Ru-based donor–acceptor photosensitizer that retards charge recombination in a p-type dye-sensitized solar cell

Abstract

We report on the synthesis and characterization of a donor–acceptor ruthenium polypyridyl complex as a photosensitizer for p-type dye-sensitized solar cells (DSSCs). The electrochemical, photophysical, and photovoltaic performance of two ruthenium-based photosensitizers were tested in NiO-based DSSCs; bis-(2,2′-bipyridine-4,4′-dicarboxylic acid)₂N-(1,10-phenanthroline)-4-nitronaphthalene-1,8-dicarboximide ruthenium(II), ([Ru(dcb)₂(NMI-phen)](PF₆)₂) and tris-(2,2′-bipyridine-4,4′-dicarboxylic acid)₃ ruthenium(II), [(Ru(dcb)₃)Cl₂]. The presence of an electron-accepting group, 4-nitronaphthalene-1,8-dicarboximide (NMI), attached to the phenanthroline of [Ru(dcb)₂(NMI-phen)]²⁺ resulted in long-lived charge separation between reduced [Ru(dcb)₂(NMI-phen)]²⁺ and NiO valence band holes; 10–50 μs. In the reduced state for [Ru(dcb)₂(NMI-phen)]²⁺, the electron localized on the distal NMI group. In tests with I₃⁻/I⁻ and Co(4,4′-di-tert-butyl-bipyridine)₃^2+/3+ electrolytes, [Ru(dcb)₂(NMI-phen)]²⁺ outperformed [Ru(dcb)₃]²⁺ in solar cell efficiency in devices. A record APCE (25%) was achieved for a ruthenium photosensitizer in a p-type DSSC. Insights on photosensitizer regeneration kinetics are included.