Pentamethylcyanine derivative (A), trimethylcyanine derivative (B) and their mixtures (AB) were used as sensitizers in nanocrystalline TiO2 solar cells to improve photoelectric conversion efficiency. It was found that the aggregates of the cyanine dyes were efficient in light harvesting and that the mixture of A and B could be employed to sensitize the solar cell over the entire visible spectrum. The sensitizing properties of three mixed dyes with different A-to-B ratio were systematically studied, and it was found that A1B3 (A:B = 1:3 V/V)-sensitized solar cell generated the highest photoelectric conversion yield of 3.4%. The effect of co-adsorption of A and B on their aggregation behavior and photosensitization was also investigated. Co-sensitization was found to suppress the aggregation and affect the sensitization performance profoundly. (c) 2004 Elsevier B.V. All rights reserved.