A real-time study of the benefits of co-solvents in polymer solar cell processing

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The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. Controlling the degree of phase separation between these components is crucial to reach efficient solar cells. In solution-processed polymer-fullerene solar cells, small amounts of co-solvents are commonly used to avoid the formation of undesired large fullerene domains that reduce performance. There is an ongoing discussion about the origin of this effect. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large fullerene-rich domains form via liquid-liquid phase separation at ~20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studied.

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### ARTICLE

Received 5 Aug 2014 | Accepted 7 Jan 2015 | Published 6 Feb 2015 DOI: 10.1038/ncomms7229
## A real-time study of the benefits of co-solvents in polymer solar cell processing

Jacobus J. van Franeker 1,2, Mathieu Turbiez 3, Weiwei Li 1,4, Martijn M. Wienk 1,2 & Rene´ A.J Janssen 1,2

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating

and electron-accepting organic semiconductors. Controlling the degree of phase separation

between these components is crucial to reach efficient solar cells. In solution-processed

polymer–fullerene solar cells, small amounts of co-solvents are commonly used to avoid the

formation of undesired large fullerene domains that reduce performance. There is an ongoing

discussion about the origin of this effect. To clarify the role of co-solvents, we combine three

optical measurements to investigate layer thickness, phase separation and polymer

aggregation in real time during solvent evaporation under realistic processing conditions.

Without co-solvent, large fullerene-rich domains form via liquid–liquid phase separation at
B20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer

aggregation below 20 vol% solids and prevent the formation of large domains. This

rationalizes the formation of intimately mixed films that give high-efficient solar cells for the

materials studied.

1 Molecular Materials and Nanosystems and Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB
Eindhoven, The Netherlands. 2 Dutch Polymer Institute P.O. Box 902, 5600 AX Eindhoven, The Netherlands. 3 BASF Schweiz AG, Schwarzwaldallee 215,
CH-4002 Basel, Switzerland. 4 Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, P.R. China. Correspondence and requests for materials should be addressed to R.A.J.J. (email: [r.a.j.janssen@tue.nl).](mailto:r.a.j.janssen@tue.nl)

ocussed research efforts on organic semiconductors have
pushed the certified efficiency of single junction organic
solar cells to over 10% (ref. 1). To become a viable
# F
technology, research is now focussing on stability and lifetime 2,3
as well as all-solution, roll-to-roll compatible production routes 4,5
and the use of environmental friendly solvents 6,7 . It is well known
that the morphology of organic solar cells is crucial to achieve
high efficiencies. New materials require new solvents and new
processing conditions to be identified that provide an optimized
morphology. This may involve thermal annealing, solvent vapour
annealing or the use of co-solvents 8,9 . In fact, the use of cosolvents has become ubiquitous but it remains a skill rather than
being based on a deep understanding of the dynamical processes
that occur during solution processing of the organic layer. This
work aims to investigate the formation of the photoactive layer in
real time during spin coating and elucidate the role of co-solvents.

Processing polymer solar cells with a co-solvent involves spin
coating of a four-component mixture: the polymer donor and
fullerene acceptor are dissolved in a main solvent together with a
small amount (1–10 vol%) of co-solvent. Empirically, it has been
established that successful co-solvents have a higher boiling point
than the main solvent and provide a higher solubility for the
fullerene derivative than for the polymer 10 . For some polymer–
fullerene combinations, it was shown that the effect of a cosolvent is to increase domain sizes 11, which is most commonly
attributed to polymer aggregation in solution 12,13 . Contrastingly,
for other combinations, the function of the co-solvent is to
decrease domain sizes. In several polymer–fullerene blends, large
(4100 nm) droplet-shaped fullerene-rich domains are formed
when the film is cast without co-solvent. It has been shown that
these large fullerene domains can originate from liquid–liquid
phase separation during solvent evaporation 14,15 . When the same
blend is processed with a suitable co-solvent, the large dropletshaped domains are not found and a much more intimately
mixed morphology is formed that provides increased
photovoltaic performance 15 . Among the materials that show
such behaviour there are several that provide very high power
conversion efficiencies (PCEs) in solar cells. A prominent
example is PTB7 that provides a well mixed blend and
PCE ¼ 9.2% when processed with co-solvents 16, but forms large
(4100 nm) fullerene-rich domains and correspondingly low
PCEs when processed from a single solvent 17–19 . Other
examples that give similar behaviour are PDPPTPT
(PCE ¼ 7.4%) (refs 20,21), PDTG-TPD (PCE ¼ 7.4%) (refs
22,23), PBnDT-FTAZ (PCE ¼ 7.0%) (ref. 24), PBDTTPD
(PCE ¼ 8.3%) (ref. 25) and P(iI-DTS) (PCE ¼ 4%) (ref. 26). In
Supplementary Fig. 1, we show the structures of these polymers to
show their widely different chemical composition.

The origin of the decrease of domain size when using a cosolvent has often been attributed to polymer aggregation, either in
the casting solution 27,28 or during solvent evaporation 18,29 . It has
also been ascribed to a reduction of fullerene aggregation in the
casting solution 30 . The lack of consensus regarding the role of cosolvents arises, in part, from the inability to study the morphology
formation during realistic device processing conditions. In situ
studies that are performed during solvent evaporation can shed
light on these processes. Liu et al. 18,29 studied the drying of a
blade-coated and drop-cast films in which a co-solvent decreases
domain sizes. However, during spin coating, the complex
morphologies develop in a few seconds, which is much faster
than the drying of blade-coated or drop-cast films, which takes
1,000 s or more. The dynamics of solvent evaporation and phase
separation govern the final structure. In situ studies have been
performed on timescales corresponding to realistic processing
conditions for practical applications 31–34 . However, in none of
these systems co-solvents are necessary to decrease domain sizes.

In the following, we monitor layer thickness, polymer
aggregation and large-scale phase separation during spin coating.
We show for the first time that without the co-solvent, polymer
aggregation occurs after large-scale liquid–liquid phase separation. Using a co-solvent, the aggregation of the polymer occurs at
higher solvent content. Enough co-solvent has to be added to
ensure that the onset of polymer aggregation occurs before largescale phase separation can occur.

Results
The effect of co-solvents on efficiency and morphology. For our
study, we use an alternating diketopyrrolopyrrole-quinquethiophene copolymer donor (PDPP5T) in combination with phenylC71-butyric acid methyl ester ([70]PCBM) as fullerene acceptor
(Fig. 1a). DPP-based copolymers have emerged as versatile small
band gap semiconductors and provide PCEs up to 8% in
combination with [70]PCBM 35 . Processing PDDP5T and

[70]PCBM (1:2 w/w) into an efficient photoactive layer requires
the use of co-solvents 36 . For the PDPP5T:[70]PCBM blend, we
studied the effect of co-solvent on device performance for two
commonly used co-solvents: ortho-dichlorobenzene (oDCB) and
diiodooctane (DIO). Figure 1b shows the distinct difference in the
current density–voltage (J–V) characteristics for layers processed
from chloroform without and with 3 vol% oDCB as co-solvent.
Figure 1c shows the PCE as a function of the used concentration
(vol%) of co-solvent (oDCB or DIO). Without co-solvent, the
PCE of this material combination is less than 2%. The PCE
reaches a maximum above 5% for oDCB concentrations above 3
vol% and remains high when the concentration is increased up to
20 vol%. For DIO, the maximum PCE is found for 2 vol% and it
decreases again rapidly above 4 vol%.

The strong effect of the co-solvent on the PCE relates to the
morphology as evidenced by transmission electron microscopy
(TEM, Fig. 1d). Using no co-solvent, large (that is, 4100 nm)

[70]PCBM-rich domains are formed in a polymer-rich matrix.
This morphology originates from the liquid–liquid phase
separation during drying: due to solvent evaporation, the mixture
enters the spinodal region of the ternary phase diagram 15, in
which rapid amplification of thermal fluctuations in composition
results in [70]PCBM-rich droplets in a polymer-rich solution,
that persist until the film is dry. The large domains that form are
detrimental for the PCE because the electrons generated in the
mixed, polymer-rich, matrix cannot be collected efficiently 15,37 .
The domain size decreases for 1 and 2 vol% oDCB, although
larger features remain visible. At 3 or more vol% oDCB these
larger structures are absent and the morphology appears to be a
finely blended bulk heterojunction, wherein most photogenerated
excitons can be dissociated and contribute to current generation.
A similar effect of co-solvent on morphology is seen when DIO
is used (Supplementary Fig. 2); however, only 2 vol% of DIO is
enough to decrease the domain size to an optimum.

Spin coating dynamics studied by real-time measurements. To
understand the formation of these blend morphologies, it is
important to investigate the dynamics of the relevant processes
during spin coating. We use three optical techniques that provide
real-time information on film thickness, liquid–liquid phase
separation and polymer aggregation during spin coating. For film
thickness, we monitor the intensity of specular reflected laser light
with a photodiode during spin coating. Owing to the decreasing
layer thickness, conditions for constructive and destructive
interference occur remittently (Fig. 2a). After determining the
final (dried) layer thickness, a peak-counting method is used to
back-calculate the wet layer thickness during drying 38 . Owing to
solvent evaporation, at some point the mixture may enter a

C6H13

C8H17

C6H13

–5

S N S O

C6H13

C6H17

PDPP5T [70]PCBM

Vol%
oDCB

without co-solvent
–10 with co-solvent

–15

–0.9 –0.6 –0.3

0.0 0.3 0.6

_V_ (V)

oDCB
DIO

0
0 1 2 3 4 5

10 20

Amount of co-solvent (vol%)

Figure 1 | The influence of co-solvent on performance and morphology. (a) Chemical structure of PDPP5T and [70]PCBM. (b) J–V curves under
simulated solar illumination of PDPP5T:[70]PCBM (1:2 w/w) solar cells processed from chloroform without and with 3 vol% oDCB. (c) The effect of the
concentration of co-solvent on the PCE for oDCB and DIO. (d) Bright-field TEM images of PDPP5T:[70]PCBM (1:2 w/w) films spin coated from chloroform
with the indicated amount of oDCB as co-solvent. Scale bar, 600 nm.

Thin-film
interference

_Wet layer_

_thickness_

Scattering

_Liquid–liquid_

_phase_
_separation_

Absorption spectrum

_Polymer_
_aggregation_

Figure 2 | Real-time optical analysis of film formation. (a) The interference of incident laser light is monitored to measure the solvent layer thickness
during drying as function of time. (b) Scattered laser light monitors the onset of liquid–liquid phase separation. (c) A change in the diffuse reflection of
white light detected the change in the absorption spectrum that accompanies aggregation of polymer chains in solution.

spinodal region and become unstable such that liquid–liquid
phase separation occurs. This causes a refractive index contrast
between the fullerene-rich droplets and the polymer-rich
solution. Consequently, the droplets will scatter light (Fig. 2b)
and the onset of liquid–liquid phase separation can be detected by
the onset of light scattering, which is measured simultaneously
with the film thickness by a second photodiode positioned at an
off-specular angle.

The results of the two combined interference and scattering
measurements are shown in Fig. 3. The top panel of Fig. 3 shows
the film thickness as a function of time, reconstructed from the
interference signal. When spin coated from pure chloroform, the
PDPP5T:[70]PCBM layer is dry within 0.8 s. The thickness curve

clearly shows two phases: first, thinning is mainly caused by radial
spreading of material. After that, the decrease in thickness
becomes linear with time, which indicates that thinning is solely
caused by solvent evaporation. The onset of a scattering signal is
readily detected in the solvent evaporation phase (indicated by
the dashed dark blue line in the bottom panel) and indicates the
onset of liquid–liquid phase separation. Using the reconstructed
thickness data and the dry layer thickness, the solvent
concentration at the onset of phase separation is calculated to
be 83 ± 3 vol% for this example.

Using 5 vol% oDCB in chloroform, the drying time increases
from 0.8 to 6 s (note the axis break in Fig. 3). After the initial flow
phase, there is a stage where mainly chloroform evaporates until

1.2

0.8

0.4

0.0
0.0 0.3 0.6
Time (s)

0 vol% oDCB

5 vol% oDCB

2 4 6

0.8

0.6

0.4 _t_ = 0.683 s

0.2

0.0

+ 0.28 s
+ 0.32 s
+ 0.35 s
+ 0.38 s
+ 7.18 s

650 700 750

Time

800 850 900

Wavelength (nm)

Figure 3 | Detecting liquid–liquid phase separation with time. Film
thickness, normalized interference and light scattering of
PDPP5T:[70]PCBM in solvent mixtures is monitored during spin coating at
2,000 r.p.m. The solvent is chloroform (blue squares) and chloroform with
5 vol% oDCB (red circles). Green arrows schematically indicate the
thickness reconstruction from the measured interference signal. The
dashed lines indicate the onset of scattering.

B0.7 s. In the final 5 s mainly oDCB evaporates. This implies that
during these last 5 s the oDCB concentration is much larger than
5 vol%. In part, this explains why such a seemingly low amount of
co-solvent can have such a dramatic effect. With oDCB, there is
no strong scattering signal, but there is a small step indicated by
the dark red dashed line. As will be shown later, this is probably
due to small inhomogeneities caused by polymer aggregation. An
interference effect can also be seen in the light scattering signal.
This is explained by the fact that interference effects cause light
intensity fluctuations in the wet layer.

Using DIO as a co-solvent, a somewhat different interference
pattern is found. When measurements are done for a characteristic spin-coating time (B60 s), only a chloroform-drying regime
is observed. A 7,000-s spin-coating experiment reveals that DIO
evaporates on much longer timescales (see Supplementary Fig. 3).
For a DIO concentration of 4 vol%, drying continues for 56 min
while spinning continuously at 2,000 r.p.m. Such long spin
coating is not commonly used and hence DIO-processed layers
typically remain wet until a high-vacuum step is employed. For
normal spin-coating times (1–2 min), it is likely that the
solids:DIO ratio in the chloroform-dried film is the same as
their initial ratio.

Monitoring polymer aggregation during spin coating. Apart
from liquid–liquid phase separation, aggregation of the polymer
chains occurs during spin coating and drying of the films. To
detect this aggregation, the substrates were modified as shown in
Supplementary Fig. 4. A white paint layer is applied on the
backside of a glass substrate. On the topside of the glass substrate

0.0 0.2 0.4 0.6 0.8

3 6 9

Vol%
oDCB

20%

10%

12 15

Time (s)

Figure 4 | Detecting polymer aggregation with time. (a) Evolution of the
absorption spectrum during spin coating of a PDPP5T:[70]PCBM film from
chloroform with 3 vol% oDCB. (b) Laser interference (thin lines) and
transmission of 780–820 nm light (thick semi-transparent lines) of
PDPP5T:[70]PCBM films during spin coating from chloroform with different
concentrations oDCB.

a titanium oxide (TiO2) layer is added to provide refractive index
contrast to enable simultaneous interference experiments. On top
of the TiO2 a PEDOT:PSS layer is used to mimic the substrate
properties used in regular solar cells. The white paint is illuminated through the wet layer and the substrate by focussed light
from a halogen lamp. The white paint scatters this light, which is
then transmitted for a second time through the whole layer stack
(Fig. 2c). The off-specular scattered light is collected by a fibre
optic cable. The fibre optic cable is connected to a spectrometer to
acquire in situ absorption spectra. Figure 4a shows the changes in
the absorption for a PDPP5T:[70]PCBM (1:2 w/w) blend spin
coated from chloroform with 3 vol% oDCB. Directly after starting

the spin coating, a decrease of absorption in the flow phase was
observed (see Supplementary Movie 1, until t ¼ 0.68 s), due to
material that is radially ejected from the substrate. In the subsequent solvent evaporation phase (Fig. 4a), a red shift of the
optical absorption can be seen (t ¼ 0.96–1.06 s). It is well known
that the aggregation of p-conjugated polymers causes a red shift
in the absorption spectrum 39, and experiments as shown in
Fig. 4a reveal when this process occurs.

To measure polymer aggregation simultaneously with layer
thickness via interference experiments, a band-pass filtered
photodiode is used which only collects light between
780–820 nm. The red shift in absorption can then be detected as
a decrease in transmitted light intensity. The results are shown in
Fig. 4b for different concentrations of oDCB, and for different
concentrations of DIO in Supplementary Fig. 5. The decrease in
transmission always occurs during the transition from the
chloroform-drying to the oDCB-drying regime. The oDCB-drying
regime takes much longer with increasing amounts of co-solvent
and shows more interference peaks. Hence, for higher oDCB
concentrations, polymer aggregation occurs in thicker wet layers
with a higher total solvent content. In addition, aggregation takes
longer and there is a larger change in transmission for higher
oDCB concentrations, indicating that the extent of aggregation
increases. As shown in Supplementary Fig. 6, this shifts the

100

absorption onset in the dry films and decreases the open-circuit
voltage, in agreement with results obtained by Kim et al. 40

The solvent content at the onset of phase separation. The total
solvent concentrations where polymer aggregation starts, as
calculated from these experiments for oDCB and DIO in
chloroform, are shown in Fig. 5a,b as blue circles; the pink stars
represent the onset of the scattering signal. Without the co-solvent, the onset of scattering occurs before the decrease in transmission. This implies that in pure chloroform liquid–liquid phase
separation precedes polymer aggregation. Raising the amount of
co-solvent to B2–4 vol% increases the solvent content at which
the transmission change occurs, while the onset of scattering still
occurs at approximately the same solvent content. The onsets of
liquid–liquid phase separation and polymer aggregation occur at
approximately the same solvent concentration, and thus time, for
co-solvent concentrations of 2 vol%. For even higher co-solvent
contents, the change in transmission and scattering signal occur
at the same time, indicating that polymer aggregation is the cause
of the (small) scattering signals.

Phase separation in other polymer–fullerene blends. To test the
generality of these results, we studied two more polymers. The

Scattering

Transmission

20
0 1 2 3
oDCB (vol%)

10 20 0 1 2 4 8 12 16
DIO (vol%)

No co-solvent With co-solvent

L-L phase
separation

Liquid–liquid

(L-L) phase

separation

Polymer
aggregation

20
0 1 2 3 5 10 15 20

Initial co-solvent content (%)

Figure 5 | Which is first: phase separation or polymer aggregation? (a) Total solvent content (concentration in vol%) at the onset of scattering (pink
stars) and at the change in transmission (blue circles at 50% change) versus concentration of oDCB in the initial chloroform solution (error bars for
scattering arise from the time-uncertainty in the scattering onset; thickness uncertainty from back-calculating the interference signals is not included. The
transmission error bars indicate 20–80% transmission change). (b) Same for DIO instead of oDCB. (c) Schematic phase diagram revealing the role of cosolvent in inducing polymer aggregation at higher solvent contents which, in turn, prevents large-scale liquid–liquid phase separation during drying and
results in optimized morphologies.

two polymers consist of a DPP unit alternating with a sequence of
thiophene–phenyl–thiophene (TPT) rings along the chain, but
differ with respect to the length of the sides chains. The shorter 2hexyldecyl (HD) side chains of PDPPTPT-HD result in a reduced
solubility compared to PDPPTPT-DT with longer 2-decyltetradecyl (DT) side chains. For PDPPTPT-HD:[70]PCBM, PCEs
over 7% have been obtained 20,35 . For both derivatives, the solvent
content at the onset of scattering and at the change in
transmission were measured (Supplementary Fig. 7) for
solutions in pure chloroform and in chloroform with an
optimized amount of co-solvent. Liquid–liquid demixing
precedes polymer aggregation in films that are processed from
solutions of PDPPTPT-HD or PDPPTPT-DT with [70]PCBM in
pure chloroform. However, when using a co-solvent, the onset of
polymer aggregation occurs before liquid–liquid phase separation
can occur. These results are fully consistent with those obtained
for PDPP5T and [70]PCBM.

Role of the co-solvents. Our interpretation of this data is clarified
in Fig. 5c. The orange arrow indicates the order of events when
no co-solvent is used: (1) when spin coating is started, the
chloroform concentration decreases due to evaporation; (2) at
B80 vol% total solvent content liquid–liquid phase separation
occurs; (3) at B50 vol% total solvent content the polymer
aggregates. The domain size is determined in step (2) by the
fullerene droplets which are formed by liquid–liquid phase
separation. When co-solvents are used, the purple arrow indicates
the order of events: (1) the total solvent content decreases mainly
due to chloroform evaporation, thus the solvent quality for the
polymer decreases; (2) at B80–95 vol% total solvent content the

polymer starts aggregating; (3) the polymer has aggregated before
liquid–liquid phase separation has occurred, and no large
domains are formed.

For both co-solvents, the onset of aggregation and scattering
seem to start overlapping around 2% co-solvent content (Fig. 5a,b).
Because the high molecular weight fraction of PDPP5T is
essentially insoluble in oDCB and in DIO, it is not surprising
that the onsets of aggregation are similar for the two co-solvents.
The 2% co-solvent contents is also the minimal amount of cosolvent that needs to be added to make high-performing solar cells
with small domain sizes (Fig. 1). In these experiments, large-scale
phase separation does not occur if the onset of polymer
aggregation occurs simultaneously with or before the unstable
spinodal regime in the phase diagram is entered.

These results qualitatively agree with in situ X-ray analyses
of a blade-coated PTB7:[70]PCBM film and a drop-cast
PDPP4T:[70]PCBM sample using a co-solvent as reported by
Liu et al. 18,29 For both systems, polymer aggregation occurs when
the fullerene is still dissolved. Liu et al. propose that the formation
of polymer networks prevents large-scale domain formation.

Effect of polymer preaggregation. Combined, these observations
seem to indicate that polymer aggregation prevents liquid–liquid
phase separation. To test this hypothesis, solar cells were made
using a solution in pure chloroform. PDPP5T and [70]PCBM
were dissolved by stirring for 60 min at 90 �C. This solution was
allowed to preaggregate for 6 days during which it remains very
fluid (not gelated) and without visible precipitation. Atomic force
microscopy (AFM) of photoactive layers of solar cells spin coated
from this preaggregated solution (Fig. 6a) clearly show that

Preaggregated

Reheated

80.0 nm

0.0 5.0 µm

80.0 nm

0.0 5.0 µm

0.8

0.6

0.4

0.2

0.0

650 700 750 800 850 900 950

Wavelength (nm)

400 500 600 700 800 900 1,000

Wavelength (nm)

Wet

Dry

Figure 6 | Aggregation alone does not prevent liquid–liquid demixing. (a) AFM height image of the photoactive layer of a solar cell made from a
PDPP5T:[70]PCBM solution in pure chloroform which had been left to preaggregate for 6 days. (b) AFM height image of a the photoactive layer of solar
cell made from the same solution as in a that had been restirred at 90 �C for 20 min and left to cool down for 10 min to room temperature before spin
coating. (c) EQE measurements of the solar cells in a and b reveal that the solar cell from the preaggregated solution has a more pronounced red-shifted
response and a slightly lower photocurrent compared to the solar cell from the reheated solution. (d) In situ absorption spectra showing wet and dry film
absorption spectra for both solar cells in a (preaggregated) and b (reheated). The measurements were vertically shifted such that the absorbance at
925 nm is set to zero. The thickness of the active layers of both solar cells was 100 nm.

liquid–liquid phase separation has occurred. The external quantum efficiency (EQE, Fig. 6c) shows a pronounced red-shifted
response, caused by the preaggregated polymer. The absorption
spectra recorded during drying of these films (Fig. 6d) evidence
that the polymer had already aggregated in the casting solution.
The onset of absorption is similar for the preaggregates in
solution and film. The same solution was then restirred for
20 min at 90 �C to redissolve the aggregates and cooled to room
temperature for 10 min. Figure 6d shows that this removes
the preaggregates from the solution and therefore results in
liquid–liquid phase separation (Fig. 6b) when spin coating a
film. Both the EQE (Fig. 6c) and the changes in the spectrum
recorded in situ (Fig. 6d) show that the polymer aggregates
formed by preaggregation have a more pronounced red-shifted
absorption onset than the aggregates formed in situ by spin
coating the reheated solution. Hence, the nature of the aggregates
might be different. These experiments demonstrate that
aggregation by itself is not enough to prevent liquid–liquid
phase separation.

To prevent the liquid–liquid phase separation, the co-solvent
induced aggregation must occur under supersaturated conditions
at 5 to 20 vol% solid content in a mixture that is enriched in the
co-solvent, which is a poor solvent for PDPP5T. We hypothesize
that under these supersaturated conditions, the polymer gelates
by forming a fibrillar network, which rapidly increases the
viscosity and thus prevents large-scale morphology development,
or that the polymer precipitates which prevents liquid–liquid
phase separation by altered interactions between the components
as a consequence of the enrichment of the solution with
co-solvent and reduced concentration of dissolved polymer.

In summary, we have shown that the combination of three
relatively simple in situ real-time measurements can elucidate
the complex processes that occur during the rapid drying of
four-component solar cells blends. For two popular co-solvents
and three different polymer–fullerene combinations, we have
shown that the co-solvent causes the onset of polymer
aggregation to occur before liquid–liquid phase separation.
When this aggregation occurs under supersaturated conditions,
the formation of large [70]PCBM-rich domains is prevented.
Because many polymer:fullerene blends exhibit very similar
morphological changes when using co-solvents 16–26, it is of
interest to study if the proposed mechanism has wider
applicability and to what extend other effects contribute. The
in situ methods outlined in our study can be used for this
purpose and can likely be transferred to the in-line monitoring
of drying processes in roll-to-roll deposition. Understanding the
mechanisms underlying the morphology formation will provide
a more straightforward optimization of solar cell processing
conditions. This is a necessary development considering the
challenges that remain for organic solar cells to become a viable
technology.

Methods
Substrate preparation. Three different substrates were used: (1) Solar cells are
made on prepatterned indium-tin-oxide substrates (Naranjo Substrates); (2)
combined interference and scattering was performed on silicon substrates (Si-Mat,
525 mm thickness with 200 nm SiO2 layer); (3) interference and absorption and
transmission measurements were done on glass slides. All substrates were thoroughly cleaned by sonication in acetone, scrubbing and sonication in a sodium
dodecyl sulfate solution (99%, Acros), rinsing with deionized water and finally
sonication in 2-propanol. Then, on substrate type (3), a titanium oxide layer was
prepared using a sol-gel method. Acetylacetone (50 ml, 99 þ %, Acros) and titanium(IV) isopropoxide (145 ml, 97%, Aldrich) were added to 2-proponol (1.0 ml),
spin coated at 2,000 r.p.m. and baked for 30 min at 400 �C. Later, a latex-based
white paint was spin coated on the backside of the slide at 800 r.p.m. Before
application of the photoactive layer, all substrates were treated with ultravioletozone and covered with a 40-nm thick poly(ethylenedioxythiophene):poly(styrene
sulfonate) (PEDOT:PSS, Heraeus Clevios PVP Al 4083) layer.

Solar cell fabrication. For the active layer PDPP5T 36 was mixed with [70]PCBM
(Solenne BV, 90–95%) and dissolved in chloroform with various amounts of cosolvents by overnight stirring at 65 �C. DIO (98%) and oDCB (99%) were supplied
Sigma-Aldrich. The mass loading was 6 mg ml � 01 for PDPP5T and 12 mg ml � 1

for [70]PCBM. Spin coating was done at 2,000 r.p.m. in air at room temperature. At
room temperature, [70]PCBM is well soluble in chloroform (61 mg ml � 1 ) (ref. 41),
in oDCB (203 mg ml � 1 ) (ref. 41) and in DIO (63 mg ml � 1 ) (ref. 42). PDPP5T is
only soluble in chloroform. In oDCB, only a low molecular weight fraction of
PDPP5T can dissolve (Supplementary Fig. 8) and it is essentially insoluble in DIO.
For solar cells, a back electrode was evaporated consisting of LiF (1 nm) and Al
(100 nm).

Solar cell characterization. Current density–voltage (J–V) characteristics were
measured by sweeping the voltage from - 2 to þ 2 V using a Keithley 2400
sourcemeter. A tungsten halogen lamp was used in conjunction with a Hoya LB120
daylight filter to simulate the solar spectrum. The total light intensity was checked
with a calibrated Si photodiode to be B100 mW cm � 2 . EQE was measured with a
chopper-modulated 50 W Philips focusline tungsten halogen lamp in combination
with an Oriel Cornerstone 130 monochromator. The modulated signal was
amplified by a current preamplifier (Stanford Research System Model SR570) and
measured by a lock-in amplifier (Stanford research Systems SR830). The measurements were converted to EQE by using a calibrated silicon reference cell.

Real-time measurements. For interference and scattering experiments, a 632.8-nm
HeNe laser (5mW, Melles-Griot) was used. The laser beam was spread to a spot of
B3 mm radius using a biconvex lens, which removes disturbances caused by the
rotating sample by averaging over a larger area. A diffuser was used to collect all
specular reflected light and to filter out a slight wobble in the spinning disk. An Si
photodiode (Thor Labs SM1PD1A) collected the light behind the diffuser. A current
preamplifier (Stanford Research System Model SR570) operating in high-bandwidth
mode amplified the short-circuit current of the photodiode. A photodiode (Hamamatsu S2281) collected off-specular scattered light under a B45� angle, which was
amplified by a second current preamplifier. A Keithley 2636 A sourcemeter simultaneously measured both amplified signals as a voltage.

For the thickness calculation, Wolfram Mathematica 9.0 was used to
automatically select peaks and valleys in the interference signals, which was always
manually checked. Using the final dry layer thickness (measured with a Veeco
Dektak 150 profilometer), the thickness was backwards reconstructed from the
interference signal by repetitive addition of l/(4n cos y) (amounting to B100 nm
per peak-valley distance). Here y is the angle of incidence of the laser on the
substrate and l is the wavelength of the laser. The approximate volume fractions of
all components were used to estimate the refractive index n.

For absorption and transmission experiments, a light from halogen lamp was
focussed on approximately the same area as the laser light. To avoid heating, a
relatively low light intensity was used. A fibre optic cable collected the light that was
transmitted through the layer stack, scattered by the white paint and again
transmitted through the whole stack. A spectrometer (Avantes Avaspec-2048 � 14)
wasspectrumused inusingstore-to-RAMAðlÞ ¼ - mode.log10�IRawmI;blankmðlðÞ �lcountsÞ �ImI;darkm;darkðlareðÞlÞ�transformed, where Im indicatesto an absorptionthe

wavelength-dependent amount of counts. The subscript dark is a dark reference and
blank is a reference measurement on the substrate before spin coating the active
layer. For photodiode-based transmission measurements, a band-pass filter (centred
at 800 nm, FWHM 40nm) was used in front of a Si photodiode (Thor Labs
SM1PD1A). This signal was measured in the same way as the scattering signal.

Morphology characterization and thickness measurement. For TEM,
PDPP5T:[70]PCBM films were floated from the PEDOT-covered indium-tin-oxide
substrates on 200 square mesh copper grids. TEM was performed with a Tecnai G2
Sphera (FEI) operating at 200 kV. Tapping-mode AFM was performed using a
Veeco MultiMode AFM with Nanosensor PPP-NCHR-50 tips.

References
1. Liu, Y. et al. Aggregation and morphology control enables multiple cases of
high-efficiency polymer solar cells. Nat. Commun. 5, 5293 (2014).
2. Grossiord, N., Kroon, J. M., Andriessen, R. & Blom, P. W. M. Degradation
mechanisms in organic photovoltaic devices. Org. Electron. 13, 432–456 (2012).
3. Gevorgyan, S. A. et al. An inter-laboratory stability study of roll-to-roll coated
flexible polymer solar modules. Sol. Energ. Mat. Sol. C 95, 1398–1416 (2011).
4. Krebs, C. K. All solution roll-to-roll processed polymer solar cells free from
indium-tin-oxide and vacuum coating steps. Org. Electron. 10, 761–768 (2009).
5. Galagan, Y. ITO-free flexible organic solar cells with printed current collecting
grids. Sol. Energ. Mat. Sol. C 95, 1339–1343 (2011).
6. Chueh, C. Non-halogenated solvents for environmentally friendly processing of
high-performance bulk-heterojunction polymer solar cells. Energy Environ. Sci.
6, 3241–3248 (2013).
7. Chen, Y., Zhang, S., Wu, Y. & Hou, J. Molecular design and morphology
control towards efficient polymer solar cells processed using non-aromatic and
non-chlorinated solvents. Adv. Mater. 26, 2744–2749 (2014).

8. Liao, H. et al. Additives for morphology control in high-efficiency organic solar
cells. Mater. Today 16, 326–336 (2013).
9. Liu, F., Gu, Y., Jung, J. W., Jo, W. H. & Russell, T. P. On the morphology of
polymer-based photovoltaics. J. Polym. Sci. B Polym. Phys. 50, 1018–1044
(2012).
10. Lee, J. K. et al. Processing Additives for improved efficiency from bulk
heterojunction solar cells. J. Am. Chem. Soc. 130, 3619–3623 (2008).
11. Peet, J. et al. Efficiency enhancement in low-bandgap polymer solar cells by
processing with alkane dithiols. Nat. Mater. 6, 497–500 (2007).
12. Gu, Y., Wang, C. & Russell, T. P. Multi-length-scale morphologies in
PCPDTBT/PCBM bulk-heterojunction solar cells. Adv. Energy Mater. 2,
683–690 (2012).
13. Peet, J., Cho, N. S., Lee, S. K. & Bazan, G. C. Transition from solution to the
solid state in polymer solar cells cast from mixed solvents. Macromolecules 41,
8655–8659 (2008).
14. Nilsson, S., Bernasik, A., Budkowski, A. & Moons, E. Morphology and phase
segregation of spin-casted films of polyfluorene/PCBM blends. Macromolecules
40, 8291–8301 (2007).
15. Kouijzer, S. et al. Predicting morphologies of solution processed
polymer:fullerene blends. J. Am. Chem. Soc. 135, 12057–12067 (2013).
16. He, Z. et al. Enhanced power-conversion efficiency in polymer solar cells using
an inverted device structure. Nat. Photon. 6, 591–595 (2012).
17. Collins, B. A. et al. Absolute measurement of domain composition and
nanoscale size distribution explains performance in PTB7:PC71BM solar cells.
Adv. Energy Mater. 3, 65–74 (2013).
18. Liu, F. et al. Understanding the morphology of PTB7:PCBM blends in organic
photovoltaics. Adv. Energy Mater. 4, 1301377 (2014).
19. Hedley, G. J. et al. Determining the optimum morphology in high-performance
polymer-fullerene organic photovoltaic cells. Nat. Commun. 4, 2867 (2013).
20. Li, W. et al. Effect of the fibrillar microstructure on the efficiency of high
molecular weight diketopyrrolopyrrole-based polymer solar cells. Adv. Mater.
26, 1565–1570 (2013).
21. Bijleveld, J. C. et al. Efficient solar cells based on an easily accessible
diketopyrrolopyrrole polymer. Adv. Mater. 22, E242–E246 (2010).
22. Amb, C. M. et al. Dithienogermole as a fused electron donor in bulk
heterojunction solar cells. J. Am. Chem. Soc. 133, 10062–10065 (2011).
23. Small, C. E. et al. High-efficiency inverted dithienogermole–
thienopyrrolodione-based polymer solar cells. Nat. Photon. 6, 115–120 (2012).
24. Wang, D. H. et al. Effect of processing additive on morphology and charge
extraction in bulk-heterojunction solar cells. J. Mater. Chem. A 2, 15052–15057
(2014).
25. Bartelt, J. A. et al. Controlling solution-phase polymer aggregation with
molecular weight and solvent additives to optimize polymer-fullerene bulk
heterojunction solar cells. Adv. Energy Mater. 4, 1301733 (2014).
26. Stalder, R. et al. An isoindigo and dithieno[3,2-b:20,30-d]silole copolymer for
polymer solar cells. Polym. Chem. 3, 89–92 (2012).
27. Schmidt, K. et al. A mechanistic understanding of processing additive-induced
efficiency enhancement in bulk heterojunction organic solar cells. Adv. Mater.
26, 300–305 (2014).
28. Gao, J. et al. Elucidating double aggregation mechanisms in the morphology
optimization of diketopyrrolopyrrole-based narrow bandgap polymer solar
cells. Adv. Mater. 26, 3142–3147 (2014).
29. Liu, F. et al. Efficient polymer solar cells based on a low bandgap semicrystalline DPP polymer-PCBM blends. Adv. Mater. 24, 3947–3951 (2012).
30. Lou, S. J. et al. Effects of additives on the morphology of solution phase
aggregates formed by active layer components of high-efficiency organic solar
cells. J. Am. Chem. Soc. 133, 20661–20663 (2011).
31. Chou, K. W. et al. Spin-cast bulk heterojunction solar cells: A dynamical
investigation. Adv. Mater. 25, 1923–1929 (2013).
32. Shin, N., Richter, L. J., Herzin, A. A., Kline, R. J. & DeLongchamp, D. M. Effect
of processing additives on the solidification of blade-coated polymer/fullerene
blend films via in-situ structure measurements. Adv. Energy Mater. 3, 938–948
(2013).

33. Schmidt-Hansberg et al. Moving through the phase diagram: Morphology
formation in solution cast polymer-fullerene blend films for organic solar cells.
ACS Nano 5, 8579–8590 (2011).
34. Pearson, A. J. et al. Morphology development in amorphous polymer:fullerene
photovoltaic blend films during solution casting. Adv. Funct. Mater. 24,
659–667 (2013).
35. Hendriks, K. H., Heintges, G. H. L., Gevaerts, V. S., Wienk, M. M. &
Janssen, R. A. J. High-molecular-weight regular alternating diketopyrrolopyrrole-based terpolymers for efficient organic solar cells. Angew. Chem. Int.
Ed. 52, 8341–8344 (2013).
36. Gevaerts, V. S., Furlan, A., Wienk, M. M., Turbiez, M. & Janssen, R. A. J.
Solution processed polymer tandem solar cell using efficient small and wide
band gap polymer:fullerene blends. Adv. Mater. 24, 2130–2134 (2012).
37. Maturova´, K., van Bavel, S. S., Wienk, M. M., Janssen, R. A. J. & Kemerink, M.
Morphological device model for organic bulk heterojunction solar cells. Nano
Lett. 9, 3032–3037 (2009).
38. Heriot, S. Y. & Jones, R. A. L. An interfacial instability in a transient wetting
layer leads to lateral phase separation in thin spin-cast polymer-blend films.
Nat. Mater. 4, 782–786 (2005).
39. Wienk, M. M., Turbiez, M., Gilot, J. & Janssen, R. A. J. Narrow-bandgap
diketo-pyrrolo-pyrrole polymer solar cells: the effect of processing on the
performance. Adv. Mater. 20, 2556–2560 (2008).
40. Kim, J. et al. A thienylenevinylene-phthalimide copolymer based polymer solar
cell with high open circuit voltage: effect of additive concentration on the open
circuit voltage. Sol. Energ. Mater. Sol. C 125, 253–260 (2014).
41. Duong, D. T. et al. Molecular solubility and Hansen solubility parameters for
the analysis of phase separation in bulk heterojunctions. J. Polym. Sci. B Polym.
Phys. 50, 1405–1413 (2012).
42. Chen, Y., Zhang, S., Wu, Y. & Hou, J. Molecular design and morphology
control towards efficient polymer solar cells processed using non-aromatic and
non-chlorinated solvents. Adv. Mater. 26, 2744–2749 (2014).

Acknowledgements
We thank Charley Schaefer (Eindhoven University of Technology) and Dr Jasper Michels
(Holst Centre) for discussions. Fruitful discussions with Dr Sjoerd Veestra (ECN, Energy
Research Centre of the Netherlands) lead to the painted substrates for in situ absorption
measurements. We appreciate the contribution of Geert Adriaans regarding the
experiments on PDPPTPT and of Dr Xianwen Lou for SEC measurements. The research
forms part of the research programme of the Dutch Polymer Institute (DPI), project
#734, and of the Solliance OPV programme, and has received funding from the European
Commission’s Seventh Framework Programme in the LargeCells project (Grant Agreement No. 261936) and from the Dutch Ministry of Education, Culture and Science
(Gravity programme 024.001.035).

Author contributions
All experiments were performed by J.J.v.F. Polymers were synthesized by M.T. and W.L.
The design of the experiments, the analysis of the results and writing was done by J.J.v.F.,
M.M.W. and R.A.J.J.

Additional information
Supplementary Information accompanies this paper at [http://www.nature.com/](http://www.nature.com/naturecommunications)
[naturecommunications](http://www.nature.com/naturecommunications)

Competing financial interests: The authors declare no competing financial interests.

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How to cite this article: van Franeker, J. J. et al. A real-time study of the benefits of
co-solvents in polymer solar cell processing. Nat. Commun. 6:6229 doi: 10.1038/
ncomms7229 (2015).

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