Electron-transfer processes in zinc phthalocyanine-phosphonic acid monolayers on ITO: Characterization of orientation and charge-transfer kinetics by waveguide spectroelectrochemistry

Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered to indium tin oxide (ITO) as a model for the donor/transparent conducting oxide (TCO) interface in organic photovoltaics (OPVs), we demonstrate the relationship between molecular orientation and charge-transfer rates using spectroscopic, electrochemical, and spectroelectrochemical methods. Both monomeric and aggregated forms of the phthalocyanine (Pc) are observed in ZnPcPA monolayers. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the monomeric subpopulation undergoes oxidation/reduction with k _s,app = 2 × 10 ² s ^-1, independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent charge-transfer rates are observed. For in-plane-oriented Pc aggregates, k _s,app = 2 × 10 ³ s ^-1, whereas for upright Pc aggregates, k _s,app = 7 × 10 ² s ^-1. The rates for the aggregates are comparable to those required for redox-active interlayer films at the hole-collection contact in organic solar cells.