Interfacial electron transfer in photoanodes based on phosphorus(v) porphyrin sensitizers co-deposited on SnO2 with the Ir(III)Cp* water oxidation precatalyst

Abstract

We introduce phosphorus(V) porphyrins (PPors) as sensitizers of high-potential photoanodes with potentials in the 1.62–1.65 V (vs. NHE) range when codeposited with Ir(III)Cp* on SnO₂. The ability of PPors to advance the oxidation state of the Ir(III)Cp* to Ir(IV)Cp*, as required for catalytic water oxidation, is demonstrated by combining electron paramagnetic resonance (EPR), steady-state fluorescence and time-resolved terahertz spectroscopy (TRTS) measurements, in conjunction with quantum dynamics simulations based on DFT structural models. Contrary to most other types of porphyrins previously analyzed in solar cells, our PPors bind to metal-oxide surfaces through axial coordination, a binding mode that makes them less prone to aggregation. The comparison of covalent binding via anchoring groups, such as m-hydroxidebenzoate (⁻OPh–COO⁻) and 3-(3-phenoxy)-acetylacetonate (⁻OPh–AcAc) as well as by direct deposition upon exchange of a chloride (Cl⁻) ligand provides insight on the effect of the anchoring group on forward and reverse light-induced interfacial electron transfer (IET). TRTS and quantum dynamics simulations reveal efficient photoinduced electron injection, from the PPor to the conduction band of SnO₂, with faster and more efficient IET from directly bound PPor than from anchor-bound PPors. The photocurrents of solar cells, however, are higher for PPor–OPh–COO⁻ and PPor–OPh–AcAc than for the directly bound PPor–O⁻ for which charge recombination is faster. The high-potentials and the ability to induce redox state transitions of Ir(III)Cp* suggest that PPor/SnO₂ assemblies are promising photoanode components for direct solar water-oxidation devices.