Phosphonic Acid Modification of Indium−Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle Studies

Abstract

Indium−tin oxide (ITO) electrodes have been modified with both fluorinated alkyl and aryl phosphonic acids [n-hexylphosphonic acid (HPA) and n-octadecylphosphonic acid (ODPA); 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (FHOPA), pentafluorobenzyl phosphonic acid (PFBPA), and tetrafluorobenzyl-1,4-diphosphonic acid (TFBdiPA)]. These are modifiers designed to control both wetting properties toward nonpolar molecular solids and to provide a wide range of tunability in effective surface work function. The molecular nature of surface attachment and changes in electronic and wetting properties were characterized by X-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), photoelastic modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and contact angle measurements using both water and hexadecane. Interface dipoles from the PA modifiers contribute to shifts in the low kinetic energy regions of UPS spectra (local vacuum level shifts, which translate into changes in effective surface work function). We show that for ITO surfaces modified with FHOPA, and to a lesser extent with PFBPA, the high work function obtained by oxygen plasma cleaning can be maintained after modification, while decreasing the polar component of surface energy. This approach to oxide surface modification is a strategy that may be beneficial for the modification of transparent conducting oxide surfaces in both organic light emitting diodes and in organic solar cells, where oxide/organic compatibility can affect device performance.

Doped Interlayers for Improved Selectivity in Bulk Heterojunction Organic Photovoltaic Devices

“…These thin layers are often called interlayers and many types of materials have been investigated, such as low work-function metals, [ 3,4 ] metal salts, [ 5 ] silane-based selfassembled monolayers (SAMs), [ 6 ] phosphonic-acid SAMs, [ 7,8 ] metal oxides, [9][10][11] polymers, [12][13][14][15] and polyelectrolytes. [ 16 ] Other examples are also available in the literature, with a thorough review provided by Ma et al [ 1 ] The majority of these interlayers improve device performance by reducing electronic barriers for extraction of the desired charge carrier and/or increasing charge selectivity by blocking the undesired charge carrier.…”

Modification of the Surface Properties of Indium Tin Oxide with Benzylphosphonic Acids: A Joint Experimental and Theoretical Study

“…These studies follow closely on earlier studies where we demonstrated the tunability of ITO surfaces with a small family of alkane and aromatic PAs. [15] We accompany these studies with a new theoretical study of PA bonding to ITO surfaces, based on density functional theory (DFT), [16] carried out to provide further insight into the relation between work function change and the choice of PA modifier.…”

Self-Assembly and Charge Transport of a Conjugated Polymer on ITO Substrates

“…Assembly of organic species on conducting substrates in the form of molecular wires and molecular assemblies have been widely studied, and within recent years, research have increased in polymer assemblies on varies conducting substrate [3,4,12]. Conjugated polymers end-functionalized with phosphonic acid anchoring groups have been grafted onto oxide substrates such as indium tin oxide (ITO) [5,[13][14][15][16], zinc oxide [17], silicon dioxide (SiO 2 ) [18], titanium dioxide (TiO 2 ) [19,20], and aluminium oxide (Al 2 O 3 ) [21]. Through a dehydration (condensation) reaction, the phosphonic acid group binds to each substrate covalently and possibly electrostatically (via hydrogen bonds), producing several binding modes [15].…”