reflect
the bulk nanomorphology. At this point, it is essential to
characterize the nanomorphology and structural evolution upon
addition of sensitizers, especially the changes in fullerene-rich
domains and nanostructures of both host and sensitizer
polymers. The driving force for the final morphology formation
needs to be further clarified beyond the surface energy
calculation prediction. Chemically sensitive characterization
methods are still needed to obtain deeper insight into the
nanomorphology of these ternary blends in order to fully
understand the structure−property relationship.
For chemical sensitive characterization, near-edge X-ray
absorption fine structure (NEXAFS) spectroscopy of light
elements like C, N, and O has been proven as a sensitive
method to differentiate polymers and fullerene derivatives27
and thus allows for superior chemically sensitive imaging or
scattering. The two complementary techniques based on
NEXAFS spectroscopy are scanning transmission X-ray microspectroscopy (STXM) in real space and resonant soft X-ray
scattering (RSoXS) in reciprocal space.28,29 STXM has been
used to measure the absolute composition distribution in
polymer:fullerene blends and the miscibility of fullerenes in
different polymers.30−33 However, state-of-the-art STXM still
faces the limitation of spatial resolution which is around 30 nm
in routine operation and thus limits its application to systems
with large-scale phase separation. To overcome this limitation
but still take advantage of the high chemical sensitivity of
NEXAFS spectroscopy, RSoXS was developed, with a q-range
corresponding to structure size down to 10 nm and all the way
up to micrometers.34,35 The high absorption and scattering
contrast at resonant energies of fullerene and polymers has
been widely used to determine the domain spacing and domain
purities in many kinds of binary systems.36−40 For real-space
imaging, conventional bright-field transmission electron microscopy (BFTEM) lacks chemical sensitivity in carbon-rich
blend materials. Therefore, energy-filtered TEM (EFTEM),
based on electron energy loss spectroscopy (EELS), is used to
visualize the material contrast.41−45
In the semicrystalline P3HT and [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) binary systems, the crystallization of
P3HT and partial miscibility are the main driving force for the
solid state nanostructure.43 In ternary systems, the polymer
crystallization and the compatibility between polymers and
polymer:fullerenes are much more complex. The resulting final
morphology could be determined by both crystallization
kinetics and equilibrium miscibility. Thus, besides crystallinity,
the compatibility/miscibility between the third component and
both host polymer and fullerenes must be considered in order
to understand the solid-state nanostructure of the ternary
blends. Many methods have been reported to determine the
miscibility. For example, traditional different calorimetric
methods are commonly used to determine the glass transition
temperature (Tg), which is a criterion to distinguish the
miscibility.46 Also, the difference in surface energy between two
components26 and the quenching effect of photoluminescence
depending on the amount of quencher can be used to deduce
the miscibility.47 In addition, dynamic secondary ion mass
spectrometry (SIMS) was explored to study the interdiffusion
of fullerenes into polymers upon annealing.48−51 NEXAFS
spectroscopy was used to directly measure the composition of
polymer:fullerene blends to achieve equilibrium concentrations
upon annealing, thus creating a miscibility phase diagram for
amorphous portions of the systems investigated.48 However,
the later two methods are more suitable to highly phaseseparated systems and are especially not practical to determine
the compatibility between polymers. We have previously
reported on a combined computational approach based on
density functional theory (DFT) and artificial neural networks
(ANN) for predicting the solubility parameters of fullerenes.52
In addition, the solubility parameters of the polymers could be
determined by the experimental binary solvent gradient
method.53,54 A numerical approach can be used to determine
relative solubility and then the Flory−Huggins intermolecular
parameter χ1,2, which is a fundamental metric of molecular
interaction and miscibility, for a polymer:polymer:fullerene
ternary systems. Several studies on correlations between the
device performances with the molecular interaction parameters
have been reported.55−57 While the importance of the
interaction parameters for the formation as well as the stability
of the complex nanostructures is recognized, a correlation of
the device performance and the parameters is not straightforward yet and needs to be studied more extensively.55−57
In this article, we utilize EFTEM and RSoXS together with
STXM as complementary chemically and/or material-sensitive
characterization techniques to follow the nanostructure
evolution of the P3HT:ICBA matrix upon addition of
PCPDTBT-based NIR sensitizers. High contrast images for
polymer:fullerene nanostructures were obtained by EFTEM
measurements. The changes in ICBA-rich domains were
revealed by RSoXS measurements. The compatibility/miscibility between polymer and fullerene as well as polymers and
polymers were determined to distinguish the driving force for
the morphology formation employing thermodynamic model
based on ab initio DFT calculations combined with an ANN.52
A detailed morphology model is proposed based on the
combined morphology studies and correlated with device
performance. The results for the prototype ternary systems
presented here pave the way to understand ternary systems in
general and further optimize the nanostructure to get functional
devices.
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Figure 1. (a) Chemical structures of the various components in the active layer and (b) corresponding normalized NEXAFS spectra recorded from
thin films of the pure materials.
■
EXPERIMENTAL METHODS
RESULTS AND DISCUSSION
Figure 1 shows the chemical structure of the component
materials in the active layer and the corresponding C K-edge
NEXAFS spectra of the single-component films as spectroscopic references. The spectra are normalized to pre-edge and
post-edge. The ICBA spectrum shows the highest C 1s to π*resonance at the lowest energy (284.5 eV) due to its high
conjugation and strong delocalization of the orbitals. The
second resonance at 285.0 eV, which is absent for PCBM, is
due to the lower symmetry of ICBA compared to PC61BM.27
The P3HT homopolymer shows a prominent resonance at
285.2 eV, while for Si-PCPDTBT and C-PCPDTBT push−pull
D−A copolymers, the energy levels are more split due to the
various chemical surroundings of C atoms. Si-PCPDTBT and
C-PCPDTBT show similar core level excitations, with three
main π*-resonance at 284.3, 284.9, and 286.0 eV. The strong
absorption contrast between ICBA and polymer compounds at
the C K-edge makes STXM measurement feasible.
In RSoXS measurements, there are two main contributions
to the scattering intensity: one is vacuum contrast originating
from surface roughness/bulk voids; the other is material
contrast originating from polymer-rich and fullerene-rich
interdomain scattering. The choice of scattering energy is
based on the material contrast function
■
Sample Preparation. The photoactive solutions were prepared
from different mixing ratios of P3HT (purchased from BASF, Sepiolid
P200, regioregularity > 98%, Mw < 50 000 g/mol) and Si-PCPDTBT
(provided by Solarmer, Mw ∼ 40 000 g/mol, PDI ∼ 1.8) or CPCPDTBT (purchased from 1-Material, Mw ∼ 38 000 g/mol, PDI ∼
2.3) with the overall polymer concentration of 1 wt % in odichlorobenzene, blended with ICBA (purchased from Nano-C) with
a total polymer:ICBA weight ratio of 1:1. The active layers were
doctor bladed from prepared solutions on top of an ultrathin
PEDOT:PSS film and annealed at 150 °C for 10 min. Then the films
were floated in water and transferred to Cu grids for EFTEM and
STXM analysis. Samples for RSoXS were floated onto a Si3N4 window
(∼100 nm thick). The films for EFTEM were approximately 50 nm
thick, while samples for RSoXS and STXM were approximately 100
nm thick.
Near-Edge X-ray Absorption Fine Structure (NEXAFS)
Spectroscopy and Scanning Transmission X-ray Microspectroscopy (STXM). NEXAFS spectroscopy and STXM were performed at
the PolLux end station of the Swiss Light Source (Paul Scherrer
Institut, Villigen, Switzerland).29 A 25 nm zone-plate focusing is used
to achieve around 30 nm lateral resolution. The transmitted X-ray
intensity was detected by means of photon multiplier tube. The
energy-dependent optical density is calculated through the Lambert−
Beer law. The absorption spectra were taken near the carbon K-edge
from 278 to 320 eV with 0.1 eV step size in the 278−292 eV range.
STXM images were obtained by raster scanning the sample through
the focal point and measuring the transmitted photons.
Resonant Soft X-ray Scattering (RSoXS). RSoXS experiments
were performed in transmission mode at Beamline 11.0.1.2 of the
Advanced Light Source (Lawrence Berkeley National Laboratory,
Berkeley, CA).34 All measurements were conducted under vacuum (1
× 10−7 Torr) to reduce the air absorption of soft X-ray beam. The size
of incident X-ray beam was set as 200 μm × 300 μm by collimating
slits. The scattering patterns were collected by a PI-MTE CCD
detector cooled down to −45 °C with 2048 × 2048 pixels. The lowand high-q patterns were collected at the sample-to-detector distance
of ∼170 and ∼50 mm, respectively. The final sample-to-detector
distance was refined by fitting the well-defined PS300 scattering
pattern. The 1D reduction of the scattering pattern was performed by
using the customized Nika analysis package.58
Transmission Electron Microscopy. The TEM investigations
were performed using an FEI Titan Themis3 300 TEM with a high
brightness field emission gun (X-FEG) operated at 200 kV equipped
with a high-resolution Gatan imaging filter (GIF Quantum) for
electron energy loss spectroscopy (EELS) and energy-filtered TEM
(EFTEM).
C = |Δδ 2 + Δβ 2|E 4 = Δn2E 4
(1)
where n = 1 − δ + iβ is a material’s complex index of refraction;
δ is dispersion, β is absorption, and E is the incident photon
energy. The calculated material contrast function for different
polymers and ICBA is shown in Figures S1 and S2 (see the
Supporting Information). The energy-dependent scattering
profiles for the P3HT:ICBA blend is shown in Figure S3,
which clearly shows that the resonant energy of ICBA gives the
highest scattering intensity in the binary blend. Based on the
calculated material contrast function, 284.2 eV was chosen for
RSoXS measurements to eliminate fluorescence background as
it is below absorption edge and provides high material contrast
at the same time. In the following, the nanomorphology of
binary as well as ternary blends will be discussed in detail.
Nanomorphology in Binary Blends. In the first instance,
the morphology of three polymer:fullerene binary blends
(weight ratio 1:1) is compared. EFTEM investigations have
been performed to directly reveal the nanomorphology in real
space. Figure 2 shows elemental maps based on EFTEM
imaging of sulfur (S) using the S L-edge and of carbon (C)
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emphasized (see Figure S1 for contrast function calculation).
It is noted that the center-to-center spacing of ICBA-rich
domains is probed considering the high material contrast at this
energy. The dominant domain spacing was represented by the
long period d = 2π/q, where q is the peak location in the spatial
frequency distribution.59 The scattering profiles at nonresonant
energy (270 eV) are also shown in Figure S4 to give mass−
thickness variation related information.
As displayed in Figure 3, the three binary blends exhibit
distinct scattering profiles with largely different scattering
maximum. The dominant domain spacing for P3HT:ICBA is
around 27 nm and for Si-PCPDTBT:ICBA around 58 nm. For
C-PCPDTBT:ICBA, the broad peak with much lower
scattering intensity represents a dominate domain spacing
around 38 nm. The domain spacing in P3HT:ICBA blends
derived from RSoXS correlates well with the distance between
P3HT fibers as observed in EFTEM. The scattering profile of
Si-PCPDTBT:ICBA is much broader than P3HT:ICBA,
implying more dispersed structures. It is noted that the
dramatically different scattering intensity for PCPDTBT-based
binary blends indicates that the Si-PCPDTBT:ICBA blend has
much larger phase separation than in C-PCPDTBT:ICBA
blend, in agreement with the EFTEM results.
Nanomorphology in Ternary Blends. The nanomorphology evolution of P3HT:ICBA host matrix upon addition of
PCPDTBT-based sensitizers were studied and discussed in this
section. The EFTEM investigation and RSoXS profiles for the
ternary blends with different concentrations of Si-/CPCPDTBT are shown in Figures 4 and 5, respectively.
For P3HT (80%):Si-PCPDTBT (20%):ICBA ternary blends,
EFTEM investigations (see Figures 4a and 4e) reveal that the
P3HT fibers are well preserved, with several structures quite
similar to those in the Si-PCPDTBT:ICBA binary blend (see
Figure 2b). From the RSoXS profile (Figure 5a), the q value of
the dominant domain spacing remains at the same position as
for the binary P3HT:ICBA host matrix. Addition of small
amounts of Si-PCPDTBT (20%, weight ratio with respect to
P3HT) does not change the P3HT:ICBA phase separation in
the films. The reduced scattering intensity is most probably due
to the reduced effective scattering volume which is influenced
by the location of semicrystalline Si-PCPDTBT in the ternary
blends. For P3HT (50%):Si-PCPDTBT (50%):ICBA ternary
blends, EFTEM reveals more structures similar to those in the
Si-PCPDTBT:ICBA binary blend (Figure 2b). In addition, the
P3HT fibrous network is not as obvious as in the case of small
Si-PCPDTBT contents. These EFTEM results further confirm
the previous speculation that the ordering of Si-PCPDTBT in
the ternary films remains or is marginally affected.26 As seen in
RSoXS profiles in Figure 5a, the dominant domain spacing
increases slightly; a second dominant peak appears around q =
0.05 nm−1, and its intensity increases when more Si-PCPDTBT
crystals are present in the blends as shown in EFTEM
micrographs. This larger domain spacing is most probably due
to the hierarchical structure related to the crystallization of SiPCPDTBT.
For P3HT (80%):C-PCPDTBT (20%):ICBA ternary blends,
the elemental maps (Figures 4c and 4g) reveal that the P3HT
fibers are still present, and no new polymer structures appear as
in the Si-PCPDTBT case. The P3HT fiber structure disappears
completely for P3HT (50%):C-PCPDTBT (50%):ICBA
ternary blends (Figures 4d and 4h). The ICBA-rich domain
spacing derived from RSoXS profiles (Figure 5b) increases
from ∼30 to ∼50 nm when 20% C-PCPDTBT is added and
Figure 2. EFTEM investigation showing elemental maps of sulfur (a−
c) and carbon (d−f) of polymer:ICBA binary blends with weight ratio
of 1:1. (a, d) P3HT:ICBA; (b, e) Si-PCPDTBT:ICBA; (c, f) CPCPDTBT:ICBA. The white arrows in Si-PCPDTBT:ICBA blends
highlight the polymer structures.
using the C K-edge of the three binary polymer:ICBA blends.
Since only the polymers comprise sulfur, the sulfur signal is
used to distinguish the polymers from ICBA. Because of the
difference of the carbon content of the polymers (CP3HT = 40.0
at. %, CSi‑PCPDTBT = 40.5 at. %, CC‑PCPDTBT = 41.9 at. %) and
ICBA (83.0 at. %), the carbon signal can be used to represent
ICBA. The three binary blends clearly show different structural
features of both polymers and ICBA-rich domains. In the
P3HT:ICBA blend (Figures 2a and 2d), P3HT forms random
fibrous networks, with fiber diameters of about 10 nm, in a
matrix of P3HT:ICBA mixed area. In the Si-PCPDTBT:ICBA
blend (Figures 2b and 2e), the bright structures in the S map
correspond to the dark features in the C map and thus are
attributed to Si-PCPDTBT domains. In the case of CPCPDTBT:ICBA (Figures 2c and 2f), no clear structure is
visible, which indicates a good intermixing of the two materials.
These results revealed distinct domains of both P3HT and SiPCPDTBT in the polymer:ICBA blends, while the CPCPDTBT:ICBA blend has no obvious structures at the
detected length scale.
In order to get further information on the ICBA-rich
domains, RSoXS was performed on these binary blends. Figure
3 shows the corresponding Lorentz corrected and thickness
normalized RSoXS scattering profiles at 284.2 eV, where the
compositional contrast between polymer and fullerene is
Figure 3. Lorentz-corrected RSoXS scattering profiles of binary blends
at 284.2 eV. The dominant domain spacing is labeled for clarity. The
intensity of C-PCPDTBT was scaled by 10 times.
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Figure 4. Elemental maps of sulfur (a−d) and carbon (e−h) of P3HT:Si-PCPDTBT:ICBA (a, b, e, f) and P3HT:C-PCPDTBT:ICBA (c, d, g, h)
ternary blends with weight ratio of 0.8:0.2:1 (a, c, e, g) and 0.5:0.5:1 (b, d, f, h). The white arrows highlight the polymer structures.
Figure 5. Lorentz corrected scattering profiles of (a) P3HT:Si-PCPDTBT:ICBA blends and (b) P3HT:C-PCPDTBT:ICBA ternary blends recorded
at 284.2 eV. The dominant domain spacing is labeled for clarity.
further increases to ∼60 nm when 50% C-PCPDTBT is added.
In addition, the scattering intensity decreases drastically
compared to host matrix. It is noted that volume fraction
normalization and contrast normalization have been performed
for accurate intensity comparison (Figure S5), and thus this
finding indicates that the ternary blends are very well mixed.
It is known that the morphology of the P3HT:PCBM system
is not thermally stable and PCBM microcrystals are often
observed after long time annealing.60 Thus, thermal instability
leads to morphology study more sensitive to film aging
problems. The thermal stability of the morphology of the
system is studied by STXM. Films prepared at device
conditions, namely, annealed at 150 °C for 10 min as well as
for long time (15 h) annealing stress, were investigated as
shown in Figure S6. The STXM micrographs of the
P3HT:ICBA blend at different energies are shown in Figure
S7. The micrograph taken at resonant energy of ICBA (284.5
eV) features the highest material contrast. It is noted that the
nanostructure could not be fully resolved in STXM; however,
inhomogeneity of ICBA-rich domains is still probed besides
thickness contrast. It is obvious that adding Si-PCPDTBT does
not severely affect ICBA aggregations on the probed length
scale. After annealing for 15 h, there are no extra micrometersized crystals observed. The coarser microstructure under
annealing stress for all three films might be due to the further
aggregation of ICBA-rich domains. The STXM images for CPCPDTBT-based ternary blends (not shown here) did not
show clear structure due to the intermixing of C-PCPDTBT
with ICBA, which is already observed in EFTEM images (see
above).
In summary, the nanostructure of the two sensitizers in
ternary blends is dramatically different compared to each other:
Si-PCPDTBT tends to form crystalline domains, while CPCPDTBT is completely amorphous. The structural features
are retained in ternary blends, indicating minor perturbation on
their structure formation by either P3HT or ICBA. The two
sensitizers feature dramatically different impact on the ICBA
aggregations: while Si-PCPDTBT has a minor influence on
ICBA-rich domain spacing at low concertation but induces
further phase separation at high concentration, C-PCPDTBT
mixes strongly with ICBA in the ternary blends. The influence
of the two sensitizers on the P3HT nanostructure is quite
similar. At low sensitizer content, the nanostructure of P3HT is
not severely affected, while at high sensitizer content, the fiber
structures of P3HT is severely reduced in both ternary systems.
It is noted that from differential scanning calorimetry (DSC)
results on P3HT:PCBM binary blends, the melting peak of
P3HT disappears when its weight ratio is less than 70%.61,62
Our previous DSC results on P3HT:C-PCPDTBT binary
blends indicate no influence on the P3HT crystallinity by
adding C-PCPDTBT.61,62 Considering the lower tendency for
ICBA to aggregate or crystallize compared to PCBM, it is
expected that this drastically reduced amount of P3HT fibers in
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of these two sensitizers with ICBA in the amorphous region.
The interaction strength between Si-PCPDTBT and P3HT is
also very close to that between C-PCPDTBT and P3HT. In
such a case, nanostructure formation is not determined by the
miscibility between polymers in the amorphous region either.
In summary, the theoretical calculations of interaction
strength based on the determination of the Hildebrand
solubility parameters through our previous thermodynamic
model suggest that the interactions between the two sensitizers
with P3HT and ICBA are quite similar. The mixing enthalpies
in the amorphous mixed domains are thus not the driving force
for the nanostructure formation. The main driving force for Si-/
C-PCPDTBT-based binary and ternary blends is thus the
crystallization enthalpies of the sensitizers. At this point, it is
important to note that the Si-PCPDTBT is known to have
much stronger intermolecular π−π interaction than CPCPDTBT.24 In consequence, Si-PCPDTBT tends to form
pure crystalline phases and forms favorable phase separation
with both ICBA and P3HT. While C-PCPDTBT is dominantly
amorphous, there is no driving force for such phase separation
but rather well intermixing with ICBA-rich domains and the
mixed P3HT:ICBA region.
Nanostructure Model and Correlation with Device
Performance. Based on the present EFTEM, RSoXS, STXM,
and theoretical calculation results, a morphology model of the
P3HT:ICBA blend with different sensitizers is proposed as
shown in Scheme 1. Here, the morphology changes are
ternary blends at high sensitizers content (50%) is kinetically
determined during the crystallization process of diluted P3HT.
Comparison of Relative Miscibility through Theoretical Calculation. It is known that the nanostructures of
P3HT:PCBM system is mainly driven by P3HT crystallization
and partial miscibility,43 which should also be the case for
P3HT:ICBA binary blends. To understand the driving force for
the nanostructure formation in ternary blends, both the
crystallinity of the third component and its compatibility/
miscibility with ICBA and the host polymer have to be
considered. For this purpose, we explored a numerical approach
to determine the relative miscibility for the material systems
discussed here. We determined a dimensionless number
according to eq 2, which indicates the strength of
interactions:63
ΔHmix
= χ1,2 ϕ(1 − ϕ)
(2)
RT
Here, ΔHmix is the enthalpy of mixing, R is the gas constant, T
is the temperature, χ1,2 is the Flory−Huggins intermolecular
parameter, and ϕ is the volume fraction. The Flory−Huggins
intermolecular parameter χ1,2 is determined according to eq 3,
ν
χ1,2 = 0 (δT1 − δT 2)2
(3)
RT
where ν0 is the lattice molar volume and δT1 and δT2 are the
Hildebrand solubility parameters for two components under
discussion. For the determination of the Hildebrand solubility
parameters, our previous thermodynamic model based on ab
initio DFT quantum chemical calculations combined with an
ANN was employed and listed in Table S2.52 The calculated
Hildebrand solubility parameters of the polymers are in good
agreement with the values determined by the experimental
binary gradient method.53 More detailed calculations can be
found in the Supporting Information. The Flory−Huggins
intermolecular parameters and the interaction strength are
shown in Table 1. It is important to note that the values were
Scheme 1. Proposed Morphology Models of P3HT:ICBA
Blends upon Addition of Low Content of Different
Sensitizersa
Table 1. Intermolecular Parameter χ1,2 and Interaction
Strength (ΔHmix/RT) for Polymer:ICBA and
Polymer:Polymer Binary Blends with Weight Ratio of 1:1
binary blends
intermolecular
parameter (χ1,2)
interaction strength
(ΔHmix/RT)
P3HT:ICBA
C-PCPDTBT:ICBA
Si-PCPDTBT:ICBA
P3HT:C-PCPDTBT
P3HT:Si-PCPDTBT
0.241 02
0.000 90
0.002 94
0.237 55
0.256 60
0.058 07
0.000 22
0.000 72
0.059 25
0.063 97
a
The semicrystalline Si-PCPDTBT crystallizes in the ternary blend
and has a minor influence on ICBA aggregations, while the amorphous
C-PCPDTBT tends to mix with ICBA. In addition, the P3HT fibers
remain in the two ternary blends.
discussed in three aspects: nanostructures of P3HT, ICBA-rich
domains, and the sensitizers. The host P3HT:ICBA matrix
contains P3HT fibers, ICBA-rich domains, and amorphous
mixed P3HT:ICBA. In ternary blends, semicrystalline SiPCPDTBT forms crystals and preferentially locates in
amorphous P3HT:ICBA mixed region, which is verified by
the unchanged ICBA-rich domain spacing and the wellpreserved P3HT fibers. Considering the high miscibility of
the Si-PCPDTBT and ICBA in amorphous state as the
theoretical calculations suggested, the minority noncrystalline
Si-PCPDTBT is incorporated in the mixed regions. In contrast,
there is no driving force for phase separation between the
amorphous C-PCPDTBT and ICBA, leading to intermixed
estimated under the same conditions, and we concentrate our
discussion on the relative trend given by the interaction
parameters rather than on the absolute values. It is worth
mentioning that the calculations are describing the transition
from the melt into the solid state for truly amorphous systems
and hence are more related to the mixing behavior of
amorphous region. For the polymer:fullerene binary blends,
the interaction strength between P3HT and ICBA is extremely
higher than that between Si-/C-PCPDTBT and ICBA,
indicating much higher miscibility in Si-/C-PCPDTBT:ICBA
blends than that in P3HT:ICBA blends. The interaction
parameters of Si-PCPDTBT and ICBA are quite close to that
between C-PCPDTBT and ICBA, indicating similar miscibility
F
DOI: 10.1021/acs.macromol.6b02699
Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
but absent in C-PCPDTBT-based ternary blends. We observe a
significant relationship of the determined nanomorphology
with the FF and Jsc.
It is noted that the characterization of the complex
nanomorphology and determination of the driving forces for
the morphology formation are of great importance in this field,
especially for studies trying to relate the solubility/miscibility
parameters with the device performance. The present
morphology studies reveal fundamental guidelines toward
designing potential sensitizers for functional organic ternary
solar cells to further expand the absorption range and enhance
the device performance.
domains and dramatically reduced purity of ICBA-rich
domains.
Here, the present findings on nanomorphology of the ternary
blends are compared with the device functionalities. The device
parameters are shown in Table S1. For devices with addition of
Si-PCPDTBT, the Jsc of all ternary cells is higher than that of
binary devices (8.0 mA cm−2) and reaches the maximum (10.0
mA cm−2) at 20% concentration. The FF increases at 20%
concentration (from 60% to 65%) and decreased at higher
concentration due to decreased charge carrier mobility and also
increases recombination as shown by previous SCLC and
transient photovoltage (TPV) measurements.16 It is worth
noting that even with 50% Si-PCPDTBT, the FF is not severely
affected (87% of the binary device), and the Jsc is higher
compared to the binary devices, which shows completely
different behavior compared to C-PCPDTBT-based ternary
solar cells. For devices with addition of C-PCPDTBT, even
with low content of sensitizer (20%), the Jsc decreases from 8.0
to 6.4 mA cm−2, and the FF decreases from 60% to 54%. The
dramatically different changes in the device performance are
related to the morphology changes in terms of P3HT and the
sensitizers nanostructures and the ICBA-rich domain spacing
and purity. For Si-PCPDTBT-based ternary devices, the
dominant ICBA-rich domain spacing around 30 nm is retained
with sensitizer concentration up to 50%, which guarantees the
efficient electron transport, while the intermixing of CPCPDTBT with ICBA as reflected in both EFTEM and
RSoXS results is detrimental to charge transport. As for the
hole transfer from sensitizers to P3HT (as suggested from
previous findings), the crystalline sensitizer in the matrix of
mixed regions is definitely better than the sensitizer well-mixed
with ICBA, since this intermixing leads to high charge
recombination, as indicated from the reduced FF in CPCPDTBT-based ternary blends. At higher sensitizer contents,
the reduced amount of P3HT fibers which reflecting the
decreased P3HT ordering is another crucial factor for low Jsc
and FF due to higher recombination. We should point out that
severe morphology destruction and device performance drop
were not observed for P3HT:PCBM at low concentration of CPCPDTBT, most probably due to higher aggregation tendency
of PCBM compared to ICBA.26,58,64
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macromol.6b02699.
Device parameters, chemical contrast function, additional
RSoXS profiles, STXM micrographs, detailed theoretical
calculation information (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: rainer.fink@fau.de (R.H.F.).
*E-mail: tayebeh.ameri@fau.de (T.A.).
ORCID
Rainer H. Fink: 0000-0002-6896-4266
Author Contributions
X.D. and X.J. contributed equally to this work. S.R. and M.M.
conducted the TEM experiments. J.D.P. performed the
theoretical calculation. N.K. helped in preparing samples.
R.H.F., E.S., C.B., H.A., and T.A. contributed to discussions
and manuscript preparation. The manuscript was written
through contributions of all authors. All authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
CONCLUSIONS
In conclusion, the difference in morphology modification of
P3HT:ICBA upon the addition of Si-/C-PCPDTBT as
sensitizers was thoroughly analyzed by combining EFTEM,
RSoXS, and STXM as complementary chemical and/or
material sensitive characterization techniques. The relative
compatibility/miscibility of the polymers and ICBA were
calculated to evaluate the driving force for the ternary
morphology. The domain spacing and purity of ICBA-rich
domains retain in Si-PCPDTBT systems but is strongly
reduced in C-PCPDTBT-based ternary systems. The two
sensitizers have similar and minor influence on P3HT fiber
structures at low sensitizer’s content. The reduced P3HT fiber
structures at high sensitizer content are most probably
kinetically determined during the crystallization process of
diluted P3HT in ternary blend solutions. The miscibility
calculations reveal that the miscibility/compatibility between
the two different sensitizers and ICBA as well as P3HT is quite
close. We thus conclude that the driving force for morphology
changes in the ternary blends is the crystallization of sensitizers
itself, which is present in Si-PCPDTBT-based ternary solar cells
■
ACKNOWLEDGMENTS
We acknowledge experimental support at the PolLux beamline
by Drs. B. Watts, J. Raabe, and A. Späth. S. Langner and N. Li
are acknowledged for inspiring discussions on the miscibility
calculations. The STXM study was funded by the Deutsche
Forschungsgemeinschaft within the Research Training Group
GRK 1896. X.Y.D. and T.A. obtained financial support through
the Bavarian initiative “Solar technologies go hybrid”. X.Y.D.
gratefully acknowledges the China Scholarship Council (CSC)
for her research grant. J.D.P. is funded by a doctoral fellowship
grant of the Colombian Agency COLCIENCIAS. T.A., S.R.,
and C.J.B. acknowledge the projects of Organic Semiconductors for NIR Optoelectronics (OSNIRO, grant
607585), DFG grant BR 4031/2-2, and Synthetic Carbon
Allotropes (SFB953). Work by X.C.J. and H.A. was supported
by ONR grant N000141410531. Soft X-ray scattering data were
acquired at Beamline 11.0.1.2 at the Advanced Light Source
(ALS), which is supported by the Director Office of Science
and Office of Basic Energy Sciences of the U.S. Department of
Energy under Contract DE-AC02-05CH11231.
■
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DOI: 10.1021/acs.macromol.6b02699
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Macromolecules
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Article
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DOI: 10.1021/acs.macromol.6b02699
Macromolecules XXXX, XXX, XXX−XXX