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Cite this: Phys. Chem. Chem. Phys.,
2014, 16, 20329
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Control of charge generation and recombination
in ternary polymer/polymer:fullerene photovoltaic
blends using amorphous and semi-crystalline
copolymers as donors†
Hannah Mangold,a Artem A. Bakulin,b Ian A. Howard,ac Christian Kästner,d
Daniel A. M. Egbe,e Harald Hoppe*d and Frédéric Laquai*a
Charge generation and recombination processes occurring in ternary photoactive copolymer:copolymer:
fullerene blends consisting of different mixing ratios between entirely amorphous and semi-crystalline PPE-PPV
copolymers are investigated by transient absorption pump–probe and pump–push photocurrent spectroscopy.
The experiments reveal that an excess of semi-crystalline polymer facilitates exciton dissociation into free charge
carriers, slows down geminate recombination, and suppresses non-geminate recombination leading to increased
short-circuit currents and high fill factors. In contrast, blends utilizing solely the amorphous polymer for their
donor phase suffer from a large fraction of sub-nanosecond geminate recombination of interfacially bound
Received 1st May 2014,
Accepted 16th June 2014
charge-transfer states and also from fast non-geminate recombination of free charges, resulting in a significantly
DOI: 10.1039/c4cp01883d
crystalline polymer increase the open-circuit voltage and the fill factor, while keeping the charge generation and
reduced photovoltaic performance. However, small fractions of the amorphous polymer blended into the semirecombination parameters largely unaltered in turn leading to an optimized device performance for the ternary
www.rsc.org/pccp
PPE-PPV copolymer:copolymer:fullerene blends.
1. Introduction
The morphology of the photoactive layer of any bulk heterojunction
polymer:fullerene solar cell is crucial for the photovoltaic device
performance.1–5 Morphology control and optimization is required
on different length-scales, starting from interfacial organization of
donors and acceptors on the sub-nanometer scale, determining the
initial step of the electron transfer process,6–11 increasing to
2–20 nanometers at which phase separation controls the size of
individual blend component domains relevant for exciton
diffusion and charge relaxation,12,13 up to several hundred
nanometers, the length scale governing charge carrier percolation/
transport and extraction.14
a
Max Planck Research Group for Organic Optoelectronics,
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz,
Germany. E-mail: [email protected]
b
FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, Netherlands
c
Institute of Microstructure Technology (IMT),
Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1,
D-76344 Eggenstein-Leopoldshafen, Germany
d
Institute of Physics, Ilmenau University of Technology, Langewiesener Str. 22,
98693 Ilmenau, Germany. E-mail: [email protected]
e
Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz,
Altenbergerstrasse 69, 4040 Linz, Austria
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cp01883d
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In the case of the heavily-researched donor polymer regioregular
poly(3-hexylthiophene), rr-P3HT, blended with fullerenes, structural
ordering, namely crystallization of the polymer, is vital for the
photovoltaic performance.15–17 Well-ordered regions of both
donors and acceptors are known to yield superior percolation
and thus charge transport properties.18,19 Several methods for
pre-aggregating P3HT in solution20–22 prior to spin-casting as
well as post-processing annealing methods, such as thermal
annealing,15,23 mechanical rubbing24 or slow drying,25 were
developed to induce and improve crystallinity and therefore
device performance. Interestingly, the best device performance
was obtained from samples that contained both ordered polymer
fibers and less-ordered P3HT domains.21,22 This can be understood in terms of an enhanced driving force of long-range charge
separation created by the mixed system containing both amorphous
and disordered as well as crystalline and ordered regions of donors
and/or acceptors as recently shown by Durrant and coworkers26,27
and by Heeger and coworkers.13 Therefore, it appears that the ideal
blend should not only contain disordered regions to facilitate
exciton dissociation but also ordered regions to promote charge
transport.28,29 It was also demonstrated that polymer crystallization
is beneficial for charge generation.30
One possible route to create both amorphous and crystalline
regions in a more controlled manner than empirically changing the
processing conditions and applying post-deposition annealing is
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Scheme 1 Chemical structures of the investigated copolymers, namely
AnE-PVab and AnE-PVba. Note the different side chain substitution
patterns of the two copolymers.
the blending of three components, i.e. two (co)polymers of different
(semi)-crystallinity mixed with PCBM as an acceptor. This has
been done, for instance, in the case of P3HT, i.e. for a mixture of
regioregular and regiorandom P3HT with PCBM.31 Khlyabich
et al. demonstrated that a ternary blend of two different P3HT
analogues mixed with PCBM achieved a considerable performance
enhancement.32
In the following we investigate ternary blends with varying
mixing ratios of amorphous and semi-crystalline anthracenecontaining poly(p-phenylene-ethynylene)-alt-poly-( p-phenylenevinylene) (PPE-PPV) copolymers (AnE-PVs) mixed with PCBM as
an acceptor. The structures of the two copolymers are depicted
in Scheme 1.
The backbones of both donor copolymers are the same,
however, they differ in their side chain substitution patterns,
which facilitates semi-crystallinity (AnE-PVab) in one case, and
leads to a completely amorphous (AnE-PVba) material in the
other case.33 Different blending ratios of semi-crystalline to
amorphous polymers were investigated as reported in a preceding
publication34 to understand the relation between morphology,
structural properties, steady-state photophysical processes and solar
cell performance. Indeed, for largely unknown reasons the optimum
performance was found for small (10%) contents of the amorphous
polymer,34 requiring a closer look at the photophysical properties by
transient optical spectroscopy. Here, the results of five different
samples are presented, containing 0, 10, 50, 90, and 100% of
amorphous AnE-PVba as a second blend component, respectively.
Table 1 summarizes the composition of the samples and
their solar cell performance. Fluctuations occurred in all photovoltaic parameters but in general a better performance was found
for the more semi-crystalline samples. In fact, the completely
Table 1 Average solar cell parameters of all devices containing different ratios
of AnE-PVab and AnE-PVba copolymers evaluated from JV-characteristics.
The total AnE-PV : PCBM ratio is 2 : 3 for all samples
AnESample PVab/% Z/%
1
2
3
4
5
100
90
50
10
0
3.66
3.75
2.49
0.55
0.37
VOC/mV
0.24
0.19
0.22
0.10
0.10
764
795
864
603
578
8
7
6
106
186
JSC/mA cm
7.05
6.71
5.26
3.00
2.25
0.18
0.37
0.42
0.06
0.20
2
FF/%
67.97
70.37
54.63
30.27
28.98
20330 | Phys. Chem. Chem. Phys., 2014, 16, 20329--20337
3.82
0.53
1.82
0.43
0.69
amorphous sample (100% ba) showed the lowest performance
and values for all figures of merit. The largest short-circuit currents
and fill factors were obtained for the samples containing 0% and
10% of the amorphous polymer. The open-circuit voltage (VOC)
increased progressively with increasing concentration of the
amorphous polymer until a maximum was reached for the
sample containing 50%, while at larger concentrations VOC
decreased again. The power conversion efficiency dropped from
4% for the blend containing 10% of the amorphous polymer to
0.4% for the sample consisting solely of the amorphous polymer.
Regarding the photoactive layer morphology it is important to
note that the amorphous sample exhibited very small scale
component demixing, while the domain sizes were considerably
larger in the mainly semi-crystalline samples, as observed by
atomic force microscopy (AFM) and grazing-incidence wideangle X-ray diffraction (GIWAXS).34 Here, we report the charge
generation and recombination properties and elucidate the
impact of semi-crystalline and amorphous polymer regions on
the free charge carrier generation efficiency. We demonstrate
that while polymer exciton quenching is efficient in all five
samples, the more amorphous samples, with 90% and 100%
of AnE-PVba, exhibit both a larger fraction and faster geminate
recombination and significantly increased non-geminate recombination, leading to a deteriorated device performance.
2. Experimental results
2.1 Sub-nanosecond exciton quenching and
charge generation
In order to compare the exciton quenching efficiency and
sub-nanosecond charge generation and recombination of the
different samples, transient absorption experiments covering
the pico- to nanosecond timescale (1 ps to 3 ns) were carried
out. The samples were excited at an excitation wavelength of
400 nm, i.e. in a wavelength region where the absorbance of all
samples was similar. We note that in this spectral region both
polymer and PCBM absorb and contribute to the photocurrent
generation. This can lead to a diffusion-limited charge generation
process, if PCBM excitons have to diffuse to the interface to undergo
dissociation. However, the dynamics of the charge-transfer (CT)
states formed upon exciton dissociation are independent of the
excitation of either the donor or the acceptor as was recently shown
for several polymer:fullerene blends.35
Fig. 1 compares the pico- to nanosecond TA spectra of all
five samples. All spectra show positive TA signals at short
wavelengths, corresponding to the ground state bleach (GSB)
of the polymer coinciding with the steady-state absorption
spectrum, followed by photo-induced absorption at longer
wavelengths extending into the NIR spectral region. The GSB
of the samples with mainly semi-crystalline polymer content
exhibits vibronic features, while the mainly amorphous samples
show a broad and entirely featureless GSB. The GSB of the samples
containing Z50% of the amorphous polymer experiences a
significant red-shift with delay time. The red-shift is most
evident for the sample consisting solely of the amorphous
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larger distances to the interface with the acceptor where quenching
by electron transfer takes place. Increasing concentrations of
the amorphous polymer decrease the presence of larger and
polymer-rich domains and thus result in more molecularlymixed blends. Therefore, exciton quenching takes place on the
sub-picosecond timescale for dominantly amorphous donor
materials, as the majority of excitons are photo-generated close
to an interface with PCBM.
2.2
Fig. 1 Picosecond to nanosecond transient absorption spectra of all
5 polymer:polymer:fullerene blends with increasing content of the amorphous
polymer (% ba) after excitation at 400 nm. The black line corresponds to the
averaged spectra between 1–1.5 ps, the red line at 10–15 ps and the blue line at
1–1.5 ns. The data around 800 nm are omitted as they are overlaid by the
second order reflection of the excitation pulse. Note that the signal remains
largely constant for 0% and 20% AnE-PVba content, while it decays significantly
for AnE-PVba concentrations over 50%. Excitation at 4.4 mJ cm 2 (0% and
10%), 12 mJ cm 2 (50% and 100%) and 14 mJ cm 2 (90%).
polymer AnE-PVba blended with PCBM. Here, the GSB maximum
red-shifts from 530 nm to 560 nm in the first 3 ns. We note that the
relaxation is more pronounced in the samples containing Z50% of
the amorphous polymer implying that the change in energy of
excited states at early times appears to increase with increasing
amorphous content. At longer wavelengths the TA spectra of all
samples are dominated by a broad photo-induced absorption (PIA)
of excited states such as excitons, free charges (SSA) and chargetransfer states. However, it appears that the TA signal in the PIA
region of the samples containing a fraction of 0 and 10% of the
amorphous polymer, respectively, is a superposition of the PIA and
a broad but distinct feature due to stimulated emission (SE)
peaking at around 680 nm, which coincides with the steady-state
fluorescence maximum of the pristine polymer film. As the ESA is
strong also in the region in which SE can be expected, a net
negative TA signal is observed. This SE feature is also apparent in
the sample with 50% of each copolymer, but it is entirely absent in
the samples containing larger fractions of the amorphous polymer.
The stimulated emission decays entirely within about 10 ps in all
samples. Hence, we conclude that exciton quenching in all blends
is efficient and close to unity, as the SE of excitons is quenched in
less than 1 ps for the amorphous samples and in less than 10 ps for
the more crystalline samples. The observation of stimulated emission in the case of the highest semi-crystalline polymer (AnE-PVab)
concentrations (0% and 10% ba) is in excellent agreement with
the morphology indicated by AFM and GIWAXS measurements.34
For dominantly semi-crystalline polymer containing samples the
domains are generally larger, which requires exciton diffusion over
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Sub-nanosecond geminate charge pair recombination
The ground-state bleach and the broad photo-induced absorption
show very little signal loss over the entire time-range up to 3 ns
in samples with high fractions of semi-crystalline copolymers
indicating that the excited-state population stays largely constant
on this timescale. This is in contrast to the more amorphous
samples, which exhibit a considerable loss of both ground-state
bleach and photo-induced absorption signals.
Fig. 2a shows the comparison of the decay dynamics of the
PIA signals integrated between 875 and 925 nm and measured
at similar excitation intensities for all five samples. The data are
normalized to 10 ps when exciton quenching is expected to be
complete. The residual signal after 2 ns clearly differs for all
samples. In the case of the samples with mainly semi-crystalline
copolymer content about 90% of the initial PIA signal remains,
in the sample with 50% of each copolymer the signal decays to
about 70%, and in the mainly amorphous samples the PIA signal
is reduced much more significantly to about 50% and 30% after
2 ns for 90% ba and 100% ba, respectively.
Fig. 2b shows the intensity dependence of the loss of the
ground-state bleach and PIA of sample 5. The signal decay is
significant, but not excitation intensity dependent. This points
to a geminate recombination process reducing the total number of
charge carriers on the nanosecond timescale. It appears that while
excitons are efficiently quenched at the interface, a significant
fraction of the excitons creates coulombically-bound geminate
charge pairs (GCPs) rather than splitting into free and mobile
charges. The GCP states then recombine geminately on a nanosecond timescale. Visual comparison with a single exponential
decay indicates a lifetime of the GCP states of approximately
2 ns. This decay rate is very similar to decay rates observed in
regiorandom or as-cast regioregular P3HT:PCBM blends,36,37
and PCDTBT:PCBM blends,38 indicating that GCP state recombination on the nanosecond timescale is a rather ubiquitous
phenomenon in polymer:fullerene blends.
The GCP dynamics investigated above contain both the
dynamics of interfacial charge-transfer (CT) states as well as
recombination dynamics of separated electrons and holes
which due to an unfavorable morphology do not escape entirely
from the polymer–fullerene interface and eventually experience
geminate recombination. To investigate the effect of polymer
nano-morphology on the dynamics of the interfacial CT states
only, we performed pump–push photocurrent (PPP) experiments
on photovoltaic devices under working conditions.10
Fig. 3a compares the kinetics of CT state recombination
and Fig. 3b shows the relative amount of such states in the
device. It can be clearly seen that the lifetime of the CT states
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Fig. 3 Results of pump–push photocurrent spectroscopy for the photovoltaic devices fabricated using AnE-PVab and AnE-PVba with various
blend ratios. The total AnE-PV : PCBM ratio is 2 : 3 for all samples/devices.
(a) Push-induced photocurrent as a function of pump–push delay. The
transients are normalized to the maximum value at 5 ps, lines are monoexponential guides to the eye convoluted with the response function of
the setup. (b) The ratio between push- and pump-induced currents for
different devices; these values reflect the ratio between the bound chargetransfer states and extracted carriers.
Fig. 2 Panel (a) shows the dynamics of the PIA signal (875–925 nm) of all
five samples in the first 2 ns after excitation at 400 nm. The fluence was
chosen in a way to ensure similar DT/T (and therefore excited state density)
of around 2.5 10 3. The initial signal was chosen to be low enough to
avoid nongeminate recombination during the first two nanoseconds, and
exciton-charge annihilation at early time for the 0% sample. Fluences
were 4.4 mJ cm 2 (0% and 10%), 7.6 mJ cm 2 (50%), 9.3 mJ cm 2 (90%),
11.9 mJ cm 2 (100%). Panel (b) shows the signal decay at multiple fluences
in the region of the ground-state bleach (540–560 nm, red lines) and two
regions in the PIA (green and blue lines) for 100% AnE-PVba. As a guide to
the eye a black line shows a monoexponential decay with a time-constant
of 2.1 ns. The excitation fluences were 2, 7 and 19 mJ cm 2. This illustrates
the significant geminate recombination in this sample.
(summarized in Table 2) shortens dramatically with the decrease
of polymer crystallinity – from 42 ns for 100% semi-crystalline
AnE-PVab to B300 ps for 100% amorphous AnE-PVba.
This can be explained by the weaker electron–hole coupling
in the samples containing the crystalline donor phase, as
polymer crystallization likely enhances the delocalization of the hole.
The observed trend is similar to the TA measurements. However, the
recombination times deduced from the PPP experiments are always
shorter. This indicates that the two techniques probe slightly
different sub-ensembles of charges, with TA being sensitive to a
wider range of weakly and strongly bound geminate charge pairs.
Interestingly, the relative amount of initially generated interfacial CT
states (Fig. 3b) seems to increase with enlargement of the
amorphous donor concentration. This indicates that the aggregated
polymer provides a channel for the hole to escape from the interface,
20332 | Phys. Chem. Chem. Phys., 2014, 16, 20329--20337
Table 2 Lifetime of CT states extracted from mono-exponential fits of
push-induced photocurrent characteristics, compare Fig. 3a
AnE-PVba/%
CT state lifetimes/ns
0
10
50
90
100
1.80
1.20
0.60
0.48
0.25
while in the amorphous material this channel appears to be
inactive. However, we note that changing the copolymer ratio
most certainly also has an additional impact on the aggregation
of the acceptor, as fullerene molecules likely get expelled from
aggregated and polymer-rich regions thereby forming larger
clusters. This in turn can further aid the charge separation
process at the interface.
Even though the early time charge carrier loss, originating
from the recombination of GCP states, can explain a large part
of the quantum efficiency loss observed in devices, we expect
further charge carrier recombination at later, namely ns–ms
times, as the measured quantum efficiency of the devices is still
lower than the total signal loss on the early timescale accounted
for. The additional loss processes on the ns–ms timescale will be
described in the next section.
2.3
Charge recombination on the ns–ls timescale
Transient absorption experiments on the ns–ms timescale were
carried out in addition to the above-mentioned ps–ns experiments.
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Fig. 4a shows a comparison of the transient absorption
spectra of all samples obtained at various delay times after
excitation at 532 nm. In good agreement with the ps–ns
experiments all TA spectra exhibit a ground-state bleach in
the wavelength region up to 620 nm, and a broad photoinduced absorption extending further into the near infrared.
Comparing the signal intensity at 1–2 ns to that at 10–20 ns
reveals a huge loss evident for all samples. Surprisingly this loss
is even more pronounced for the more crystalline samples
containing 0–50% ba, which show better solar cell performance. The spectral signature of the bleach and PIA also varies
between the samples indicating that the amorphous content
has an impact on the nature of the ground and excited-state
transitions and on the way they are affected by the presence of
excited states such as charges. In the following we analyze the
dynamics of the photo-induced absorption more quantitatively
by fitting the intensity dependent recombination to our previously reported two-pool model,36 where the kinetics of two
independent pools of charge-transfer states and free charges
formed on the sub-100 fs time scale are given by dCCT/dt =
kCCT and dCFC/dt = gCFCl+1, where CCT and CFC are the
concentrations of charge-transfer states and free charges, k is
the lifetime of the charge-transfer states, g is the bimolecular
recombination coefficient, and l + 1 is the order of the
recombination.
Fig. 4b shows an overview of the dynamics of the photoinduced absorption integrated between 700 and 900 nm for all
five samples at various excitation intensities ranging from 4.5
to 80 mJ cm 2. Clearly, all samples exhibit intensity-dependent
recombination dynamics indicating non-geminate recombination is the main charge carrier loss channel on the nanosecond
Fig. 4 (a) Nanosecond to microsecond transient absorption spectra of all
5 samples (excitation wavelength 532 nm, excitation intensity 22 mJ cm 2)
at 1–2 ns (black line), 10–20 ns (red line) and 100–200 ns (blue line). All
samples exhibit a significant signal loss in the first 20 ns. (b) Normalized
decay dynamics (ns–ms time-range) of the charge-induced absorption
between 700 and 900 nm for pump fluences ranging from 4.5 to
80 mJ cm 2. The experimental data are shown as coloured dots. At higher
fluences the decay is faster due to accelerated non-geminate recombination of free charge carriers. The solid lines represent fits to our analytical
model of charge recombination as described in the text.
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to microsecond timescale. The solid black lines represent fits
to the above-mentioned model of concomitant recombination of
coulombically-bound charge pairs and spatially-separated (free)
charge carriers. The applicability of this model has previously been
shown for various other polymer:fullerene systems, for instance
P3HT:PCBM,36 PCDTBT:PCBM38 and PCPDTBT:PCBM blends.39
Briefly, the model assumes that after 1 ns, i.e. at the earliest times
observed in the ns–ms TA experiment, the charge carrier population
consists of two pools: geminate charge pairs (GCP) and free (mobile)
charge carriers. The former recombine exclusively via geminate
recombination to the ground state (GS) as there is no additional
driving force for their separation, while the spatially-separated (free)
charge carriers recombine non-geminately. In the model interconversion between the two pools, specifically the dissociation of
GCPs into free charges, is excluded deliberately, since after 1 ns the
initial (primary) photo-generated excitons have either fully separated
into free charges or collapsed into GCPs that cannot dissociate
anymore under the measurement conditions applied in the TA
experiment. We note, however, that we do not exclude the formation
of electron–hole encounter complexes during the non-geminate
recombination of free charges and emphasize that we have very
recently demonstrated their relevance in PCDTBT:PCBM blends.40
The analytical model parameterizes the experimental recombination
data and allows us to extract values for (i) the branching ratio of
geminate versus non-geminate recombination f (which is equal to
the branching ratio between the geminate charge pair state and the
free charge carrier), (ii) the geminate recombination rate constant
kCT-GS, (iii) the non-geminate recombination coefficient g, and
(iv) the order of the non-geminate recombination process l + 1.
A further limitation of the model is that we have to assume that
geminate charge pair states and free charge carriers have equal
absorption cross-sections in the wavelength region used to monitor
the dynamics. However, this assumption is applicable in the present
case, as the spectral shape did not change during the entire ns–ms
measurement.
The recombination dynamics were globally fit across the TA
experiments for all five samples. Except for the initial excitation
density, all parameters were shared among the different intensities for each sample. As we can see in Fig. 4b the experimental
data are well described by the fit.
Table 3 presents the fitting parameters along with their
standard deviations as extracted from the global fit to the experimental data. For a better comparison, the effective (bimolecular)
Langevin recombination coefficient, i.e. g for which l + 1 = 2 at a
charge carrier density roughly corresponding to an illumination
density of one sun (n = 5 1015 cm 3) was calculated. Solving
geff n2 = gfit nl+1 for geff we obtain geff = gfit nl 1 and with
this equation we extract the effective solar cell recombination
coefficients, which are also listed in Table 3.
3. Discussion and implications for
device performance
We begin the discussion with the least efficient sample, which
contains only the amorphous AnE-PVba copolymer as a donor
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Table 3 Fitting parameters along with their standard deviation extracted from a global fit of the photoinduced absorption kinetics (700–900 nm) in the
long delay measurement according to the model described above. For the quality of the fits see Fig. 4b. The parameters were shared for all excitation
intensities of each sample. f is the fraction of non-geminate recombination, g is the non-geminate decay constant, l + 1 is the order of the non-geminate
recombination, k is the geminate recombination rate, and geff is the effective (l + 1 = 2) bimolecular recombination constant at a charge density of 5 1015 cm 3
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Parameter
f
g/(cm3)l s
l+1
k/s 1
geff/cm3 s
1
1
0% ba
10% ba
50% ba
90% ba
100% ba
0.59 0.01
(8 2) 10 30
3.03 0.01
(1.9 0.1) 108
(1.2 0.5) 10 13
0.57 0.01
(7 2) 10 30
3.04 0.01
(1.7 0.1) 108
(1.5 0.7) 10 13
0.51 0.01
(5 3) 10 27
2.99 0.02
(2.4 0.1) 108
(1.8 1.6) 10 11
0.58 0.01
(2.8 1.8) 10 12
2.08 0.02
(4.3 0.3) 108
(5 4) 10 11
0.68 0.01
(1.5 0.4) 10 17
2.39 0.01
(4.2 0.2) 108
(2 1) 10 11
material mixed with PCBM. The fraction f of non-geminate
recombination is highest for this sample with a value of
approximately two thirds. This is a direct consequence of the
fast (sub-)nanosecond CT state decay leading to the situation
that most of the interfacial CT states have already decayed
within the instrument response time of our ns–ms TA setup
and are thus not observed on this timescale anymore. Moreover, in solar cell devices using only the amorphous polymer,
non-geminate recombination is expected to further reduce the
photocurrent, in addition to the significant fraction of geminate
recombination, as a comparably large value of 2 10 11 cm3 s 1
for the effective Langevin recombination coefficient is observed.
This is in line with the low fill factor (only about 0.3) found for
devices using the amorphous polymer (as it is unlikely that CT
states recombining on the sub-nanosecond timescale can be
split by the weak internal electric field of the solar cell, the low
fill factor can probably not be explained by a bias dependence of
free charge generation). Hence, the field dependence of the
photocurrent can be explained by the competition between
extraction of free charge carriers and their non-geminate
recombination.
In the case of the samples containing a higher amount of
the semi-crystalline polymer, we observe that the fraction of
geminate recombination, extracted from fitting the experimental data to the two-pool model, is increased. However,
virtually no GSB and PIA signal decay on the sub-nanosecond
timescale was observed in the TA experiments. Therefore, we
conclude that both interfacial CT states and geminate charge
pairs are longer-lived than in the more amorphous samples.
This is also confirmed by a smaller rate of geminate recombination, with k = 1.7 108 s 1, which equals an inverse rate or
lifetime of 6 ns. The effective Langevin recombination coefficient is about two orders of magnitude smaller than in the
amorphous samples, approximately 1 10 13 cm3 s 1. This
indicates an effective suppression of non-geminate recombination of free charge carriers which in turn shifts the competition
between recombination and extraction in favor of the latter in
an operating solar cell device. This is also in line with the large
fill factors (around 0.7) observed for these devices, indicating
that the free carriers are swept out efficiently even at rather
weak internal fields in a device.
The nongeminate recombination coefficients (effective Langevin
coefficients) determined for the semi-crystalline and amorphous
samples compare well with values previously determined for as-cast
20334 | Phys. Chem. Chem. Phys., 2014, 16, 20329--20337
(and thus amorphous) and annealed (and thus semi-crystalline)
P3HT:PCBM films.36 For the annealed P3HT:PCBM sample geff was
determined to be 2.2 10 13 cm3 s 1, which is one order of
magnitude smaller than geff = 1.5 10 12 cm3 s 1 determined for
as-cast P3HT:PCBM.
The smaller effective Langevin recombination coefficients
of the semi-crystalline samples could be explained by the
additional thermodynamic driving force created between the
amorphous and crystalline polymer domains. The higher
HOMO energy of the latter aids the separation of charge
carriers at the interface and facilitates their transport from
the interface towards the more crystalline bulk polymer as
previously discussed for other polymer:fullerene blends.29 In
the more amorphous samples the larger disorder reduces the
energetic difference between the interface and the bulk or even
cancels it. Therefore charge carriers can approach the interface
more easily thereby also recombining faster.
The analysis of the ns–ms recombination dynamics indicates
that a larger fraction of free charge carriers is left in the
samples with a higher content of amorphous polymer, however,
their extraction appears to be severely limited by the large
Langevin recombination coefficient, i.e. the comparably fast
non-geminate recombination is strongly competing with extraction.
In the samples consisting of mainly semi-crystalline polymer,
geminate charge pairs decay on a timescale of 3–10 ns, resulting
in a greater overall signal loss as compared to the amorphous
samples on the same intermediate timescale. However, free charge
carriers can be extracted more efficiently due to the smaller
Langevin recombination coefficient, i.e. the slower non-geminate
recombination.
The experiments demonstrate that geminate recombination
is a highly relevant charge carrier loss mechanism on the subnanosecond and nanosecond timescales depending on the
blend’s composition. One can further estimate the total geminate charge carrier loss by adding the losses occurring on both
timescales and then compare the total fraction of geminate
recombination to the quantum efficiencies of the different
samples. As a rough estimate, we determine the charge carrier
loss on the sub-1 ns timescale from the height of the DT/T
signal remaining after 1 ns in the short delay measurement (see
Fig. 2a). The geminate charge pairs that recombine on the subnanosecond timescale are strongly-bound and thus do not
contribute to the extracted photocurrent in an organic solar
cell device. The obtained value accounts for the total amount of
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Table 4 Comparison of the ratio of free charge carrier formation and the
maximum external quantum efficiency. The DT/T value after 1 ns is extracted
from the PIA kinetics of the short delay measurement shown in Fig. 2a. This
value accounts for the geminate recombination of geminate charge pairs on
the sub-nanosecond timescale, which is not resolved in the ns–ms TA
measurement due to the limited instrument response. The fraction of free
charge carriers on the long timescale (f) is obtained from the fit described in the
preceding section. Both values are multiplied to obtain the total amount of free
charge carriers and compared to the maximum EQE values (see ref. 34)
DT/T after 1 ns
f
fDT/T (1 ns)
Max EQE
0% ba
10% ba
50% ba
90% ba
100% ba
0.95
0.59
0.56
0.60
0.95
0.57
0.54
0.55
0.8
0.51
0.41
0.45
0.63
0.58
0.37
0.28
0.51
0.68
0.35
0.20
charge carriers (both bound and free) left after 1 ns. This value
is then multiplied with f, the fraction of non-geminate recombination that was extracted by fitting the ns–ms dynamics to the
two-pool model, to get the total fraction of free charge carriers.
The obtained number is a measure of the amount of excitons
separating into free charge carriers that can potentially be
extracted, that is, it is a measure of the internal quantum
efficiency excluding non-geminate recombination processes
on the ns–ms timescale.
Table 4 summarizes the obtained values and compares them
with the maximum EQE value for all samples, which was
measured under bias illumination with an intensity corresponding to one sun. To illustrate the comparison, the values
are also shown graphically in Fig. 5.
The EQE values clearly follow qualitatively the fraction of free
charge carrier formation in all samples. We therefore conclude
that geminate charge pair recombination is the main and
dominant quantum efficiency loss mechanism in these samples.
The amount of geminate charge pair recombination depends on
the blending ratio of the two polymers and clearly increases with
increasing fraction of the amorphous polymer. However, the
estimated fraction of free charge carriers is distinctly higher than
the EQE values in the case of samples containing the highest
fractions of the amorphous polymer (90% and 100% ba) and is
accompanied by a significantly lower fill factor of these devices.
Hence, we conclude that in these samples non-geminate recombination of free charge carriers adds an additional loss channel
further reducing the quantum efficiency at solar illumination
intensities. This is a likely explanation due to the comparably
larger effective Langevin recombination coefficients for higher
concentrations of the amorphous polymer as already discussed
above, which shift the competition between non-geminate
recombination and extraction in favor of the former.
4. Conclusions
The efficiency of the investigated AnE-PV copolymer:copolymer:fullerene ternary blends is largely determined by their photoactive layer
morphology. Blends containing mainly the amorphous polymer
exhibit ultrafast exciton dissociation, but also a large fraction of
sub-nanosecond geminate recombination of interfacial chargetransfer states and bound geminate charge pairs, as charges are
not fully separated. This suggests that high amorphous content in
organic solar cells does not in general lead to the energy cascade
morphologies favorable for charge separation.41 Furthermore, fast
non-geminate recombination of free carriers is observed as demonstrated by our transient absorption experiments, which competes
with charge carrier extraction in a solar cell. Both effects reduce the
power conversion efficiency due to a decreased short-circuit current
and fill factor. In samples that contain larger amounts of semicrystalline polymer we observe diffusion-limited polymer exciton
dissociation due to large polymer-rich domains and an increased
fraction of free charge generation plus significantly reduced nongeminate recombination in turn leading to a higher power conversion efficiency when compared to the more amorphous blends. The
results demonstrate that polymer crystallinity is an important parameter that determines the device performance. Intriguingly, best
device efficiencies are obtained if small amounts of amorphous
polymer are blended into the semi-crystalline polymer, which
increases the open-circuit voltage while keeping the charge generation and recombination parameters similar to those obtained for a
purely semi-crystalline polymer:fullerene blend.
5. Experimental section
Transient absorption spectroscopy
Fig. 5 The ratio of free charge carriers is calculated from the combined
PIA kinetics of the short and long delay measurements and shown on the
left-hand axis (in black). The maximum EQE of the samples (measured with
background illumination of 1 sun) is shown on the right-hand axis (in red).
Note that the values are very similar, indicating that geminate charge pair
recombination is the dominant loss mechanism. For AnE-PVba concentrations of 90% and 100% non-geminate recombination of free charge
carriers adds an additional loss channel at solar illumination intensities.
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Transient absorption (TA) measurements were performed using
a home-built pump–probe setup. To measure in the time range
of 1 ps to 4 ns with a resolution of B100 fs, the output of a
commercial titanium:sapphire amplifier (Coherent LIBRA HE,
3.5 mJ, 1 kHz, 100 fs) was split into two beams. One beam was used
to pump an optical parametric amplifier (Coherent OPerA Solo) to
generate a 1300 nm seed pulse for white-light generation with a
sapphire window in the visible. The other beam was frequency
doubled and used as an excitation pulse (400 nm). The variable
delay of up to 4 ns between the pump and the probe was
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introduced using a broadband retroreflector mounted on a
mechanical delay stage. Only reflective elements were used to guide
the probe beam to the sample to minimize chirp. The excitation
pulse was chopped at 500 Hz, while the white light pulses were
dispersed onto a linear photodiode array which was read out at
1 kHz. Adjacent diode readings corresponding to the transmission
of the sample after an excitation pulse and without an excitation
pulse were used to calculate DT/T. For measuring in the time range
of 1 ns to 1 ms with a resolution of 600 ps, the excitation pulse was
provided using an actively Q-switched Nd:YVO4 laser (AOT Ltd
MOPA) at 532 nm. The delay between the pump and the probe in
this case was controlled using an electronic delay generator (Stanford Research Systems DG535). TA measurements were performed
at room temperature under dynamic vacuum at pressures lower
than 10 5 mbar. For sample preparation thin films of the binary
and ternary blends were spin cast on quartz glass slides.
Pump–push photocurrent spectroscopy
A regenerative 1 kHz Ti:Sapphire amplifier system (Coherent, Legend
Elite Duo) was used to pump a broadband non-collinear optical
amplifier (Clark) and a 3-stage home-built optical parametric
amplifier (OPA) to generate visible pump pulses (B540 nm
central wavelength, 20 nm bandwidth) and infrared push
pulses (2000 100 nm), respectively. B1 nJ pump and B1 mJ
push pulses were focused onto a B1 mm2 spot on the device. The
reference photocurrent from a photodiode was detected at a pump
repetition frequency of 1 kHz using a lock-in amplifier. The push
beam was mechanically chopped at B370 Hz, and its effect on the
photocurrent was also detected using a lock-in amplifier.
Acknowledgements
D.A.M.E., H.H. and F.L. are grateful to the Deutsche Forschungsgemeinschaft (DFG) for funding within the framework of
the priority program 1355 ‘‘Elementary Processes in Organic Photovoltaics’’. F.L. furthermore acknowledges funding of the Max Planck
Research Group by the Max Planck Society and thanks the Chemical
Industry Fund (FCI) for the Hoechst Dozentenpreis of the Aventis
Foundation. C.K. is grateful to the Thüringer Landesgraduiertenschule für Photovoltaik (PhotoGrad) for financial support. I.A.H.
thanks the Humboldt Foundation and the Max Planck Society for
scholarships. A.A.B. acknowledges a VENI grant from the Netherlands Organization for Scientific Research (NWO).
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