XPS Studies

1. Phosphonic Acid Modification of Indium-Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle Studies Sergio A. Paniagua, Peter J. Hotchkiss, Simon C. Jones, Seth R. Marder, Anoma Mudalige, F. Saneeha Marrikar, Jeanne E. Pemberton, Neal R. Armstrong Thrust 2: Light Sources and Organic Electronics, STA 4: Organic electronics and energy harvesting devices, Project 4.2: Organics for portable power generation One of the most common bottom contact electrodes in organic PVs is indium-tin oxide (ITO), whose hydrophilic nature makes it incompatible with the usual nonpolar organic layers used as hole-transport layers, with the consequence of reduced charge collection, and device instability due to delamination of the organic layer. ~100 nm Surface modification with phosphonic acids ~10 nm ~40 nm ~20 nm ITO >100 nm Phosphonic Acid Modified DSC/OP ITO Oxygen plasma (OP) treatment can be used to “activate” the ITO surface. Surface modification with phosphonic acid (PA) monolayers following this activation may result in a more favorable interaction with the hole-transport layer through better surface energy matching, and a more homogeneous contact while retaining some of the large work function increase seen in OP-ITO. Schematic view of a detergent/solvent “cleaned” (DSC) ITO surface; contamination reduces work function, electroactivity, and homogeneity. Glass Illustration of an excitonic planar bilayer heterojunction cell. UPS / Summary XPS Studies PM-IRRAS / Contact Angle Measurements PAs used in this project: PM-IRRAS gives insight into the binding mode of the PAs to the oxide surface. The Secondary electron edge (SEE) and Valence band maximum (VBM) of the ITO can be tuned by choosing different phosphonic acids ODPA powder TFBdiPA powder Analysis of the modified surfaces is necessary to explain device behavior ODPA modified OP ITO TFBdiPA modified OP ITO XPS shows the benefits of OP treatment for activation towards modification. XPS spectra acquired without charge neutralizer: Surface band energy diagrams constructed from UPS data arising from the monolayer-ITO surface system. The high work function Ф of OP treatment is maintained with FHOPA and to a lesser degree with PFBPA. Significant surface dipoles are seen for the modifications with alkyl PAs and TFBdiPA. Reaction scheme proposed for the modification protocol used: • Surface energy components for each treatment are obtained from contact angle data probing with water and hexadecane • By changing the surface modifier, the polar component of the surface energy can be changed while keeping the dispersive component relatively the same The expected monolayers are obtained. Upper column→ Polar Lower column→ Dispersion This research was made possible through a grant from the NSF Science and Technology Center of Materials and Devices for Information Technology Research, No. DMR - 0120967