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

Kryman, Mark W., Nasca, Justin N., Watson, David F., Detty, Michael R.
Langmuir 2016 v.32 no.6 pp. 1521-1532
absorption, desorption, dyes, hydrochloric acid, lighting, phosphorous acid, photosensitizing agents, photovoltaic cells, solvents, spectral analysis, titanium dioxide
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.