The synthesis and application of novel copper redox mediators for dye-sensitised solar cells David Bradford and Abigail Watkins Newcastle University, UK Resurgence of dye-sensitised solar cells (DSCs) in recent years has been encouraged by the adoption of the earth abundant copper-based complexes. The historically used iodide/triiodide mediator observes inbuilt losses of open circuit voltages (Voc) due to its intrinsic formal redox potential +0.35V vs NHE and large reorganisation energies for electron transfer. Shifting to copper-based complexes impressive efficiencies of 15% have been achieved by Y. Ren et al. 1 with the state-of-the-art Cu(Tmby) 2 TSFI/TSFI 2 redox couple (Tmby = 4,4’,6,6’-tetramethyl-2,2’bipyridine). 2 Copper complexes with preorganised ligands possess more positive redox potentials and observe extraordinarily low reorganisation energies by minimising the geometrical changes between oxidation states, which permits high photovoltages in devices. Voc is determined by the difference between the quasi-fermi level of TiO 2 (E F ) and redox potential of the mediator. The quasi-fermi level is influenced by the electron population of the conduction band which depends on recombination mechanisms at the interface of the semiconductor/dye/mediator. Cu(Tmby) 2 over the previous seven years has sat upon the “Copper-Throne” for DSC device performance. This work looks to expand the library of complexes available through structural modification of the ligand environment, to challenge its dominance. New synthetic approaches have permitted control over the alkyl substituents in both 4 th and 6 th positions of 2,2’-bipyridine ligand system, opening the prospect of tuning properties of the copper complexes. All novel copper redox mediators, Cu(D-Bmby) 2 TSFI/TSFI 2 , Cu(Bmby) 2 TSFI/TSFI 2 and Cu(P-Tmby) 2 TSFI/TSFI 2 have been designed, via three various parameters, to obtain higher Voc in devices. Cu(D-Bmby) 2 and Cu(Bmby) 2 present tert-butyl blocking groups in 4th positions to reduce parasitic recombination reactions, furthermore, with one less alkyl substituent Cu(Bmby) 2 observes a 30mV more positive redox potential. Another approach of fine-tuning is explored by Cu(P-Tmby) 2 with a single n-propyl chain introduced to 6 th position, which positively shifted the redox potential through further restrictions of the Cu II geometry to pseudo-tetrahedral.
Results have demonstrated reduced recombination reactions for Cu(D-Bmby) 2 TSFI/TSFI 2 , confirmed by longer electron lifetimes and leading to the greater Voc in DSC devices, especially for low light conditions, due to the introduced tert-butyl blocking groups. Cu(P-Tmby) 2 exhibits increased recombination reactions due to the diminished stability of the copper(II) species outweighing increase in potential of the mediator. Cu(Bmby) 2 in an earlier stage of development promises to combine both increased redox potential and incorporated blocking groups. References 1. Y. Ren, et al. Nature, 2023, 613 (7942), 60-65. 2. Y. Saygili, et al. Journal of the American Chemical Society, 2016, 138 (45), 15087-15096S. 3. H. Tsao, et al . ChemSusChem, 2011, 4 , 591-594.
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