Yiqun Ma
SUPERVISOR: Dr. Gu Xu
1

Outline
• Background and introduction
I.
Dye-sensitized solar cells
II.
Mass transport in electrolyte
III.
Problem: pore-size dependence of ion diffusivity
• Experimental
I.
Device fabrication and pore-size variation
II.
DC polarization measurement
• Results and discussion
I.
Unification of two opposite views
II.
Unexpected surface diffusion
III.
Significance of results
• Conclusion
2

Introduction to Dye-sensitized Solar Cells
• Electrochemical cells utilizing dye molecules to harvest sunlight
• First published in Nature in 1991
• 7% overall power conversion efficiency was achieved, now has exceeded 12%
• New generation solar cell with possible low cost and high stability
Oregan, B.; Gratzel, M., Nature 1991, 353 (6346), 737-740
3

Mesoporous TiO
2
Thin Film
• Monolayer Dye molecules for light absorption  High surface area required  mesoporous structure gives rise of 700 times of nominal surface area
• Working electrochemical Junction formed at the interface
TiO
2
Dye
I /I
3
-
4

Device Physics of Dye-sensitized Solar Cells
FTO
Mass transport of ions
 Bottleneck of performance
5

Three Possible Mechanisms of Mass Transport
Diffusion • Concentration gradient dominant mechanism in DSSCs
Migration • Electric field
Convection
• Mass movement
• Due to temperature difference etc.
 In standard DSSCs, the mass transport rate is determined by the diffusion of minority ions (I
3
) i.e. [I
3
] <<[I ]
Kalaignan, G. P.; Kang, Y. S., J. Photochem. Photobiol. C-Photochem. Rev. 2006, 7 (1), 17-22.
6

Two Conflicting Views from Literature:
A) Pore-size Independent Diffusion
• Diffusion is pore-size independent when λ<0.1 (λ = r molecule
/r pore
)
Based on the short mean free path of inter-molecular collision in
1 liquid : 𝑙 𝑡𝑜𝑡𝑎𝑙
1
= 𝑙 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒
1
+ 𝑙 𝑝𝑜𝑟𝑒
• 𝐷 𝑚𝑎𝑡𝑟𝑖𝑥
= 𝐷 𝑏𝑢𝑙𝑘
×
ε
(ε: porosity; τ:tortuosity)
τ
• Tortuosity: ratio of the length of the curve (L) to the distance between the ends of it (C)
A
C
B
𝑳 𝝉 =
𝑪
L
Karger, J.; Ruthven, D. M., Diffusion in zeolites and other microporous solids. : Wiley: New York, 1992; pp 350-365.
7

Two Conflicting Views from Literature:
B) Pore-size Dependent Diffusion
• Frequently observed impeded diffusion in much larger pores (λ ≈ 0.01)
• In this case ion diffusivity heavily depends on pore diameter
•
•
Possibly due to the surface interaction or bonding mechanisms
Decreases effective free pore volume
Mitzithras, A.; Coveney, F. M.; Strange, J. H., J. Mol. Liq. 1992, 54 (4), 273-281.
40nm
8

Debating in Dye-sensitized Solar Cells
• Remains controversial in dye-sensitized solar cells
• Yet critical in estimation of the limiting current and design of efficient devices
• Because various fabrication processes lead to pore shrinking
I.
Dye loading
II. TiCl
4 post-treatment
9

Experimental:
Device Fabrications
 To focus on ion diffusion, a modified version of DSSC is fabricated
Injection hole
1.
Coating of Pt on FTO glass by heat treatment of chloroplanitic acid
(H
2
PtCl
6
)
2.
Deposition of TiO
2 thin film by screen printing process
3.
Sealing the cell with Surlyn film as spacer(25μm)
4.
Injecting electrolyte (I /I
3
redox couple in acetonitrile) from the hole at the top
10

Pore-size Variation by
TiCl
4
Treatment
• TiCl
4 post-treatment is widely used in DSSC fabrication
• Chemical bath which forms TiO
2 on top of TiO
2 mesoporous film by epitaxial growth – growing overlayer with the same structure
• Reduce recombination rate and improve charge injection from dye molecules to the CB of TiO
2
• Also reduce average pore size of TiO
2 film
11

Pore-size Variation by
TiCl
4
Treatment
1. Immerse for 30 mins
2. Rinse with DI water
3. Anneal at 450 o C for 30 mins
TiCl
4 treated TiO
2 with smaller pores film
TiO
2 film on FTO/Pt glass
0.1M TiCl
4 aqueous solution at 70 o C
Hot Plate
TiCl
4
+ 2 H
2
O → TiO
2
+ 4 HCl
Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel, M., Thin Solid Films
2008, 516 (14), 4613-4619.
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BET Characterization
B
C
D
Sample Number of TiCl
4 treatments
A 0
E
1
2
3
4
Average pore diameter (nm)
20.91
±
1.83
16.92
±
2.32
11.33
±
2.57
7.97
±
1.7
5.7
±
1.35
Porosity ε
0.616
±
0.018
0.497
±
0.010
0.404
±
0.014
0.339
±
0.008
0.287
±
0.006
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BET Characterization
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Pore-size Distribution
Sample A, C and E underwent 0, 2 and 4 times of TiCl
4 treatments respectively
 Curves follow more or less the normal distribution
 Distribution shape remains almost unchanged after treatments
 Average pore diameter decreases
 Error bars of pore diameters are obtained from the FWHM values
15

DC Polarization Measurement
• The DC measurement was conducted in Dark
• Mass transport limited current
In this case, diffusion limited current
• IV curve will reach plateau at limiting current value
• In this case, the current will increase
I lim after the plateau
I
Ionic diffusion
Charge injection from the TiO
2 to electrolyte
Charge injection starts
V
T
V
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Model Construction
• First consider neat electrolyte between two electrodes
• Assuming diffusion layer thickness = cell thickness, and 𝑑
2 𝑑𝑥 𝑐
2
= 0
(since the current flow is independent of x)
• General equation of diffusion limited current
𝐼 𝑙𝑖𝑚
=
2𝑛𝐹𝑐𝐷 𝑑
• F is the Faraday constant, c is the I
3
concentration and n is the stoichiometry constant which equals to 2 for I /I
3
redox couple
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Model Construction
• Continuity of current in the device:
I = 2F 𝐷
𝑇𝑖𝑂
2
𝐶 𝑡
−𝐶
0 𝑡
= 2FD bulk
𝐶 𝑙
−𝐶 𝑙−𝑡 𝑡
• The conservation of I
3
ions:
(1) c[ εt + (l – t)] = ε
𝐶 𝑡
+𝐶
0
2 t+
𝐶 𝑡
+𝐶 𝑙
2
(l – t) (2)
• Combine (1) and (2) with boundary condition c 0 =0:
𝑰 𝒍𝒊𝒎
= 4Fc
𝑫
𝑻𝒊𝑶𝟐
𝑫
𝑻𝒊𝑶𝟐
𝑫 𝒃𝒖𝒍𝒌
(𝜺𝒕−𝒕+𝒍)
(𝒍−𝒕) 𝟐 +𝑫 𝒃𝒖𝒍𝒌
(𝜺𝒕−𝟐𝒕+𝟐𝒍)
(3) t = 12 μ m; 𝒍 = 25 μ m
Kron, G.; Rau, U.; Durr, M.; Miteva, T.; Nelles, G.; Yasuda, A.; Werner, J. H., Electrochem. Solid State Lett.
2003, 6 (6), E11-E14.
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DC Measurement Results a) IV characteristic of control sample without TiO
2 thin film; b) Typical IV curves of samples A to E after 0 to 4 times of TiCl respectively
4 treatments
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DC Measurement Results
Sample I
A lim
(mAcm -2 )
D
TiO2
(10 -5 cm 2 s -1 )
35.25
±
1.25
0.747
±
0.038
B 24.80
±
0.60
0.513
±
0.016
C
D
E
21.10
±
0.45
0.437
±
0.012
16.67
±
0.35
0.343
±
0.009
10.33
±
0.50
0.207
±
0.011
D eff
(10 -5 cm 2 s -1 )
1.22
±
0.09
1.03
±
0.05
Tortuosity
( τ )
1.05
±
0.09
1.24
±
0.06
1.08
±
0.07
1.01
±
0.05
1.18
1.26
±
±
0.08
0.06
0.721
±
0.055
1.78
±
0.13
D
TiO2
: ion diffusivity in matrix
D eff
: effective ion diffusivity normalized with porosity
ε
τ : tortuosity calculated from 𝐷
𝑇𝑖𝑂2
= 𝐷 𝑏𝑢𝑙𝑘
×
τ
, expected to range from 1.2 to 1.8 *
20

Surprising Pore-size Dependence
E
D
C
B
A
D – E:
Pore-size dependent region, D eff heavily depends on pore diameters;
B – D:
Pore-size independent region, almost forms a platform;
Transition:
Critical point of transition is located at 5 – 7 nm;
A – B:
? What is going on here?
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Two Opposite Views Are Now Unified……
Pore-size dependent
D
E
C
B
Distinctive Regions of each diffusion mode
I.
Pore-size dependent region
• < 5 – 7 nm
• Significant steric hindrance
effect of pore walls.
Pore-size independent
II. Pore-size independent region
• > 5 – 7 nm
• Negligible collision between liquid molecules and pore walls
Observed in DSSCs for the first time!
22

……by the Critical Point of Transition
• λ value at the transition ≈ 0.1 (550pm/5nm), which bears
remarkable agreement to the theoretical prediction
• The range of pore-size independent region(>5-7nm) suggests fabrication processes of DSSCs will NOT cause transition of diffusion behavior
• Not likely those processes will impede ion diffusivity significantly
23

Significance of Our Results
Smaller Larger
• Large interfacial
Area for efficient light harvesting
• May impede mass transport rate
Pore Size
• High mass transport limiting current
• Not enough interfacial area
 Our results suggest the minimum pore-size without hindering the diffusion.
 The balance between mass transport of electrolyte and interfacial area can be optimized
24

Unexpected Rise from B to A
• The tortuosity in A ≈ 1(unrealistic)  Other diffusion mechanism is involved
• Surface diffusion
⁻ Hopping mechanism of surface-adsorbed molecules between adsorption sites.
⁻ Suppressed by the surface modification after
TiCl
4 treatments
⁻ Act as a passivation process and decrease the number of available adsorption sites
I
3
I
3
-
B
A
Surface diffusion
TiO
2 25

Conclusion
• Both pore-size dependent and independent diffusion were observed under the same scheme by altering the average poresize of TiO
2 matrix.
• The critical point of transition was located in the range of 5 – 7 nm. Thus standard fabrication processes will not cause transition of diffusion mode.
• Surface diffusion mechanism was observed in untreated TiO
2 and suppressed after the surface modification of TiCl
4 posttreatment.
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Acknowledgements
• Dr. Gu Xu
• Dr. Tony Petric and Dr. Joey Kish
• Dear group mates: Cindy Zhao, Lucy Deng
• Mr. Jim Garret
• Dr. Hanjiang Dong
• NSERC
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Any questions?
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