Steric and Electronic Effect in Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

Abstract

Functionalizing the light harvesting sensitizers with additional electron-donating or -withdrawing groups is a potential approach to modulate the photophysical and electrochemical properties which in-turn optimizes the driving force associated with the charge injection and dye-regeneration processes at the dye-TiO 2 /electrolyte interface and the photovoltaic device performance in dye-sensitized solar cells (DSSCs). Furthermore, modulated electronic levels of the dyes provide an opportunity to reduce the overpotential associated with the dye-regeneration process and make the dye-TiO 2 interface compatible with various electrolytes. Furthermore, an in-built steric feature by means of introducing linear/branched alkyl groups in the sensitizer is important in controlling the aggregation of dyes on the TiO 2 surface. Hence, to integrate both steric and electronic properties, a series of alkyl group-wrapped unsymmetrical squaraine dyes ( SQ-X ) with electron-donating and -withdrawing groups have been designed, synthesized, and utilized for DSSC device fabrication. These dyes are functionalized with alkyl groups at both sp 3 -C and N-atoms of the indoline donor moiety at the nonanchoring side to have a similar steric feature. Photophysical and electrochemical studies revealed that the HOMO and LUMO energy levels of the SQ-X series of dyes have been modulated systematically with sufficient driving forces for both charge injection and dye-regeneration processes with iodolyte (I – /I 3 – ) electrolyte. In the presence of electron-donating groups in SQ-X (where X = –NPh 2 and –OMe ), the HOMO energy levels are less positive than SQ-H, whereas the presence of electron-withdrawing groups such as –CO 2 Me, –CN, and –NO 2 pushed the HOMO energy levels toward more positive potentials. Enhanced photovoltaic performances have been obtained for the dyes containing electron-donating groups, where the dye with the –NPh 2 group showed a maximum of η 7.03% ( V OC 708 mV, J SC 13.16 mA cm –2, and ff 78%). The dye with the strong electron-withdrawing group –NO 2 showed an efficiency of 1.49% ( V OC = 634 mV, J SC = 3.13 mA cm –2, and ff 75%). As the dyes with the electron-withdrawing group possess deep positive HOMO energy levels, the DSSC device characterization has been investigated with the Cu +/2+ redox shuttle. The reduced device performance of electron-withdrawing-group-containing dyes is due to the unfavored charge distribution in the LUMO compared to the presence of electron-donating-group-containing dyes, and it was supported by the difference in the charge injection efficiency.

Topics & Concepts