IR, Raman and UV–vis spectra of the Ru(II) cyano complexes studied by DFT

Abstract

Harvesting energy directly from sunlight using photovoltaic technology is an essential component of the future global energy production programmes (A. Hagfeldt and M. Gratzel, Molecular photovoltaic, Acc. Chem. Res. 33 (2000), pp. 269–277). Photovoltaic devices, or solar cells, are based on the photoelectric effect, in which the incoming solar photons are absorbed in a sensitised semiconductor material releasing electric charges that are used to energise an external circuit (J.E. Monat, J.H. Rodriguez and J.K. McCusker, Ground- and excited-state electronic structures of the solar cell sensitiser bis(4,4′-dicarboxylato-2,2′-bipyridine)bis(isothiocyanato)ruthenium(II), J. Phys. Chem. A 106 (2002), pp. 7399–7406). The solar cells on the bases of nanocrystalline TiO₂ are usually sensitised by metal-organic dyes such as Ru-complexes with phenylpyridine and bipyridine ligands. The incident photon-to-current conversion efficiency (IPCCE) in the wavelength region 400–600 nm can be rather high (A. Hagfeldt and M. Gratzel, Molecular photovoltaic, Acc. Chem. Res. 33 (2000), pp. 269–277; J.E. Monat, J.H. Rodriguez and J.K. McCusker, Ground- and excited-state electronic structures of the solar cell sensitiser bis(4,4′-dicarboxylato-2,2′-bipyridine)bis(isothiocyanato)ruthenium(II), J. Phys. Chem. A 106 (2002), pp. 7399–7406). In order to improve the IPCCE value, the heavier metal ions can be tested. Sensitisers such as coumarin, porphyrins, chlorophyll derivatives, antenna-sensitiser polynuclear complexes and eosin have been studied and reported to be less efficient than the most effective Ru-based sensitising dyes, called the black dye (BD) and N3 dye (J.E. Monat, J.H. Rodriguez and J.K. McCusker, Ground- and excited-state electronic structures of the solar cell sensitiser bis(4,4′-dicarboxylato-2,2′-bipyridine)bis(isothiocyanato)ruthenium(II), J. Phys. Chem. A 106 (2002), pp. 7399–7406; B. Minaev, V. Minaeva and O. Vahtras, Electronic structure and spectra of dyes for sensitising nanocrystalline solar cells, Nano-Sol-Net International Symposium: Trends in Organic Electronics and Hybrid Photovoltaic's, Constanţa, Romania, 2008; F. Aiga and T. Tada, Molecular and electronic structures of BD; An efficient sensitising dye for nanocrystalline TiO₂ solar cells, J. Mol. Struct. 658 (2003), pp. 25–32), used for nanocrystalline TiO₂ solar cells. In this paper, we aim to understand the IPCCE of a new dye-sensitiser in order to predict a higher absorption efficiency of the solar spectrum and higher electron transfer rate from the redox systems to the oxidised dye. A simple Ru(II)(bpy)₂(CN)₂ complex is studied by the density functional method with optimised structure and vibrational analysis in order to predict the role of vibronic perturbations in the spectra and in the interface electron transfer rate. Intermolecular interactions with solvent and nanocrystalline surface are also discussed in terms of vibronic mixing of states.

Keywords:

• solar cell
• density functional
• Ru-complexes
• IR and Raman spectra