Pore-size Dependence of Ion D iffusivity in Dye-sensitized S olar C ells

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

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

3. 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

4. Mesoporous TiO2 Thin Film TiO2 I-/I3- Dye 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

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

6. Three Possible Mechanisms of Mass Transport dominant mechanism in DSSCs • In standard DSSCs, the mass transport rate is determined by the diffusion of minority ions (I3-) i.e. [I3-] <<[I-] Kalaignan, G. P.; Kang, Y. S., J. Photochem. Photobiol. C-Photochem. Rev. 2006,7 (1), 17-22.

7. Two Conflicting Views from Literature:A) Pore-size Independent Diffusion B A C L • 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) Karger, J.; Ruthven, D. M., Diffusion in zeolites and other microporous solids. : Wiley: New York, 1992; pp 350-365.

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

9. 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 • Dye loading • TiCl4post-treatment

10. Experimental:Device Fabrications • To focus on ion diffusion, a modified version of DSSC is fabricated Injection hole Coating of Pt on FTO glass by heat treatment of chloroplanitic acid (H2PtCl6) Deposition of TiO2 thin film by screen printing process Sealing the cell with Surlyn film as spacer(25μm) Injecting electrolyte (I-/I3- redox couple in acetonitrile) from the hole at the top

11. Pore-size Variation by TiCl4Treatment TiCl4 post-treatment is widely used in DSSC fabrication Chemical bath which forms TiO2on top of TiO2mesoporousfilm 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

12. Pore-size Variation by TiCl4Treatment 2. Rinse with DI water 3. Anneal at 450oC for 30 mins 1. Immerse for 30 mins TiCl4treated TiO2film with smaller pores TiO2film on FTO/Pt glass 0.1M TiCl4aqueous solution at 70 oC TiCl4 + 2 H2O → TiO2 + 4 HCl HotPlate Ito, S.; Murakami, T. N.; Comte, P.; Liska, P.; Gratzel, C.; Nazeeruddin, M. K.; Gratzel, M., Thin Solid Films 2008,516 (14), 4613-4619.

13. BET Characterization

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 TiCl4 treatments respectively

16. DC Polarization Measurement • The DC measurement was conducted in Dark I Charge injection starts Ilim Ionic diffusion V VT • 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 TiO2to electrolyte

17. Model Construction 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 couple

18. Model Construction t = 12 μm;= 25 μm 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) 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 TiO2 thin film; b) Typical IV curves of samples A to E after 0 to 4 times of TiCl4 treatments respectively

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 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? A C B D E

22. Two Opposite Views Are Now Unified…… Pore-size dependent • Distinctive Regions of each diffusion mode • Pore-size dependent region • < 5 – 7 nm • Significant steric hindrance effect of pore walls. • Pore-size independent region • > 5 – 7 nm • Negligible collision between liquid molecules and pore walls • Observed in DSSCs for the first time! C B D Pore-size independent E

23. ……by the Critical Point of Transition λvalue 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

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

25. Unexpected Rise from B to A Surface diffusion A B I3- I3- TiO2 • 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 TiCl4treatments • Act as a passivation process and decrease the number of available adsorption sites

26. Conclusion Both pore-size dependent and independent diffusion were observed under the same scheme by altering the average pore-size of TiO2matrix. 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 TiCl4post-treatment.

27. Acknowledgements Dr. GuXu Dr. Tony Petric and Dr. Joey Kish Dear group mates: Cindy Zhao, Lucy Deng Mr. Jim Garret Dr. HanjiangDong NSERC

28. Thanks for the attention! Any questions?