The photoactive component of a Dye-Sensitized Solar Cell (DSC) consists of a dye molecule bound (either electrostatically or covalently) to a mesoporous TiO₂ surface through functional groups such as phosphonic acids.^[1] Here, we investigate how the protonation state of such phosphonic acids affects their ability to anchor to TiO₂ substrates, and how the operational parameters of the resultant DSCs are influenced in turn.

Stepwise titration of bases into a solution of LH₄ alters the ligands protonation state and the nature of its associated cations to give [LH_n][X]_n-4 (Fig. 1). The formation of Cu(I) dye molecules is achieved by sequentially exposing TiO₂ electrodes to [LH_n][X]_n-4, followed by the homoleptic Cu(I) complex of the ancillary ligand. Our results demonstrate that the addition of a small amount of base to LH₄ (~1 eq) can afford up to a 30% increase in DSC efficiency. These results are rationalized through consideration of proton / cation transfer from the ligand to the surface, deprotonation of surface hydroxides, and changes in the photochemical and electrochemical properties of ligand LH₄ upon deprotonation.

Figure 1. The sequential formation of a bis(diimine)copper(I) dye on a TiO₂ electrode using phosphonic acids in different protonation states.

[1] C. E. Housecroft, E. C. Constable, Chem. Soc. Rev., 2015, 44, 8386–8398.