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

Abstract 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.