Organic dyes containing 1H-phenanthro[9,10-d]imidazole conjugation for solar cells

Series of dyes based on 1H-phenanthro[9,10-d]imidazole chromophores were developed and characterized. Dye-sensitized solar cells (DSSCs) using the dyes as the sensitizers exhibited good efficiencies, ranging from 2.95 to 4.68%, which reached 45-70% with respect to that of an N719-based device fabricated under similar conditions. Incorporation of arylamine moieties at 1H-phenanthro[9,10-d]imidazole was found to retard charge recombination of the electrons in the conduction band of TiO2 with the oxidized dyes. In contrast, incorporation of hydrophobic dodecanyl chains at 1H-phenanthro[9,10-d]imidazole reduced the dark current and improved the device efficiency. Results of TDDFT computation on the compounds are in support of the charge-transfer character of the compounds and consistent with the device performance. A cascade of charge separation is possible for compounds with arylamine moieties via an internal conversion from a bright higher excited state to a long-range charge separated state. Our results indicate that the extent of charge separation in the excited state is an important factor for DSSC efficiency.