Paolo G. Bomben

A Trisheteroleptic Cyclometalated RuII Sensitizer that Enables High Power Output in a Dye-Sensitized Solar Cell†

The low embodied energy and high power-conversion efficiency (h) over disparate light intensities renders the dyesensitized solar cell (DSSC) a promising alternative to conventional photovoltaic technologies. Significant penetration of the DSSC into the photovoltaic market, however, is hindered predominantly by the long-term stability of dyes and electrolytes under practical conditions. The instability of champion (i.e., h> 10%) dyes (which, until recently, all were derivatives of [Ru(dcbpy)2(NCS)2] (N3 ; dcbpy= 4,4’dicarboxy-2,2’-bipyridine)) in the DSSC is caused primarily by desorption of the dyes from the surface and/or liberation of the NCS ligands from the metal centre. While the rate of dye desorption from TiO2 can be manipulated by replacing the CO2H moiety with other anchoring groups, this strategy typically compromises electron injection into the TiO2. [8] An alternative approach is to replace the dcbpy ligands that comprise N3 with bidentate ligands bearing aliphatic substituents (e.g., Scheme 1a), which serve to hinder water from reaching the surface to hydrolytically cleave the TiO2–dye ester linkage. These groups provide the additional benefit of suppressing recombination between the electrolyte and the electrons in TiO2, thus leading to higher efficiencies (Scheme 1a). Chemical strategies for avoiding the labile Ru NCS bond have been realized recently; indeed, we and others have now documented remarkably high h values for DSSCs containing NCS -free Ru sensitizers. Cyclometalated Ru complexes such as [Ru(dcbpy)2(ppy)] 1+ (ppy= 2-phenylpyridine) provide a versatile platform in this respect because: 1) the highest occupied molecular orbital (HOMO) is extended over the metal and anionic ring thus enabling its modulation through judicious installation of substituents at the R2 site in Scheme 1b; and 2) the low-lying excited states, which contain orbital character that resides on the p* framework of the dcbpy ligand(s), are poised for electron injection into the TiO2. [10,11,15–18] This scenario leaves open the opportunity to replace one dcbpy with a bidentate ligand capable of suppressing recombination and enhancing the optical properties as per the aforementioned protocol (Scheme 1). While we recently demonstrated synthetic access to trisheteroleptic Ru sensitizers (e.g., 1 and 2 ; Scheme 2), we learned that removing the acid linkers