Self-Assembled Nanocomposite Polymer Light-Emitting Diodes with Improved Efficiency and Luminance

Abstract

Hybrid organic and nanoparticle-based systems have been recently studied as prospective materials for optoelectronics applications because they combine the advantages of organic polymers with those of inorganic clusters. For photonic applications, nanoparticles enable a wider variation of the dielectric constant (refractive index) and charge transport properties than polymers. The mesoscale character of nanoparticle dimensions, corresponding to the wavelength of visible light, allows for the assembly of superlattice structures with possible optical band gap properties or with microcavity effects for polymer lasing. Highly efficient photovoltaics can be made through organic dye sensitization of TiO2 and related nanoparticles. [4,5] More recently, 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. In this paper, we show that nanoparticles assembled at a semiconducting polymer/electrode interface can also affect charge injection into polymer lightemitting diodes. For the case of negatively charged dielectric SiO2 monolayers assembled at the anode interface, external electroluminescence quantum efficiencies approaching theoretical limits for radiative singlet decay can be achieved. Moreover, nanoparticle monolayers result in enhanced 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 improving performance of polymer optoelectronic devices. Since the discovery of electroluminescence in polymers, charge transport and injection in semiconducting polymers has been actively studied with the goal of achieving bright and highly efficient polymer light-emitting diodes operating at low electric fields. A key requirement for high electroluminescent efficiencies is balanced injection of holes and electrons at the anode and cathode interfaces, respectively. Such balance can be dramatically affected by controlling injection through modification of the interface between the semiconducting polymer and the electrodes. Although both electrode surfaces can be modified in principle, the modification of the indium tin oxide (ITO) anode is more common due to the atmospheric stability of the ITO surface that enables aweto preparation stages (cleaning, chemical modification, and spinning) before the vacuum stages such as evaporation of a metal cathode and protective coatings. For materials limited by holeinjection, the device efficiency can be improved by inserting a hole transporting layer that enables smaller tunneling barriers, or Ohmic injection, from the anode into the highest occupied molecular orbital (HOMO) of the polymer. 11] For electron-limited materials, the quantum efficiency can be improved by inserting a layer that effectively blocks electrons from reaching the anode. In this work, modification of the ITO transparent anode was achieved using self-assembled monolayers and electrostatically assembled SiO2 nanoparticles. This modification was performed in two stages. First, the 3-aminopropyltriethoxysilane molecules were attached to the ITO surface via chemo-adsorption from an ethanol solution. This procedure is similar to modification of Si described before, allowing NH3 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) measurements. In the second step of modification, nanoparticles 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 protonation of NH2 groups to NH + 3 charged groups and the formation of a complete monolayer of negatively charged nanoparticles consisting of SiO2 (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 H2O/ isopropanol bath and with polyaniline-PSS (PAni) conducting polymer layer. Table 1 describes the device structures, electrode modificaton and acronyms contained in the figures. The semiconducting polymers used in these study were poly(2-methoxy-5-(2¢-ethyl-hexoxy)-p-phenylene vinylene)