Avoiding Diffusion Limitations in Cobalt(III/II)-Tris(2,2′-Bipyridine)-Based Dye-Sensitized Solar Cells by Tuning the Mesoporous TiO2 Film Properties

• Published in: ChemPhysChem (Print) Vol. 13; no. 12; pp. 2976 - 2981
• Main Authors: Tsao, Hoi Nok, Comte, Pascal, Yi, Chenyi, Grätzel, Michael
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Weinheim WILEY-VCH Verlag 27-08-2012; WILEY‐VCH Verlag; Wiley
• Subjects: Applied sciences; Chemistry; cobalt; Colloidal state and disperse state; diffusion limitation; dye-sensitized solar cells; Energy; Exact sciences and technology; General and physical chemistry; Natural energy; Photovoltaic conversion; Porous materials; redox couples; Solar cells. Photoelectrochemical cells; Solar energy; titanium dioxide; Dye-sensitized solar cell; Binary compound; Dyes; redox couples; diffusion limitation; Film; Transition element compounds; Organic ligand; Bipyridines; cobalt; Cobalt complex; Porous material; Mesoporosity; dyd-sensitized solar cells; Solar cell; Transport process; Redox couple; Cobalt II; titanium dioxide; Diffusion; Titanium oxide
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201200435

Dye‐sensitized solar cells based on electrolytes containing cobalt complexes as redox shuttles typically suffer a major limitation in terms of slow diffusion of those couples through the mesoporous TiO2 film. This results in a drop of the photocurrent density, particularly at high incident light intensities, reducing the overall cell performance. This work illustrates how tuning the four characteristic parameters of the mesoporous TiO2 layer, namely film thickness, particle size, pore size and porosity, by simply optimizing the TiCl4 post‐treatment, completely eliminates diffusion problems of cobalt(III/II) tris(2,2′‐bipyridine) and at the same time maximizes the short‐circuit photocurrent density. As a result, a power conversion efficiency of 10.0 % at AM 1.5 G 100 mW cm−2 was reached in conjunction with an organic sensitizer.
At the limit: It is illustrated how diffusion limitations of cobalt redox couples in dye‐sensitized solar cells (see picture) can be eliminated by tuning the porosity, particle and pore sizes of the mesoporous TiO2 layers through simple TiCl4 post‐treatment. Thus, thicker TiO2 films required for higher photocurrent densities can be used without encountering mass transport limitations, leading to power conversion efficiencies of 10 %.