Communications Self-Assembled Nanocomposite Polymer Light- Emitting Diodes with Improved Efficiency and Luminance** By Valery Bliznyuk, Beat Ruhstaller, Phil J. Brock, Ulli Scherf , and Sue A. Carter* Hybrid organic and nanoparticle-based systems have been recently studied as prospective materials for optoelec- tronics applications because they combine the advantages of organic polymers with those of inorganic clusters. For photonic applications, nanoparticles enable a wider varia- tion of the dielectric constant (refractive index) and charge transport properties than polymers. [1] The mesoscale char- acter of nanoparticle dimensions, corresponding to the wavelength of visible light, allows for the assembly of super- lattice structures with possible optical band gap properties [2] or with microcavity effects for polymer lasing. [3] Highly effi- cient photovoltaics can be made through organic dye sensi- tization of TiO 2 and related nanoparticles. [4,5] More re- cently, dielectric oxide nanoparticles have been shown to modify the charge transport in polymer light-emitting diodes, resulting in an increase in both the current density and light emission. [6] In this paper, we show that nanopar- ticles assembled at a semiconducting polymer/electrode in- terface can also affect charge injection into polymer light- emitting diodes. For the case of negatively charged dielec- tric SiO 2 monolayers assembled at the anode interface, ex- ternal electroluminescence quantum efficiencies approach- ing theoretical limits for radiative singlet decay can be achieved. Moreover, nanoparticle monolayers result in en- hanced luminances at lower drive voltages, similar to what has been achieved with conducting polymer layers. These results indicate that interfacial nanoparticle layers offer a general method for enhancing the local electric field across the polymer/anode interface, providing versatility for im- proving performance of polymer optoelectronic devices. Since the discovery of electroluminescence in poly- mers, [7] charge transport and injection in semiconducting polymers has been actively studied with the goal of achiev- ing bright and highly efficient polymer light-emitting diodes operating at low electric fields. A key requirement for high electroluminescent efficiencies is balanced injec- tion of holes and electrons at the anode and cathode inter- faces, respectively. Such balance can be dramatically af- fected by controlling injection through modification of the interface between the semiconducting polymer and the electrodes. Although both electrode surfaces can be modi- fied in principle, the modification of the indium tin oxide (ITO) anode is more common due to the atmospheric sta- bility of the ITO surface that enables ªwetº preparation stages (cleaning, chemical modification, and spinning) be- fore the vacuum stages such as evaporation of a metal cath- ode and protective coatings. For materials limited by hole- injection, the device efficiency can be improved by insert- ing a hole transporting layer that enables smaller tunneling barriers, or Ohmic injection, from the anode into the high- est occupied molecular orbital (HOMO) of the polymer. [8± 11] For electron-limited materials, the quantum efficiency can be improved by inserting a layer that effectively blocks electrons from reaching the anode. [12] In this work, modification of the ITO transparent anode was achieved using self-assembled monolayers and electro- statically assembled SiO 2 nanoparticles. This modification was performed in two stages. First, the 3-aminopropyl- triethoxysilane molecules were attached to the ITO surface via chemo-adsorption from an ethanol solution. This proce- dure is similar to modification of Si described before, [13] al- lowing NH + 3 functionalization of the ITO surface. The thickness of a self-assembled organic layer was estimated to be 0.9 nm from atomic force microscopy (AFM) mea- surements. [13] In the second step of modification, nanopar- ticles were attached to the surface of ITO via electrostatic physical adsorption from water solution. Adjustment of the pH conditions on the surface of the SAM resulted in pro- tonation of NH 2 groups to NH + 3 charged groups and the formation of a complete monolayer of negatively charged nanoparticles consisting of SiO 2 (Nissan Chemicals Co., 20 nm diameter) or polystyrene latexes spheres (Interfacial Dynamics Corp., 30 nm diameter) as demonstrated by AFM. The amine functionality also enables attachment of metallic gold nanoparticles (BBI International Co., 40 nm) through chemical tethering to the surface. For comparison, devices were made with bare ITO surfaces cleaned in H 2 O/ isopropanol bath and with polyaniline-PSS (PAni) conduct- ing polymer layer. Table 1 describes the device structures, electrode modificaton and acronyms contained in the fig- ures. The semiconducting polymers used in these study were poly(2-methoxy-5-(2¢-ethyl-hexoxy)-p-phenylene vinylene) Adv. Mater. 1999, 11, No. 15 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,1999 0935-9648/99/1510-1257 $ 17.50+.50/0 1257 Communications ± [*] Prof. S.A. Carter, Dr. V. Bliznyuk, Dr. B. Ruhstaller Physics Department, University of California Santa Cruz, CA 95064 (USA) Dr. P. J. Brock IBM Almaden Research Center San Jose, CA 95120-6099 (USA) Dr. U. Scherf Max Planck Institute for Polymer Research D-55021 Mainz (Germany) [**] This work was supported by an NSF Goali Grant DMR#9704177 and the NSF MRSEC Center for Polymer Interface and Macromolecular Assembly.