Alternative bases to 4-tert-butylpyridine for dye-sensitized solar cells employing copper redox mediator

Novel facile synthetic protocol is developed to prepare electrochemically and optically clean Cu(tmby)2TFSI and Cu(tmby)2TFSI2 in a mixture (tmby = 4,4,6,6-tetramethyl-2,2-bipyridine; TFSI = trifluoromethylsufonylimide). This pure Cu(II/I) redox mediator exhibits improved charge-transfer rate at the counterelectrode (PEDOT) and faster diffusion transport in the solution. Four pyridine derivatives: 4-tert-butylpyridine, 2,6-bis-tert-butylpyridine, 4-methoxypyridine and 4-(5-nonyl)pyridine are evaluated as electrolyte additives. Base-specific electrochemical properties of the redox mediator are found for Cu(tmby)22+/+, but not for Co(bpy)33+/2+ which is used as control system. Due to steric hindrance, 2,6-bis-tert-butylpyridine has the smallest influence on the mediator's electrochemistry, but is also ineffective for the VOC enhancement through TiO2 conduction band upshift. Charge-transfer rates at PEDOT surface and diffusion resistances correlate with the basicity (pKa) of the used pyridine derivatives. The dye (Y123)-sensitized solar cells are evaluated by solar conversion performance in addition to electron lifetime, charge extraction and long-term stability tests. The optimization of pyridine bases for the Cu-mediated solar cells represents interplay of basicity and coordination ability. In turn, this allows for tuning of the charge transfer rate at counterelectrode and the mass transport in the electrolyte solution. The 4-(5-nonyl)pyridine is outperforming all the remaining bases in performance metrics of the corresponding solar cells.