Tuning copper dynamic dimers’ redox potential Iacopo Benesperi 1,2 , David Bradford 2 , Marina Freitag 2 1 Niversità di Torino, Dipartimento di Chimica, Italy, 2 Newcastle University, School of Natural and Environmental Sciences, UK Historically, iodide/triiodide has been the redox couple of choice to regenerate dyes in dye-sensitised solar cells (DSSCs). However, this redox couple presents several drawbacks, such as a two-electron redox process (which lowers the maximum potential output attainable by the cell), corrosiveness towards other cell components, and competing light absorption with the dye. 1 Metal complex-based electrolytes based on cobalt and especially on the non-toxic and more environmentally-friendly copper metal centres have in recent years replaced iodide in high-efficiency and high-potential devices. The use of Cu I/II (tmby) 2 (tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine) in particular has recently allowed DSSCs to reach 15% efficiency. 2 One of the reasons for this high efficiency is the minimisation of reorganisation energy losses between the Cu(I/II) species via steric hindrance, as the 6,6' methyl groups lock the complex in a (pseudo)tetrahedral geometry for both copper oxidation states, preventing the Cu(II) species to reach the preferred planar configuration. Although this material is highly efficient, its (pseudo) tetrahedral geometry favours the stabilisation of the Cu(I) species, which facilitates unwanted recombination processes during DSSC operation. In this work, 3 we present copper complexes based on tetradentate ligands (Cu(Ctetra) and Cu(Stetra))that stabilise the Cu(II) species of the complex. Interestingly, the Cu(II) species of these complexes are planar molecules, while the Cu(I) species dimerise in order to allow this oxidation state to reach its preferred tetrahedral geometry (Figure 1). DFT computations show that Cu(I) monomers are highly energetically unfavoured and therefore do not form, thus hindering recombination processes in DSSCs. The proposed mechanism for the redox couple is that of disproportionation: after regenerating the dye with a one-electron process, two mixed-valence dimers disproportionate to form one Cu(I) dimer and two Cu(II) monomers. This disproportionation process theory is supported by impedance and cyclic voltammetry measurements. Despite the quite complex geometrical changes involved in the redox process, DFT calculations show that the reorganisation energy required is relatively small and is not detrimental to the cell’s potential output. The reduced recombination processes of these tetradentate complexes compared to Cu(tmby) 2 are supported by longer electron lifetimes and smaller open- circuit voltage losses compared to the complexes’ redox potential. Despite their performance, Cu(Stetra) and Cu(Ctetra) have a relatively low redox potential, which does not allow them to fully reach the high efficiency of which organic dyes such as Y123 and XY1 are capable. For this reason, the bipyridyl groups in the tetradentate complexes have been modified with functional groups such as –CF 3 and –F, in order to raise their redox potential and bring it closer to that of the state-of-the-art Cu(tmby) 2 .
Figure 1: Structures of Cu(I/II) species of Cu(Ctetra) and Cu(Stetra). References 1. A. B. Muñoz-García, I. Benesperi, et al., Chem. Soc. Rev. , 2021, 50 , 12450–12550. 2. Y. Ren, D. Zhang, et al., Nature , 2023, 613 , 60–65. 3. I. Benesperi, H. Michaels, et al., Chem , 2022, 8 , 439–449.
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