6th

Hybrid Organic Photovoltaic Conference - Lausanne 2014

P53 The end of Pt counter-electrodes: Transparent graphene-based cathodes for

iodine and cobalt-based liquid-junction (electrolytes in) DSCs

Rui Cruza, Luísa Andrade

a, Adélio Mendes

a

LEPABE – Laboratório de Engenharia de Processos, Ambiente, Biotecnologia e Energia, Faculdade de Engenharia da Universidade

do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, PT

Dye-sensitized solar cells (DSCs) are a promising technology among the so-called third

generation of photovoltaic cells. DSCs use materials fairly abundant/environmentally friendly

for lowering production costs and increase sustainability. This technology is now emerging due

to its superior performance under more demanding atmospheric conditions and semi-

transparency properties[1]

. Nevertheless, to turn DSCs a truly cost-competitive alternative to

conventional photovoltaic devices in the energy market it is necessary new cost-effective

materials. Amongst them it is the replacement of the catalyst material used for the

photoelectrochemical reaction that takes place at the counter-electrode side of DSCs.

To date, there was no other material capable of matching both the electrocatalytic activity

for the I3−/I

− redox couple (the most used electrolyte system), and the high optical

transparency of the traditionally used Pt electrodes. Although transparency is not mandatory

for a DSC, it is highly appreciated as it increases the product value by enabling its use in

building integrated applications (BIPV), as well as in other solutions such as tandem cells.

Here we present the first graphene-based CE capable of replacing Pt (yielding

simultaneously high efficiency and transparency) for iodine-based electrolyte systems in liquid-

junctions DSCs. When nickel nanoparticles are placed between a FTO substrate and oxidized

graphene platelets, they are capable of partially restoring the electronic double bonds along

the platelets as well as improving ohmic resistance between the catalytic material and the FTO

layer (Figure 1). Both materials and processes should enable the fabrication of a cheaper DSC.

The microstructure and morphology of the prepared counter-electrodes werecharacterized

by FE-SEM, TEM, AFM and Raman analysis. TGA/DTA and XPS were used to evaluate the

oxygen content at the graphene platelets. The optimization of the graphene-based counter-

electrodes involved maximizing its catalytic activity (efficiency) and transparency by creating a

sufficient number of active sites while ensuring enough conductivity in the graphene platelets

and a good electrical contact with the substrate[2]

. This evaluation was carried out using half-

cells and complete DSCs [3]

; characterization included I-V and EIS analysis.

The use of the graphene/Ni CE was also validated to cobalt complexes electrolytes/

porphyrin dyes systems. This was done because conventionally used Pt/iodide-

triiodide/ruthenium dye systems have efficiency limitations[4]

; the cobalt complexes/ porphyrin

dyes systems have the potential to yield high-efficiency (> 12 %) cells, with graphene being the

best suitable material to act as the catalytic layer[5-9]

.

6th

Hybrid Organic Photovoltaic Conference - Lausanne 2014

Figure 1 Highly transparent, low cost and easily scalable CEs for DSCs, comprised of a structured graphene film over

nickel nanoparticles (graphene/Ni CE), matched the energy conversion efficiencies of the reference platinum CE for

liquid-junction iodine and cobalt-based electrolytes.

References

[1] O'Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2

Films. Nature, 1991. 353(6346): p. 737-740.

[2] Kavan, L.; Yum, J.H.; Grätzel , M. Optically Transparent Cathode for Dye-Sensitized Solar Cells Based on

Graphene Nanoplatelets. Acs Nano, 2010. 5(1): p. 165-172.

[3] Cruz, R.; Pacheco Tanaka, D.A.; Mendes, A. Reduced graphene oxide films as transparent counter-

electrodes for dye-sensitized solar cells. Solar Energy, 2012. 86(2): p. 716-724.

[4] Hamann, T.W., The end of iodide? Cobalt complex redox shuttles in DSSCs. Dalton Transactions, 2012.

41(11): p. 3111-3115.

[5] Roy-Mayhew, J.D. et al., Functionalized graphene sheets as a versatile replacement for platinum in dye-

sensitized solar cells. ACS Applied Materials and Interfaces, 2012. 4(5): p. 2794 -2800.

[6] Kavan, L. et al., Graphene nanoplatelet cathode for Co(III)/(II) mediated dye-sensitized solar cells. Acs

Nano, 2011. 5(11): p. 9171-9178.

[7] Yella, A. et al., Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12

Percent Efficiency. Science, 2011. 334(6056): p. 629-634.

[8] Stefik, M. et al., Carbon-graphene nanocomposite cathodes for improved Co(ii/iii) mediated dye-

sensitized solar cells. Journal of Materials Chemistry A, 2013. 1(16): p. 4982 -4987.

[9] Ju, M.J. et al., N-Doped Graphene Nanoplatelets as Superior Metal-Free Counter Electrodes for Organic

Dye-Sensitized Solar Cells. ACS Nano, 2013. 7(6): p. 5243-5250.

• Liquid-junction DYE SENSITIZED SOLAR CELLS ( DSCS ) commercial devices need new cost-effective materials

• Requirements for alternative catalyst materials: EFFICIENCY and STABILITYTRANSPARENCY ads product value for applications such as BIPV

Acknowledgements: This work was supported by FCT and Efacec – Engenharia e Sistemas S.A. (grant no.

(SFRH/BDE/33439/2008) and projects WinDSC SI&IDT (ref. 21539/2011), SolarConcept (ref. PTDC/EQU-EQU/120064/2010)

and BI-DSC (ERC Advanced Research Grant ref. 321315). The authors would like to acknowledge Efacec – Engenharia e

Sistemas S.A. for the facilities and extra financial support. Rui Cruz also acknowledges project REELCOOP (co-funded by

the SP7 framework programme grant ref. 608466) for funding.

Rui Cruza, Luísa Andradea Adélio Mendesa*

a LEPABE - Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias /n, 4200-465

Porto, Portugal

*Corresponding author: Tel.: +351 225081695; Fax: +351 225081449; E-mail addresses:

mendes@fe.up.pt; landrade@fe.up.pt; rui.cruz@fe.up.pt

� The developed mGOM5/Ni CE matches the performance,

stability and transparency of Pt for iodine-based systems in

DSCs. Extension to cobalt systems should result in similar

results

� The mGOM5/Ni CE is a low-cost and easily assembled solution

enabling the fabrication of cheaper liquid-junction DSCs

The end of Pt counter-electrodes: Transparent graphene-based cathodes

for iodine and cobalt-based liquid-junction (electrolytes in) DSC

INTRODUCTION

RESULTS

CONCLUSIONS

DyeGlassTiO2

Electrolyte

-3I

-Ie-

e-

e-

Light

e-

e-

GlassFTOFTO Nickel

Graphene

FTO

Gla

ss

�↓Rct ⇒↑Cataly6c ac6vity ≡ ↑ ηηηη �↓Rs ⇒↓Ohmic resistance ≡ ↑ ηηηη

FTO

mGOM5/-, 20 ⁰C

mGOM5/-, 550 ⁰C, inert gas

mGOM5/Ni, 550 ⁰C, inert gas

CE (Solvent) VOC / mA cm-2 JSC / mA cm-2 FF ηηηη / % ττττ550 nm / % Rs (Ωcm2) Rct,CE (Ω cm2)

Pt 0.76 16.2 0.61 7.45 92.0 5.27 5.34

mGOM5/Ni (H 2O) 0.74 15.8 0.64 7.51 80.6 5.81 2.76

mGOM5/Ni (EtOH) 0.74 15.7 0.64 7.54 91.8 5.37 2.30

CE ID/IG

mGOM5/-, 20 ºC 1.153

mGOM5/-, 550 ºC 1.070

mGOM5/Ni, 20 ºC 1.181

mGOM5/Ni, 550 ºC 0.954

CE Ra / nm

FTO 13.5

mGOM5/-, 20 ºC 9.7

mGOM5/- 12.5

Pt mGOM5/Ni

0

10

20

30

40

50

0.1 1 10 100 1000 10000 100000

Pha

se(d

eg)

Frequency (Hz)

Pt

Ni

Au

mGOM5/-

mGOM5/Ni

mGOM5/Au

0

20

40

60

80

100

0 20 40 60 80 100-Z

'' (Ω

cm2 )

Z' (Ω cm2)

Pt Ni Au mGOM5/- mGOM5/Ni mGOM5/Au

0

2

4

6

5 10 15

-Z''

(Ωcm

2 )

Z' (Ω cm2)

0

4

8

12

5 15 25

-Z''

(Ωcm

2 )

Z' (Ω cm2)

0 0.2 0.4 0.6 0.8

Cur

rent

den

sity

(m

A c

m-2)

Voltage (V)

PtNiAumGOM5/-mGOM5/NimGOM5/Au

SEM/AFM

RAMAN

�Annealing causes ↓ ID/IG & ~ Ra (FTO)

Restoration of sp2 bonds due to elimination of O2 groups

Better contact with FTO irregular surface

�Ni particles cause extra ↓ ID/IG

Extra restoration of sp2 bonds

(elimination of O2 groups & repairing of defects)

≡ C-O group

≡ graphene

nanoplatelet

I-V / EIS / STABILITY

� Underneath surface of graphene platelets

& connection to the FTO surface - more

electrically conductive (↓ Rs)

� Upper surface of graphene platelets

retains sufficient number of catalytic sites

for high electrocatalytic activity (↓ Rct)

TEM

• A NEW TRANSPARENT COUNTER-ELECTRODE (CE) ARCHITECTURE ISPROPOSED, that suites both iodine and cobalt-based electrolytes for

liquid-junction DSCs

• The CE is comprised of a structured GRAPHENE FILM (comprised of modified commercial graphene nanoplatelets) OVER

electrophoretically deposited NICKEL NANOPARTICLES (mGOM5/Ni)

subjected to annealing under an INERT ATMOSPHERE AT 550 ⁰C

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 200 400 600 800 1000 1200

η/η

,i

analysis time / h

Pt

mGOM5/Ni

Modified graphene nanoplatelets (mGOM5)

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