Alkyl chain-functionalized hole-transporting domains in zinc(II) dye-sensitized solar cells

FTO/TiO₂ electrodes have been functionalized with {Zn(tpy_anchor)(tpy_ancillary)}²⁺ dyes (tpy = 2,2′:6′,2″-terpyridine) using a stepwise method to sequentially introduce (i) the anchoring ligand tpy_anchor (either a dicarboxylic acid or a diphosphonic acid), (ii) Zn²⁺ ions, and (iii) chromophoric ancillary (4-([2,2′:6′,2″-terpyridin]-4′-yl)-N,N-bis(4-alkoxyphenyl)aniline ligands. A comparison of unmasked and fully masked DSSCs containing representative dyes shows a significant drop in photon-to-current efficiency upon masking. Solid-state absorption spectra of the dye-functionalized electrodes confirm that the intensity of absorption decreases with the steric demands of the ancillary ligand. DSSC measurements show that the {Zn(tpy_anchor)(tpy_ancillary)}²⁺ dyes give poor photon-to-current efficiencies, values of the short circuit current density (J_SC) and the external quantum efficiency (EQE) spectra are consistent with very poor electron injection. Introducing longer alkoxy chains in place of methoxy substituents in the hole-transporting domains in tpy_ancillary is beneficial, resulting in increased J_SC and V_OC, although values remain low despite the 'push-pull' design of the sensitizers.