300
Rev.Adv.Mater.Sci. 10 (2005) 300-305
Emmanuel Kymakis and Gehan A.J. Amaratunga
CARBON NANOTUBES AS ELECTRON ACCEPTORS IN
POLYMERIC PHOTOVOLTAICS
Emmanuel Kymakis1,2 and Gehan A.J. Amaratunga2
Electrical Engineering Department, Technological Educational Institute of Crete, Estavromenos, P.B. 1939,
Heraklion, GR-71004, Crete, Greece
2
Engineering Department, University of Cambridge, Cambridge CB2 1PZ, UK
1
Received: April 27, 2005
Abstract. Photovoltaic properties of functionalised single-walled carbon nanotubes (SWNT)conjugated polymer, poly(3-octylthiophene) (P3OT), blend composites are reported. Devices
were fabricated by spin cast from the solution of composite onto ITO coated glass. The carbon
nanotubes were functionalised in order to increase their solubility and dispersion. Diodes (ITO/
PEDOT:PSS/P3OT-SWNTs/Al) with low nanotube concentration (1%) show photovoltaic behaviour, with a short circuit current of 0.25 mA/cm2, an open circuit voltage of 0.75 V and a fill factor of
0.48, resulting in an AM 1.5 power conversion efficiency of 0.1%. It is proposed that the photovoltaic response of the device is based on the introduction of internal polymer-nanotube junctions within the polymer matrix, which due to a photoinduced electron transfer from the polymer
to the nanotube contribute to enhanced charge separation and collection. It is shown that the
carbon nanotubes represent an alternative class of electron acceptor materials for applications
in polymeric photovoltaics.
1. INTRODUCTION
Organic photovoltaic devices have attracted much
attention since their discovery [1], due to their potential of creating low cost solar cells in large area,
flexible and remarkably light substrates [2,3].
Recent research demonstrates that the photovoltaic properties of a conjugated polymer, which is
normally an electron donor, can be drastically enhanced by being blended with another polymer or
an organic molecule with different electronic structures. Having a mixture of donor and acceptor material in these devices, charge separation is achieved
due to a band offset at the interface, and collection
because of the existence of a bi-continuous network along which electrons and holes can travel
through the electron acceptor and the electron donor, respectively towards their respective contacts.
That is, the electrons are energetically liable to move
into the phase with greater electron affinity (acceptor), while the photoinduced holes are energetically
liable to move into the polymer phase with smaller
ionization potential (donor). By carefully controlling
the morphology of the D and A phases, one can
achieve high interfacial area within a ‘bulk D/A
heterojunction’ material. A bicontinuous network
morphology provides pathways with the ability to
collect the separated carriers at external electrodes.
Since the discovery of photoinduced charge
transfer between conjugated polymers (as donors)
and buckminsterfullerene C60 and its derivatives (as
Corresponding author: Emmanuel Kymakis, e-mail: [email protected]
© 2005 Advanced Study Center Co. Ltd.
Carbon nanotubes as electron acceptors in polymeric photovoltaics
301
Scheme 1. Molecular structures of SWNTs used as acceptors in this study.
acceptors) [4], several efficient photovoltaic systems
based on the donor-acceptor principle using a combination of polymer and fullerenes have been fabricated [5]. In this respect, the charge transfer mechanism in a polymer matrix containing fullerene provides the motivation for investigating the use of the
longest fullerene molecule, the carbon nanotubes
as the electron transport material. The nanotubes
consist of one (singlewalled) or more (multiwalled)
sheets of graphite wrapped around each other in
concentric cylinders (Scheme 1). Individually, they
could be metallic or semiconducting depending on
their chirality and diameter, making them ideal reinforcing fillers in composite materials [6,7].
In previous investigations [8,9], we demonstrated
that the introduction of a small amount of carbon
nanotubes in the polymer matrix, significantly enhance the photovoltaic properties of the device. This
effect was attributed to the formation of internal polymer/nanotube junctions resulting in better exciton
dissociation and balanced bipolar transport throughout the entire volume of the polymer-nanotube composite.
Aiming at evaluating the role of the carbon
nanotubes as electron acceptors in organic photovoltaics, a systematic analysis is taken place. In
this paper, solar cells based on composites of poly(3octylthiophene) (P3OT) acting as the photoexcited
electron donor is blended with SWNTs, which act
as the electron acceptors. Efficient exciton dissociation is utilized at the polymer/nanotube interface.
Charge transfer then follows by the transport of electrons through the nanotube length to the electron
contacting contact (Al) and holes through the polymer to the hole collecting contact (ITO).
2. EXPERIMENTAL DETAILS
The single-walled nanotubes (SWNTs) were produced by the arc discharge method (Carbolex) us-
ing a Ni–Y catalyst with a purity of 60%, as estimated by transmission and scanning electron microscopy [9,10].The arc prepared (AP) ASWNTs consist of single-walled, closed-ended, carbon
nanotubes. These nanotubes have a diameter 1.4
nm and are bundled together, forming 1-3 µm long
ropes, with diameters in the range of 10-20 nm. Impurities include approximately 30 wt.% residual catalyst metal nanoparticles, which are encapsulated in
carbon shells and amorphous carbon typically found
on the outer surfaces of the ropes.
Purification of this material was performed using
the hydrothermal (HIDE) method [11]. The first step
involves refluxing 100 mg of the raw soot in distilled
water for 12 hours, followed by filtering and drying.
This treatment removed some of the graphitic particles and amorphous carbon. Next, the soot was
heated at 470 °C in air for 20 minutes, with the aim
of oxidising more of the amorphous carbon and removing the graphitic covering from the metal particles. Finally, the remaining soot was treated with
hydrochloric acid in order to dissolve all the metal
particles.
The photovoltaic devices reported were fabricated
on 25 mm by 25 mm indium–tinoxide (ITO) glass
substrates with a sheet resistance of about 10 Ω/
sq. The device configuration of a polymer-nanotube
dispersed heterojunction photovoltaic cell is shown
in Scheme 1, as well as the materials used. As a
buffer layer, a PEDOT:PSS, poly (ethylenedioxythiophene) doped with poly(4-styrenesulfonate), purchased from Bayer AG, was spin cast
from an aqueous solution (0.5 wt.%, PEDOT: PSS
2:3) on the ITO substrate, giving an average thickness of 40 nm layer. The PEDOT:PSS layer improves
the quality of the ITO electrode, the surface roughness of ITO is minimized and the resistance between the photoactive layer and the ITO is expected
to be decreased. The photoactive layer was prepared
by spin coating of P3OT-SWNT solution in chloro-
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Emmanuel Kymakis and Gehan A.J. Amaratunga
form (SWNT concentration: 1 wt. %) to a thickness
of ~100 nm on the PEDOT:PSS buffer layer.
Current–voltage (I–V) measurements were performed at room temperature using a HP 4140B voltage source-monitor unit. For photovoltaic characterization the cells were illuminated with 100 mW/
cm2 power intensity of white light by a solar simulator (active area ~5 mm2) through the glass/ITO side.
The calculation of the power conversion efficiency,
η has been performed using the equation:
η=
Voc I sc FF
Pin
,
where Voc, Isc, FF and Pin are the open circuit voltage, the short circuit current density, the fill factor
and the incident light power. The value of the fill factor (FF) of the device is determined from the maximum power point (Vmax, Imax) in the 4th quadrant of
the I-V characteristics with the maximum electrical
power according to:
FF =
Vmax Imax
Vos I sc
.
3. RESULTS AND DISCUSSION
All the photovoltaic cells were produced in a sandwich geometry as mentioned above, i. e. between
two metal electrodes with different work functions,
ITO (ΦITO= 4.7 eV) as anode (hole collector) and
aluminium (ΦAl=4.3 eV) as cathode (electron collector). The Fermi level energies for the electrodes
(ITO, Al) and highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) levels relative to vacuum level for the
PEDOT:PSS, P3OT and SWNTs are indicated in
Fig. 1. Considering the energy diagram, it is clear
that the heterojunction, formed at the interface between the P3OT and the SWNTs, should function
as a diode with rectifying current–voltage characteristics. The inherent polarity of the device results
in very low reversed bias current densities. Electron
transfer onto the nanotubes and hole transport from
the P3OT are energetically favourable and this creates conducting species on both sides and relatively high current densities under forward bias.
In a short circuit condition (Fig. 1b), the negative
electrode will form ohmic contacts to the nanotube
percolation paths and the positive electrode to the
polymer [12], this implies that the Fermi energy is
constant throughout the system. In this way, the
built-in potential of the device is the built-in potential of the polymer–nanotube junction. Therefore, the
upper limit of the Voc (built-in potential Vbi of the
polymer–nanotube junction), must be equal to the
difference in the P3OT HOMO and the SWNTs work
function, which is 0.75 eV. Under illumination, electrons and holes travel to opposite contacts due to
an internal electric field. Thereby, a collection of a
short circuit photocurrent is guaranteed.
The current-voltage characteristics of the ITO/
PEDOT:PSS/P3OT-SWNTs/Al device with 1 wt.%
of SWNTs under white light illumination (AM 1.5,
100 mW/cm2) from the ITO side, is depicted in Fig.
2. Forward bias is defined as positive voltage ap-
Scheme 2. Schematic description of a polymer-nanotube heterojunction solar cell.
Carbon nanotubes as electron acceptors in polymeric photovoltaics
303
Fig. 1. Potential energy diagrams relative to vacuum level of a P3OT-SWNT bulk heterojunction under (a)
flat band conditions and (b) under short circuit conditions, assuming no interfacial layer at the metal contacts and pinning of the Al and ITO to the energy states of the polymer and SWNT, respectively.
plied to the ITO electrode. The composite device
shows short-circuit photocurrent density (Isc) of 0.25
mA/cm2, an open-circuit voltage (Voc) of 0.75 V and
a fill factor of 0.48, while the pristine just P3OT device shows Isc =0.7 µA/cm2, Voc=0.35V and FF=0.3
[8]. The power efficiency of the blend device is dramatically increased from 2.5 10-5 to 0.1% with respect to the pristine one. Thus, an enormous improvement of photovoltaic effect is observed upon
the use of nanotubes as electron acceptors. The Isc
in the composite device is larger than that in the
pristine device by about two orders of magnitude.
Moreover, the Voc and the FF in the composite device are also significantly larger than those in the
pristine diode. It is proposed that the enhancement
in the photovoltaic properties of the composite device is due to the introduction of internal polymer/
nanotube junctions within the polymer matrix. These
junctions act as dissociation centers, which are able
to split up the excitons and also create a continuous pathway for the electrons to be efficiently transported to the negative electrode. This results in an
increase in the electron mobility and hence balances
the charge carrier transport to the electrodes.
According to the polymer–nanotube composite
percolation characteristics [6], the composite keeps
its insulating properties only for weight fractions
between 0.1% and 5%. Photodiodes were constructed by varying the nanotube concentration for
Fig. 2. I–V characteristics of the 1 wt.% device in
the structure of an ITO/PEDOT:PSS/P3OT:SWNTs/
Al photovoltaic cell under simulated solar light (AM
1.5, 100 mW/cm2).
0.25%, 0.5%, 1%, 2%, 3%, and 5%. The active
layer thickness was the same in each case. The
short circuit current was found to be significantly
dependent of the nanotube concentration. Fig. 3
shows the dependence of Isc and Voc on the weight
percentage of nanotubes in the blend. Four or more
samples were measured in each case using identical preparation procedures. The error bars represent the scatter of the experimental values and ob-
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Emmanuel Kymakis and Gehan A.J. Amaratunga
that there are bi-continuous conducting paths to
provide percolation of electrons and holes to the
appropriate electrode. However, as the nanotube
content further increases, the photocurrent decreases, confirming the proportion of collected photons decreases, indicating that the nanotubes do
not contribute to the photocurrent.
4. CONCLUSIONS
Fig. 3. Short circuit current and open circuit voltage
dependence on carbon nanotubes concentration.
Error bars represent the scatter of at least four measurements for each nanotube composition.
tained from at least four samples. The wide range of
the error bars suggest that the distribution of the
junctions through the thickness of the film is not
consistent in the four samples, since it is impossible to efficiently control the nanotube orientation
within the polymer matrix. Therefore, the internal
heterojunctions are expected to be randomly distributed, resulting in a partial discontinuity of the
donor or acceptor phases. Furthermore, the likely
increased energy level disorder in both phases may
result in an increase of the charge trap density and
in a reduction of the electron and hole mobilities.
Therefore, a degradation of the overall device performance should be expected. The photocurrent increases with increasing nanotube concentration of
up to 1% and then decays. On the other hand, the
open circuit voltage increases until 1% and then
tends to saturate at higher concentrations. The
maximum efficiency was obtained from the composite containing 1% of carbon nanotubes. For
higher concentrations, the photocurrent is believed
to be limited due to a lower photogeneration rate,
since the exciton generation takes place only in
the polymer.
The maximum intensity of the emitted solar energy occurs at a wavelength of about 555 nm (2.2
eV), which falls within the band of green light. Upon
increase in nanotube concentration, the distance
between individual nanotubes becomes smaller than
555 nm, which results in a significant decrease in
the absorption, and thus in the photogeneration rate.
It can be concluded that there are sufficient interfaces to ensure efficient exciton dissociation and
In this paper, we review the prospect of using carbon nanotubes as the electron acceptor materials
in polymer based solar cells. A photovoltaic device
based on SWNTs and a conjugated polymer, P3OT
is demonstrated. The operating principle of this device is that the interaction of the carbon nanotubes
with the polymer, allows charge separation of the
photogenerated excitons in the polymer and efficient
electron transport to the electrode through the
nanotubes. SWNTs doping was found to dramatically improve the photovoltaic performance of P3OT
devices revealing a photocurrent larger than two orders of magnitude compared to that of the pristine
diodes, and a doubling of the open circuit voltage.
These results suggest that the metal negative electrode forms ohmic contacts to the nanotube percolation paths. It is believed that the Voc can be estimated from the energy differences between the work
function of the nanotubes and the HOMO of the polymer.
The results show that the conjugated polymerSWNTs composite represents an alternative class
of organic semiconducting material that can be used
to manufacture organic photovoltaic cells with improved performance. Further improvements in device performance are expected with more controlled
film preparation and polymer doping.
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