Factors influencing the photoelectrochemical device performance sensitized by ruthenium polypyridyl dyes

Abstract

The dye-sensitized photoelectrochemical cells (DSPECs) incorporating a family of ruthenium complexes [Ru^II(bipyP)(bipy)₂] (P1), [Ru^II(bipyP)(dmb)₂] (P2), [Ru^II(bipyP)(dtbb)₂] (P3) and [Ru^II(bipyP)(dnb)₂] (P4), where bipyP = 2,2′-bipyridine-4,4′-diphosphonic acid, bipy = 2,2′-bipyridine, dmb = 4,4′-dimethyl-2,2′-bipyridine, dtbb = 4,4′-di-tert-butyl-2,2′-bipyridine, and dnb = 4,4′-dinonyl-2,2′-bipyridine, were fabricated in a dye-only system (do-DSPEC) and in a system where the electrolyte solution was loaded with EDTA sacrificial agent (sa-DSPEC). The increasing number of the alkyl chains of the ancillary bipy ligand shifts the ground- and excited-state potentials to the more negative values, although the introduction of the longer nonyl chain in P4 shows the opposite effect. In do-DSPECs, the photocurrent and hydrogen production performance follows the order P4 > P3 > P2 > P1, which correlates well with the degree of the excited-state quenching by electron injection to the conduction band of TiO₂. The photoelectrochemistry of the sa-DSPECs reveals 10 times as many photocurrents as that measured in do-DSPECs, suggesting the ability of the hole to oxidize EDTA molecule. The hydrogen production performance of sa-DSPECs over five hours follows the order P2 > P1 > P3 > P4, which is consistent with the Ru^III/Ru^II reorganization energies and the hole mobility on the TiO₂ surface. The present study provides evidence that the subtle alkyl chain variation of the ruthenium photosensitizers can fine tune the electron injection capacity, Ru^III/Ru^II self-exchange energetics and photostability of the complexes, which significantly influence the performance of the DSPECs.