Characterizing and Modifying the ITO/Organic Interface: Organic Solar Cells

1. CT-AFM Current-Voltage Curves Characterizing and Modifying the ITO/Organic Interface: Organic Solar Cells ITO ITO 60 nm CuPc CuPc Pt Pt 0 nm -V +V I0 Height K Aluminum Aluminum C60 C60 ITO ITO Ratio of K/Io Diode Quality Factor Aluminum Quinolate BCP Copper Phthalocyanine TiOPc Diogenes Placencia, Alexander Veneman, Neal Armstrong Thrust 2, project 4.2 Transparent conducting oxides (TCO) are the primary bottom contacts in a wide variety of organic solar cells and organic light emitting diodes. Research from this group has recently shown that for indium-tin oxide (ITO) surface composition, morphology and electrical activity are extremely heterogeneous, and difficult to reproduce. Less than 5% of the geometric area may be electroactive, and responsible for carrying 100% of the current in one of these devices. We have focused recently on the characterization of electrical properties of ITO and ITO coated with thin organic layers, and 20 nm2 size scales, using conductive tip AFM (C-AFM), and a new technique – Current-Volume AFM has been developed to give qualitative information on the variation in electron injection barrier heights, organic layer thickness, and electrode active area. We have also explored surface modification techniques, through the introduction of Au nanoparticles, at coverages which significantly improve electroactivity, but do not sacrifice optical transparency. Characterizing the ITO/Organic Interface Au NP Modification of ITO Substrates and Pre-treatment: Effect on Photovoltaic Performance Surface Modification Approaches MOx Substrate Gold-Doped MOx TiOPc/C60 planar heterojunction OPVs showed a significant improvement in device efficiency as Au-NPs were added to the ITO surface: Both exposure time to the Au-precursor solution, and solution pH during this exposure, were important in this optimization. The increased conversion efficiency is attributed to greater absorption in the near-IR from Phase II TiOPc. The Effect of ITO Modification on OPV Performance HAuCl4 Basic pH Rinse ∆ Substrates are pre-treated via detergent-solvent cleaning followed by solution-phase deposition of a hydroxylated gold species. Oxygen-plasma treatment of the Au-NP modified surface is conducted prior to device fabrication. Heating Stirring + Al BCP C60 TiOPc Measurements of I-V curves yield information about organic layer thickness, the effective area of the electrode and the effective barrier height for injection 100nm 10nm 40nm 18nm Surface Composition; Optical/XPS Characterization Current-Volume Mapping: A New C-AFM Approach to Organic/TCO Characterization The Effect of ITO Treatment on OPV Performance Detergent Cleaned ITO O2 Plasma Cleaned ITO HCl-FeCl3 Etched ITO The surface of ITO is a tin-doped indium oxide (bixebyte) lattice, with grains of various texture. Tin dopants are supposed to be uniformly distributed, but are likely to be clustered as SnO or Sn3O4-like domains. The clean ITO surface is reactive and in air rapidly (~1 second) forms hydroxides. Over time, these hydroxides thicken and carbonaceous contaminates build up. This results in electrodes in which less than ~5% of the geometric surface area is electrically active. The performance of organic photovoltaics can be drastically affected by the treatment the ITO receives. These treatments can change the ITO’s work function, remove carbon and/or hydroxides and change the relative ratio of In/Sn on the surface. This variability affects device repeatability and the performance of large-area devices A sol-gel type pre-cursor is proposed as the predominate Au species at the ITO surface prior to annealing. A corrosion-cell is hypothesized to form near tin-rich sites. Annealing produces reduced gold; a final O2-plasma etch produces a mixture of metallic and oxide species. Pre-Annealing: mainly AuxOy and Au(OH)z species Optical Characterization showed small transparency loss in the visible region. A phase change (Phase I to Phase II) is induced by the presence of gold nanoparticles on the surface of the oxide. Annealed: Au° Intensity (Cts/s) OP-etched Binding Energy (eV) Work supported by the NSF-Center for Materials and Devices for Information Technology Research – DMR-0120967, the National Science Foundation, CHE-0517963 , and the Office of Naval Research