Self-assembled conjugated polymer–surfactant–silica mesostructures and their integration into light-emitting diodes

Abstract

A self-assembly process for the preparation of functional mesoscopically ordered semiconducting polymer–silica nanocomposite thin films is reported. The nanocomposites are prepared by introducing pre-synthesized semiconducting polymers into a tetrahydrofuran (THF)–water homogeneous sol solution containing silica precursor species and a surface-active agent. Depending on the concentration of the surface-active agent, it was possible to prepare materials with three different types of mesostructural order: i) a 2D hexagonal mesophase silica with conjugated polymer guest species incorporated within the hydrophobic cylinders organized in domains aligned parallel to the substrate surface plane; ii) a lamellar mesophase silica with the layers oriented parallel to the substrate surface and the conjugated polymer guest species incorporated in the hydrophobic layers; or iii) an apparent intermediate phase consisting of a mixture of the hexagonal and lamellar phases in addition to worm-like aggregates with no appreciable orientational order. The continuous through-film conductive pathway provided by the intermediate phase has allowed the integration of ordered semiconducting polymer–silica nanocomposites into opto-electronic devices. By comparison, the lamellar mesostructure prevents through-film conduction, with the result that no light emission occurs. Blue-, green- and red-emitting diodes comprising blue-emitting poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), green-emitting poly(9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-(2,1′,3)-thiadiazole) (F8BT), and red-emitting poly[2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV) confined within the 2D hexagonal silica nanostructure were fabricated with luminances of ca. 3 cd m^–2 at 15 V. Device performances provide criteria for optimizing the selection of synthesis chemistries, processing conditions, compositions, and structures, for light-emission properties sought.