Pore-size Dependence of Ion Diffusivity in Dye-sensitized Solar Cells Yiqun Ma SUPERVISOR: Dr. Gu Xu 1

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 Outline

Electrochemical cells utilizing dye molecules to harvest sunlight First published in Nature in % 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),

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 TiO 2 Thin Film TiO 2 Dye I - /I 3 -

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), C o n c e nt ra ti o n gr a di e nt Diffusion Electric field Migration Mass movement Due to temperature difference etc. Convectio n dominant mechanism in DSSCs  In standard DSSCs, the mass transport rate is determined by the diffusion of minority ions (I 3 - ) i.e. [I 3 - ] <<[I - ]

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 A B L C

Frequently observed impeded diffusion in much larger pores (λ ≈ 0.01) In this case ion diffusivity heavily depends on pore diameter 8 Mitzithras, A.; Coveney, F. M.; Strange, J. H., J. Mol. Liq. 1992, 54 (4), nm 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.TiCl 4 post-treatment 9 Debating in Dye-sensitized Solar Cells

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 Experimental: Device Fabrications Injection hole  To focus on ion diffusion, a modified version of DSSC is fabricated

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

12 Pore-size Variation by TiCl 4 Treatment Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel, M., Thin Solid Films 2008, 516 (14), TiO 2 film on FTO/Pt glass 1. Immerse for 30 mins 2. Rinse with DI water 3. Anneal at 450 o C for 30 mins Hot Plate 0.1M TiCl 4 aqueous solution at 70 o C TiCl 4 treated TiO 2 film with smaller pores TiCl H 2 O → TiO HCl

13 BET Characterization SampleNumber of TiCl 4 treatments Average pore diameter (nm) Porosity ε A020.91± ±0.018 B116.92± ±0.010 C211.33± ±0.014 D37.97± ±0.008 E45.7± ±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 values Sample A, C and E underwent 0, 2 and 4 times of TiCl 4 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 TiO 2 to electrolyte 16 DC Polarization Measurement I V I lim Ionic diffusion Charge injection starts VTVT The DC measurement was conducted in Dark

17 Model Construction

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.

19 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 4 treatments respectively

SampleI lim (mAcm -2 ) D TiO2 (10 -5 cm 2 s -1 ) D eff (10 -5 cm 2 s -1 ) A35.25± ± ± ±0.09 B24.80± ± ± ±0.06 C21.10± ± ± ±0.08 D16.67± ± ± ±0.06 E10.33± ± ± ± DC Measurement Results D TiO2 : ion diffusivity in matrix D eff : effective ion diffusivity normalized with porosity

21 Surprising Pore-size Dependence A B C D E 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?

22 Two Opposite Views Are Now Unified…… Distinctive Regions of each diffusion mode I.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 B C 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 TiCl 4 treatments ⁻Act as a passivation process and decrease the number of available adsorption sites 25 Unexpected Rise from B to A TiO 2 I3-I3- 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 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 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?