Copper Piperazine Complex with a High Diffusion Coefficient for Dye-Sensitized Solar Cells

Copper complexes are considered to be environmentally friendly and are high-quality redox mediators for use in dye-sensitized solar cells (DSSCs), but their low diffusion coefficient impedes them from obtaining a significant short-circuit current. To solve this problem, two sulfur-copper complexes, Cu-(II/I)(mpdt)(2)TFSI2/1 and Cu-(II/I)(ppdt)(2)TFSI2/1 (mpdt = 1,4-dimethylpiperazine-2,3-dithione and ppdt = 1,4-diisopropylpiperazine-2,3-dithione), are synthesized and applied in DSSCs as redox mediators. The small volume of ligand allows the copper complexes to have a high diffusion ability, which is conducive for obtaining a high short-circuit current in the devices. Photocurrent density-voltage (J-V) characteristics of the DSSCs containing different concentrations of the monovalent copper complex show that the devices achieve an optimal photoelectric conversion efficiency of 9.9% with a lower redox mediator concentration compared with the other copper bipyridyl complexes. This is the highest efficiency for devices that use sulfur-copper complexes as the redox couple. Owing to its excellent diffusion ability, a small amount of redox mediator can meet the requirements of electron transfer and photosensitizer regeneration, which not only reduces the raw material dosage and cost but also avoids the complex crystal precipitation phenomenon at high concentration. This study provides a case for the sulfur-copper complex and points out a direction for the industrialization of DSSCs, which can benefit from easy synthesis and low cost of the complex.