Investigation of dye-doped red emitting organic electroluminescent devices with metal-mirror microcavity structure

The first soluble hyperbranched tetrahedral polymers were prepared by Suzuki coupling polycondensation reaction between tetrabromoarylmethane/silane and 9,9-dihexylfluorene-2,7-diboronic acid at low concentrations. The polymers exhibited high thermal stability with their decomposition temperatures (T ds) in the range 352−449 °C. The polymers emitted blue light highly efficiently in both solution and the solid state. The photoluminescence quantum efficiencies of the polymers in THF solution were in the range 73−99%, and those in thin films were 38−82%. The film PL spectra of the polymers exhibited similar spectral patterns to those determined in solutions, with 1−8 nm red shift in their emission maxima and 0−8 nm increase in their full width at half-maximum (fwhm) values being observed. No long wavelength excimer-like emissions at 500−600 nm, which were typical for polyfluorenes due to their self-aggregation in the solid state, were observed. Thus, the polymers were less prone to self-aggregation in the solid state due to their hyperbranched structures. A double-layer polymer light-emitting diode (PLED) device with a configuration of ITO/PEDOT/polymer/LiF/Ca/Ag was fabricated. The device showed bright blue emission peaking at 415 nm with an external quantum efficiency of 0.6% and a turn voltage at 6.0 V. The synthetic simplicity, good solubility and solution processability, high PL quantum efficiencies in solution and the solid state, and nonaggregating property in the solid state would make the present polymers a novel class of blue emitters.

The wettability and evaporation of water-ethanol binary droplets on the substrate with micropyramid cavities are studied by controlling the initial ethanol concentrations. The droplets form octagonal initial wetting areas on the substrate. As the ethanol concentration increases, the side ratio of the initial wetting octagon increases from 1.5 at 0% ethanol concentration to 3.5 at 30% ethanol concentration. The increasing side ratio indicates that the wetting area transforms from an octagon to a square if we consider the octagon to be a square with its four corners cut. The droplets experience a pinning-depinning transition during evaporation. The pure water sessile droplet evaporation demonstrates three stages from the constant contact line (CCL) stage, and then the constant contact angle (CCA) stage, to the mixed stage. An additional mixed stage is found between the CCL and CCA stages in the evaporation of water-ethanol binary droplets due to the anisotropic depinning along the two different axes of symmetry of the octagonal wetting area. Droplet depinning occurs earlier on the patterned surface as the ethanol concentration increases.

A series of hyperbranched alternating copolymers were synthesized by either Suzuki or Grignard coupling polycondensation reactions between two AB4-type tetrahedral monomers [tetra(4-bromobiphenyl)silane (1) and tetra(2-bromo-9,9-dihexylfluorenyl)silane (2)] with AB2-type dibromo or diboronic acid monomers of 9,9-dihexylfluorene or oligothiophenes. Polymers (3, 5−8) (derived from 1 and 9,9-dihexylfluorene-2,7-diboronic acid, 2,2‘-bithiophene-5,5‘-diboronic acid, 2,2‘:5‘,2‘ ‘-terthiophene-5,5‘ ‘-diboronic acid, 5,5‘-dibromo-3,3‘-dihexyl-2,2‘-bithiophene, and 5,5‘ ‘-dibromo-3,3‘-dihexyl-2,2‘:5‘,2‘ ‘-terthiophene, respectively) emitted blue to red light in the solid state and their absorption and emission maxima were red shifted with the increase in conjugation length. Polymer 4 (derived from 2 and 9,9-dihexylfluorene-2,7-diboronic acid) emitted blue light highly efficiently in both solution and the condensed state, but its absorption and emission maxima were slightly blue-shifted from those of 3, revealing that no effective electron delocalization occurred among the fluorene units. Polymer 9 (derived from 2 and 5,5‘-dibromo-3,3‘-dihexyl-2,2‘-bithiophene) emitted intense blue light in solution with only the structured peaks from the fluorene units being observed in its PL spectra. The film of 9 emitted green light and emissions from both the fluorene and bithiophene chromophores were observed in the PL spectra with the latter being stronger, due to the intramolecular energy transfer from fluorene units to bithiophene units. The ab initio studies were carried out to elucidate the intrigue optical properties of the present hyperbranched polymers. Double layer devices with configurations ITO/PEDOT/polymer (4 or 7)/LiF/Ca/Ag, which emitted blue and green light, respectively, were fabricated and preliminary data on the device performances are reported. We present two strategies for the design and synthesis of RGB-emitting hyperbarched polymers:  (1) using a structurally not crowded core 1 and adjusting the emission by conjugation length; (2) using a structurally crowded core 2 and adjusting the light emission by intramolecular energy transfer.