The effect of N-substitution and ethylthio substitution on the performance of phenothiazine donors in dye-sensitized solar cells

Three new organic sensitizers with phenothiazine chromophores were synthesized and used in dye-sensitized solar cells. N-substituents were added to the phenothiazine to suppress dye aggregation and an ethylthiol was added as an additional donor function. The effects of these substituents on the performance of the dyes were investigated by assessing the photophysical, electrochemical, and photovoltaic properties. The introduction of an N-methoxyphenyl unit improved the open-circuit voltage; the addition of the ethylthiol unit decreased the amount of dye adsorbed on the TiO₂. A very high open-circuit voltage of 818.5 mV was achieved by the phenothiazine with added N-methoxyphenyl and ethylthiol groups. However, the best overall conversion efficiency was achieved by a cell fabricated with a phenothiazine devoid of the ethylthiol group (an overall conversion efficiency = 6.02%, short-circuit current = 10.20 mA/cm², open-circuit voltage = 781.7 mV, fill factor = 75.50% under AM 1.5G conditions) due to the high open-circuit voltage and greater dye adsorption.