Competing Effects of Fluorination on the Orientation of Aromatic and Aliphatic Phosphonic Acid Monolayers on Indium Tin Oxide

Transparent conductive oxides such as indium tin oxide (ITO) are common substrates for optoelectronic devices, including organic light-emitting diodes and organic solar cells. Tailoring the interface between the oxide and the active layer by adjusting the work function or wettability of the oxide can improve the performance of these devices in both emissive and photovoltaic applications. The use of carefully designed surface modifiers that form self-assembled monolayers (SAMs) can allow the tuning of the surface of one oxide material to optimize its properties for use with a variety of different organic semiconductors or for different applications. Fluorinated phosphonic-acid-based SAMs can affect the interface dipole and the work function of a metal oxide. Fluorination may also affect the molecular packing and the orientation of the SAM once bound to the surface. We utilize angle-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to determine the molecular orientations of octylphosphonic acid, phenylphosphonic acid, and fluorinated derivatives on ITO and correlate the molecular orientations derived from these studies with predictions from density functional theory (DFT). We account quantitatively for the effect of surface roughness on the measured orientations. We observe that fluorination of the octylphosphonic acid SAM results in a more upright orientation, an effect we attribute to intermolecular forces and increased steric bulk. In contrast, fluorination of the phenylphosphonic acid SAM leads to a less upright orientation that we associate with changes in binding mode

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 <i>k</i>_s,app = 2 × 10² s^–1, independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent charge-transfer rates are observed. For in-plane-oriented Pc aggregates, <i>k</i>_s,app = 2 × 10³ s^–1, whereas for upright Pc aggregates, <i>k</i>_s,app = 7 × 10² 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