Photoinduced electron injection in black dye sensitized nanocrystalline TiO₂ films

Photoinduced electron injection in nanocrystalline TiO₂ films sensitized with black dye [trithiocyanato(4,4′,4″-tricarboxy-2,2′: 6′,2″-terpyridine)ruthenium(ii), Ru(tcterpy)(NCS)₃] was studied by means of transient absorption (TA) spectroscopy. We examined the effect of the additive 4-tert-butylpyridine (TBP) in acetonitrile, which is known to markedly improve the performance of solar cell devices. The efficiency of electron injection for the film immersed in acetonitrile was 50% higher than that for the film dried in air. Femtosecond TA measurements indicated that this efficiency enhancement was due to the opening of an additional injection pathway from a lower-lying energy level. Upon the addition of 1 M TBP, which is known to raise the level of the conduction band of TiO₂, the enhancement induced by acetonitrile disappeared. We discuss the mechanism of electron injection by comparing our results with results previously reported for the N3/TiO₂ system.