Thiocyanate ligand substitution kinetics of the solar cell dye Z-907 by 3-methoxypropionitrile and 4-tert-butylpyridine at elevated temperatures

The dye sensitized solar cell dye Z-907, [RuLL´(NCS)₂] may loose a thiocyanate ligand at elevated temperatures (80-100 ºC) by ligand exchange with the solar cell additive 4-tert-butylpyridine

(4-TBP) or the electrolyte solvent 3-methoxypropionitrile (3-MPN). The mechanism in homogeneous solution proceeds via three equilibrium reactions (eqs. 1-3) with the solvent complex [RuLL´(NCS)(3-MPN)] as an intermediate:

 

[RuLL´(NCS)₂] + 3-MPN = [RuLL´(NCS)(3-MPN)]⁺ + NCS^- (1)

[RuLL´(NCS)(3-MPN)]⁺ + 4-TBP = [RuLL´(NCS)(4-TBP)]⁺ + 3-MPN (2)

[RuLL´(NCS)₂] + 4-TBP = [RuLL´(NCS)(4-TBP)]⁺ + NCS^- (3)

 

 

A similar mechanism describes the heterogeneous substitution reactions of Z-907 attached to the surface of TiO₂ particles with 3-MPN and 4-TBP. All the six homogeneous and heterogeneous rate constants were obtained at 100 ºC by monitoring the decay of Z-907 and product formation in test tube experiments by HPLC coupled to UV/Vis and electrospray mass spectrometry.

A half life time t_1/2 = 150 h was obtained for the Z-907 dye bound to TiO₂ nano crystalline particles at 85 ºC in presence of 4-TBP and 3-MPN. Dye sensitized solar cells (DSC) with Z-907 as sensitizer and application of the so called “non robust” electrolytes containing 4-TBP and 3-MPN is therefore not expected to be able to pass a 1000 h thermal stress test at 85 ºC. Addition of thiocyanate to the cell electrolyte may however, eliminate or reduce the problems caused by dye thiocyanate ligand substitution in DSC cells.