DFT study of electronic structure and optical properties of some Ru- and Rh-based complexes for dye-sensitized solar cells

Abstract

The paper reports Time Dependent Density Functional Theory (TD DFT) calculations providing the structure, electronic properties and spectra of [Ru(II)(bpy)_{3− n} (dcbpy) _n ]²⁺ and [Rh(III)(bpy)_{3− n} (dcbpy) _n ]³⁺ complexes, where bpy = 2,2′-bipyridyl, dcbpy = 4,4′-dicarboxy-2,2′-bipyridyl, and n = 0, 1, 2, 3, studied as possible pigments for dye-sensitized solar cells. The role of the metallic ion and of the COOH groups on the optical properties of these complexes are compared and contrasted and their relevance as dyes for hybrid organic–inorganic photovoltaic cells is discussed. It was found that the optical spectra are strongly influenced by the metallic ion, with visible absorption bands for the Ru(II) complexes and only ultraviolet bands for the Rh(III) complexes. Upon excitation, the extra positive charge of the Rh³⁺ centre tends to draw electrons towards the metal ion, facilitating some charge transfer from the ligand to the metal, whereas in the case of the Ru²⁺ ion the electron transfer is clearly from the metal to the ligand. The carboxyl groups play an important role in strengthening the absorption bands in solution in the visible region. Of the complexes studied, the most suited as pigments for dye-sensitized solar cells are the [Ru(II)(bpy)_{3− n} (dcbpy) _n ]²⁺ complexes with n = 1 and 2. This is based on the following arguments: (i) their intense absorption band in the visible region, (ii) the presence of the anchoring groups allowing the bonding to the TiO₂ substrate and the charge transfer, and (iii) the good energy level alignment with the conduction band edge of the semiconducting substrate and the redox level of the electrolyte.

Keywords:

• ruthenium complexes
• rhodium complexes
• 2,2′-bipyridyl
• 4,4′-dicarboxy-2,2′-bipyridyl
• time-dependent density functional theory, optical spectra
• dye-sensitized solar cells

Acknowledgements

M.A.G and B.F.M acknowledge the joint financial support of the Romanian Authority for Scientific Research, grant PN2-Capacitati-M3 116/2008, together with the Ministry of Education and Science of Ukraine. C.I.O. acknowledges the financial support from CNCS/UEFISCDI grant code PNII-RU-PD-603/2010, contract no. 172/2010.