Molecular level photovoltaics: the electrooptical properties of metal cyanide complexes anchored to titanium dioxide

Abstract

The preparation, surface attachment, optical, and photoelectrochemical properties of metal cyanide complexes, cis-[(4,4[prime]-(CO[sub 2]H)[sub 2]bpy)[sub 2]M(CN)[sub 2]] where M = Ru(II) and Os(II), attached to sol-gel processed TiO[sub 2] electrodes are reported. The specific sol-gel process provides a route to stable, high surface area anatase TiO[sub 2] films. With surface anchored [(4,4[prime]-(CO[sub 2]H)[sub 2]bpy)[sub 2]Ru(CN)[sub 2]]RB we report incident-photon-to-current efficiencies exceeding 90%. The net photocurrent scales linearly over 5 decades of incident irradiance. The molecular level similarity of the two polypyridyl complexes allows for a meaningful comparison of the electrooptical properties of Ru(II) and Os(II) derivatized surfaces. A kinetic interpretation with different sacrificial electron donors provides insight into the rate limiting step(s) in the photosensitization process. An increased rate of electron tunneling from the semiconductor conduction band to the oxidized dye and a more sluggish halide oxidation rate are believed to be responsible for a low photocurrent efficiency observed with surface-anchored [(4,4[prime]-(CO[sub 2]H)[sub 2]-bpy)[sub 2]Os(CN)[sub 2]]. The results and interpretation suggest methods for developing more efficient photovoltaic devices. 35 refs., 8 figs., 2 tabs.