Beneficial effect of electron withdrawing groups on the sensitizing action of squaraines for p-type dye sensitized solar cells

This works analyzes the employment of three n-type squaraines VG1-C8, VG10-C8, VG11-C8 asbdye sensitizers of screen-printed NiO photocathodes for p-type dye-sensitized solar cells (p-DSCs). The photoelectrochemical performance of the three squarine-based p-DSCs is evaluated in terms of the molecular structure and successively compared to that of an analogous p-type solar cell utilizing the benchmark colorant erythrosin b (ERY). VG1-C8, VG10-C8 and VG11-C8 differ for the extent of electronic conjugation and the nature of the substituents on the central squarainic core. These three squaraines have alkyl chains of the same length, with the intention of excluding any eventual influence of the pending groups on the performances of the corresponding p-DSCs. From the analysis of electrochemical impedance spectra of the variously sensitized p-DSCs the cell with VG11-C8, i.e. bearing an electron withdrawing dicyanovynil group instead of a simple carbonyl, resulted the most efficient device in terms of charge photoinjection under the actual conditions of operation. This result accounted for the observed trend of the overall conversion efficiencies (from JV characteristic curves) and IPCE (incident photon-to-current conversion efficiency) spectral profiles of the three squaraine based p-DSCs. The effect of chenodeoxycholic acid (CDCA) as anti-aggregating agent of the anchored dyes on the performance of the p-DSCs was also evaluated. The optimal composition of the dipping solution was not dependent on the nature of the dye within the set of three squaraines. The best cell performances were recorded when the sensitization solution had the same concentration ratio [CDCA]/[dye]=10:1. In the conclusive remarks the correlation between p-DSC performance and the molecular structure of the three squaraines here proposed was analyzed. Moreover, some guidelines for the molecular design of new optimized squaraines for NiO-based p-DSCs will be defined taking into account the molecular features of the most efficient dye-sensitizers so far reported in the literature.