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COMMENT UTILISER DESNANOPARTICULES D’OR POUR

PRODUIRE DE L’ÉNERGIE ?Julien Barrier, Simon Lottier, Baptiste Michon

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OHO

OH

OH

OH

H

OH

H

HHO H

OHO

O Ti

O

e-e-

e-e-e-

e-

Light

Glass

Glass

Iodide

+

Dye

Electrolyte(Iodide/Triiodide)

TiO 2 nanocrystals(Diameter ≈ 20nanometers)

Transparentconductingcoating and catalyst

Transparentconductingcoating

Electrolyte(Iodide/Triiodide)

Dye and TiO 2

Light

Light

Magni�ed1,000 x

Magni�ed1,000,000 x

Cycle

e-

2 CoatedElectrode

Counter Electrode

Electron Injection

I_

e-

–

Load

Iodide TriiodideI_

3

Triiodide

Demonstrating Electron Transfer and Nanotechnology:A Natural Dye–Sensitized Nanocrystalline Energy Converter, Greg P. Smestad et Michael Grätzel, Journal of Chemical Education, 1998

INTRODUCTION

EF

D*/D+

D/D+

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VOChυ

VB

CB

contre-électrodeélectrolytecolorantphotoanodesemiconducteur

ITO

1

2

34

ITO

Pt

TiO2

e−

e−

5

6

I−/I3−

PEDOT:AuNPs

e− i

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COMMENT ON EN EST ARRIVÉ LÀ ?

Electrochimica Acta 125 (2014) 601–605

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

Gold nanoparticles and poly(3,4-ethylenedioxythiophene) (PEDOT)hybrid films as counter-electrodes for enhanced efficiency indye-sensitized solar cells

Sana Koussi-Daoud, Delphine Schaming, Pascal Martin, Jean-Christophe Lacroix ∗

Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France

a r t i c l e i n f o

Article history:Received 13 November 2013Received in revised form 22 January 2014Accepted 25 January 2014Available online 7 February 2014

Keywords:Dye-sensitized solar cells–PEDOT–goldnanoparticles–photoanode–conductivity

a b s t r a c t

PEDOT films incorporating gold nanoparticles have been used for counter-electrodes in dye-sensitizedsolar cells. An increase in efficiency of about 130% has been observed compared to the use of PEDOT alone.Electrochemical investigations of the CoIII(phen)3/CoII(phen)3 used as redox shuttle on the PEDOT-AuNPsand PEDOT electrodes point out towards neither an enhanced catalytic activity nor a plasmonic effectbut strongly suggest that this enhanced power efficiency is due to an increase in the conductivity of thecounter electrode. On a more general point of view, this study shows the important impact of internalohmic drop inside a DSSC on its efficiency.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, solar energy appears as a green and renewable alter-native energy source to fossil fuels. Photovoltaic cells are among thebest devices of harvesting energy directly from sunlight. In partic-ular, solid-state semiconductor cells based on p-n junctions andusually made of silicon have dominated the field of photovoltaicsince its inception. Nevertheless, dye-sensitized solar cells (DSSCs),based on molecular components and proposed over two decadesago,[1] have emerged as a competent alternative technology toconventional semi-conductor devices. Indeed, DSSCs possess keyattractive features such as their low cost, their ease of fabrication,and their powerful sunlight harvesting efficiency.[2,3] Moreover,their improved performance with diffused light and at high tem-peratures, and their negligible dependence on the incident lightangle, are additional advantages over traditional semiconductortechnologies.[3]

Basically, the principle of photovoltaic devices is based on theconcept of charge separation at an interface of two materials, underlight excitation. In the case of DSCCs,[2] the photo-anode of choiceconsists of a transparent conducting oxide (TCO) coating glasssubstrate covered with a nanocrystalline TiO2 layer in which a pho-tosensitive dye is adsorbed. Photo-excitation of this dye resultsin the injection of an electron into the conduction band of TiO2.The original state of the dye is subsequently restored by electrondonation from an electrolyte, usually an organic solvent containing

∗ Corresponding author.E-mail address: lacroix@univ-paris-diderot.fr (J.-C. Lacroix).

a redox shuttle. Finally, the electrolyte is regenerated in turn byreduction at a counter-electrode (namely the cathode), the cyclebeing completed via electron migration through the external cir-cuit. The voltage generated under illumination corresponds to thedifference between the Fermi level of the electron in the TiO2 layerand the redox potential of the electrolyte.

In many studies, the counter-electrode was a platinum-coatedTCO electrode and the redox shuttle was the triiodide/iodide (I3–/I–)redox couple.[1] Indeed, TCO electrodes have a very high chargetransfer resistance, and it is necessary to reduced it, which isachieved by introducing a layer of catalyst on top of the TCO.[4]Platinum layers were currently used because of their high catalyticactivity toward the reduction of I3–. However, platinum is a noblemetal which is rare on earth and very expensive. Fabrication ofcounter-electrodes with alternative cheaper and more abundantmaterials is expected to bring down the production cost of DSSCs.

One decade ago, Saito et al. had reported for the first time the useof poly(3,4-ethylenedioxythiophene) (PEDOT) to substitute the Ptfor counter-electrodes of DSSCs.[5] Indeed, PEDOT is known to havegood conductivity and remarkable stability.[6] In addition, PEDOTcan also catalyze the reaction of triiodide/iodide redox couple,[7]due to the formation of charge-transfer complexes betweeniodine species and polymeric rings.[8] Afterwards, counter-electrodes with catalytic layers based on conducting polymerssuch as PEDOT[9,10], PEDOT doped with poly(4-styrene sulfonate)(PSS)[11], polypyrrole (PPy)[12] and polyaniline (PANI)[13,14]have been studied: More recently, cooperation between conduct-ing polymers and other materials has been also investigated.For instance, PEDOT-graphene[15], PEDOT-carbon nanotubes[16],PEDOT-nanoparticles[17,18] or PANI-Pt nanoparticles[19] hybrid

0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.electacta.2014.01.154

S. Koussi-Daoud et al. / Electrochimica Acta 125 (2014) 601–605 603

Figure 1. (A) Optical absorption spectrum of a PEDOT-AuNPs FTO electrode (FTOplate overcoated with PEDOT alone was used as reference to record the spectrum).(B) SEM image of a PEDOT-AuNPS FTO electrode.

and 0.2 M of LiClO4 in acetonitrile. Finally, the holes were closedwith sealed thin glass plates.

2.6. Characterization of DSSCs

DSSCs were studied under 1 sun illumination (AM 1.5; 100mW/cm2) using a solar simulator (Newport) calibrated with astandard silicon cell (Oriel). A potentiostat/galvanostat (CH Instru-ments 660 C) was used to record J–V curves.

3. Results and discussion

3.1. Preparation and characterization of PEDOT-AuNPs hybridfilms

After dipping of the PEDOT-modified FTO electrodes in the col-loidal solution of AuNPs during 2 hours, the initially blue films ofPEDOT became violet due to the adsorption of AuNPs. Fig. 1A showsthe optical absorption spectrum of such a PEDOT-AuNPs hybrid FTOelectrode (in using a FTO plate overcoated with PEDOT alone asreference to record the spectrum). The typical plasmon band cor-responding to AuNPs is observed around 515 nm. PEDOT-AuNPshybrid electrodes have also been scrutinized by scanning electronmicroscopy (SEM, Fig. 1B), in order to verify the NPs size and toestimate the surface concentration of AuNPs.

Figure 2. J–V curves (full lines) and power curves (dashed lines) of DSSCs usingPEDOT alone (blue curves – a) and PEDOT-AuNPs hybrid films (red curves – b) ascounter-electrodes.

3.2. Photovoltaic performances of DSSCs

Typical current density-voltage (J–V) characteristics of theDSSCs under 1 sun illumination (AM 1.5; 100 mW/cm2) are shownin Fig. 2. Important cell parameters derived from J–V curves are theshort-circuit current density Jsc and the open-circuit voltage (Voc).The fill factor (FF) characterizing the resistivity of the cell and thepower conversion efficiency (�) can be determined by the followingequations:

FF = Pmax

JscVoc

and:

� = Pmax

Pin

where Pmax is the maximum power output from the cell and Pin isthe input optical power (100 mW/cm2). Table 1 summarizes thesephotovoltaic parameters for the three different DSSCs investigated.Values reported are average values from about ten cells for eachkind of counter-electrodes investigated.

Cells using a counter-electrode in platinum exhibited a short-circuit current density around 3.3 mA/cm2, whereas when PEDOTwas used, this value was found around 3.6 mA/cm2, leading to effi-ciencies around 1.3 and 1.4%, respectively. While many works havealready shown that platinum and PEDOT lead to similar efficien-cies when the triiodide/iodide redox couple was used as shuttle,this preliminary result shows that PEDOT can also efficientlyreplace platinum when cobalt complexes are employed as redoxshuttle, these two kinds of counter-electrode leading to similarresults. Moreover, the efficiency obtained with the PEDOT counter-electrode is comparable to the one reported very recently for a DSSCin a similar configuration (same CoIII(phen)3/CoII(phen)3 redoxshuttle and same Z907 ruthenium dye).[41] Thus, results obtainedhere with the PEDOT film can serve as reference when AuNPs will be

Table 1Photovoltaic parameters of the DSSCs with platinum, PEDOT alone and hybridPEDOT-AuNPs films as counter-electrodes.

Platinum PEDOT alone PEDOT with 15-nm AuNPs

Jsc(mA/cm2) 3.3 ± 0.3 3.6 ± 0.5 7.6 ± 0.4Voc(V) 0.71 ± 0.03 0.70 ± 0.02 0.71 ± 0.01FF 0.59 ± 0.02 0.56 ± 0.03 0.60 ± 0.03�(%) 1.3 ± 0.1 1.4 ± 0.2 3.2 ± 0.2

PEDOTAuNPs

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0.8

0.9

1

Abso

rban

ce(u

a)

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1:01:0.011:0.11:10:1

RÉSULTATS

figure 1 : Absorbance UV-visible de plusieurs solutions PEDOT:AuNPs

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f (Hz)

103

104

105

106

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)

conductivity 1V

PEDOTor (10-11 M) in PEDOTor (10-10 M) in PEDOTor (10-9 M) in PEDOTor (10-8 M) in PEDOTor (10-7 M) in PEDOT

102 103 104 105 106

f (Hz)

100

101

102

r

permittivity 1V

PEDOTAuNPs (10-11 M) in PEDOTAuNPs (10-10 M) in PEDOTAuNPs (10-9 M) in PEDOTAuNPs (10-8 M) in PEDOTAuNPs (10-7 M) in PEDOT

figure 2 : Conductivité et permittivité de films de PEDOT:AuNPs en différentes proportions

[AuNPs] ↑

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102

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permittivity 1V

0I0/4I0/2I0

102 103 104 105 106

f (Hz)

104

105

106

107

(S/m

)

conductivity 1V

0I0/4I0/2I0

figure 3 : Conductivité et permittivité de films PEDOT:AuNPs pour différents éclairements

I ↑

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CONCLUSION

OHO

OH

OH

OH

H

OH

H

HHO H

OHO

O Ti

O

e-e-

e-e-e-

e-

Light

Glass

Glass

Iodide

+

Dye

Electrolyte(Iodide/Triiodide)

TiO 2 nanocrystals(Diameter ≈ 20nanometers)

Transparentconductingcoating and catalyst

Transparentconductingcoating

Electrolyte(Iodide/Triiodide)

Dye and TiO 2

Light

Light

Magni�ed1,000 x

Magni�ed1,000,000 x

Cycle

e-

2 CoatedElectrode

Counter Electrode

Electron Injection

I_

e-

–

Load

Iodide TriiodideI_

3

Triiodide

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REMERCIEMENTS

Yvette Tran, pour l’accompagnement ;Emmanuel Fort, pour nous avoir guidé sur l’effet plasmonique ;Heng Lu, pour les manipulations à l’IPGG ;Frédéric Kanoufi, pour nous avoir orienté à l’ITODYS ;Pascal Martin, pour les discussions sur les DSSCs ;Mickaël Pruvost, pour les mesures d’impédance complexe ;Émilie Verneuil, pour l’encadrement ;Jean-Baptiste d’Espinose, pour les conseils ;Raymond Even, pour le matériel.