1) The document summarizes organic solar cells, which use a bulk heterojunction of a conjugated polymer donor and fullerene acceptor. When light is absorbed, excitons are formed that must dissociate at the donor-acceptor interface into free charges. 2) The bulk heterojunction morphology, consisting of an interpenetrating network of the donor and acceptor materials, allows more excitons to dissociate since the interface is throughout the volume. This leads to higher efficiencies than simple bilayer cells. 3) Efficiencies of over 6% have been achieved but further work is needed to improve stability and lower costs for organic solar cells to become commercially viable. Optimization of

1. BΦ – Belgian Physical Society Magazine
FEATURED ARTICLE
ORGANIC SOLAR CELLS: THE EXCITING INTERPLAY OF
EXCITONS AND NANO-MORPHOLOGY
K. Vandewal, L. Goris, G. Krishna & J.V. Manca
Universiteit Hasselt, Instituut voor Materiaalonderzoek, Wetenschapspark 1,
B-3590 Diepenbeek
jean.manca@uhasselt.be
Photovoltaic energy conversion in nanostructured organic donor:acceptor bulk
heterojunctions is a very promising concept towards future renewable energy generation.
This article provides a brief introduction into the field of organic ‘excitonic’ solar cells.
1. Organic electronics
In 1990 researchers from the Cavendish
Laboratory in Cambridge (UK) discovered that
a thin layer of the conjugated polymer Poly(p-­‐‑
phenylene vinylene) sandwiched between a
hole-­‐‑injecting electrode (transparent ITO) and
an electron-­‐‑injecting electrode (e.g. aluminium)
yielded light emission under voltage bias1. The
injected electrons and holes meet in the bulk of
the polymer film and emit light as the result of
radiative charge carrier recombination. The
discovery of electroluminescence in polymer
films was rapidly followed by a wave of
breakthroughs in the development of light
emitting diodes, thin film transistors, (bio-­‐‑)
sensors and solar cells based on organic
materials, e.g. conjugated polymers or small
organic molecules.
Conjugated polymers possess a delocalized π-­‐‑
electron system along the polymer backbone.
In general they are constructed from aromatic
units and/or multiple bonds alternating with
single bonds. The overlap of adjacent atomic
pz-­‐‑orbitals yields lower energy bonding (π)
and higher energy anti-­‐‑bonding (π*) molecular
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orbitals. The difference in energy between the
highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital
(LUMO) – as in inorganic semiconductors
termed bandgap (Eg) – is typically between 1-­‐‑4
eV. The chemical structures of some the most
known conjugated polymers is shown in
Figure 1.
Figure 1: Chemical structures of
several common conjugated polymers:
poly(acetylene)
(PA),
poly(aniline)
(PANI), poly(pyrole) (PPy), poly(pphenylene)
(PPP),
poly(pphenylenevinylene)
(PPV)
and
poly(thiophene) (PT).
Conjugated polymers combine properties of
classical macromolecules, such as low weight,
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good mechanical behaviour and an easy
processing with (semi)-­‐‑conductor properties
arising from their electronic structure. From a
technological point of view, these polymers
yield a potential to develop a large-­‐‑scale and
low cost, roll-­‐‑to-­‐‑roll production of solid state
electro-­‐‑optical devices on flexible substrates
using wet-­‐‑solution processing techniques such
as spincoating, screenprinting or inktjet
printing.
The scientific community has explicitly
acknowledged the importance of this class of
materials by awarding the pioneers in this field
Alan Heeger, Alan MacDiarmid and Hideki
Shirakawa with the year 2000 Nobel Prize for
chemistry. In the seventies, they observed an
increase in conductivity by several orders of
magnitude for a poly(acetylene) film, oxidized
with iodine vapour2.
2. Organic solar cells
As compared to inorganic materials used in
solar cells nowadays (e.g. silicon), typical
organic small molecules and conjugated
polymers have high absorption coefficients. A
100 nm thick device of such a material is
sufficient to absorb virtually all the light with
energy higher than its optical gap. Therefore it
is no surprise that already in the beginning
days of photovoltaic research, people have
attempted to prepare devices from strongly
absorbing organic materials3. The power
efficiency η of single layer organic materials
sandwiched between two electrodes however,
is disappointing (η < 1 %)4. This originates
from the low dielectric constant of organic
materials, causing the optical excitations to
consist of an electron and hole which are still
mutually attracting – termed as excitons-­‐‑, with
a typical binding energy of 0.5 eV5. This
binding energy is much too large for the
internal fields in the device to break the
excitons within their ~1 ns lifetime. This causes
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organic solar cells consisting of a single
organic material sandwiched between two
electrodes to generate low photocurrents
resulting in low overall performances.
Figure 2-­‐‑a : Schematic representation of architecture of bulk
heterojunction solar cell.
A breakthrough came in 1985 when Tang6
presented a two layer organic photovoltaic
device with a power conversion efficiency η of
~1%. In such bilayer devices, the interface
between the two organic layers is crucial in
determining its photovoltaic properties.
Excitons created in either of the two material
phases are dissociated at the interface. The
material in which the electron ends up after
dissociation is named the electron acceptor,
accepting the electron from the donor material.
Today, the bilayer cell concept is still used for
devices using evaporated organic small
molecules7.
One of the most successful and most studied
electron accepting material is the C60
buckminsterfullerene. The discovery of
ultrafast (~100 fs) electron transfer between
C60 and conjugated polymers8 stimulated
interest in these systems for photovoltaic
applications. In bilayer devices comprising
conjugated polymers and C60, however, only
excitons created within their diffusion length
from the interface, can contribute to the
photovoltaic effect. For conjugated polymers, a
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typical exciton diffusion length of ~5-­‐‑7 nm is
not sufficient to absorb a large fraction of the
light, in such a bilayer configuration9.
of research on organic photovoltaics therefore
is to improve device efficiency together with
device stability, while keeping the cost of the
technology low.
Figure 2-b : Transmission Electron Miscroscopy (TEM) micrograph of bulk morphology of MDMOPPV:PCBM (1:4 weight fraction) solar cell prepared from respectively toluene (left) and chlorobenzene
(right) solvents, yielding a clear difference in both morphology and in photovoltaic performance.
Today, the highest efficiencies reached using
this approach are about 6 % by using the
polymer PCDTBT as donor material12. The
most successful soluble acceptor materials up
to date are the C60 derivative PCBM (depicted
in Figure 2) and the C70 derivative PC71BM.
PCBM is a weak absorber while PC71BM
contributes to sunlight absorption when used
in polymer:fullerene solar cells. Alternative
electron accepting materials, such as n-­‐‑type
conjugated polymers and inorganic metal
oxides are currently under investigation. With
inorganic metal oxides so-­‐‑called hybrid Dye
Sensitized Solar Cells (DSSC) are being
developed (will be discussed in paragraph 4).
Table 1-­‐‑1 summarizes the confirmed power
conversion efficiencies of several photovoltaic
technologies13. It reveals that, as compared to
the other technologies, the organic solar cells
still have a modest efficiency. One of the goals
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Photovoltaic technology
η (%)
Silicon (Si)
Mono-crystalline
Silicon (Si)
Multi-crystalline
Silicon (Si)
Amorphous
Gallium arsenide (GaAs)
25.0
Copper indium gallium
diselenide (CIGS)
Dye sensitized
19.4
Organic
20.4
9.5
26.1
10.4
5.2
Table 1-­‐‑1: Confirmed submodule power conversion efficiencies
(η) measured on a 1 cm2 cell surface, under the standardized
global AM1.5 spectrum (1000 W.m-­‐‑2) at 25 °C for several
photovoltaic technologies13. The highest efficiency measured
for organic solar cells is 5.2 %. However for cells smaller than
1 cm2, efficiencies higher than 6 % have been reported.12
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3. Working principle
In the past years, many reviews on organic
solar cells have been written (see ref. 14). In
most of them, the following scheme (Figure 3
(a)) is presented, depicting the simplified
mechanism by which the incident photon flux
is converted into an electrical current in
organic donor/acceptor based devices. It has 4
fundamental steps. While the efficiency of the
exciton creation (step 1) and diffusion (step 2)
depend strongly on sample thickness and bulk
heterojunction morphology, the crucial charge
generation mechanism (step 3), is believed to
depend on the energetic interfacial structure
and can be highly efficient in some well
performing BHJ solar cells. However, up to
now, this step is not fully understood and
under vivid discussion. Once the electron on
the acceptor material and the hole on the
donor material have escaped each other’s
evidences indicate that an intermediate charge-­‐‑
transfer (CT) state exists between the excitons
created upon light absorption in the polymer
and the long-­‐‑lived, free charge carriers. Highly
sensitive studies of the absorption spectra of
polymer:fullerene blends by our research
group in Universiteit Hasselt, have revealed
the presence of a long wavelength absorption
band characteristic for a weak ground state CT
complex (CTC), formed by the interaction of
the lowest unoccupied molecular orbital of the
fullerene acceptor LUMO(A) with the highest
occupied molecular orbital of the polymer
donor HOMO(D)15–18. Illumination with
wavelengths in this CT band results in the
direct creation of bound electron-­‐‑hole pairs or
CT excitons. The highly sensitive techniques
used by our group to study these low signal
sub-­‐‑band gap features are Photothermal
Deflection Spectroscopy (PDS)15,16 and Fourier
Transform
Photocurrent
Spectroscopy
Figure 3: (a) General mechanism for photo-­‐‑energy conversion in donor/acceptor organic solar cells. The four steps are: (1)
Absorption of light, creating an exciton in the donor (acceptor) phase. (2) Diffusion of excitons to the donor/acceptor interface. (3)
Dissociation of excitons yielding charge carriers. (4) Charge transport and collection at the electrodes. (b) A scheme of the energy of
relevant pairs of electrons and holes: the donor excitonic state (D*) and the charge transfer state (CT). The energy of a free electron
on the acceptor phase and a free hole on the donor phase is equal to the difference between their respective molecular orbital energy
levels.
Coulomb binding energy, they are transported
to the collecting electrodes (step 4).
Charge-­‐‑transfer states
As far as the exciton dissociation process is
concerned, recent theories and experimental
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(FTPS)17,18. It has been demonstrated that FTPS
allows measuring the spectral dependence of
the absorption coefficient of organic thin film
material systems and also of the external
quantum efficiency (EQE) of photovoltaic
devices with high resolution (< 1 nm) in just a
matter of seconds. FTPS has the required
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sensitivity to measure the low signal sub-­‐‑band
gap photocurrent produced by the direct
creation of CT excitons upon long wavelength
illumination of the CTC’s.
Radiative decay of CT excitons is sometimes
observed in photoluminescence measurements
of polymer:fullerene blends16,19,20 and can be
more easily detected in electroluminescence
spectra obtained by applying a forward
voltage over polymer:fullerene photovoltaic
devices21.
CT excitons play a major role in the operation
of polymer:fullerene photovoltaic devices.
These weakly bound electron-­‐‑hole pairs at the
polymer:fullerene interface are mainly
populated via a photoinduced electron transfer
after excitation of polymer or fullerene. Due to
the low oscillator strength of polymer:fullerene
CTCs only a very small fraction of CT excitons
is populated by direct optical excitation of the
CTCs. The major contribution to the
photocurrent originates from polymer or
fullerene excitation. However, the efficiency of
CT exciton formation and their dissociation
into free carriers determines the photocurrent.
Both formation and dissociation efficiencies
depend on the blend morphology and
donor:acceptor energetics.
and recombination processes in the active
layer. These recombination processes can
proceed through the formation of a CT exciton
with subsequent emission of low energy
photons,
visible
in
sensitive
electroluminescence experiments. In order to
quantitatively investigate the role of CTC
formation
on
the
photovoltage
polymer:fullerene photovoltaic devices, a
reciprocity relation between Voc and the
photovoltaic and electroluminescent actions of
a generalized solar cell is used21-­‐‑23. As
predicted by the reciprocity relations, a linear
correlation between Voc and the spectral
position of the CT band is observed for a range
of polymer:fullerene blends, comprising
different donor polymers. The energy of the
CT state (ECT) is known to correlate with the
difference between the HOMO energy of the
polymer donor and the LUMO energy of the
fullerene acceptor. This explains the widely
observed, but partly unexplained, empirical
linear correlation between Voc and this
energetic difference24.
4. Challenges
The general challenges for organic based solar
cells are the increase of both performance and
lifetime. From a technological point of view,
an important challenge is to develop cost
efficient large area production techniques
using environmentally friendly solvents.
Figure 4 – Micrograph of ZnO nanorods as highways for
electrons in hybrid polymer: ZnO solar cells.
Open Circuit Voltage
Also the open-­‐‑circuit voltage Voc of the
photovoltaic cells is determined by the spectral
properties of the CT excitons, again being
morphology dependent. Voc is determined by
the balance between free carrier generation
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Towards ‘green’ organic based solar cells
Dye sensitized solar cells (DSSCs) are
considered as a promising low-­‐‑cost alternative
to conventional inorganic semiconductor
photovoltaic
devices.
DSSCs,
using
nanoporous TiO2 electrodes, ruthenium-­‐‑based
complexes dyes and liquid electrolytes, reach
power conversions up to 10% under AM 1.5
(100 mW/cm2) solar illumination. The presence
of the liquid electrolytes requires special
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attention regarding sealing, stability and
multi-­‐‑cell module manufacturing. As an
alternative to the liquid electrolyte, conjugated
polymers and in particular PTs attract much
interest because of their – higher mentioned -­‐‑
good processability, low cost and high hole
mobilities. Other advantages of PTs are the
low bandgap and high absorption coefficient,
which make them good photosensitizers.
Through the use of PTs, light absorption and
hole transport are combined in one single
material.
From an environmental point of view, an
important drawback when upscaling the
production process of PT-­‐‑based (e.g. P3HT)
solar cells is the need for toxic organic solvents
such as chlorobenzene or chloroform.
Therefore, a water-­‐‑soluble PT (P3SHT) is used
to allow a safe and environmentally friendly
processing. By using an aqueous route for both
the dense titania hole-­‐‑blocking layer and the
nanoporous TiO2 network it is possible to
develop fully ‘green’ solid-­‐‑state solar cells in
which photosensitizer, electron and hole
conductor are achieved from a water-­‐‑based
preparation method.
Recent activities include the controlled growth
of nanocolumnar ZnO25 -­‐‑ as highways for
electrons -­‐‑ which is studied in combination
with organic semiconductors for photovoltaic
applications.
Interdisciplinarity
The field of organic solar cells is a truly
interdisciplinary field of research involving
chemists, physicists and engineers working on
materials
synthesis,
device
physics,
characterization, modeling, device technology
and reliability. A further strengthening of this
interdisciplinary approach is the only road for
organic based or nanostructured solar cells to
contribute towards an intelligent and
sustainable future.
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Figure 5: Interdisciplinary approach towards novel generation
organic based solar cells.
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