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

Abstract

We present an investigation of the effects of a change in the protonation state of the phosphonic acid anchoring ligand in the dye [Cu(H₄1)(2)][PF₆] (H₄1 = ((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/TiO₂ electrodes were immersed in solutions of H₄1 in the presence of base (0–4 equivalents). TiO₂-anchored heteroleptic copper(I) sensitizers were subsequently formed by ligand exchange between the homoleptic complex [Cu(2)₂][PF₆] and the anchored ligand [H_4−n1]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 H₄1 has been investigated using ¹H and ³¹P 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.