Dithiafulvenyl-triphenylamine organic dyes with alkyl chains for efficient coadsorbent-free dye-sensitized solar cells

Dithiafulvenyl-triphenylamine (DTF-TPA) based organic dyes with different alkyl chains attached on the DTF unit were examined to investigate the effect of alkyl chain lengths on the photovoltaic performances of dye-sensitized solar cells (DSSCs). Relative to WD9 with two methyl groups, the power conversion efficiency (eta) increased significantly from 3.18% to 4.29% (WD12 with two hexyl chains) and 4.62% (WD13 with two decyl chains) in the absence of the chenodeoxycholic acid (CDCA). An increase in eta of about 45.2% was obtained from WD9 to WD13. The attached longer alkyl chains in DTF-TPA based organic dyes are effective to suppress the electron recombination and reduce the interactions between dye molecules. Electrochemical impedance spectroscopy studies indicate that both the resistance for charge recombination and the electron lifetime are increased after the increase of alkyl chains length to the dye molecules. The eta of DSSC based on WD9 was markedly increased by about 28.6% in the presence of CDCA as compared to that in the absence of CDCA. However, the eta of WD13-sensitized DSSC was not dependent on CDCA and decreased by about 5.4% in the presence of CDCA. This work indicates that the incorporation of longer alkyl chains into DTF-TPA organic dyes is a promising way for efficient coadsorbent-free DSSCs.