Pore-size Dependence of Ion Diffusivity in Dye-sensitized Solar Cells

Yiqun Ma

SUPERVISOR: Dr. Gu Xu1

• Background and introductionI. Dye-sensitized solar cellsII. Mass transport in electrolyteIII. Problem: pore-size dependence of ion diffusivity

• ExperimentalI. Device fabrication and pore-size variationII. DC polarization measurement

• Results and discussionI. Unification of two opposite viewsII. Unexpected surface diffusionIII. Significance of results

• Conclusion

2

Outline

• 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

3

Introduction to Dye-sensitized Solar Cells

Oregan, B.; Gratzel, M., Nature 1991, 353 (6346), 737-740

• 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

4

Mesoporous TiO2 Thin Film

TiO2

DyeI-/I3

-

5

Device Physics of Dye-sensitized Solar Cells

Mass transport of ions Bottleneck of performance

FTO

6

Three Possible Mechanisms of Mass Transport

Kalaignan, G. P.; Kang, Y. S., J. Photochem. Photobiol. C-Photochem. Rev. 2006, 7 (1), 17-22.

•Concentration gradient

Diffusion

•Electric field

Migration

•Mass movement

•Due to temperature difference etc.

Convection

dominant mechanism in DSSCs

In standard DSSCs, the mass transport rate is determined by the diffusion of minority ions (I3

-) i.e. [I3-] <<[I-]

• Diffusion is pore-size independent when λ<0.1 (λ = rmolecule/rpore)- Based on the short mean free path of inter-molecular collision in

liquid : = +• (ε: porosity; τ:tortuosity)• Tortuosity: ratio of the length of the curve (L) to the distance

between the ends of it (C)

7

Two Conflicting Views from Literature:A) Pore-size Independent Diffusion

Karger, J.; Ruthven, D. M., Diffusion in zeolites and other microporous solids. : Wiley: New York, 1992; pp 350-365.

A B

L

C𝝉=

𝑳𝑪

• Frequently observed impeded diffusion in much larger

pores (λ ≈ 0.01)

• In this case ion diffusivity heavily depends on pore diameter

8Mitzithras, A.; Coveney, F. M.; Strange, J. H., J. Mol. Liq. 1992, 54 (4), 273-281.

40nm

• Possibly due to the surface interaction or bonding mechanisms

• Decreases effective free pore volume

Two Conflicting Views from Literature:B) Pore-size Dependent Diffusion

• 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. TiCl4 post-treatment

9

Debating in Dye-sensitized Solar Cells

1. Coating of Pt on FTO glass by heat

treatment of chloroplanitic acid

(H2PtCl6)

2. Deposition of TiO2 thin film by

screen printing process

3. Sealing the cell with Surlyn film as

spacer(25μm)

4. Injecting electrolyte (I-/I3- redox

couple in acetonitrile) from the hole

at the top10

Experimental:Device Fabrications

Injection hole

To focus on ion diffusion, a modified version of DSSC is fabricated

• TiCl4 post-treatment is widely used in DSSC fabrication

• Chemical bath which forms TiO2 on top of TiO2 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 TiO2

• Also reduce average pore size of TiO2 film

11

Pore-size Variation by TiCl4 Treatment

12

Pore-size Variation by TiCl4 Treatment

Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel, M., Thin Solid Films 2008, 516 (14), 4613-4619.

TiO2 film on FTO/Pt glass

1. Immerse for 30 mins2. Rinse with DI water3. Anneal at 450oC for 30 mins

Hot Plate

0.1M TiCl4 aqueous solution at 70 oC

TiCl4 treated TiO2 film with smaller pores

TiCl4 + 2 H2O → TiO2 + 4 HCl

13

BET Characterization

Sample Number of TiCl4 treatments

Average pore diameter (nm)

Porosity ε

A 0 20.91±1.83 0.616±0.018

B 1 16.92±2.32 0.497±0.010

C 2 11.33±2.57 0.404±0.014

D 3 7.97±1.7 0.339±0.008

E 4 5.7±1.35 0.287±0.006

14

BET Characterization

15

Pore-size Distribution

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 valuesSample A, C and E underwent 0, 2 and 4 times of TiCl4 treatments respectively

• 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

after the plateau

- Charge injection from the TiO2 to electrolyte

16

DC Polarization Measurement

I

V

Ilim

Ionic diffusion

Charge injection starts

VT

• The DC measurement was conducted in Dark

• First consider neat electrolyte between two electrodes

• Assuming diffusion layer thickness = cell thickness, and

(since the current flow is independent of x)

• General equation of diffusion limited current

• F is the Faraday constant, c is the I3- concentration and n is

the stoichiometry constant which equals to 2 for I-/I3- redox

couple17

Model Construction

• Continuity of current in the device:I = 2F = 2FDbulk (1)

• The conservation of I3- ions:

c[εt + (l – t)] = ε t+ (l – t) (2)• Combine (1) and (2) with boundary condition c0=0:

= 4Fc (3)

18

Model Construction

Kron, G.; Rau, U.; Durr, M.; Miteva, T.; Nelles, G.; Yasuda, A.; Werner, J. H., Electrochem. Solid State Lett. 2003, 6 (6), E11-E14.

t = 12 μm; = 25 μm

19

DC Measurement Results

a) IV characteristic of control sample without TiO2 thin film;

b) Typical IV curves of samples A to E after 0 to 4 times of TiCl4 treatments respectively

Sample Ilim

(mAcm-2)DTiO2

(10-5cm2s-1)Deff

(10-5cm2s-1)Tortuosity()

A 35.25±1.25 0.747±0.038 1.22±0.09 1.05±0.09

B 24.80±0.60 0.513±0.016 1.03±0.05 1.24±0.06

C 21.10±0.45 0.437±0.012 1.08±0.07 1.18±0.08

D 16.67±0.35 0.343±0.009 1.01±0.05 1.26±0.06

E 10.33±0.50 0.207±0.011 0.721±0.055 1.78±0.13

20

DC Measurement Results

DTiO2: ion diffusivity in matrix

Deff: effective ion diffusivity normalized with porosity: tortuosity calculated from , expected to range from 1.2 to 1.8*

21

Surprising Pore-size Dependence

A

BC

D

E

D – E: Pore-size dependent region, Deff 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?

22

Two Opposite Views Are Now Unified……

Distinctive Regions of each diffusion modeI. Pore-size dependent region• < 5 – 7 nm• Significant steric hindrance

effect of pore walls.

II. Pore-size independent region• > 5 – 7 nm• Negligible collision between

liquid molecules and pore walls

Observed in DSSCs for the first time!

Pore-size dependent

Pore-size independent

BC

D

E

• 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

……by the Critical Point of Transition

24

Significance of Our Results

Pore Size

Smaller• Large interfacial

Area for efficient light harvesting

• May impede mass transport rate

Larger• 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

• 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

TiCl4 treatments⁻ Act as a passivation process and decrease the

number of available adsorption sites

25

Unexpected Rise from B to A

TiO2

I3- I3

-

Surface diffusion

A

B

• Both pore-size dependent and independent diffusion were

observed under the same scheme by altering the average pore-

size of TiO2 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 TiO2

and suppressed after the surface modification of TiCl4 post-

treatment.26

Conclusion

• Dr. Gu Xu

• Dr. Tony Petric and Dr. Joey Kish

• Dear group mates: Cindy Zhao, Lucy Deng

• Mr. Jim Garret

• Dr. Hanjiang Dong

• NSERC

27

Acknowledgements

28

Thanks for the attention!

Any questions?