A Fundamental Investigation of Electron Transport and Recombination Characteristics of Titania Nanowire/Nanoparticle Hybrid Structures

Wednesday, October 19, 2011: 5:05 PM
102 F (Minneapolis Convention Center)
Venkat Kalyan Vendra1, Delaina A. Amos1, Mahendra K. Sunkara1 and Thad Druffel2, (1)Chemical Engineering, University of Louisville, Louisville, KY, (2)Conn Center for Renewable Energy Research, Louisville

The electron transport and recombination of long titania nanowires and titania nanoparticle/nanowire composites are investigated. Understanding charge transport and recombination in 1D nanostructures of titania is critical for improving the performance of DSSC employing these 1D nanostructures as photoanodes in dye sensitized solar cells. 1D titania nanostructures are expected to have short transport times and high electron lifetimes. However, most of the 1D titania nanostructures have not shown these desirable properties. For instance, the electron diffusion coefficients and transport time constants in titania nanotubes were found to be very similar to that of titania nanoparticles.[1] But, the recombination time constants for titania nanotubes were 10 times higher than then nanoparticle based DSSCs. In contrast, DSSCs fabricated using titania nanowires showed both shorter transport time scales and higher electron lifetimes.[2]  In a seminal paper, Richter and Schmuttenmaer showed that the lower the electron mobility in TiO2 nanotubes was due to the existence of exciton like trap states formed from Ti+3 donor states resulting from oxygen deficiencies, fluoride impurities and excess titanium introduced during synthesis.[3] Although thermal annealing is expected to remove the oxygen deficiencies, the structural integrity of the nanotubes would be compromised and hence a new synthesis technique is needed to prepare defect free TiO2 nanotubes for enhanced DSSC performance.

Several approaches such as electrochemical anodization, solvothermal techniques, hydrothermal methods, thermal evaporation and template based approaches have been widely explored. However, most the methods produce short titania nanotubes (~ 2 µm in length) which is undesirable considering the low dye loading and slow electron transport in these architectures. In addition, all these synthesis techniques discussed above are very time consuming with the reaction time scales ranging from several hours to even days. To our knowledge there have not been any studies which have focused on the fast synthesis of single crystalline titania nanowires. Here we present a novel, scalable approach for making single crystalline titania nanowires arrays for dye sensitized solar cells.[4]  Electron transport and recombination in these single crystalline nanowires and in single crystalline titania nanowires coated with titania nanoparticles are discussed.

[1] Zhu, K., Neale, N.R., Miedaner, A., Frank, A.J.  Enhanced charge collection efficiencies and light scattering in dye sensitized solar cells using oriented TiO2 nanotubes.  Nanoletters 2007, 7, 69-74.

[2] Tetreault, N., Horvath, E., Moehl, T., Brillet, J., Smajda, R., Bungener, S., Cai, N., Peng, W., Zakeeruddin, S.M., Forro, L., Magrez, A., Gratzel, M. High-Efficiency Solid-State Dye- Sensitized Solar Cells: Fast Charge Extraction through Self-Assembled 3D Fibrous Network of Crystalline TiO2 Nanowires. ACS Nano 2010, 4, 7644-7650.

[3] Richter, C., Schmuttenmaer, C.A., Exciton like trap states limit electron mobility in TiO2 nanotubes, Nature Nanotechnology 2010, 5, 769–772

 [4] Kumar, V., Kim, J.H.  Jasinski, J.B., Clark, E.L., Sunkara, M.  Alkali metal assisted gas phase bulk production of titania nanowires. Journal of Crystal Growth and Design 2011, Article Accepted.

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