Thin Film Actinometers for Transient Absorption Spectroscopy:  Applications to Dye-Sensitized Solar Cells

Abstract

The chromophores Ru(bpy)3(PF6)2 and Os(bpy)3(PF6)2 were immobilized within poly(methyl methacrylate) (PMMA) thin films on glass substrates for applications as actinometers for nanosecond flash photolysis. Transient absorption difference spectra of M(bpy)3(PF6)2 (M = Ru, Os), at ambient temperature and in an argon atmosphere, were the same when imbedded in PMMA films as in solution, within experimental error. Linear ranges of ΔA versus 532 nm pulsed laser energy where these actinometers were applicable were identified, up to 25 mJ/(cm2 pulse) for Ru(bpy)32+/PMMA and up to 5 mJ/(cm2 pulse) for Os(bpy)32+/PMMA. Laser energy measurements were used to estimate the difference between the excited- and ground-state extinction coefficients at 450 nm for Os(bpy)32+, Δε450nm, which is −7300 M-1 cm-1. The Ru(bpy)32+/PMMA actinometer was useful from 300 to ∼550 nm, while the Os(bpy)32+/PMMA actinometer extends the sensitivity to ∼700 nm. An application of these actinometers for dye-sensitized solar cells is described, wherein the quantum yield for electron injection from Ru(dcbH2)(bpy)22+*, where dcbH2 is 4,4‘-(CO2H)2-2,2‘-bipyridine, into mesoporous nanocrystalline (anatase) TiO2 thin films was quantified as a function of ionic strength