Optical Properties Engineering for Organic Solar Cells

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Int. J. Materials and Product Technology, Vol. 34, No. 4, 2009 469

Copyright 2009 Inderscience Enterprises Ltd.

Optical properties engineering for Organic Solar Cells

Jean-Jacques Simon*, Ludovic Escoubas,Florent Monestier and Philippe Torchio

Aix-Marseille Universit, IM2NP,

CNRS, IM2NP UMR 6242 (Marseille Toulon),

Facult des Sciences et Techniques,

Campus de Saint-Jrme,

Avenue Escadrille Normandie Niemen Service 231,

F-13397 Marseille Cedex, France

E-mail: [email protected]: [email protected]

E-mail: [email protected]


*Corresponding author

Franois Flory

Directeur de Centrale Marseille Recherche et Technologies,

and IM2NP UMR 6242,

Ecole Centrale Marseille,

Technopole de Chateau-Gombert,

38 rue Joliot Curie, 13451 Marseille Cedex 20, France


Abstract: Organic Solar Cells (OSC) have received increasing attention overthe last few years for their potential technological applications as well asfundamental science. To increase the part of the incident light which isabsorbed in the photoactive layer, optical properties of the cell are critical andshould be optimised. In this paper, we review how photonics has stimulatedresearches and news experimental approaches to improve photovoltaicconversion in OSC. Solutions used to confine the light in the device aredescribed and new architectures of organic solar cell are presented.

Keywords: solar cells; optics; organic materials.

Reference to this paper should be made as follows: Simon, J-J., Escoubas, L.,

Monestier, F., Torchio, P. and Flory, F. (2009) Optical properties engineeringfor Organic Solar Cells, Int. J. Materials and Product Technology, Vol. 34,No. 4, pp.469487.

Biographical notes: Jean-Jacques Simon received his PhD at the University ofMarseille in 1996 for a thesis on Electrical activity of dislocations in silicon.From 1997 to 2000, he worked as a process engineer for IBM Semiconductorat Corbeil-Essonnes, France. He joined the University Paul Cezanne ofMarseille in 2000. Working at the IM2NP laboratory since 2007, his currentresearch interests are photonics for organic solar cells and more generallymicrostructured optical components. He has authored more than 20 papers.


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470 J-J. Simon et al.

Ludovic Escoubas was graduated from Centrale Marseille (a French Grande

Ecole of Engineer), and received a PhD in Optics in 1997. He is now Professorat Paul Cezanne University (Marseille France) and leader of the OPTO-PVTeam (Optoelectronics Components and Photovoltaics) of IM2NP (CNRSLaboratory). His current research interests are micro and nano opticalcomponents and solar cells. He has authored more than 150 papers andcommunications and holds six patents.

Florent Monestier was graduated from the Engineer School of Dijon (France)in 2004. After he did receive his PhD Degree (2008) from the UniversityPaul Cezanne of Marseille, he joined the Astron-Fiamm-safety companyas engineer.

Philippe Torchio received his PhD in materials sciences in 1992. He hasworked in the fields of semiconductor materials for photovoltaic applications,infrared absorption mapping, thin film deposition techniques, and optical

coatings. He is now Senior Lecturer at the Paul Czanne Aix-Marseillethree University and member of the OPTO-PV Team (OptoelectronicsComponents and Photovoltaics) of IM2NP (CNRS Laboratory). His currentresearch interests are photovoltaic cells, in particular organic solar cells,and micro or nano structured optical components. He has authored more than100 papers and communications.

Franois Flory received his PhD in 1978 and his thse d'Etat in 1985 in lightscattering and in the relation between optical properties and the microstructureof thin films, respectively. He is now Professor in Optics at the Ecole Centralede Marseille. He is editor of the book Thin Films for Optical Systems andof more than 150 papers and communications. He has been chairman ormember of the scientific committee of more than 15 international conferences.His current research interest is now in the field of micro/nano photonics.

1 Introduction

Among the variety of renewable energies, photovoltaics is a proven technology.

Today silicon is still the leading technology in the world market of photovoltaic solar

cells, with power conversion efficiencies reaching 25% for mono-crystalline devices.

Organic Solar Cells (OSC) have received increasing attention over the last few years.

Indeed, the materials used in photovoltaic polymer domain offer many practical

advantages over conventional photovoltaic materials such as silicon thanks to the solution

processing techniques used to fabricate the cells (low cost, flexible and large scale

applications). The power conversion efficiency of polymer-based solar cells has beensignificantly increased during the last years from 1% in 1986 (Tang, 1986) to 5% in 2005

(Xue et al., 2005) and more recently to 6% in 2007 (Kim et al., 2007). With such

efficiencies, OSC will possibly be a competitive alternative way to inorganic solar cells

in the near future. Generally, the different OSC are distinguished by the production

technique (spin-coating, doctor blade or vacuum deposition techniques), by the

photoactive materials used (polymer or small molecules) and by the device architecture

(single layer, bilayer heterojunction or bulk heterojunction).


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Optical properties engineering for Organic Solar Cells 471

The aim of this paper is to review how photonics has stimulated researches and news

experimental approaches to improve photovoltaic conversion in OSC. After a shorthistoric review of OSC, we will described, in the first part of this paper, the general

architecture of OSC and the physical mechanisms leading to the generation and collection

of charge carriers in such devices. Differences between OSC and conventional solar cells

(silicon based) will be underlined. In a second part, we will focus on the optical

optimisation of a standard organic solar cell (modulation of the electromagnetic field

inside the device implementation of periodic structures realisation of tandem

cells use of plasmon properties). The third part will be devoted to news concepts and

architectures of OSC. We will present recent efforts and promising routes to increase the

interaction between light and OSC.

2 Organic solar cells background

In 1959, the first investigation of an organic photovoltaic cell related to an anthracene

single crystal was performed with a Schottky diode structure. The cell exhibited

a photovoltage of 200 mV with an extremely low efficiency (Kallmann and Pope, 1959).

Since then, many years of research have shown that the typical power conversion

efficiency of PV devices based on single (or homojunction) organic materials will remain

below 0.1%. The difficulty was to find an organic material presenting both best electrical

properties (both for excitons, electrons and holes) and a sufficiently high optical

absorption. A major breakthrough in the cell performance came in 1986 with the

introduction of the bilayers device concept. In this device architecture an electron donor

layer (D) and an electron acceptor (A) layer are stacked on top of each other in one cell,

like a p-n junction in silicon solar cells. In the pioneering work of Tang (1986),

a CuPc/perylene derivative bilayer system was sandwiched between In2O3 and silver

electrodes and reached a power conversion efficiency of around 1%. Due to the

low exciton diffusion length, the efficiency of bilayer solar cells is limited by the

charge generation which occurs only in a region of 1020 nm around the Donor-Acceptor

(D-A) interface. The revolutionary development in OSC came in the mid 1990s with

the introduction of the bulk heterojunction devices (Yu and Heeger, 1995), where the

donor and acceptor material are blended together. In such a nanoscale interpenetrated

network, each exciton created could reach a dissociating site (interface D/A) within

a distance lower than the exciton diffusion length. Nowadays, the bulk heterojunction

is still the most promising concept for all-organic PV cells. Various combinations

of donor and acceptor materials have been studied. Using regioregular poly

(3-hexylthiophene) (RR-P3HT) as donor and 6,6-Phenyl C61-Butyric Acid Methyl

ester (PCBM) as accceptor bulk heterojunction solar cells have been realised withexternal quantum efficiencies of around 75% and power conversion efficiencies up to 5%

(Kim et al., 2006).

3 Working principles of an organic solar cell

The structure of an OSC is depicted in Figure 1. As observed, the cell is illuminated

through a transparent substrate. A conductive and transparent anode (usually a Indium

Tin Oxide (ITO)) is deposited on the substrate and usually five layers are then deposited


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on top of this anode: a first interfacial layer (typically a film of PEDOT:PSS

(poly(ethylene-dioxythiophene) doped with polystyrenesulfonic acid)), a photoactivelayer, a second ultrathin interfacial layer (LiF (lithium fluoride) or BCP) and a thick

cathode. The organic materials composing the photoactive layers are generally split into

two groups: small molecules and conjugated polymers. Small molecules, which have

a well-defined molecular weight, are deposited by low or high pressure vapour methods.

To create interpenetrating D-A networks co-evaporation techniques can be applied.Polymers are usually processed from solution (spin-coating, doctor blading or

screen-printing techniques (Shaheen et al., 2001).

Figure 1 Schematic of an Organic Solar Cell (see online version for colours)

There is a fundamental difference between solar cells based on inorganic or on organic

semiconductors. In solar cells made from inorganic semiconductors, photons are directly

converted at room temperature into free charge carriers (electrons and holes) which

can be collected at their respective electrodes. In OSC, a photon absorption leads to the

creation of a bound electron hole pair called exciton, which binding energies are ranging

from 50 meV up to >1 eV (Gregg, 2003). Then excitons have to diffuse to be dissociated

into free charges. This additional stage of excitons diffusion completely changes the scale

of the physical phenomen involved in OSC. Indeed, in silicon photovoltaic technology,

the electron diffusion length could reach the wafer thickness (>200 m) even in

polycrystalline silicon. In organic materials, both the exciton diffusion length and the

charge carrier diffusion lengths are in submicronic range (1020 nm for excitons and

100200 nm for charge carriers). The most visible consequence of such low values is the

design of the organic photovoltaic devices where the active layer thicknesses do not

exceed 200 nm. In addition, the control of the morphology of each layer at the nanometer

scale strongly impact the performance of OSC.

Before going more into the details, we recall here the four steps of the photovoltaic

conversion process in OSC (see Figure 2):


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Figure 2 Energetic diagram and photovoltaic conversion process in OSC (see online version

for colours)

3.1 Photon absorption (photon absorption efficiency: A)

Upon absorption of a photon having an energy larger than the gap, an electron is

promoted from the Highest Occupied Molecular Orbital (HOMO) band to the Lowest

Unoccupied Molecular Orbital (LUMO) band of the donor material. The majority of

organic materials have band gaps higher than 2 eV which limits to less than 30% the

possible harvesting of solar photons when a gap of 1.1 eV (silicon) makes possible to

absorb 77% of solar radiations. Moreover, due to low charge carrier mobilities, the activelayer thickness should be restricted to around 150 nm to avoid serie resistances.

Fortunately, absorption coefficient are high in organic materials (exceeding 105 cm1) and

make possible to absorb the main part of the incident light even with a thicknesses of

around 100 nm. Thicknesses of the films are not the bottleneck.

3.2 Exciton diffusion

The strong local electrical field at a D-A material interface will be used to dissociate

excitons. For efficient dissociation of excitons, the distance between the photon

absorption and the first interface should be lower than the exciton diffusion length

(between 10 nm and 30 nm).

3.3 Exciton dissociation or charge separation (exciton dissociation

efficiency: D)

When an exciton reaches the interface between donor and acceptor materials, the charge

separation takes place in an ultrafast timescale of about 45 femtoseconds (Brabec et al.,

2001). However, a necessary condition for efficient dissociation of the created excitons

is that the difference between the LUMO of the donor and the acceptor is higher than the

exciton binding energy. If this is the case, electrons are transferred to the acceptor. Up to

now, in conjugated polymer based OSC the most efficient electron acceptors found are


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C60 based fullerenes. Because the separation is faster than any other competing process,

its efficiency is about 100%.

3.4 Charge transport (charge carrier collection efficiency: CC)

The separated charges then need to diffuse to the respective device electrodes,

holes to the anode and electrons to the cathode to provide voltage. In conjugated

polymer, transport charge carriers is done by successive jumps (hopping) form a located

state to another, which is a notable difference with the inorganic semiconductors,

where the conduction electrons move freely through delocalised states. Mobilities

in organic materials are low and could also be reduced by traps. Values are typically

104

cm2/V.s. but higher mobilities have been reported in pentacene (Unni et al., 2004;

Jurchescu et al., 2004).

In the following, we will focus on optical properties engineering in OSC.More details on the other properties of OSC are available in many recent reviews:

4 Optical optimisation of an organic solar cell

The basic idea developed in this paragraph is to increase the part of the incident light

which is absorbed in the photoactive layer.

An example of the spectral redistribution of the incoming light (on reflection

and layer absorption) for a standard bulk heterojunction solar cells (pentacene:

PTCDI-C13H27 blend) is shown in Figure 3. If most of the incoming light is absorbed in

the blend in the 400700 nm range, average reflection is about 10%, ITO absorption is

about 15% while PEDOT absorbs mainly in the blue range (4%). Although a remarkablyhigh external quantum yield measured in this device (83%), the best power conversion

efficiency reported for this couple of materials does not exceed 2% (Pandey et al., 2006).

Figure 3 Electromagnetic field distribution in OSC (see online version for colours)

The first approach to improve the performance of the cell is to increase the thickness of

the absorbing layer. However, this thickness is generally restricted (


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limited charge carrier mobility in conjugated polymers (highly efficient devices with

thickness around 200250 nm have been recently demonstrated (Irwin et al., 2008)).Thus, to overcome this problem and from the optical point of view the following

solutions can be considered:

optimisation of the electromagnetic field distribution inside an OSC

fabrication of tandem cells

implementation of periodic structures (such as gratings or photonic crystals) in the

thin layers stacking

investigation of plasmons effects for increasing absorption in OSC.

4.1 Optimisation of the electromagnetic field distribution

A solar cell can be represented by stacked thin films surrounded by a semi infinite

substrate and a semi infinite transparent medium (Figure 3). Each layer i is defined by

its thickness di and by its refractive index ni and its extinction coefficient ki as a function

of wavelength . In such a device, when layer thicknesses are thin compared to the

coherence of the incident light, interference effects are not neglected and the one

dimensional transfer matrix formalism (Berning and Berning, 1963) has to be applied

to modelise the distribution of the electromagnetic field inside the solar cell (Pettersson

et al., 1999). An example of the distribution of the squared modulus of the

electromagnetic field IEI, in the depth of the device and for wavelengths between

300600 nm, is given in Figure 4. To go further into details and to link the opticalproperties to the electrical properties in OSC, the key equation is the following one:

solarlight

22 ( )0

000

1( , ) ( )2

zii

n EQ z n E z I En

= = (1)

where Q is the local energy dissipated in the material, 0 is the permittivity of vacuumand 0 is the permeability of vacuum, is the absorption coefficient, n0 is the real part ofthe complex refractive index of the glass substrate, and Isolar light is the polychromatic

incoming light with standard AM1.5 distribution.Indeed,from equation 1 and by takinginto account values (A,D and CC), the generation rate of free carriers G(z) andthe short current density Jsc can be computed (Peumans et al., 2003) and used to predict

the organic solar cell efficiency. Thus, a large number of recent papers concerns the

comparison between experimental results and optical modelisation of OSC (Sievers et al.,

2006; Monestier et al., 2007b; Sylvester-Hvid et al., 2007). For example, it has been

shown that the dependency with the active layer thickness of the short circuit currentdensity of OSC based on blend heterojunction (Figure 5) follows a rather complicated

behaviour (Moul and Meerholz, 2007; Hoppe and Sariciftci, 2007; Monestier et al.,

2007b). Furthermore, based on the electromagnetic field optimisation, Monestier et al.

(2007a) has developed an automatic software for optimising the design of OSC.

As regards organic tandem cells, this software is set to find simultaneously optimal

thickness of each layer and to balance the photocurrent in the two active regions.

In summary, optical modelling is essential to understand the behaviour of OSC devices

consisting of multilayer structures.


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Figure 4 3D representation of the electromagnetic field distribution vs. incident wavelength

inside an organic solar cell (see online version for colours)

Figure 5 Comparison of computed short circuit current density (solid line) with experimentaldata (circles) as a function of blend thickness

4.2 Realisation of tandem cells

Because of the narrow absorption spectra of most organic materials used in solar cells,

tandem structures must be considered. The idea of a tandem cell is to achieve a better

absorption efficiency by using materials having different band gap. Then one material

collect the higher energetic photons and the other, one with a lower band gap than the

first one, absorbs photons with lower energy. In a tandem cell, two OSC are deposited on

top of each other and can be connected in serie or in parallel. In serial connection,

the intermediate electrical contact layer is of a major importance:

it should acts as a recombination centre to prevent cell charging

it should be as transparent as possible

it should act as a optical spacer to confine the energy in the two active layers.

As regards the fabrication of polymer tandem cells, an other criterium is required:

the intermediate layer has to protect the bottom cell during the deposition of the top cell

(preventing dissolution or damaging the bottom cell specially when similar solvents are

used). This intermediate layer is regarded as a bottleneck for the development of polymer


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tandem cells. For more details please refer to the review of organic tandem cells

which has been recently published by Hadipour et al. (2008). The first significantpower conversion efficiency of an organic tandem cell (2.5 %) has been described by

Yakimov and Forrest (2002). The two cells were connected by a silver layer. In the work

of Xue et al. (2004) related to tandem cells based on evaporated layers of small molecules

(CuPc and C60), a power conversion efficiency higher than 5% is achieved using Ag

nanoclusters with a typical thickness 0.5 nm instead of a very thin Ag interlayer as used

before for the separating layer. The role of the Ag nanoclusters is not yet clearly

understood, in addition to recombination centres, silver particles can also serve as

scattering centres for incident light and can also excite plasmons as it will be shown in

the part three of this paper. Then, various concepts of intermediate layer were

successively introduced in order to allow the fabrication of polymer tandem cells:

by Kawano et al. (2006) with ITO deposited by sputtering

by Hadipour et al. (2006) with a composite layer of Al, Au, Lif and PEDOT:PSS

by Gilot et al. (2007a) with ZnO nanoparticles deposited by spin coating and covered

by a neutral PEDOT layer.

Finally, with a power conversion of 6.7% the recent work of Kim et al. (2007) is the best

power conversion efficiency reported until now and not only for organic tandem solar

cells but for all OSC. To reach this value, the authors have used a highly transparent

sol-gel processed titanium oxide (TiOx) as intermediate layer and a second TiOx layer

between the top cell and the Al electrode (Figure 6). As explained by the authors, this

second layer acts as an optical spacer that redistributes the light intensity to optimise the

efficiency of the back cell. This work confirms that inserting a transparent electron

transporting layer between the active layer and the Al top electrode allows us to confine

the electromagnetic field inside the active layer. The same effect has also been

demonstrated by Gilot et al. (2007b) with a ZnO layer as the optical spacer.

Figure 6 Tandem polymer solar cell as describe in by Kim et al. (2007) (see online versionfor colours)

It is important to note that more than 200 individual tandem cells were necessary,

as reported by the authors, to optimise the fabrication procedure and device architecture


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(optimising and balancing the current in each subcell), proving that the optical

modelisations described previously are essential.In summary, the organic tandem cell research has developed during the last five years

with a scientific interest triggered by a rapid increase in power conversion efficiencies.

This was achieved by the introduction of new materials, but mainly by the improvement

of technical processes (sol-gel technique) and the improvement of the understanding

of the optical properties of the multilayer structure. To go further, the knowledge coming

from optical interference coatings (generally inorganic) may allow the development

of new concepts.

4.3 Implementation of periodic structures in the multilayer stack

Periodic structures for light trapping have been used extensively to enhance absorption

in silicon solar cells in addition to standard anti-reflection coatings. Silicon wafer randomtexturing is usually achieved in commercial solar cells using anisotropic etching of the

silicon network. More complex techniques, like reactive ion etching, have been

performed to obtain high-efficiency structures based on pyramids (Zechner et al., 1998).

Most of the above structures usually have feature sizes larger than 10 m and are

efficient because they use the geometric optics phenomenon of ray trapping. To reach

un-polarised reflectances as low as 14% submicrometric structures are needed

(Hava and Auslender, 2000).In OSC the context is different: reflectivity losses at the air-device interface are lower

than in silicon, around 4% for an air-glass interface instead of 30% for an air-silicon

interface, but the absorption of organic solar cell photoactive layers is too weak as

explained above (mainly the high organic materials band-gap). Thus, the challenge in

OSC is to implement periodic structures inside the multilayer stack to confine the

incident light in the active layer (see Figure 7). Optical simulations and realisation

of diffractive optical structures in OSC have been first investigated by Niggemann

(2004). Their work is base on two concepts described in Figure 8: diffraction gratings (a)

and buried nano-electrodes (b). The Rigorous Coupled Wave Analysis (RCWA) method

was used in both architectures. In case (a), the sinusoidal grating leads to a resonant

coupling of the light into the active polymer film ((MDMO-PPV:PCBM) blend 200 nm

thick) and the substrate acts as a light-guide for propagating diffraction orders.

The diffraction grating has been realised by embossing technique (period of 720 nm and

a depth of 190 nm). In case (b) the acrylic substrate is nanostructured to enhance the

charge carriers collection at the electrodes. Dimensions of the nanostructure are a period

of the lamellae of 720 nm, a depth of approximately 400 nm and a width of the cavities of

400 nm. Although some difficulties to correlate computations and initial experimental

results, the potential of diffractive optical structures for OSC has been evaluated for thefirst time by this researchers team. More recently, Yang et al. (2007) has evaluated the

short-circuit photocurrent given by an OSC containing nanograting heterojunctions using

a two-dimensional analytic exciton transport theory. In this work the optical properties of

the nanogratings are neglected (homogeneous theory) because the authors assumed that

the dimension of the gratings (period of 100 nm and height ranging between 10 nm and

140 nm) are much smaller than the incident wavelength (300800 nm).


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Figure 7 Diffractive optical structures in OSC (diffraction gratings (a) and buried

nano-electrodes (b)) as describe by Yang et al. (2007) (see online version for colours)

Figure 8 Distribution of the incoming energy in a bulk heterojunction solar cellITO/(pentacene/PTCDI-C13H27 blend)/BCP/Al. (The thin BCP layer isincluded but not represented)

Still to increase the interaction of the light with the photoactive layer, an original study of

nanometric periodic patterning for enhancement of OSC power efficiency has been

reported by Cocoyer et al. (2006). As illustrated in Figure 9, a nanostructured azopolymer

layer is used as a substrate in a classical CuPc/C60 bilayer organic solar cell

(ITO/PEDOT/CuPc/C60/BCP/Ag). Sinusoidal periodic structures are realised in one step

using optical interferences. A mass transport process occurs in azopolymer films when

illuminated. According to the grating formula, new absorption bands (which depends on

the grating period) appear in the external quantum efficiency spectra leading to an

enhancement of the short-circuit photocurrent (Jsc) of about 15%. Indeed 2D gratings

can increase the interaction of the light with the photoactive layers in OSC by coupling a

part of the solar spectrum into a quasi-guided mode. This is clearly one of the solution

to increase the overlapping between the absorption range of OSC and the solar spectrum.

Before closing this part, we would like to mention a preliminary study of Escarreet al. (2005) devoted to the realisation of random roughness surfaces on plastic substrates

by means of hot embossing processes. Different random roughnesses (nanometer and

micrometer sizes) have been transferred on poly(methylmethacrylate) (PMMA) using

two kind of master (commonly available frosted glass and commercial transparent

conductive oxide). After stamping, the PMMA rough surfaces have been optically

characterised by measuring the transmitted and reflected scattered light (red-diode laser

source (= 633 nm)). With a roughness RMS value around 2.6 m, the PPMA layer

embossed with a frosted glass exhibits the best optical behaviour (much closer to an ideal

diffuser). Beyond the results, this work demonstrates that a low cost technique


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(hot embossing) with available and cheap masters (frosted glass) can be used to produce

random textures on plastic surfaces. This can be of a great interest to enhance theabsorption an OSC, especially in the near infra-red domain.

Figure 9 Schematic of an organic solar cell with a nanostructured substrate as describeby Cocoyer et al. (2006) (see online version for colours)

4.4 Plasmons for increasing absorption in OSC

An optically generated oscillation of free electrons that takes place along

a metal/dielectric interface is a surface plasmon. By tuning the excitation light,

a resonance occurs when the incident photon frequencies match the collective oscillations

of the conduction electrons of the metallic particles. In photonics, this field of interest is

known as plasmonics. Among a wide range of applications, these surface plasmons

properties can be used in photovoltaic domain in order to improve performances of solarcells. Noble metal particles such as gold or silver can exhibit an enhanced absorption in

the visible range.

Thin film amorphous silicon solar cells were manufactured by Derkacs et al. (2006)

in which gold nanoparticles were used to engineer the transmission and spatial

distribution of the electromagnetic fields in visible wavelengths inside a-Si:H layer,

resulting in an increase of 8.1% in short-circuit current density and 8.3% in energy

conversion efficiency compared to values achieved in reference devices without gold

particles. Pillai et al. (2007) found that surface plasmons can increase response of silicon

cells over the visible as well as near-infrared spectra. They reported a significant

enhancement of the absorption for both thin-film and wafer-based crystalline silicon

structures including silver nanoparticles. This light trapping approach also based on the

effect of scattering by particles in silicon devices allows clearly an amplication of theinteraction between light and material. This work was initiated by Schaadt et al. (2005).

Similar studies were carried out with organic materials to enhance light

absorption, subsequently leading to an increase in the amount of excitons. In the systems

ITO/metal-clusters/CuPc/In, it was shown by Stenzel et al. (1995) that the incorporation

of copper or gold increases the photocurrent by a factor of more than 2. This work was

also performed with ITO/silver-clusters/ZnPc/Ag components by Westphalen et al.

(2000). A schematic of such devices using surface plasmons effect is given in Figure 10.

Optical properties of silver nanoparticles used in tandem thin-film OSC were investigated

by Rand et al. (2007a, 2007b). The multiplayer stack consists in a serie connection of two


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D-A heterojonctions including a very thin metallic nanocluster layer separating each

subcell. It was shown an optical-field enhancement due to plasmon generation on theultra-thin cluster surfaces which involved higher efficiencies.

Figure 10 Schematic of the simple structure with the intermediate metallic clusters layer

5 New organic solar cell architectures for the enhancementof light harvesting

At the same time, apart the development of new photovoltaic organic materials,

many studies concern new architectures of organic solar cell which are alternative to the

typical thin film structures described in the first part of this paper. This research area can

be illustrated by four examples of architectures in which new optical properties of OSC

are demonstrated:

an organic solar cell manufactured on the model of an optical fibre (Liu et al., 2007)

a folded reflective tandem organic solar cell (Tvingstedt et al., 2007)

an OSC without ITO (Tvingstedt and Inganas, 2007)

a luminescent concentrator coupled to an organic solar cell (Koeppe 2007).

5.1 Optical fibre based architecture

In this work, Caroll and co-workers have investigated the possibility to convert photons

into electrons using a modified standard multimode optical fibre. As shown in Figure 11,

the basic idea is to replace the conventional hard polymer cladding by a photovoltaic

material layer (P3HT/PCBM). In such a structure, electrons are collected by an ITO layer

inserted between the fibre core and the active layer while the holes are collected by an

aluminium layer recovering the fibre. The working principle is as following: a part of the

incident light is coupled into the fibre (illumination of the cleaved face at normal angle

of incidence) and is attenuated during propagation because of the absorption in the active

layer. High current densities near 28 mA/cm2

are obtained, proving that the internal

conversion efficiency is very high in such architecture. Conversion efficiency of 1.1%

is achieved after optimisation of the fibre diameter.


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Figure 11 Optical fibre based architecture for OSC as describe by Liu et al. (2007)

(see online version for colours)

5.2 A folded reflective tandem organic solar cell

An original OSC architecture has been developed by Swedish researchers (Tvingstedt

et al., 2007), as shown in Figure 12 the cell exhibits a V shape geometry and combines

two different cells realised on each face of the V structure. The two cells are based on

alternating copolymers of fluorene (APFO3 and APFO-Green9) as donors and PCBM as

acceptor. The both materials are first deposited on a flat substrate which is then folded.

The first advantage is the broad spectra of absorption of this structure: the APFO-Green9

absorbs mainly in the red range while the APFO3 absorbs until 850 nm. In addition, the

folded structures cause light trapping at high angles. And finally, this architecture allows

to use tandem or multiple bandgap solar cells in optical and electrical series or parallelconnection. Added to the power conversion efficiencies demonstrated in this work

(between 3% and 4%), process allows a large scale production of folded reflective cells

by roll to roll inkjet printing.

Figure 12 Folded reflective tandem organic solar cell as describe by Tvingstedt et al. (2007)(see online version for colours)

5.3 An organic solar cells without ITO

The same Swedish group has also developed a very interesting method to eliminate the

ITO layer from the classical OSC architecture. Because the amount of Indium is limited

on the earth, the idea of the authors is to replace ITO layer by metallic micro grids

(Figure 13). It could be an alternative to new transparent conductive oxydes (TCO) or to


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more complex stacks like a thin metallic layer embedded between two transparent layers.

Metallic grids are already used to realised the electrode at the front side of silicon solarcells. The thickness of the silver grips for OSC is much smaller than in the case of silicon

solar cells (around 100 nm instead of more than 10 m) but the working principle is the

same: dimensions of the grids are optimised to reach the conductivity needed (around

2 105 S.cm1) and the period of the structure is calculated to minimise the shadow effect

of the metallic grids. The power conversion efficiency (1%) of this free ITO solar cell is

found to be better than the ITO based OSC (0.83%). This remarkable result is obtained

by using a new high conductive layer: a PEDOT:PSS with diethylene glycol (DEG) for

which conductivity is 4000 times higher than conventional PEDOT:PSS. However, if the

concept of such architecture without expensive ITO is demonstrated, the process to

realise the grids remains complex with a fluidic deposition method exploiting PDMS

stamps. For manufacturing, a cheaper grids deposition technique should be developed.

Figure 13 Architecture of an OSC without ITO as describe by Tvingstedt and Inganas (2007)(see online version for colours)

5.4 A luminescent concentrator coupled to an organic solar cell

As underlined before, an important feature of OSC is the limited spectral range of

absorption of OSC. To overcome this limitation, low energy band gap materials are

intensively investigated. Koeppe et al. (2007) have chosen another way: authors report

the application of luminescent concentrators on OSC. As in the concept of

antenna systems, in this work, optical absorption and charge transport are dissociated.

Indeed, as shown on Figure 14 the light is absorbed in a luminescent concentrator made

of polymethyl methacrylate plate doped with red dye. Such dyes absorb in the

550750 nm range and are luminescent in the 400600 nm range. The luminescent light

is collected by two OSC based on a ZnPC:C60 blend and fixed onto the respective sides

of the concentrator plate which acts as a waveguide. In the future, the most important

application of this architecture for an OSC mass production could be to used the

concentrator as a substrate.


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Figure 14 A luminescent concentrator coupled to an OSCas describe by Koeppe et al. (2007)

(see online version for colours)

6 Conclusion

Recently, OSC have broken the 6.5% power conversion efficiency barrier. In this paper

we have reviewed how photonics has stimulated researches and news experimental

approaches to improve photovoltaic conversion in OSC. If it seems to be sure that new

materials are needed to push the efficiencies into the 10% range, developing new optical

concepts (including photonic crystal for example) might be one promising way for OSC

development.

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