Beneficial Effect of Electron-Withdrawing Groups on the Sensitizing Action of Squaraines for p-Type Dye-Sensitized Solar Cells

The three squaraines (SQs) here indicated as VG1-C8, VG10-C8, and VG11-C8 (Figure 1) have been synthesized and employed as dye sensitizers of p-type dye-sensitized solar cells (p-DSCs) when screen-printed NiO is the nanoporous photocathode. The photoelectrochemical performance of the three differently sensitized p-DSCs is compared in terms of the structural diversities among the three SQs. The p-DSC with VG11-C8 as sensitizer, i.e., the sole SQwith a dicyano vinyl substituent on the central squaric ring, results in the most efficient device because of the highest values of overall conversion efficiency (eta > 0.04% for VG11 vs eta < 0.02% for VG1 and VG10). Moreover, VG11 is the sole SQ of the C8 series which exerts a favorable synergistic effect on the external quantum efficiency (EQE) of screen-printed NiO. The antiaggregating agent CDCA (= chenodeoxycholic acid) in the sensitizing solution of NiO is beneficial on the performance of the corresponding p-DSCs when the ratio [CDCA]/[dye] is 1:10 for all three SQs. Electrochemical impedance spectroscopy (EIS) of the SQs-sensitized p-DSCs under illumination reveals that the best performing photoelectrochemical cell, i.e., the one with VG11 sensitizer, is characterized by relatively lower resistances of charge transport (similar to 30 Omega) and interfacial recombination (similar to 90 Omega), shorter hole diffusion times (similar to 2 ms), and a larger hole diffusion coefficient (similar to 2 X 10(-7) cm(2) s(-1)) with respect to the p-DSCs with VG1 and VG10 dyes.