Electron transfer kinetics of phosphonic acid functionalized zinc phthalocyanine monolayers on ITO by potential modulated attenuated total reflectance

Phthlocyanines are candidate building blocks for organic photovoltaic (OPV) devices. Crucial to the performance of OPV devices is the charge-injection barrier between the indium tin oxide (ITO) electrode and the organic semiconductor that arises from poor wettability of ITO surface towards the organic layer. Phosphonic acid (PA)-linked Pcs are an excellent candidate in this context because of the strong affinity of PAs towards ITO. Here monolayer films of zinc phthalocyanine (ZnPc-PA), tethered to ITO via the PA moiety, were characterized by polarized attenuated total reflectance (ATR) spectroscopy. The monomeric form of ZnPc-PA exhibit a more flat orientation, whereas the aggregated form of ZnPc-PA is oriented in a more upright arrangement. The electron transfer rate constants across the monolayer-ITO interface are measured using potential modulated attenuated total reflectance (PM-ATR) in TE and TM polarizations. This allows rate constants to be determined for sub-populations of molecules that are oriented predominately in-plane (monomers) and out-of-plane (aggregates), respectively.