Unsymmetrical Squaraines Incorporating Cabazole as a Donor for Dye-Sensitized Solar Cells

Three unsymmetrical squaraines, where the electron-rich thiophene or bithiophene conjugated fragment was used to link unconventionally the squaraine core and carbazole (or methyl) group, were prepared. JYL-SQ1 with thiophene as a donor and indoline-carboxylic acid as an acceptor has lambda max of 568 nm which is much longer than the general metal-free organic dyes. Adding a carbazole unit in JYL-SQ1 (forms JYL-SQ2) can red-shift the lambda max to 603 nm. However, cabazole is not a strong donor (and indoline-carboxylic acid is a weak acceptor) the LUMO of all three dyes are delocalized majorly on squaraine unit: the excited electrons of dye molecules injecting into TiO2 are not so efficiency, resulting in low short-circuit current and conversion efficiency. The corresponding DSSC devices have the conversion efficiency of 0.71%, 2.59% and 2.22% for JYL-SQ1, JYL-SQ2 and THL-SQ3, respectively.