Reducing mass-transport limitations in cobalt-electrolyte-based dye-sensitized solar cells by photoanode modification

Mass transport has been identified as a limiting problem in the photovoltaic performance of dye-sensitized solar cells based on electrolytes consisting of ionic liquids or cobalt complexes. A mixed TiO₂ macroporous-mesoporous morphology employed as photoanode is demonstrated to assist the diffusion of electrolytes with higher viscosity or consisting of bulky redox mediators, such as cobalt di-tert-butyl bipyridine [Co(dtb) ₃]^2+/3+. This morphology with large pores improves the non-linearity of photocurrent response to light intensity indicating better diffusion. The incorporated sub-micrometer pores also reduce recombination and decrease diffusion resistance, as revealed by electrochemical impedance spectroscopy. Sub-micrometer pores are incorporated in the mesoporous TiO ₂ film of a dye-sensitized solar cell to create a network of bimodal pore sizes. The large pores assist the diffusion of bulky Co(dtb) ^2+/3+ ions in the cobalt electrolyte, reduce the diffusion resistance, and improve the photocurrent at high light intensity.