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

• Published in: Sustainable energy & fuels 2018, Vol.2 (4), p.786-794
• Main Authors: Stephens, Alexander J, Malzner, Frederik J, Constable, Edwin C, Housecroft, Catherine E
• Format: Article
• Language: English
• Subjects: Acids; Anchoring; Copper; Dye-sensitized solar cells; Dyes; Electrochemical impedance spectroscopy; Electrochemistry; Electrodes; Equivalence; Ligands; NMR; Nuclear magnetic resonance; Photovoltaic cells; Protonation; Solar cells; Spectroscopy; Titanium dioxide
• ISSN: 2398-4902; 2398-4902
• DOI: 10.1039/c7se00586e

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. The effects of a change in the protonation state of the phosphonic acid anchoring ligand in a heteroleptic copper( i ) sensitizer on the performance of n-type dye-sensitized solar cells are presented.