Nanowire Architectures for Iodide Free Dye-Sensitized Solar Cells

In this work, we demonstrate that the performance of dye-sensitized solar cells (DSCs) can be improved by engineering the electron dynamics and surface properties of the semiconductor photoanode. Specifically, we find that titania nanoparticles coated tin oxide nanowires yield DSCs with ten times higher short-circuit current density than the widely employed titania nanoparticle, even when commercially available Ru dye is used for sensitization.¹

Literature reports on improving the performance of DSCs using alternate redox electrolytes have only focused on using new organic dyes instead of the conventional Ru based dyes. Analysis of the literature data showed that Ru based dyes have higher reorganization energies compared to organic dyes and hence Ru based dyes are associated with high driving forces for dye-regeneration.¹ No attention has been paid to engineering the photoanode architecture for improving the performance with alternate redox electrolytes. Prior studies using tin oxide nanowires in conjunction with iodide/triiodide redox electrolyte has shown that tin oxide nanowires have two orders of magnitude higher electron lifetimes and  fast electron transport properties when compared to titania nanoparticles.^2,3 In this study, the electron transport and recombination properties of different architectures involving nanoparticles, nanowires and nanoparticle/nanowire hybrid architectures of tin oxide and titania are compared to gain insight into the factors that contribute to the improved performance when alternate redox couples such as ferrocene/ferrocenium and TEMPO/TEMPO⁺ are used.

The location of surface traps states and their passivation was found to have a significant impact the electron dynamics and photovoltaic characteristics of the DSC.  The trap states in tin oxide nanowires are located very close to the conduction band (0.071 eV) when compared to titania nanoparticles that have much deeper trap states (0.24 eV). The shallow trap state in tin oxide nanowires allow faster detrapping of the electrons from the trap resulting in improved the electron lifetimes over titania nanoparticles. In addition, the high diffusion lengths in tin oxide nanowires thicker photoanode films when alternate redox electrolytes are used thereby allowing increased sensitizer loading and light harvesting.

Figure 1. Schematic showing that the difference in the location of surface trap states in tin oxide nanowires and titania nanoparticles influences the electron recombination properties in a DSC.

Ongoing research efforts are focused on replacing the Ru based dye with semiconductors as absorbers.

References

1. Vendra, V.K., Nguyen, T., Druffel, T., Amos, D.A., Sunkara, M.K. Nanowire architectures for iodide free dye-sensitized solar cells, Submitted 2013.
2. Gubbala, S., Chakrapani, V., Kumar, V., Sunkara, M.K., Band edge engineered hybrid architectures for dye-sensitized solar cells. Adv. Func. Mater. 2008, 16, 2411-2418.
3. Gubbala, S., Russell, H.B., Shah, H., Deb, B., Jasinski, S., Rypkema, H., Sunkara, M.K. Surface properties of tin oxide nanowires for enhanced performance with dye-sensitized solar cells. Energy Environ. Sci., 2009, 2, 1302–1309.