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1754-5692(2009)2:3;1-2 Energy& Environmental Science www.rsc.org/ees Volume 2 | Number 3 | March 2009 | Pages 241–332 COVER ARTICLE Bernard Kippelen and Jean-Luc Brédas Organic photovoltaics: recent science, engineering results and future challenges PERSPECTIVE Y.-H. Percival Zhang A sweet out-of-the-box solution to the hydrogen economy: is the sugar-powered car science fiction? ISSN 1754-5692

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Page 1: Organic Photovoltaics

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

Page 2: Organic Photovoltaics

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

Page 3: Organic Photovoltaics

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

Page 4: Organic Photovoltaics

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

Page 5: Organic Photovoltaics

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

Page 6: Organic Photovoltaics

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

Page 7: Organic Photovoltaics

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

Page 8: Organic Photovoltaics

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

Page 9: Organic Photovoltaics

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

Page 10: Organic Photovoltaics

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

Page 11: Organic Photovoltaics

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.

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