The influence of phosphonic acid protonation state on the efficiency of bis(diimine)copper(I) dye-sensitized solar cells

We present an investigation of the effects of a change in the protonation state of the phosphonic acid anchoring ligand in the dye [Cu(H 4 1 )( 2 )][PF 6 ] (H 4 1 = ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid), 2 = 4,4'-bis(4-bromophenyl)-6,6'-dimethyl-2,2'-bipyridine) on the performance of n-type dye-sensitized solar cells (DSCs). FTO/TiO 2 electrodes were immersed in solutions of H 4 1 in the presence of base (0–4 equivalents). TiO 2 -anchored heteroleptic copper(I) sensitizers were subsequently formed by ligand exchange between the homoleptic complex [Cu( 2 ) 2 ][PF 6 ] and the anchored ligand [H 4– n 1 ] n – . The results demonstrate that the addition of one equivalent of base during the initial surface functionalization can afford up to a 26% increase in DSC efficiency, while the addition of ≥3 equivalents of base significantly hinders DSC performance. Deprotonation of H 4 1 has been investigated using 1 H and 31 P NMR spectroscopic titrations. Further insight into DSC performance has been gained by using electrochemical impedance spectroscopy, and a comparison is made between DSCs in which the working electrodes are either pre-treated with a base, or exposed to a base post heteroleptic copper(I) dye-assembly.