Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance.

A library of six selenorhodamine dyes (4-Se-9-Se) were synthesized, characterized, and evaluated as photosensitizers of TiO2 in dye-sensitized solar cells (DSSCs). The dyes were constructed around either a bis(julolidyl)- or bis(half-julolidyl)-modified selenoxanthylium core functionalized at the 9-position with a thienyl group bearing a carboxylic, hydroxamic, or phosphonic acid for attachment to TiO2. Absorption bands of solvated dyes 4-Se-9-Se were red-shifted relative to the dimethylamino analogues. The dyes adsorbed to TiO2 as mixtures of monomeric and H-aggregated dyes, which exhibited broadened absorption spectra and increased light-harvesting efficiencies relative to the solvated monomeric dyes. Carboxylic acid-bearing dyes 4-Se and 7-Se initially exhibited the highest incident photon-to-current efficiencies (IPCEs) of 65-80% under monochromatic illumination, but the dyes desorbed rapidly from TiO2 into solutions of HCl (0.1 M) in a CH3CN:H2O mixed solvent (120:1 v:v). The hydroxamic acid- and phosphonic acid-bearing dyes 5-Se, 6-Se, 8-Se, and 9-Se exhibited lower IPCEs (49-65%) immediately after preparation of DSSCs; however, the dyes were vastly more inert on TiO2, and IPCEs decreased only minimally with successive measurements under constant illumination. Power-conversion efficiencies (PCEs) of the selenorhodamine-derived DSSCs were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given anchoring group, the bis(half-julolidyl) dyes exhibited higher open-circuit photovoltages and PCEs than the corresponding bis(julolidyl) dyes. The hydroxamic acid- and phosphonic acid-bearing dyes are intriguing photosensitizers of TiO2 in light of their aggregation-induced spectral broadening, high monochromatic IPCEs, and relative inertness to desorption into acidic media.

Relating structure and photoelectrochemical properties: electron injection by structurally and theoretically characterized transition metal-doped phenanthroline-polyoxotitanate nanoparticles.

Whereas a large number of sensitized polyoxotitanate clusters have been reported, information on the electrochemical properties of the fully structurally defined nanoparticles is not available. Bridging of this gap will allow a systematic analysis of the relation between sensitizer-cluster binding geometry, electronic structure and electron injection properties. Ti17O28(O(i)Pr)16(Fe(II)Phen)2 is a member of a doubly-doped series of nanoclusters in which the phenanthroline is attached to the surface-located transition metal atom. The visible spectrum of a dichloromethane solution of the studied sample shows a series of absorption bands in the 400-900 nm region. Theoretical DOS and TDDFT calculations indicate that the bands in increasing wavelength order correspond essentially to metal-to-core charge transfer (MCCT) at ∼460 nm, metal-to-ligand charge transfer (MLCT) at ∼520 nm and d-d metal-atom transitions. Exposure of a thin layer of the sample to light in a photoelectrochemical cell produces an electric current in the 400 to ∼640 nm region. The fit of the wavelength range of the electron injection with the results of the calculations suggests that charge injection into the FTO anode occurs both from the TiO cluster and from the phenanthroline ligand. Injection from the phenanthroline via the cluster orbitals is ruled out by the lower energy of the phenanthroline orbitals.