Phthalocyanine aggregates on metal dichalcogenide surfaces: Dye sensitization on SnS₂ semiconductor electrodes by ordered and disordered InPc-Cl thin films

Photoelectrochemical dye sensitization processes have been explored for various coverages of ultrathin films of chloroindium phthalocyanine (InPc-Cl), on single crystal metal dichalcogenide (SnS₂) surfaces. Films were prepared by conventional vacuum deposition and by a process which led mainly to flat-lying, epitaxial Pc layers, exhibiting a "layer-by-layer" growth mode. These experiments were carried out in parallel with optical characterization of InPc-Cl layers grown on nonconductive transparent SnS₂ thin films, where both the Pc and SnS₂ thin films were grown by a molecular beam epitaxy (MBE) process on freshly cleaved mica. Surface electron diffraction data were collected during the organic/inorganic-molecular beam epitaxy (O/I-MBE) experiments, on both bulk and MBE-grown SnS₂ and on bulk MoS₂. These data suggest that (a) InPc-Cl can deposit in a flat-lying ordered monolayer, consisting of one or two sets of 3-fold rotations of coincident 3 × 2 square lattice domains aligned along the principal axes of the metal dichalcogenide, the ordering of which can be extended to multilayers of InPc-Cl, or (b) the InPc-Cl can deposit in a strict three-dimensional growth mode, which produces less ordered multilayers. Low coverages of both types of InPc-Cl ultrathin films sensitize the SnS₂ photoelectrochemical response with quantum yields for charge injection per absorbed photon (QYAP) ≥ 10%. Low coverage (below monolayer) InPc-Cl deposits produce photocurrent yield spectra nearly as narrow as the solution absorption spectra (fwhm 35 nm). Both the highly ordered and less ordered InPc-Cl multilayers continue to produce sizeable QYAP values out to coverages of tens of monolayers. The "epitaxial" deposits show both absorbance and photocurrent yield spectra which are much narrower (fwhm ≤ 60 nm) than those for polycrystalline deposits, consistent with the more homogeneous Pc environment in these ultrathin films.