Effect of thiophene-based π-spacers on N-arylphenothiazine dyes for dye-sensitized solar cells

To study the effect of π-spacers on dye performance in DSSC, five novel phenothiazine dyes have been prepared where the number of successive thiophenes in the π-spacer was increased from zero to four. Additionally, a dye bearing thieno[3,2-b]thiophene was synthesized, and compared to its unfused bithiophene analogue. When the number of thiophenes were two or more the light absorption properties was significantly better in the region 400–550 nm compared to the dye without a π-spacer. DSSC device testing revealed that the reference dye without a π-spacer gave the best overall performance producing a power conversion efficiency of 5.7% (JSC = 10.1 mA cm􀀀 2, VOC = 0.83 V, FF = 0.68), attributed to the superior VOC of this dye. The incorporation of one and two thiophenes resulted in a similar performance as the reference, while incorporation of larger oligothiophenes proved to be deleterious to DSSC-performance. Electrochemical impedance spectroscopy indicates that the voltage drops and performance loss associated with these dyes are due to more facile recombination.