organic photovoltaics
TRANSCRIPT
1754-5692(2009)2:3;1-2
Energy&Environmental Sciencewww.rsc.org/ees Volume 2 | Number 3 | March 2009 | Pages 241–332
COVER ARTICLEBernard Kippelen and Jean-Luc Brédas Organic photovoltaics: recent science, engineering results and future challenges
PERSPECTIVEY.-H. Percival ZhangA sweet out-of-the-box solution to the hydrogen economy: is the sugar-powered car science fiction?
ISSN 1754-5692
REVIEW www.rsc.org/ees | Energy & Environmental Science
Organic photovoltaics
Bernard Kippelen*a and Jean-Luc Br�edasb
Received 21st July 2008, Accepted 4th December 2008
First published as an Advance Article on the web 7th January 2009
DOI: 10.1039/b812502n
Organic photovoltaics, the technology to convert sun light into electricity by employing thin films of
organic semiconductors, has been the subject of active research over the past 20 years and has received
increased interest in recent years by the industrial sector. This technology has the potential to spawn
a new generation of low-cost, solar-powered products with thin and flexible form factors. Here, we
introduce the energy and environmental science community to the basic concepts of organic
photovoltaics and discuss some recent science and engineering results and future challenges.
The photovoltaic effect—the conversion of light into electrical
power—can be traced back to Becquerel’s 1839 pioneering
studies in liquid electrolytes1 and has since been studied in a wide
range of materials. In the modern era, the tipping point that
transformed photovoltaics into a technology to convert sun light
into electricity was the 1954 report by Chapin et al.2 of a silicon-
based single p–n junction device with a solar power conversion
efficiency of 6%. Recognized initially as a sustainable power
source for geostationary communications satellites, photovoltaic
cells have now gained in efficiency and found many applications
in the consumer market.3–5 More importantly, in recent years,
they are emerging as a clean and sustainable source of energy and
are expected to play a major role in meeting the global energy
challenge.6
Solar technologies are currently dominated by wafer-size
single-junction solar cells based on crystalline silicon that are
assembled into large area modules. However, other semi-
conductor materials and devices are under active investigation in
order to further reduce the cost of produced electricity by:
increasing the power conversion efficiency, reducing the amount
of absorbing material needed, and lowering the assembly cost of
modules. Thin-film photovoltaic technologies,7 referred to as
second-generation photovoltaics, are based on inorganic semi-
conductor materials that are more absorbing than crystalline
silicon and can be processed directly onto large area substrates.
Such semiconductors include amorphous silicon, II–VI semi-
aSchool of Electrical and Computer Engineering, Center for OrganicPhotonics and Electronics, Georgia Institute of Technology, Atlanta,Georgia, 30332, USA. E-mail: [email protected] of Chemistry and Biochemistry, Center for Organic Photonics andElectronics, Georgia Institute of Technology, Atlanta, Georgia, 30332,USA
Broader context
Organic photovoltaics, the technology to convert sun light into ele
been the subject of active research over the past 20 years and has re
This technology has the potential to spawn a new generation of low-
This review introduces the energy and environmental science commu
some recent science and engineering results and future challenges.
This journal is ª The Royal Society of Chemistry 2009
conductors such as CdS or CdTe, and chalcogenides such as
CuInSe2 (CIS) or CuInGaSe2 (CIGS).8 Despite the laboratory
demonstration of cells with high efficiencies (19% for CIGS9 and
16% for CdTe10), the controlled manufacturing of second-
generation cells remains a challenge and their commercial use is
growing but not as widespread yet.
Simultaneously, over the past two decades, the science and
engineering of organic semiconducting materials have advanced
very rapidly, leading to the demonstration and optimization of
a range of organics-based solid-state devices, including organic
light-emitting diodes (OLEDs),11 field-effect transistors,12,13
photodiodes,14 and photovoltaic cells. Seeded in the 1960s by
fundamental studies on the optical and electronic properties of
model organic molecules such as acenes15—molecules based on
up to five fused benzene rings—this area of research gained
significant momentum in the late 1970s and in the 1980s when
high-purity small organic molecules with tailored structure and
properties were synthesized and processed at room temperature
into thin films using physical vapor deposition techniques.
Building on such advances, Tang16 developed single hetero-
junction organic photovoltaic cells and reported in 1986 a power
conversion efficiency of about 1%. This result represented
a major milestone and a significant improvement in efficiency
over the first report of a device with similar geometry by Kearns
and Calvin17 in 1958. The advent in the 1990’s of high-purity
conjugated polymers allowed the fabrication of organic photo-
voltaic cells with materials simply processed from solution.18,19
The low-temperature processing of either organic small
molecules from the vapor phase or polymers from solution
confers organic semiconductors with a critical advantage over
their inorganic counterparts, as the high-temperature processing
requirements of the latter limit the range of substrates on
which they can be deposited. Particularly attractive for organic
ctricity by employing thin films of organic semiconductors, has
ceived increased interest in recent years by the industrial sector.
cost, solar-powered products with thin and flexible form factors.
nity to the basic concepts of organic photovoltaics and discusses
Energy Environ. Sci., 2009, 2, 251–261 | 251
semiconductors are flexible plastic substrates that can lead to
applications and consumer products with lower cost, highly
flexible form factors, and light weight. Furthermore,
low-temperature processing cuts on energy use during
manufacturing, further reducing the energy payback time which
is defined as the operating life of a power-generating device
needed to produce the amount of energy invested during
manufacturing, installation and maintenance. These attributes,
combined with the ability to tune the physical properties of
organic (macro)molecules by fine tuning their chemical structure,
constitute the main drivers boosting research and industrial
interest in organic photovoltaics.
The organics-based approaches and those that do not rely on
conventional single p–n junctions are often referred to as third-
generation technologies. They include: (i) the dye-sensitized solar
cells pioneered by Gr€atzel,20,21 which are electrochemical cells
that require an electrolyte; (ii) multijunction cells fabricated from
group IV and III–V semiconductors;22 (iii) hybrid approaches
in which inorganic quantum dots23–25 are doped into a semi-
conducting polymer matrix or by combining nanostructured
inorganic semiconductors such asTiO2withorganicmaterials;26–30
(iv) and all-organic solid-state approaches. In this article we will
focus mainly on the latter.
While the physics of conventional p–n junctions is reasonably
well understood and solar cell properties can be derived from
materials parameters and the nature of the electrical contacts
with electrodes, the understanding of the underlying science of
organic solar cells is far less advanced and remains an intense
subject of research. For instance, a key difference in the physics
of organic semiconductors compared to their inorganic coun-
terparts is the nature of the optically excited states. The
absorption of a photon in organic materials leads to the forma-
tion of an exciton, i.e., a bound electron–hole pair. The exciton
binding energy is typically large, on the order of or larger than
500 meV (such binding energies represent twenty times or more
the thermal energy at room temperature, kT(300 K) ¼ 26
meV,31,32 to be compared with a few meV in the case of inorganic
semiconductors. Consequently, optical absorption in organic
Bernard Kippelen received his
PhD in solid-state physics in
1990 from the University Louis
Pasteur, Strasbourg, France. He
was Charg�e de Recherches at the
CNRS. In 1994 he joined the
Optical Sciences Center at The
University of Arizona where he
became an Assistant Professor
in 1998 and an Associate
Professor in 2001. Since 2003,
he has been Professor of Elec-
trical and Computer Engi-
neering at the Georgia Institute
of Technology, Atlanta, USA.
He is a Fellow of the Optical Society of America, a Fellow of
SPIE, and a Senior Member of IEEE.
252 | Energy Environ. Sci., 2009, 2, 251–261
materials does not lead directly to free electron and hole carriers
that could readily generate an electrical current. Instead, to
generate a current, the excitons must first dissociate. The exci-
tonic character of their optical properties is a signature feature of
organic semiconductors, which has impacted the design and
geometry of organic photovoltaic devices for the past decades.
Our objective in this review is to introduce organic photovol-
taics to the general energy and environmental science commu-
nity. We discuss the science and engineering of solid-state
organic photovoltaic materials and devices with an emphasis on
the physical processes involved in the operation of these devices
and the comparison with traditional inorganic p–n devices. Thus,
we do not intend to provide a comprehensive survey of the wide
range of materials and device geometries that have been devel-
oped; to get more familiar with the latter topics, we refer the
reader to excellent recent reviews.14,33–37
Materials and device structures
Organic molecular and polymeric semiconductors can form films
with complex morphologies and varying degrees of order and
packing modes through the interplay of a variety of non-covalent
interactions. Their molecular structure consistently presents
a backbone along which the carbon (or nitrogen, oxygen, sulfur)
atoms are sp2-hybridized and thus possess a p-atomic orbital.
The conjugation (overlap) of these p orbitals along the backbone
results in the formation of delocalized p molecular orbitals,
which define the frontier (HOMO and LUMO) electronic levels
and determine the optical and electrical properties of the
(macro)molecules. The overlap of the frontier p molecular
orbitals between adjacent molecules or polymer chains charac-
terizes the strength of the intermolecular electronic couplings
(also called transfer integrals or tunneling matrix elements),
which represent the key parameter governing charge carrier
mobilities. In crystalline inorganic semiconductors, the three-
dimensional character and rigidity of the lattice ensure wide
valence and conduction bands and large charge carrier mobilities
(typically on the order of several 102 to 103 cm2 V�1 s�1). In
Jean-Luc Br�edas received his
PhD in Chemistry from the
University of Namur, Belgium,
in 1979. In 1988, he was
appointed Professor at the
University of Mons-Hainaut,
Belgium. He joined the Univer-
sity of Arizona in 1999 before
moving in 2003 to the Georgia
Institute of Technology where he
is a Professor of Chemistry and
Biochemistry. He is the recipient
of the 1997 Francqui Prize, the
2000 Quinquennial Prize of the
Belgian National Science Foun-
dation, the 2001 Italgas Prize (shared with Richard Friend), and
a member of the team laureate of the 2003 Descartes Prize of the
European Union.
This journal is ª The Royal Society of Chemistry 2009
Fig. 2 Comparison of the energy level diagrams for inorganic and
organic solar cells. (a) Energy levels in an inorganic p–n junction under
illumination. 3Fe and 3Fh denote the quasi-Fermi levels in the n-type and
p-type semiconductors. The difference between the quasi-Fermi level
energies determines the maximum open-circuit voltage (VOC) under
illumination. Absorption of photons with an average photon energy
larger than the band gap on either side of the junction in the n-type and
contrast, in organic semiconductors, the weakness of the elec-
tronic couplings (due to their intermolecular character), the large
electron–vibration couplings (leading to pronounced geometry
relaxations), and the disorder effects all conspire to produce
more modest carrier mobilities due to charge-carrier localization
and formation of polarons; transport then relies on polarons
hopping frommolecule to molecule (here and in the remainder of
the text, ‘‘molecule’’ should also be taken as meaning ‘‘polymer
chain segment’’ where appropriate).38 As a result, the charge
carrier mobilities strongly depend on the morphology and can
vary over several orders of magnitude when going from highly
disordered amorphous films (typically, 10�6 to 10�3 cm2 V�1 s�1)
to highly ordered crystalline materials (>1 cm2 V�1 s�1).38 To
describe the operation of organic solar cells, energy diagrams can
be drawn in which the valence and conduction band energies are
replaced by the HOMO and LUMO (polaron level) energies,
respectively.39
The general structure of an organic photovoltaic device similar
to that reported by Tang16 in 1986 is shown in Fig. 1. It consists
of a transparent electrode, typically a conducting oxide such as
indium-tin oxide (ITO), two organic light-absorbing layers, and
a second electrode. The two organic layers are made of different
organic semiconductors, one with an electron-donor character
and the other with an electron-acceptor character. Electron-
donor molecules (D) exhibit a low ionization potential (and thus
a high-lying HOMO energy), while electron-acceptor molecules
(A) possess a high electron affinity (and thus a low-lying LUMO
energy) (see Fig. 1). TheD and A layers must provide for efficient
hole and electron transport, respectively.
To compare the operation of an organic solar cell with
conventional cells and to highlight their fundamental differences,
it is useful to review the operation of inorganic solar cells based
on p–n junctions. When an abrupt p–n junction is formed from
doped inorganic semiconductors, the carrier distributions and
concentrations in the materials under equilibrium conditions can
be modeled and derived from the semiconductors parameters
and the properties of their interfaces with the electrodes.40,41 The
condition of invariance of the equilibrium Fermi level leads to
the conventional band energy diagram used to describe a p–n
junction in which the energies of the valence band and conduc-
tion band are position dependent near the junction (see Fig. 2a).
Fig. 1 Cross-section of a bilayer organic solar cell. Chemical structure of
examples of donor and acceptor molecules used in the seminal work of
Tang. Donor: copper-phthalocyanine; acceptor: perylene tetracarboxylic
derivative.
This journal is ª The Royal Society of Chemistry 2009
The charge redistribution that takes place near the junction
(in the depletion region) leads to a built-in potential and to an
electric field that is confined to the vicinity of the junction.
Assuming constant doping on each side of the junction, the
electric field outside the depletion region vanishes. Illumination
of the materials leads to photogenerated electron/hole pairs, for
p-type semiconductors (step 1) is followed by thermalization of the holes
and electrons near the top of the valence and conduction bands,
respectively (step 2). Minority carriers (electrons in the p-type semi-
conductor; holes in the n-type semiconductor) diffuse to the junction
where they are swept away and accumulate on the other side of the
junction where they become majority carriers (step 3). For the sake of
simplicity, these three steps have been drawn for electrons, but the same
applies to holes. (b) Energy level diagram of an organic heterojunction
under illumination. IP(D) and EA(A) denote the ionization potential
(HOMO level energy) of the donor molecular layer and the electron
affinity (LUMO level energy) of the acceptor molecular layer, respec-
tively. Absorption of photons with an average photon energy larger than
the optical band gap on either side of the heterojunction (step 1) is fol-
lowed by thermalization and the formation of excitons (step 2). Excitons
diffuse to the heterojunction (step 3) where they dissociate and transfer an
electron [hole] into the acceptor [donor] layer (step 4). The difference
between IP(D) and EA(A) determines the maximum open circuit voltage
(VOC) under illumination. The D arrows denote the energy offsets
between the ionization potential values (HOMO energies) and electron
affinities (LUMO energies).
Energy Environ. Sci., 2009, 2, 251–261 | 253
Fig. 3 Cross-sectional representations of organic photovoltaic cell
geometries. (a) Geometry of a multilayer structure in which a mixed
interlayer D:A is sandwiched between D and A layers. (b) Geometry of
a bulk heterojunction. The chemical structures are examples of
commonly used donor-like polymers (regio-regular poly(3-hexyl-
thiophene): P3HT) and acceptor-like soluble molecules (methano-
fullerene [6,6]-phenyl C61-butyric acid methyl ester: PCBM). (c) Energy
level diagram of a bulk heterojunction device. The HOMO and LUMO
levels of the donor and acceptor materials are shown by solid and dot–
dashed lines, respectively.
which excitonic effects can be ignored at room temperature. The
excess of photogenerated electrons in the n-doped semiconductor
and holes in the p-doped side of the junction are negligible
compared to the densities of majority carriers on both sides at
thermal equilibrium. In contrast, electron and hole densities
created in the p-doped and n-doped semiconductors, respec-
tively, are significant relative to the densities of minority carriers
at equilibrium in the dark. Carrier transport generally occurs
inside the semiconductors mainly through a combination of drift
and diffusion. At the junction, the minority carriers are swept
away through a drift process due to the junction potential. Away
from the junction, the transport of the minority carriers is mainly
governed by diffusion. In such conditions, the photocurrent
density of a conventional inorganic solar cell can be derived from
the minority carrier diffusion equations obtained by combining
the continuity and current equations. Solutions are found by
considering the boundary conditions in which the photo-
generated minority carriers at the junction are zero and where the
interfaces of the semiconductors with the front and back elec-
trodes are characterized by surface recombination velocities.40,41
For the organic solar cell shown in Fig. 1, a parallel can be
drawn between the two organic light-absorbing layers and the n-
and p-doped semiconductors in an inorganic solar cell. However,
unlike their inorganic counterparts, organic semiconductors in
the device structure shown in Fig. 1 are in most cases essentially
intrinsic (although some amounts of uncontrolled impurities are
likely to be present). The interface between the two layers—the
D/A heterojunction—is responsible for efficient exciton dissoci-
ation. Therefore, it plays a role for excitons similar to that played
by the junction for minority carriers in inorganic cells. The
electrons and holes created at the interface can be transported
through the A andD layers, respectively, and collected at the two
electrodes, thereby contributing to an electrical current in the
external circuit. Organic semiconductors have large extinction
coefficients compared to crystalline Si leading to efficient light
harvesting in layers that are relatively thin with thicknesses in the
range of 100–200 nm. Since the organic layers are sandwiched
between electrodes with different work functions, a built-in
potential appears and results in an electric field that assists the
transport of charges.42 Thus, the electron and hole currents in the
device under illumination after exciton dissociation are governed
by an interplay of drift and diffusion.43
While a critical step in inorganic solar cells is to collect minority
carriers before they recombine by having them migrate to the
opposite side of the junction, the challenge in organic cells is to
dissociate the excitons before they decay to the ground state; this
requires that they rapidly diffuse to the D/A heterojunctions,
which is the only location where dissociation is efficient in pure
materials (see Fig. 2b). Hence, the thickness of the organic layers
has to be comparable to the exciton diffusion lengthL (L¼ (Dt)1/2
where D is the diffusion coefficient and t the lifetime of the
exciton). An optimal compromise regarding the thickness of the
organic layers has to be found between allowing for efficient
exciton diffusion to the heterojunction and efficient sun light
absorption. The latter requires that the total absorbance of the
film be preferably in the range 2–3 (corresponding to 86–95%
absorption efficiency). However, if the layers of organic semi-
conductors have to be too thin as a result of small L values
(e.g., d ¼ 10–20 nm), the incident light does not get absorbed
254 | Energy Environ. Sci., 2009, 2, 251–261
efficiently. For an organic semiconductor such as the copper-
phthalocyanine compound (see structure in Fig. 1) used by Tang,
the peak extinction coefficient at 620 nm is k ¼ 0.74, which leads
to an absorption coefficient (a ¼ 4pk/l where l is the wave-
length) of a ¼ 0.015 nm�1; for a 10 nm-thick film, this translates
into an absorbance ad [absorption efficiency] of only 0.15 [14%]
for a single pass and 0.3 [26%] for a double pass assuming the
transmitted light gets reflected by the back electrode. Although
devices based on pentacene have been shown recently to exhibit
larger exciton diffusion lengths (70 nm),44 this issue remains
a limitation—often referred to as the ‘‘exciton bottleneck’’—and
has triggered the design of devices that are based on bulk
heterojunctions in which the A and D components are mixed
together and form an interpenetrating, phase-separated network
D:A with a nanoscale morphology (see Fig. 3). This layer is then
sandwiched between hole and electron transport (undoped or
doped) layers. Bulk heterojunction devices can be fabricated
by codeposition of two molecular materials45–51 as pioneered by
Yokoyama and co-workers52 (see Fig. 3a) or by blending two
materials in solution as pioneered by the Heeger18 and Friend
groups19 (see Fig. 3b). In such devices, the D/A heterojunction is
distributed throughout the bulk of the composite film, enabling
efficient exciton dissociation and leaving holes in the D compo-
nent and electrons in the A component of the phase-separated
This journal is ª The Royal Society of Chemistry 2009
network; control of the morphology53–57 is critical to extract the
carriers efficiently.
Device operation
We now turn to a more specific discussion of each of the
processes taking place during device operation and highlight
again what is particular to p-conjugated organic materials with
respect to conventional inorganic materials.
Fig. 4 Electronic state diagram. S0 denotes the singlet ground state of
the donor or the acceptor and S1, the first singlet excited state reached
after optical excitation (exciton state); at the D/A interface, intermolec-
ular charge transfer leads to charge-transfer (CT) states where the hole is
on donor molecule(s) and the electron on acceptor molecule(s): CT1 is the
lowest energy charge-transfer state; the CTn states represent higher-
energy, more diffuse charge-transfer states; the final state relevant for
photovoltaics operation is a charge-separated state (CS) where the hole in
the donor layer and the electron in the acceptor layer have become free
from one another; the ki terms indicate competing transfer rates between
various electronic states.
Optical absorption
Conjugated oligomers and polymers display absorption bands
that are usually: (i) very intense, as a result of the large wave-
function overlap between the ground state and the lowest excited
states; and (ii) broad, due to the significant geometry relaxations
that take place in the excited states (the width of the absorption
bands can reach over 1 eV). In p-conjugated systems, the bond
lengths are primarily modulated by the p-electron densities on
the bonds; any change in electronic state, due to excitation or
ionization, modifies the p-electron bond densities and thus the
bond lengths.58 This strong connection between electronic
structure and geometric structure is referred to as strong elec-
tron–vibration (phonon) coupling.
The widths of the absorption bands allow for good matching
with a sizeable portion of the solar spectrum. Once promoted to
an excited state, the p-conjugated system relaxes down to the
bottom of the potential energy surface of the lowest excited state,
the excited state reaches its equilibrium geometry, and an exciton
forms (we note that this relaxation/thermalization process
constitutes a first source of power loss). We recall that the exciton
is a bound electron-hole pair with a typical binding energy on the
order of a few tenths of an eV. This high value is a reflection not
only of the rather low dielectric constant of p-conjugated organic
materials but also of the significant electron correlation and
geometry relaxation effects present in these materials (because of
the geometry relaxation associated to its formation, the exciton is
sometimes referred to as a polaron-exciton). Thus, in contrast to
inorganic semiconductors, the absorption of a photon at room
temperature in conjugated materials does not lead to free charge
carriers but to neutral, bound electron-hole pairs. This is the
reason why two components, an electron donor and an electron
acceptor, are required to promote the generation of charge
carriers.
In general, the ground state of the p-conjugated system is
singlet (total spin multiplicity is zero) and is denoted S0; the
lowest singlet excited state, S1, is usually one-photon allowed.58
In pure hydrocarbons with a coplanar conformation (such as
pentacene), the spin–orbit coupling to triplet states (with total
spin multiplicity of one) is vanishingly small and intersystem
crossing between the singlet and triplet manifolds can be
neglected; in systems with heavy atoms or far from planarity (for
instance, metal phthalocyanines or fullerenes), this is no longer
true and intersystem crossing to triplet excitons can be efficient.
The lowest-energy triplet exciton, T1, often lies a few tenths of an
eV below S1.
Since organic solar cells are composed of several thin layers of
materials with different optical properties, mismatch of the
complex refractive index at multiple interfaces leads to multiple
This journal is ª The Royal Society of Chemistry 2009
reflections that produce optical interference effects. As a result,
the light distribution inside the solar cell is highly inhomoge-
neous and determined by a complicated interplay of the relative
optical constants of the materials and their thickness.59,60
Knowledge of the light distribution in the solar cell is important
to model the steady-state exciton distribution and consequently
the photocurrent produced in the solar cell.
Exciton diffusion
In order to generate separated negative and positive charges, the
excitons need to diffuse to the donor–acceptor interface where
they can dissociate. Since excitons are neutral species, their
motion is not influenced by any electric field and they diffuse via
random hops; importantly, they need to reach the interface prior
to their decay back to the ground state.
The hopping of singlet excitons is usually described via
a generalized F€orster mechanism, which involves the long-range
electrostatic coupling between the excitation transition dipoles at
the initial and final sites (we note that the traditional point-dipole
F€orster model is totally inappropriate here61 as it is based on
resonance energy transfer between distant molecules); in the case
of triplet excitons, hops are restricted to adjacent sites, as they
depend on a short-range exchange (Dexter-type) mechanism.
Thus, singlet excitons can move more quickly than triplets but
decay more quickly as well (on a ns scale vs. ms or ms scale—
which represents the difference between the fluoresecence and
phosphorescence lifetimes). As a result, the efficiency with which
singlets and triplets reach the interface is very much system
dependent.
Exciton dissociation at the donor–acceptor interface
At the D/A interface, excitons can dissociate provided their
energy is higher than that of charge-transfer or charge-separated
states; here, we refer to charge-separated (CS) states as states
where the electron and the hole have been completely freed from
one another, while in charge-transfer (CT) states the electron and
Energy Environ. Sci., 2009, 2, 251–261 | 255
hole are still somewhat bound to one another. Interestingly, at
the present time, no clear picture has emerged to describe the
exciton dissociation process. Some of the most relevant elec-
tronic states are depicted in Fig. 4.
In most instances, the dissociation process is described as
involving a transition from the exciton state down to the lowest
CT state, which corresponds to the situation where the hole sits
on the HOMO level of a D molecule and the electron on the
LUMO level of an adjacent A molecule (see Fig. 4). However, in
such a case, since they remain in close proximity, the electron and
the hole are still rather strongly Coulombically bound, which is
precisely the reason why that CT state is lowest in energy. Several
scenarios have been proposed to explain the eventual separation
of the charges from the lowest CT state, for instance, the presence
of disorder or dipoles at the interface or the assistance of
phonons,62,63 which would make the kCS1charge-separation rate
larger than the kCR charge-recombination rate.
Another proposition has been recently advanced.64 It involves
the efficient coupling of the exciton arriving at the interface to
higher-lying CTn states. By definition, such states are more
diffuse than the lowest CT state and could be delocalized over
a few sites; as a result, the electron and the hole could become
more distant and more easily screened from one another, which
would lead to easier charge separation. For that process to be
relevant, the kCTnand kCSn
rates have to be larger than those
bringing the system down to CT1.
Note that many factors can complicate the description of the
D/A interfacial processes. Suppose that the exciton reaching the
interface has formed in the donor. First, instead of direct electron
transfer from D to A, there could occur energy transfer leading
to the formation of an exciton on A, followed by hole transfer
from A to D; this process has been demonstrated in the case of
oligophenylene-fullerene dyads.65The final state is the same as for
the direct electron-transfer process; however, the rates involved in
the energy-transfer and hole-transfer processes can be markedly
different. Secondly, even when singlet excitons are exclusively
formed in D, triplet excitons can appear at the interface. For
instance in bis-dicyanovinyl-oligothiophenes/fullerene blends, it
has been observed66 that, for certain thiophene oligomer lengths,
excitons can efficiently transfer to C60 where the large intersystem
crossing leads to the formation of triplet excitons, which then
hop back to the donor; such processes do not result in charge
separation and constitute a loss mechanism.
Thus, the situation is much more complex than what the
simple HOMO-LUMO diagrams often found in the literature
(and illustrated in Fig. 3c) would lead one to believe. In addition,
charge separation can also be influenced by concentration and
morphology gradients near the heterojunction that take place
during the formation of the organic films, or can be assisted by
local electric fields.67 New theoretical methodologies are being
developed to provide better descriptions and understanding of all
these competing mechanisms.
Charge carrier mobility and collection at electrodes
Once the charges have separated, they can drift and diffuse
towards their respective electrodes with efficiency depending on
their mobilities. Because of the large electron-vibration coupling
in p-conjugated materials and of disorder effects, each charge is
256 | Energy Environ. Sci., 2009, 2, 251–261
associated to a local geometry relaxation and constitutes
a polaron (radical-ion in chemical terminology) which hops
from molecule to molecule.68 The corresponding polaronic
electronic state has an energy defined by the (adiabatic) ioniza-
tion potential (IP) of the donor or electron affinity (EA) of the
acceptor (note that in the simple HOMO–LUMO diagrams,
Efinal would be crudely approximated by the difference between
the energies of the donor HOMO and acceptor LUMO). The
sum of the energies of the polaron states for the donor and
acceptor, Efinal ¼ IP(D) + EA(A), represents the energy of the
final state of the system, see Fig. 4. To a large extent, Efinal
defines the upper limit for the open-circuit voltage of the solar
cell, as discussed below.
The nature of the electrode/organic layer interfaces is complex.
The efficiency of the charge collection process cannot be simply
determined from the difference between the workfunction of
the isolated electrode and the donor IP or acceptor EA. The
deposition of organic layers on electrodes (or vice versa) lead to
interfacial charge-density redistributions and/or geometry
modifications that strongly affect the alignment of the organic
frontier electronic levels vs. the electrode Fermi level.38 Much
remains to be done to understand the intricate details of these
interfaces. Surface modification of the electrodes via deposition
of self-assembled monolayers69 is an efficient way to enhance
the quality of the electrical contact as well as, in particular
when dealing with conducting oxide electrodes, to promote
compatibilization between the (hydrophilic) oxide surface and
(hydrophobic) organic layer.
Device performance and modeling
Here, we discuss the electrical characteristics of organic solar
cells and their performance. In the dark the solar cell works as
a diode. As for conventional p–n solar cells, an organic photo-
voltaic device can be approximated by an equivalent circuit, see
Fig. 5d, comprised of: (i) a diode with reverse saturation current
density J0 (current density in the dark at reverse bias) and ideality
factor n; (ii) a current source (Jph), which corresponds to the
photocurrent upon illumination; (iii) a series resistance (RS),
which has to be minimized and takes account of the finite
conductivity of the semiconducting material, the contact resis-
tance between the semiconductors and the adjacent electrodes,
and the resistance associated with electrodes and interconnec-
tions; and (iv) a shunt (RP) resistance, which needs to be maxi-
mized and takes into account the loss of carriers via possible
leakage paths; the latter include structural defects such as
pinholes in the film, or recombination centers introduced by
impurities. Solving for this simple circuit provides the following
analytical expression for the current–voltage characteristics,
referred to as the Shockley equation:3–5
J ¼ 1
1þ RS=RP
"J0
�exp
�V � JRSA
nkT=e
�� 1
���Jph �
V
RPA
� #
(1)
where e denotes the elementary charge, kT the thermal energy,
and A the area of the cell. Analysis of eqn (1) in various regimes
of photocurrent shows that the series resistance is the critical
factor, especially in regimes of high photocurrents, Jph. From
This journal is ª The Royal Society of Chemistry 2009
Fig. 5 Optical and electrical properties of solar cells. (a) Spectral photon flux density in the standardized AM 1.5 G illumination conditions and
corresponding integrated current that would be produced if each photon contributes to current with unity efficiency. (b) Current–density voltage
characteristics of a solar cell in the dark and under illumination. (c) Semi-logarithmic plot of the same electrical characteristics, illustrating the effects of
the parasitic resistances RS and RP in forward and reverse bias. (d) Equivalent circuit used to model solar cells. The notations are defined in the text.
eqn (1), equations for the open-circuit voltage VOC and the short-
circuit current density JSC can be derived:
VOC ¼ nkT
eln
(1þ Jph
J0
�1� VOC
JphRPA
�)z n
kT
eln
�1 þ Jph
J0
�
(2)
JSC ¼ � 1
1þ RS=RP
(Jph � J0
�exp
���JSC��RSA
nkT=e
�� 1
�)z� Jph
(3)
Eqns (1)–(3) usually need to be solved numerically except for
cases where RS is very small and/or RP sufficiently large so that
the effect of RS or RP can be ignored; in such instances, the
approximate expressions on the right hand side of eqns (2)–(3)
apply. When the device is under illumination, two quantities
can be easily determined experimentally: the intersects of the
electrical characteristics with the vertical and horizontal axes,
which correspond to JSC and VOC, respectively (see Fig. 5b). At
any point on the electrical characteristic in the fourth quadrant
(JSC negative and VOC positive), the solar cell produces an elec-
trical power density given by the product of voltage and current
density. This product is maximized at a point that corresponds to
voltage Vmax and current density Jmax. The power conversion
efficiency h, which represents the most important metric for
a photovoltaic cell, is then defined as:
h ¼ JmaxVmax
Pinc
¼ FFJSCVOC
Pinc
(4)
where Pinc is the incident power density and FF denotes the fill
factor. These parameters are illustrated in Fig. 5b. The effects of
This journal is ª The Royal Society of Chemistry 2009
the parasitic resistances on the shape of the current density/
voltage characteristics are illustrated in Fig. 5c. A finite value of
the series resistance RS limits the current density in forward bias,
while a finite shunt resistance RP is responsible for a dark current
increase in reverse bias. To characterize quantitatively the
performance of solar cells for terrestrial applications, standard-
ized illumination conditions are used in which the spectrum of the
source simulates the solar spectrum (AM 1.5 G, see Fig. 5a) and
has an intensity on the order of 100mW cm�2; this corresponds to
the average intensity of sun lightwith an angle of incidence q¼ 48�
relative to the normal to the earth’s surface (AM denotes the air
mass ¼ 1/cosq; G stands for global and refers to a small contri-
bution of diffuse light to the direct incident light). Importantly,
according to eqn (4), the power conversion efficiency of a solar cell
is determined by three parameters: the fill factor, the short-circuit
current density, and the open-circuit voltage.Wenow examine the
limits of these three parameters in organic solar cells and discuss
how they relate to materials and contacts properties.
Fill factor
The maximum value for the fill factor is a function of the open-
circuit voltage VOC and the ideality factor of the diode n
(optimally equal to 1). As for inorganic solar cells based on p–n
junctions, its maximum value can be described by the empirical
expression:3
FF0 ¼ vOC � lnðvOC þ 0:72ÞvOC þ 1
(5)
where VOC is a normalized voltage defined as VOC ¼ eVOC/nkT.
Eqn (5) is a good approximation for VOC values > 10. For
Energy Environ. Sci., 2009, 2, 251–261 | 257
organic solar cells with VOC ¼ 0.5–1 V, and ideality factors in the
range of n¼ 1.5–2, this condition is satisfied. We note that in p–n
junction-based cells, the various recombination mechanisms
are taken into account by drawing equivalent circuits with two
diodes, where the first diode (with n ¼ 1) describes radiative
band-to-band recombinations while the second (with n ¼ 2)
describes recombination via impurities with energy states within
the band gap. In organic solar cells, little is known at this stage
about the specific recombination processes of excitons and
carriers. In view of the higher disorder and impurity levels
present in amorphous organic semiconductors compared to the
pure grades of silicon wafers that can be fabricated, ideality
factors that deviate from unity are expected; this negatively
impacts both the maximum fill factor and the maximum open-
circuit voltage (for instance, for VOC ¼ 0.5 V, FF0 drops from
0.80 to 0.69 when n goes from 1 to 2, to be compared with FF0 ¼0.85 in monocrystalline silicon solar cells). It is worth mentioning
that the impact of the parasitic resistances RS and RP in reducing
the fill factor varies with cell performance, cell area, and opera-
tion conditions. A rule of thumb is that the value of RS must
be small compared to the characteristic resistance defined as
RCH¼VOC/JSCA (whereA is the cell area) whileRP must be large
compared to RCH.
Short-circuit current density
The maximum JSC is given by:
JSC ¼ð
AM 1:5
ehEQEðlÞNphðlÞdl (6)
Here, Nph(l) is the photon flux density in the incident AM 1.5 G
spectrum (see Fig. 5a) at wavelength l and a total intensity of
100 mW cm�2 (integrated over the full spectrum). The external
quantum efficiency hEQE(l) is defined by how efficiently an
incident photon gives rise to an electron flowing in the external
circuit. In organic cells, hEQE can be broken down into the
product of efficiencies associated with each of the steps discussed
previously: absorption, exciton diffusion, exciton dissociation
into free carriers, charge transport, and charge collection. Upper
limits on short-circuit current density are obtained according to
eqn (6) by integrating from the high photon energy side (short
wavelength) of the spectrum to the wavelength corresponding to
the optical band gap of the material (as shown in Fig. 5a). Hence,
the smaller the optical band gap, the larger the maximum short-
circuit current. In the case of silicon, the band gap is 1.1. eV
(�1130 nm), which yields a maximum value JSC ¼ 43.6 mA cm�2
for AM 1.5 G. It is worth mentioning that it is the fraction of
photons that are harvested in the AM 1.5 G spectrum that
matters rather than the fraction of the intensity contained in that
same portion of the spectrum. Since the high-energy side of the
spectrum gets harvested, the average photon energy in the
absorbed portion of the spectrum is larger than the band gap
energy. In silicon, the average photon energy is 1.8 eV, to be
compared with the 1.1 eV band gap. As mentioned earlier, this
results in a significant loss mechanism since energy goes down
during thermalization of the electron-hole pairs. These losses can
be somewhat minimized by using tandem cell geometries22 in
which materials with different optical band gaps are stacked on
258 | Energy Environ. Sci., 2009, 2, 251–261
top of one another and absorb different parts of the spectrum.
Tandem cell geometries have recently been demonstrated with
organic cells.70–77
Open-circuit voltage
As in the case of conventional solar cells, maximization of the
short-circuit current density by using organic semiconductors
with decreasing optical absorption gap is not overall an effective
strategy since the maximum open-circuit voltage presents an
opposite trend with optical absorption gap. It follows that the
determination of the optimum light-harvesting conditions to
maximize the efficiency of organic solar cells depends largely on
the understanding of the origin of VOC and its dependence on
materials properties, in particular the relative energies of the
relevant energy levels at the organic heterojunction.
Neglecting the effects of the parasitic resistances, according to
eqn (2), VOC is a logarithmic function of the ratio of the short-
circuit current and the reverse saturation current. In the dark, in
the absence of carriers, the device is in equilibrium and no
photovoltage or VOC is observed since the dark J–V character-
istics cross the origin. Upon illumination, absorbed photons
generate charge carriers, whose distributions can be described by
non-equilibrium quasi-Fermi levels (see Fig. 2a). For illumina-
tion levels such that the produced photocurrent is larger than the
reverse saturation current (which holds true under average illu-
mination conditions), VOC is observed experimentally to increase
logarithmically with intensity. Since the maximum short-circuit
current can be estimated using eqn (6), reaching the maximum
VOC value requires that the reverse saturation current density
J0 be kept at a minimum. Recent studies by Scharber and
co-workers78 on bulk heterojunction cells and Rand and
co-workers79 on multilayer solar cells, indicate that VOC in
organic solar cells depends on the energy difference between the
ionization potential of the D component and the electron affinity
of the A component forming the heterojunction. Studies of the
temperature dependence of the reverse saturation current J0 by
Waldauf et al.80 in bulk heterojunctions and Rand79 in multilayer
structures of small molecules, show that the reverse saturation
current can be approximated by:
J0 ¼ B exp
��Efinal
n 0 kT
�(7)
where B is a coefficient with a value in the range of 1000 A cm�2
and Efinal ¼ IP(D) + EA(A). Eqn (7) shows that the reverse
saturation current is thermally activated with a barrier height
equal to Efinal/n0 where n0 in an ideality factor that corrects for
effects such as vaccum level misalignments at the heterojunction
caused by energy level bending and interfaces dipoles and the
formation of charge-transfer states. By combining eqns (2), (3),
and (7), the maximum open-circuit voltage VOC for an organic
cell can then be written as:
VOC ¼ 1
e
n
n 0 Efinal � nkT ln
�B
JSC
�!(8)
From the energy diagram in Fig. 2b, Efinal ¼ IP(D) + EA(A) is
increased as the energy offsets D between the D and A molecular
states are decreased. However, too strong a reduction in D
This journal is ª The Royal Society of Chemistry 2009
compromises the efficiency of exciton dissociation at the heter-
ojunction and thus decreases the photocurrent; it is generally
accepted that D has to be on the order of �0.3–0.5 eV to over-
come the exciton binding energy. In this case, optimized condi-
tions to maximize the power conversion efficiency in an organic
single heterojunction cell are obtained for a band gap in the range
EG ¼ 1.6–1.9 eV (775–650 nm).
It is worth mentioning that molecules with EA energies larger
than 4.2 eVare less sensitive to oxidation in the presence of oxygen
and moisture.81 Hence, optimized material combinations should
not only satisfy conditions associated with relative state energies
but also absolute energies referenced to the vacuum level.
Based on this analysis, a limit of the maximum power
efficiency of an idealized single junction organic solar cell can be
estimated. With an optimized optical band gap of EG ¼ 1.6 eV
(775 nm) and a hypothetical average external quantum efficiency
of hEQE ¼ 0.8, a maximum short-circuit current of JSC,max ¼20.2 mA cm�2 is calculated from eqn (6) (see also Fig. 5a). A
required energy offset of D ¼ 0.5 eV would translate into Efinal ¼IP(D) + EA(A) ¼ EG � D ¼ 1.1 eV. From eqn (8) and assuming
that the solar cell has an ideality factor of n ¼ 1.5 and with n0 zn, the maximum open-circuit voltage is estimated to be VOC,max
¼ 0.68 V. Such a VOC,max leads to a normalized voltage VOC ¼eVOC/nkT¼ 17.4 V, from which, according to eqn (5), an ideal
maximum fill factor FF0 ¼ 0.79 is obtained. Following eqn
(4), these values translate into a power conversion efficiency
h ¼ 10.8%. Table 1 collects a summary of these estimates and
provides examples of parameters measured in solar cells fabri-
cated from different materials.
Future prospects and challenges
The future of organic solar cells as a pervasive technology for
portable power will largely rely on their economic potential.86
This depends on a number of intricate factors such as efficiency,
manufacturing cost, lifetime, form factor, weight, scalability,
and sustainable manufacturing. At this point, two main
manufacturing techniques can be foreseen, vacuum processing
and wet processing. Vacuum processing presents the advantages
of relatively easy fabrication of, on one hand, high-quality thin
Table 1 Selected examples of device parameters for solar cells measured un
Device type JSC/mA cm�2
Monocrystalline Si PERLa cell 42.2Multicrystalline Si cell 35.6Monocrystalline Si commercial module —a-Si cellb 8.11a-Si commercial module —CdTe cell 26.08CIGS cell 35.7Gr€atzel cell 20.53Nanocrystal hybrid cell 13.2OPV: 1986 Tang cell 2.3OPV: Small molecule cell 15.4OPV: P3HT:PCBM single cell 9.5OPV tandem cell 7.8Ideal single heterojunction OPV cell 20.2
a PERL: passivated emitter, rear locally diffused; fabricated from high qualityc PCE: Power conversion efficiency.
This journal is ª The Royal Society of Chemistry 2009
films from highly purified materials with well-controlled thick-
ness and, on the other hand, devices with complex multilayer
architectures. The limiting factors are the deposition rates
and the tooling costs associated with vacuum techniques.
Wet processing allows for high throughputs using various
printing techniques and holds the long-term promise of lowest
manufacturing costs. However, printing of organic semi-
conductors into pinhole-free and 100 nm-thick films over large
areas remains a major challenge. Semiconductor inks generally
have low viscosity which limits the range of adequate printing
techniques. Furthermore, substrates and electrodes have surface
energies that are significantly different from those of organic
semiconductor inks, which can lead to wetting issues. Vertical
segregation87 and other rheological effects also make it difficult
to control the nanoscale phase-separated morphology88 neces-
sary for good operation of bulk heterojunctions. Stability of
the inks after formulation is another challenge that vacuum
processing does not face.
A cost factor common to all material platforms is packaging.
Coatings with small transmission rates to oxygen and moisture
are necessary to protect the organic semiconductors from
undergoing photo-oxidative reactions that limits their stability
over time. Packaging techniques must be compatible with the
substrate and active materials89 and keep the overall
manufacturing cost low. Since highly flexible form factors are
possible, much work remains to be done to understand how
flexing affects the operation of the devices and their packaging.
Most laboratory cells with efficiencies in the range of 2–6% have
been fabricated over small areas, and area scaling without loss of
efficiency is required. A clear advantage of organics over crys-
talline silicon is the relative ease of monolithic integration,90
which reduces the module assembly cost considerably.
If organic photovoltaic technologies mature beyond niche
consumer market applications and become players in power
generation, their composition must be based on materials
available on large scales at low cost. Also, concerns about the
limited supply of indium ($900 per kg) are currently fueling
research efforts to replace ITO as the transparent electrode.91–93
Promising results have been demonstrated, for instance, with
polymers doped with carbon nanotubes.
der the standard global spectrum (AM 1.5G, 100 mW cm�2)
VOC/V FF PCEc (%) Reference
0.706 0.828 24.7 820.631 0.808 18.2 83— — 16.9 SunPower
2.297 0.697 13.0 84— — 6.3 United Solar
0.840 0.731 16.0 100.689 0.781 19.2 90.721 0.704 10.4 850.45 0.49 2.9 240.45 0.65 0.9 160.50 0.46 3.5 450.63 0.68 5.1 561.24 0.67 6.5 760.68 0.79 10.8 See text
fusion zone (FZ) monocrystalline silicon substrates. b Triple junction cell.
Energy Environ. Sci., 2009, 2, 251–261 | 259
Last but not least, organic solar cells must demonstrate
lifetimes of several years. While organic cells might not show
twenty years of operational lifetime like crystalline silicon cells,
the recent demonstration of 100 000 h operational lifetime in
organic light-emitting diodes is indicative that long lifetimes are
within reach with organic semiconductors.
To summarize, organic photovoltaics provides an exciting
playground at the frontiers of science, engineering, and tech-
nology. Advances in the near term are likely to lead to solar cells
with efficiencies close to 10% in single heterojunction geometries
and efficiencies up to 15% in tandem-cell geometries. If organic
photovoltaics holds its promise, it can soon become an ubiqui-
tous, clean and sustainable technology for portable power and
potentially provide large-scale energy production for future
generations.
Acknowledgements
This material is based upon work supported in part by the STC
Program of the National Science Foundation under Agreement
Number DMR-0120967, the Office of Naval Research, the
Department of Energy, the Georgia Research Alliance, and the
AtlanTICC Alliance.
References
1 E. Becquerel, M�emoire sur les effets �electriques produits sousl’influence des rayons solaires, C. R. Acad. Sci., 1839, 9, 561–567.
2 D. M. Chapin, C. S. Fuller and G. L. Pearson, A new silicon p-njunction photocell for converting solar radiation into electricalpower, J. Appl. Phys., 1954, 25, 676.
3 M. A. Green, Solar Cells, The University of New South Wales,Kensington, Australia, 1998.
4 P. Wurfel, Physics of Solar Cells, Wiley VCH, Weinheim, Germany,2005.
5 R. H. Bube, Photovoltaic Materials, Imperial College Press, London,UK, 1998.
6 For general information about solar energy, consult the InternationalEnergy Agency Photovoltaic Power Systems Program at www.iea-pvps.org.
7 S. Hegedus, Thin film solar modules: The low cost, high throughputand versatile alternative to Si wafers, Prog. Photovoltaics, 2006, 14,393–411.
8 A. Slaoui and R. T. Collins, Advanced inorganic materials forphotovoltaics, MRS Bull., 2007, 32, 211.
9 K. Ramanathan et al., Properties of 19.2% efficiency ZnO/CdS/CuInGaSe2 thin-film solar cells, Prog. Photovoltaics, 2003, 11, 225.
10 T. Aramoto et al., 16.0% efficient thin-film CdS/CdTe solar cells, Jpn.J. Appl. Phys., Part 1, 1997, 36, 6304.
11 R. H. Friend et al., Electroluminescence in conjugated polymers,Nature, 1999, 397, 121.
12 F. Garnier, R. Hajlaoui, A. Yassar and P. Srivastava, All-polymerfield-effect transistor realized by printing techniques, Science, 1994,265, 1684.
13 C. D. Dimitrakopoulos and P. R. L. Malenfant, Organic thin filmtransistors for large area electronics, Adv. Mater., 2002, 14, 99.
14 P. Peumans, A. Yakimov and S. R. Forrest, Small molecular weightorganic thin-film photodetectors and solar cells, J. Appl. Phys.,2003, 93, 3693.
15 M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystalsand Polymers, Oxford University Press, New York, 1999.
16 C. W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett.,1986, 48, 183.
17 D. Kearns and M. Calvin, Photovoltaic effect and photoconductivityin organic laminated systems, J. Chem. Phys., 1958, 29, 950–951.
18 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Polymerphotovoltaic cells – enhanced efficiencies via a network of internaldonor–acceptor heterojunctions, Science, 1995, 270, 1789.
260 | Energy Environ. Sci., 2009, 2, 251–261
19 J. J. M. Halls et al., Efficient photodiodes from interpenetratingpolymer networks, Nature, 1995, 376, 498.
20 M. Gr€atzel, Photoelectrochemical cells, Nature, 2001, 414, 338.21 M. K. Nazeeruddin et al., Conversion of light to electricity by cis-
x2bis(2,20-bipyridyl-4,40-dicarboxylate)ruthenium(II) charge-transfer
sensitizers (x ¼ Cl�, Br�, I�, CN�, and SCN�) on nanocrystallineTiO2 electrodes, J. Am. Chem. Soc., 1993, 115, 6382.
22 F. Dimroth and S. Kurtz, High-efficiency multijunction solar cells,MRS Bull., 2007, 32, 230.
23 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Hybrid nanorod-polymer solar cells, Science, 2002, 295, 2425.
24 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Air-stableall-inorganic nanocrystal solar cells processed from solution,Science, 2005, 310, 462.
25 D. J. Milliron, I. Gur and A. P. Alivisatos, Hybrid organic–nanocrystal solar cells, MRS Bull., 2005, 30, 41.
26 A. Petrella et al., Colloidal TiO2 nanocrystals/MEH:PPVnanocomposites: Photo(electro)chemical study, J. Phys. Chem. B,2005, 109, 1554.
27 K. Sayama et al., Efficient sensitization of nanocrystalline TiO2 filmswith cyanine and merocyanine organic dyes, Sol. Energy Mater. Sol.Cells, 2003, 80, 47.
28 A. C. Arango et al., Efficient titanium oxide/conjugated polymerphotovoltaics for solar energy conversion, Adv. Mater., 2000, 12,1689.
29 K. M. Coakley, Y. X. Liu, C. Goh and M. D. McGehee, Orderedorganic-inorganic bulk heterojunction photovoltaic cells, MRSBull., 2005, 30, 37.
30 K. M. Coakley, Y. X. Liu, M. D. McGehee, K. L. Frindell andG. D. Stucky, Infiltrating semiconducting polymers into self-assembled mesoporous titania films for photovoltaic applications,Adv. Funct. Mater., 2003, 13, 301.
31 J. L. Br�edas, J. Cornil and A. J. Heeger, The exciton binding energy inluminescent conjugated polymers, Adv. Mater., 1996, 8, 447.
32 S. F. Alvarado, P. F. Seidler, D. G. Lidzey and D. D. C. Bradley,Direct determination of the exciton binding energy of conjugatedpolymers using a scanning tunneling microscope, Phys. Rev. Lett.,1998, 81, 1082.
33 J. Nelson, Organic photovoltaic films, Curr. Opin. Solid State Mater.Sci., 2002, 6, 87.
34 Organic Photovoltaics: Mechanisms, Materials and Devices, ed. S. S.Sun and N. S. Sariciftci, Taylor and Francis, Boca Raton, FL, 2005.
35 S. E. Shaheen, D. S. Ginley and G. E. Jabbour, Organic-basedphotovoltaics. Toward lowm-cost power generation, MRS Bull.,2005, 30, 10.
36 H. Hoppe and N. S. Sariciftci, Organic solar cells: An overview,J. Mater. Res., 2004, 19, 1924.
37 H. Spanggaard and F. C. Krebs, A brief history of the development oforganic and polymeric photovoltaics, Sol. Energy Mater. Sol. Cells,2004, 83, 125.
38 V. Coropceanu et al., Charge transport in organic semiconductors,Chem. Rev., 2007, 107, 926.
39 Conjugated Polymers and Molecular Interfaces, ed. W. R. Salaneck,K. Seki, A. Kahn and J.-J. Pireaux, Marcel Dekker, New York,2002.
40 R. F. Pierret Semiconductor Device Fundamentals, Addison Wesley,Reading, Massachusetts, 1996.
41 S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, WileyInterscience, Hoboken, New Jersey, 2007.
42 I. H. Campbell, T. W. Hagler, D. L. Smith and J. P. Ferraris, Directmeasurement of conjugated polymer electronic excitation energiesusing metal/polymer/metal structures, Phys. Rev. Lett., 1996, 76,1900.
43 P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster and D. E. Markov,Device physics of polymer: Fullerene bulk heterojunction solar cells,Adv. Mater., 2007, 19, 1551.
44 S. Yoo, B. Domercq and B. Kippelen, Efficient thin-film organic solarcells based on pentacene/C60 heterojunctions, Appl. Phys. Lett., 2004,85, 5427.
45 S. Uchida, J. G. Xue, B. P. Rand and S. R. Forrest, Organic smallmolecule solar cells with a homogeneously mixed copperphthalocyanine:C60 active layer, Appl. Phys. Lett., 2004, 84, 4218.
46 P. Peumans, S. Uchida and S. R. Forrest, Efficient bulkheterojunction photovoltaic cells using small-molecular-weightorganic thin films, Nature, 2003, 425, 158.
This journal is ª The Royal Society of Chemistry 2009
47 J. Drechsel et al., Efficient organic solar cells based on a double p-i-narchitecture using doped wide-gap transport layers, Appl. Phys. Lett.,2005, 86, 244102.
48 F. Yang, M. Shtein and S. R. Forrest, Controlled growth ofa molecular bulk heterojunction photovoltaic cell, Nat. Mater.,2005, 4, 37.
49 A. K. Pandey, S. Dabos-Seignon and J. M. Nunzi, Pentacene:PTCDI-C13H27 molecular blends efficiently harvest light for solarcell applications, Appl. Phys. Lett., 2006, 89.
50 K. Suemori, T. Miyata, M. Yokoyama and M. Hiramoto, Three-layered organic solar cells incorporating a nanostructure-optimizedphthalocyanine:fullerene codeposited interlayer, Appl. Phys. Lett.,2005, 86.
51 B. Maennig et al., Organic p-i-n solar cells, Appl. Phys. A, 2004, 79, 1.52 M. Hiramoto, H. Fujiwara and M. Yokoyama, 3-layered organic
solar-cell with a photoactive interlayer of codeposited pigments,Appl. Phys. Lett., 1991, 58, 1062.
53 E. Moons, Conjugated polymer blends: Linking film morphology toperformance of light emitting diodes and photodiodes, J. Phys.:Condens. Matter, 2002, 14, 12235.
54 F. Padinger, R. S. Rittberger and N. S. Sariciftci, Effects ofpostproduction treatment on plastic solar cells, Adv. Funct. Mater.,2003, 13, 85.
55 J. Peet et al., Efficiency enhancement in low-bandgap polymer solarcells by processing with alkane dithiols, Nat. Mater., 2007, 6, 497.
56 W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Thermallystable, efficient polymer solar cells with nanoscale control of theinterpenetrating network morphology, Adv. Funct. Mater., 2005, 15,1617.
57 G. Li et al., High-efficiency solution processable polymerphotovoltaic cells by self-organization of polymer blends, Nat.Mater., 2005, 4, 864.
58 L. Salem, The Molecular Orbital Theory of Conjugated Systems,Benjamin, New York, 1966.
59 L. A. A. Pettersson, L. S. Roman and O. Inganas, Modelingphotocurrent action spectra of photovoltaic devices based onorganic thin films, J. Appl. Phys., 1999, 86, 487.
60 J. Gilot, I. Barbu, M. M. Wienk and R. A. J. Janssen, The use of ZnOas optical spacer in polymer solar cells: Theoretical and experimentalstudy, Appl. Phys. Lett., 2007, 91, 113520.
61 H.Wiesenhofer et al., Limitations of the F€orster description of singletexciton migration: The illustrative example of energy transfer toketonic defects in ladder-type poly(para-phenylenes), Adv. Funct.Mater., 2005, 15, 155.
62 V. I. Arkhipov, P. Heremans and H. Bassler, Why is excitondissociation so efficient at the interface between a conjugatedpolymer and an electron acceptor?, Appl. Phys. Lett., 2003, 82, 4605.
63 T. Offermans, S. C. J. Meskers and R. A. J. Janssen, Chargerecombination in a poly(para-phenylene vinylene)-fullerenederivative composite film studied by transient, nonresonant, hole-burning spectroscopy, J. Chem. Phys., 2003, 119, 10924.
64 A. C. Morteani, P. Sreearunothai, L. M. Herz, R. H. Friend andC. Silva, Exciton regeneration at polymeric semiconductorheterojunctions, Phys. Rev. Lett., 2004, 92, 247402.
65 E. Peeters et al., Synthesis, photophysical properties, andphotovoltaic devices of oligo(p-phenylene vinylene)-fullerene dyads,J. Phys. Chem. B, 2000, 104, 10174.
66 R. Schueppel et al., Enhanced photogeneration of triplet excitons inan oligothiophene fullerene blend, ChemPhysChem, 2007, 8, 1497.
67 M. Koeler, M. C. Santos and M. G. E. da Luz, Positional disorderenhancement of exciton dissociation at donor/acceptor interface,J. Appl. Phys., 2006, 99, 053702.
68 G. Heimel et al., Toward control of the metal-organic interfacialelectronic structure in molecular electronics: A first-principles studyon self-assembled monolayers of pi-conjugated molecules on noblemetals, Nano Lett., 2007, 7, 932.
69 S. Khodabakhsh, B. M. Sanderson, J. Nelson and T. S. Jones, Usingself-assembling dipole molecules to improve charge collection inmolecular solar cells, Adv. Funct. Mater., 2006, 16, 95.
70 M. Hiramoto, M. Suezaki and M. Yokoyama, Effect of thin goldinterstitial-layer on the photovoltaic properties of tandem organicsolar-cells, Chem. Lett., 1990, 327.
This journal is ª The Royal Society of Chemistry 2009
71 B. P. Rand, P. Peumans and S. R. Forrest, Long-range absorptionenhancement in organic tandem thin-film solar cells containingsilver nanoclusters, J. Appl. Phys., 2004, 96, 7519.
72 J. G. Xue, S. Uchida, B. P. Rand and S. R. Forrest, Asymmetrictandem organic photovoltaic cells with hybrid planar-mixedmolecular heterojunctions, Appl. Phys. Lett., 2004, 85, 5757.
73 A. Hadipour, B. de Boer and P. W. M. Blom, Solution-processedorganic tandem solar cells with embedded optical spacers, J. Appl.Phys., 2007, 102, 074506.
74 J. Gilot, M. M. Wienk and R. A. J. Janssen, Double and triplejunction polymer solar cells processed from solution, Appl. Phys.Lett., 2007, 90, 143512.
75 A. G. F. Janssen, T. Riedl, S. Hamwi, H. H. Johannes andW. Kowalsky, Highly efficient organic tandem solar cells using animproved connecting architecture, Appl. Phys. Lett., 2007, 91,073519.
76 J. Y. Kim et al., Efficient tandem polymer solar cells fabricated by all-solution processing, Science, 2007, 317, 222.
77 K. Tvingstedt, V. Andersson, F. Zhang and O. Inganas, Foldedreflective tandem polymer solar cell doubles efficiency, Appl. Phys.Lett., 2007, 91, 123514.
78 M. C. Scharber et al., Design rules for donors in bulk-heterojunctionsolar cells - towards 10 % energy-conversion efficiency, Adv. Mater.,2006, 18, 789.
79 B. P. Rand, D. P. Burk and S. R. Forrest, Offset energies at organicsemiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells, Phys. Rev. B, 2007, 75,115327.
80 C. Waldauf, M. C. Scharber, P. Schilinsky, J. A. Hauch andC. J. Brabec, Physics of organic bulk heterojunction devices forphotovoltaic applications, J. Appl. Phys., 2006, 99, 104503.
81 D. M. De Leeuw, M. M. J. Simenon, A. R. Brown andR. E. F. Einerhand, Stability of n-type doped conducting polymersand consequences for polymeric microelectronic devices, Synth.Met., 1997, 87, 53.
82 J. H. Zhao, A. H. Wang and M. A. Green, 24.5% efficiency siliconPERT cells on MCZ substrates and 24.7% efficiency PERL cells onFZ substrates, Prog. Photovoltaics, 1999, 7, 471.
83 A. Rohatgi, S. Narasimha, S. Kamra and C. P. Khattak, Fabricationand analysis of record high 18.2% efficient solar cells onmulticrystalline silicon material, IEEE Electron Device Lett., 1996,17, 401.
84 J. Yang, A. Banerjee and S. Guha, Triple-junction amorphous siliconalloy solar cell with 14.6% initial and 13.0% stable conversionefficiencies, Appl. Phys. Lett., 1997, 70, 2975.
85 M. K. Nazeeruddin et al., Engineering of efficient panchromaticsensitizers for nanocrystalline TiO2-based solar cells, J. Am. Chem.Soc., 2001, 123, 1613.
86 C. J. Brabec, J. A. Hauch, P. Schilinsky and C. Waldauf, Productionaspects of organic photovoltaics and their impact on thecommercialization of devices, MRS Bull., 2005, 30, 50.
87 A. C. Arias, Vertically segregated polymer blends: Their use inorganic electronics, Polym. Rev., 2006, 46, 103.
88 J. S. Huang et al., Investigation of the effects of doping andpost-deposition treatments on the conductivity, morphology, andwork function of poly (3,4-ethylenedioxythiophene)/poly (styrenesulfonate) films, Adv. Funct. Mater., 2005, 15, 290.
89 W. J. Potscavage, S. Yoo, B. Domercq and B. Kippelen,Encapsulation of pentacene/C60 organic solar cells with Al2O3
deposited by atomic layer deposition, Appl. Phys. Lett., 2007, 90,253511.
90 S. Yoo et al., Integrated organic photovoltaic modules with a scalablevoltage output, Appl. Phys. Lett., 2006, 89, 233516.
91 J. van de Lagemaat et al., Organic solar cells with carbon nanotubesreplacing In2O3:Sn as the transparent electrode, Appl. Phys. Lett.,2006, 88, 233503.
92 M. W. Rowell et al., Organic solar cells with carbon nanotubenetwork electrodes, Appl. Phys. Lett., 2006, 88, 233506.
93 A. D. Pasquier, H. E. Unalan, A. Kanwal, S. Miller andM. Chhowalla, Conducting and transparent single-wall carbonnanotube electrodes for polymer-fullerene solar cells, Appl. Phys.Lett., 2005, 87, 203511.
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