Halogen functionalized D–A–D-type unsymmetrical squaraine dyes for dye-sensitized solar cells

Abstract

The development of sensitizers with the D–A–D architecture with an indoline donor and squaryl acceptor is attractive towards achieving the high-efficiency dye-sensitized solar cells (DSSCs) due to an intense absorption (ε > 10⁵ M⁻¹ cm⁻¹) in the far-red region and dye aggregation behaviour on the mesoporous TiO₂ surface, which broadens the light harvesting properties. To further enhance the photovoltaic parameters, features that help to enhance the charge injection and dye-regeneration processes must be built into the sensitizers, and the charge recombination must be diminished. Here, a series of squaraine dyes (ISQ) in which an alkyl-group-wrapped indoline donor functionalized with a halogen atom, which is separated from another indoline donor that contains an anchoring group, have been designed and synthesized. Photophysical, electrochemical and photovoltaic characterizations have been carried out to understand the effect of the halogen atom on their electronic, photo-excited and DSSC device properties. Photophysical characterization revealed the singlet nature of the photo-excited state despite the presence of a heavy atom such as iodine, bromine and chlorine in the ISQ dyes. The singlet excited state lifetime and the fluorescence quantum yield for the ISQ dyes were almost equal to or slightly higher than those of the ISQ-H dye. The molecular electrostatic potential obtained from DFT studies indicated the presence of a σ-hole in the halogen-functionalized ISQ dyes, which assists in the dye regeneration process through halogen bonding. DSSC devices were fabricated with iodolyte electrolytes in high (Z-150) and low (Z-50) redox concentrations in order to determine the importance and effect of halogen functionalization in squaraine dyes on the DSSC performance. Different photovoltaic performances were observed for the ISQ dyes when the devices were fabricated with the Z-50 and Z-150 iodolyte electrolytes. A maximum DSSC efficiency of 6.48% was achieved using SQS4 dye in Z-50 without CDCA, and an efficiency of 7.80% was achieved using ISQ-I dye in Z-150 with CDCA.