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17998 |Phys. Chem. Chem. Phys., 2014, 16, 17998--18003 This journal is ©the Owner Societies 2014
Cite this: Phys. Chem. Chem. Phys.,
2014, 16, 17998
Ternary hybrid systems of P3HT–CdSe–WS
2
nanotubes for photovoltaic applications†
A. Bruno,*
ab
C. Borriello,
a
S. A. Haque,
b
C. Minarini
a
and T. Di Luccio
a
Hybrid heterojunctions of conjugated polymers and inorganic nanomaterials are a promising combination
for obtaining high performance solar cells (SC). In this work we have explored new possible uses of the
WS
2
nanotubes (NTs) both as the only acceptor material blended with a polymer and in ternary systems
mixed with a polymer and quantum dots (QDs). In particular we have spectroscopically investigated
binary blends of poly(3-hexylthiophene) (P3HT) and WS
2
NTs, P3HT and CdSe QDs, and ternary blends
of P3HT, CdSe QDs and WS
2
NTs. We report fluorescence quenching effects of the QD signal in the
P3HT–CdSe–WS
2
system with the increase of NT concentration. Static and time-resolved fluorescence
studies reveal efficient resonant energy transfer from the QDs to the NTs upon photoexcitation. The
evidence of energetic interaction between WS
2
NTs and QDs opens new fields of application of WS
2
NTs and holds very promising potential for improving charge transfer phenomena in the active layer of
hybrid solar cells.
1 Introduction
The elevated cost of silicon-based photovoltaics and depletion
of material stocks have given a strong input to search for new
and less expensive materials for solar energy conversion. A valid
alternative is represented by purely organic or hybrid organic–
inorganic solar cells as they do not require expensive high
temperature processes and can be easily processed on a large
scale.
1
In particular, bulk heterojunctions of conjugated polymers,
as donor materials, and inorganic nanomaterials, as acceptor
materials, can offer numerous advantages. On one hand, poly-
mers having large absorption coefficients permit us to realize
efficient thin film based devices. Polymeric thin films can be
deposited by different solution based deposition techniques
2
on rigid or flexible substrates also with special morphologies
targeted to enhance the cell performances. However, much
effort is being devoted to overcome major limitations of poly-
mer based photovoltaic devices i.e. degradation and limited
time stability. To this purpose air stable polymers, efficient
encapsulation materials and other solutions are under develop-
ment and have been extensively discussed by Jo
¨rgensen’s review.
3
On the other hand, inorganic nanomaterials do not easily degrade
over time
4
providing a longer lifetime to the hybrid blends and
related electronic devices.
5–7
Inorganic quantum dots (QDs) are
the most used inorganic nanomaterials in hybrid blends for
photovoltaics because of their tunable band-gap over visible and
infra-red spectral ranges. In addition, multiple exciton generation
effects occurring in several types of QDs can potentially lead to
high photovoltaic conversion efficiency.
8
Despite these promising properties one of the main factors
limiting the performances of polymer–QD heterojunction solar
cells is the poor charge transport among QDs in blends, due to
the low carrier mobility leading to cell efficiencies of still
around 4%.
9
The low carrier mobility of QDs is often ascribed
to the inefficient charge transfer processes between the nano-
crystals themselves due to the presence of organic capping
agents, which are used during their synthesis in order to avoid
nanocrystal aggregation and the loss of quantum properties.
These organic ligands are usually long alkyl chains that need to be
replaced by shorter ligands after the synthesis through suitable
ligand exchange processes that improve charge mobility while
retaining the good miscibility of the dots in the polymer solution
for a homogeneous blend.
Alternative non-toxic inorganic nanomaterials that do not
require the use of ligands during and after their synthesis are
tungsten disulfide (WS
2
) nanotubes (NTs). Moreover, they can
extend the absorption region of the blend into the infrared
region. Bulk WS
2
is a metal dichalcogenide compound char-
acterized by a direct bandgap at B1.95 eV and a small indirect
bandgap at B1.3 eV.
10
WS
2
nanotubes were firstly synthesized
in 1992
11
and nowadays pure multiwall WS
2
nanotubes are
a
ENEA, Italian National Agency for New Technologies, Energy and Sustainable
Economic Development, P.le Enrico Fermi 1, Portici (NA), Italy.
E-mail: annalisa.bruno@enea.it
b
Department of Chemistry, Imperial College London, South Kensington Campus,
London SW7 2AZ, UK
†Electronic supplementary information (ESI) available: The optical scheme for
the fluorescence upconversion system, fluorescence decays of the P3HT–WS
2
and
P3HT–CdSe blends and the fluorescence peak position vs. QD concentrations.
See DOI: 10.1039/c4cp00594e
Received 10th February 2014,
Accepted 15th July 2014
DOI: 10.1039/c4cp00594e
www.rsc.org/pccp
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available in macroscopic quantities on the market (about
0.5 kg per day).
12
WS
2
nanotubes are known mainly for their
excellent mechanical behavior
13
and reinforcing properties of
polymer composites
14
based on resins,
15
thermoplastic polymers,
16
and elastomers.
17
Just recently the first nanocomposites of WS
2
NTs and conductive polymers have been reported. They have
shown to increase polyaniline conductivity by efficient doping
18
and preserve OLED device performances until high percentage
loading(about10%).
19
Anyway,theiruseinhybridblendsisquite
challenging and unexplored.
In this work we have investigated the possibility of using
WS
2
both as the sole acceptor material blended with a polymer
and in systems of a polymer and QDs to improve the perfor-
mances of this better known system. We report a series of both
static and time-resolved spectroscopic studies on binary blend
systems, composed of poly(3-hexylthiophene) (P3HT) and WS
2
NTs (P3HT/WS
2
), and P3HT and cadmium selenide (CdSe) QDs
(P3HT/CdSe), and a ternary blend system (P3HT/CdSe/WS
2
). The
ternary blends were prepared keeping a fixed ratio between
P3HT and CdSe, and varying the concentrations of WS
2
nano-
tubes in a range between 9 and 33 wt% with respect to the total
weight of the blend.
2 Methods
2.1 Materials
The P3HT polymer was purchased from Sigma-Aldrich, WS
2
nanotubes from NanoMaterials Ltd, and CdSe QDs (absorbing
at 620 nm) from NN-Labs. All the solvents were furnished by
Sigma-Aldrich and used as received.
2.2 Binary and ternary blend preparation
WS
2
nanotubes were purified and disagglomerated before use.
1 g of material was dispersed in 1 l of 2-propanol and sonicated
for 2 hours at low power. Successively they were centrifuged at
1500 rpm for 15 minutes, and the supernatant was collected
and dried in a vacuum to give purified nanotubes. To prepare
the P3HT–WS
2
binary blends, P3HT was dissolved in chloro-
benzene at the concentration of 20 mg ml
1
and the solution
was stirred for 15 minutes at 60 1C and for one hour at room
temperature. It was then filtered through a 0.2 mm PTFE filter
and mixed with different amounts of WS
2
chlorobenzene
suspension (20 mg ml
1
) to obtain blends with WS
2
loading
of 5, 10, 25, 50, and 75 wt%.
The QDs were provided in toluene solution by the company.
In order to remove the excess ligand they were treated before
usage. Indeed after precipitation and washing in acetone, they
were dried and re-dissolved in chlorobenzene at a concentration
of 20 mg ml
1
. P3HT–CdSe binary blends were prepared by
mixing different amounts of CdSe QD chlorobenzene solution
with the polymer solution to obtain blends with QD loading of 25,
50, and 70 wt%. The resulting mixtures were stirred for 2 hours at
room temperature to obtain homogenous nanocomposites.
For the preparation of ternary blends we followed a two step
process. First a fixed volume of CdSe QD chlorobenzene solution
(20 mg ml
1
) was mixed with the appropriate amount of solid WS
2
nanotubes to obtain the CdSe : WS
2
weight ratio of 1:0.2, 1:0.5,
1:0.75, and 1:1. The mixtures were sonicated for 40 minutes.
Then the same volume of chlorobenzene polymer solution at
the same concentration (20 mg ml
1
) was added to each new
solution. In this way it was possible to maintain the P3HT:CdSe
weight ratio constant while varying the weight concentration of
the nanotubes in the ternary mixture (9, 20, 27, and 33 wt%).
All the solutions were deposited by spin coating on glass
substrates at 1000 rpm for 30 s, at room temperature. The
substrates were pre-cleaned by sonication in d.i. water and the
deconex detergent, acetone and subsequently in 2-propanol.
The films were homogeneous and their thickness was in the
range of 80–100 nm as measured using a Tencor profilometer.
2.3 Optical measurements
The pure polymer and the nanocomposite blend layers were
characterized by UV-Vis absorption spectroscopy using a Perkin
Elmer Lambda 900 Spectrophotometer. Photoluminescence
emission (PL) of the same films was measured using a Fluorolog
3 instrument, Horiba Jobin Yvon Instruments SA.
The ultra-fast fluorescence emission experiments were per-
formed using a femtosecond laser based system. The Second
Harmonic output of a mode-locked Ti:Sapphire oscillator with the
wavelength fixed at 800 nm was used as the excitation beam. The
pulse duration was 70 fs and the repetition rate 80 MHz. A portion
of this fundamental beam was frequency doubled to create the
excitation beam with a wavelength of 400 nm. Fluorescence from
the sample was focused on a beta barium borate (BBO) crystal
along with the 800 nm fundamental beam. The fluorescence
emitted at different emission wavelengths was mixed in the BBO
crystal with the gate beam to generate sum frequency photons,
which were detected using a photomultiplier tube. Data were
acquired using Lab-View software and subsequently analyzed.
The temporal reconstruction of the signal was obtained through
a micrometrical precise delay line on the gate beam. The resolu-
tion of the system was measured to be 150 fs. Sample degradation
was avoided by performing the measurements under flowing
nitrogen and using a translation stage to move the sample within
the beam, removing the effect of photobleaching and providing
data averaged across the whole sample. The system has been
already described in previous studies
20,21
and the optical scheme is
reported in the ESI†section (Fig. S1).
3 Results and discussion
In P3HT–WS
2
blends we have investigated the possibility of
using WS
2
NTs as an electron acceptor due to the favorable
band alignment between the polymer and the NTs. In Fig. 1(a)
the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels are reported both
for WS
2
and P3HT.
10,22
Fig. 1(b) shows the UV-visible absorption spectra of P3HT/
WS
2
nanocomposites with an increasing amount of WS
2
NTs,
ranging from 5 to 50 wt%, together with those of pristine P3HT
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and WS
2
films for comparison. As expected the absorption
signal between 400 and 630 nm, mainly due to the polymer,
reduces as a function of the NT concentration because the
relative amount of P3HT in the blend decreases accordingly.
Moreover, a clear effect of the NT addition is an increase of the
absorption of nanocomposites in the region between 630 and
800 nm, around the maximum absorption of NTs.
The emission spectra of pure P3HT and P3HT/WS
2
nano-
composite films are shown in Fig. 1(c). All the emissions have
been collected, after excitation at 400 nm, in whole visible and
near infrared regions. The fluorescence signals of the blends
are essentially dominated by the polymer contribution, since
the NTs are not emissive, being an indirect bandgap material.
23
The fluorescence spectra have been corrected by the number
of photons absorbed at 400 nm where the sample was excited.
It is clear that the fluorescence intensity of the P3HT is just very
slightly quenched by the presence of the WS
2
NTs indicating
that no transfer mechanism is taking place between the two
materials in the blends. Indeed, the quenching values are quite
small (around 10%) for all NT concentrations. This suggests
that such small variation can be attributed more to the different
thickness and uniformity of the films and/or experimental
uncertainty than to a real quenching effect. This result has
also been confirmed by the time resolved measurements per-
formed on all the blends showing that the lifetime of the
polymer does not change due to the presence of the NTs. These
data are reported in the ESI.†
An alternative way to exploit the properties of WS
2
NTs could
be using them in ternary blends of polymer and inorganic QDs.
In previous studies ternary hybrid systems composed of polymers
blended with carbon nanotubes (CNT) and QDs have shown
interesting physical phenomena such as long-lived charge transfer
and luminescence quenching via energy transfer and reduced
blinking.
24
In these systems the NT backbone is expected to
promote carrier mobility
25
and QD dispersion that are both very
relevant factors for an efficient bulk heterojunction photovoltaic
solar cell.
Starting from these observations, we wish to explore the
spectroscopic properties of the ternary blend system P3HT–
QDs–WS
2
, with the aim of testing its application as an active layer
in hybrid solar cells with respect to the binary system P3HT–CdSe.
P3HT–CdSe hybrid blends are among the most studied
hybrid nanocomposites where the polymer photoluminescence is
quenched by the CdSe quantum dots.
26,27
Nevertheless, there is
room for addressing some critical points in this binary system. In
particular the efficiency of charge separation at the donor/acceptor
interface is crucial to the photocurrent generation in hydrid solar
cells, and a complete understanding of this process is essential to
optimize it.
28–30
In order to follow this route, different amounts of
WS
2
nanotubes have been added to the P3HT–CdSe blends. The
idea is to facilitate the charge generation at the interface and make
possible the energy transfer between the QDs and the NTs. In our
case, upon adding NTs, P3HT is the donor material and both CdSe
QDs and WS
2
NTs could act as the acceptors.
In the framework of Fo
¨rster theory of fluorescence resonance
energy transfer, the energy transfer phenomenon is mediated
by a long-range dipole–dipole interaction between donor and
acceptor molecules. The occurrence of an energy transfer event
requires spectral overlap of the donor emission spectrum with
the acceptor absorption spectrum for energy conservation.
The maximum distance over which Fo
¨rster energy transfer can,
typically, occur is 30–50 Å.
31,32
Fig. 2(a) clearly shows that the alignment of the energy levels
of P3HT, CdSe and WS
2
is very favorable for this kind of
transfer. In the same way CdSe QDs and P3HT emission and
WS
2
nanotube absorption peaks, Fig. 2(b), nicely overlap satis-
fying the requirement for Fo
¨rster energy transfer. Indeed the
WS
2
NTs can in principle efficiently absorb at around 650 nm
both the less intense light emitted by the P3HT and the more
intense light emitted by the CdSe QDs. Because the NTs are not
emissive the occurrence of this energy transfer could not be
Fig. 1 (a) P3HT and WS
2
energy levels. Pure P3HT, WS
2
and P3HT–WS
2
blends. (b) UV-Vis absorption spectra and (c) emission spectra after excitation
at 400 nm. The intensity of the excitation beam is kept constant for all the
measurements and the data are corrected for the number of absorbed
photons.
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probed directly by monitoring the WS
2
emission as is usually
done.
10,33
In an equivalent way we have proceeded to monitor
the changes of the static and dynamic emission of the QDs
induced by the presence of different amounts of NTs.
In Fig. 3(a) the normalized static emissions of the pure QDs,
pure P3HT and the P3HT–CdSe blends are reported. For the
blends and the polymer the normalization was done with
respect to the P3HT peak emission at 720 nm. This value has
been chosen because at this wavelength there is no contribu-
tion from the QDs or the nanotubes. As expected, the emission
from the QDs becomes more pronounced as their concen-
tration in the blends increases, as indicated by the arrow in
Fig. 3(a) even if the total emission is quenched (data are reported
in the ESI†section).
26–35
Interestingly, upon increasing the QD
concentrations in the blend the emission peak linearly blue shifts
from 660 nm, the position of the pure P3HT peak, to 640 nm,
where the bare CdSe emission is centered (data are reported in
the ESI†section). At this point we fixed the CdSe amount with
respect to the P3HT in the blend at 50 wt% and varied the
NT concentration. In Fig. 3(b) the emission spectra of the
P3HT–CdSe blend without NTs (namely sample CdSe–WS
2
0)
together with all the ternary blends P3HT–CdSe–WS
2
containing
different amounts of nanotubes (from 9 to 33%), but the same
amount of polymer and QDs, are reported. Also in this case the
spectra are shown to be normalized with respect to the polymer
emission at 720 nm. It is clear that the QD emission peak is
strongly reduced by the presence of the nanotubes. This result
suggests that the emission of the QDs is reabsorbed by the NTs
and so an energy transfer process is efficiently taking place
between the CdSe and the WS
2
nanotubes. Therefore, following
the exciton dynamics is a powerful method for evaluating these
processes inside the blends. In order to probe the nature of this
transfer in greater detail and evaluate the quenching effects,
also in the lifetime of the exciton, time resolved fluorescence
measurements have been performed on the ternary blend samples
containing different amounts of WS
2
nanotubes. Energy transfer
processes occur typically on time scales ranging from picoseconds
to nanoseconds for singlet energy transfer.
The study of the fluorescence signals in the femtosecond (fs)
to picosecond (ps) time range allows following the dynamics of
the fluorescent excitons generated immediately after absorption
right up until charge separation. The fluorescence decay time
strongly depends on the nanocomposition of the blends and the
energy transfer process inside the blends.
The emission decays have been measured exciting the
sample at 400 nm and collecting the fluorescence decays at
650 nm where the contribution of both the polymer and the
QDs is present. The fluorescence decays for the ternary blend
samples are reported in Fig. 4(a) with WS
2
contents varying
from 9 wt% to 33 wt% with respect to the P3HT–CdSe 50 wt%
blend (indicated as CdSe–WS
2
_0). Pristine P3HT has also been
reported for clarity.
It can be observed that the fluorescence decays are faster in
the ternary blends with respect to pure P3HT, and the lifetimes
decrease for increasing concentrations of nanotubes in the blends.
Fig. 2 (a) P3HT, CdSe and WS
2
energy levels and (b) CdSe and P3HT
emission spectra and WS
2
absorption peak. Fig. 3 Emission spectra normalized at 720 nm after excitation at 400 nm
of (a) P3HT–CdSe blends and (b) P3HT–CdSe–WS
2
blends.
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It appears that for concentrations higher than 27 wt% the
values of lifetimes reach a saturation level.
The kinetics is well described by a double exponential decay,
a fast and a slow phase correlated with different phenomena as
described in ref. 34. Indeed the emission at this wavelength
(650 nm) is given by a contribution from both CdSe QDs and
P3HT polymer emission. The results of the double fitting procedure
(starting from the maximum of the signal, thus excluding processes
occurring within the response function) are reported in Fig. 4(b).
Lifetimes rapidly decrease as a function of nanotube concentration
from about 100 to 40 ps. These results indicate that the quenching
effect is most probably due to an energy transfer between the QDs
and the WS
2
nanotubes.
4 Conclusions
In this work we have reported unprecedented fluorescence
quenching effects in the hybrid ternary system P3HT–CdSe–WS
2
due to WS
2
NT addition to P3HT–CdSe QD blends. Static and
time-resolved fluorescence decays give a strong indication of an
efficient energy transfer process from the QDs to the WS
2
NTs in
the ternary blend, responsible for fluorescence quenching. On the
other hand, in the binary blend P3HT–WS
2
no fluorescence
quenching has been detected suggesting the absence of energy
or electron transfer processes in this system. The evidence of
energetic interaction between WS
2
NTs and QDs, observed in
purely inorganic systems,
23
and here established for the first time
in hybrid nanocomposites with P3HT opens new possible fields of
application to metal dichalcogenide nanomaterials, which are
non-toxic and nowadays available in large amounts. Moreover,
our results provide new insights into the physics of hybrid blends
based on conjugated polymers and QDs and promising improve-
ments of the corresponding hybrid solar cell performances.
Acknowledgements
The authors wish to thank Prof. R. Tenne for useful discussion
and suggestions and for reading the manuscript. MIUR Public–
Private Laboratory Project (PO2_00556_33069378 RELIGHT) and
the COST Action MP0902 Composites of Inorganic Nanotubes
and Polymers (COINAPO) are acknowledged for partial financial
support.
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