Jason U. Wallace

Evaluation of propylene-, meta-, and para-linked triazine and tert-butyltriphenylamine as bipolar hosts for phosphorescent organic light-emitting diodes

Three representative bipolar hybrids – tBu-TPA-p-TRZ, tBu-TPA-m-TRZ, and tBu-TPA--TRZ with triplet energies, ET = 2.5, 2.7 and 3.0 eV, respectively – were synthesized and characterized for a comprehensive evaluation of their potential as a host for phosphorescent organic light-emitting diodes (PhOLEDs) using red-emitting Ir(piq)3 as the dopant with an ET value of 2.1 eV. Formation of charge transfer complexes, CTCs, was diagnosed by fluorescence bathochromism in increasingly polar solvents. Both intra- and inter-molecular charge transfer processes are invoked to explain CTC formation in all three hybrids. The ppp-hybrid is by far the most susceptible to CTC formation both in solution and neat solid film, resulting in PhOLEDs with reduced external quantum efficiency, EQE, despite the best balance between charge fluxes across the emitting layer, EML, as revealed by the electron- and hole-only devices in addition to PhOLEDs containing a sensing layer. The highest EQE is achieved with the mm-hybrid thanks to the compromise between balanced charge fluxes and CTC formation. The -hybrid is the least prone to CTC formation while suffering charge flux imbalance to yield an EQE intermediate between those of the mm- and ppp-hybrids. The least CTC formation involving the -hybrid is advantageous in accommodating the most singlets and triplets readily transferrable to both red and blue phosphors on account of its relatively high ET value. Furthermore, the -hybrid offers the best morphological stability of the desired glassy EML, thus holding promise for the fabrication of superior PhOLEDs overall.

Temporal stability of blue phosphorescent organic light-emitting diodes affected by thermal annealing of emitting layers

Emitting layers were thermally annealed at 20 to 100 °C for varying durations without causing phase transformation in the rest of the PhOLEDs. Heating EMLs above their Tgs with a free surface created pinholes filled by the underlying TAPC melt with concurrent interlayer mixing to emit satellite peaks accompanying FIrpics phosphorescence. With a robust glassy TmPyPB layer on top of the EML, pinholes and fortuitous fluorescence could be prevented. Annealing of mCP:SiPh4:FIrpic induced crystallization in 1 h, while mCP--PhSiPh3:FIrpic consistently resisted crystallization under all conditions. Crystallization or pinhole formation diminished EQE and driving voltage at the same time. Without incurring pinhole formation in the absence of a free surface presented by the EML, annealing of mCP:SiPh4:FIrpic at 60 °C for 1 h led to about 50% loss in EQE. In contrast, the pristine devices EQE persisted with mCP--PhSiPh3:FIrpic annealed at 60 °C for up to 24 h, beyond which other sources of device failure took over. The concept of bipolar hybrids holds promise for mitigating morphological instability as part of the challenge to the PhOLED device lifetime.

Photochromic glassy liquid crystals comprising mesogenic pendants to dithienylethene cores

Photochromic glassy liquid crystals were synthesized using dithienylethenes as the volume-excluding cores to which liquid crystalline mesogens were chemically bonded through alkyl spacers. Nematic, smectic, and cholesteric glassy liquid crystals were demonstrated with glass transition temperatures above 90 °C and clearing points up to 220 °C without traces of crystallization on cooling or crystalline melting on heating. A monodomain cholesteric glassy liquid crystalline film containing an enantiomeric 2-methylpropylene chiral spacer was characterized as a left-handed helical stack, exhibiting a selective reflection band centered at 686 nm, an orientational order parameter of 0.65 for the quasi-nematic layers, and a combination of reflective coloration with photoswitchable absorptive coloration.

Charge-retraction time-of-flight measurement for organic charge transport materials

This letter describes an all-electrical technique, charge-retraction time-of-flight (CR-TOF), to measure charge carrier mobility through an organic layer. Carriers are injected and accumulated at a blocking interface, then retracted. The retraction current transient is nearly indistinguishable from a traditional time-of-flight photocurrent. The CR-TOF technique is validated by measurement of the hole mobility of two well-known compounds, 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl, utilizing 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene as a hole-blocking layer. A sample layer thickness of less than 300nm can be used for the measurement.

Quantitative analysis of photoalignment of liquid crystals on coumarin-containing polymer films

Polymers containing 6- and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV-irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of the extent of dimerization, X. The experimental data for X as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. The kinetic model was employed to describe the evolutions of coumarin dimers and monomers orientational order. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and the late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation was successfully interpreted by considering three factors: the relative abundance of coumarin dimers to monomers, their orientational order parameters, and the energetics of molecular interaction.

64.1: Invited Paper: Polarized OLEDS as Backlight for Liquid Crystal Displays

This work aims at the design, synthesis, and characterization of novel conjugated glassy-nematic liquid crystals for highly efficient and strongly polarized OLEDs. In addition to deep blue emission using oligofluorenes, emission of standard green, red, and white-light has also been accomplished through chemical modification and physical doping.

Novel Organic Light-Emitting Materials Capable of Variable Charge Injection and Transport

Novel organic materials were designed and synthesized by attaching monodisperse oligofluorenes to a hole- and an electron-conducting core through a flexible spacer. These material class holds promise for the realization of efficient and stable OLEDs. Article not available.

Temporal Stability of Blue OLEDs: Effect of Hole Mobility through the Emissive Layer

Anthracene-containing model compounds were synthesized to investigate causes of instability in blue OLEDs. Transient OLED measurements revealed that an emissive layer with higher hole mobility resulted in a longer lifetime at the expense of efficiency. Article not available.

Novel light-emitting organic materials with variable electron and hole conductivities

Novel organic materials are constructed by attaching monodisperse oligofluorenes to a hole- or an electron-conducting core through a flexible spacer. These materials exhibit desirable properties for use in polarized and unpolarized light-emitting diodes, such as the ability to form morphologically stable glassy liquid crystalline and amorphous films with elevated glass transition temperatures, capability for full-color emission, tunability of charge injection and transport, and ultimately achieving superior OLED device efficiency and lifetime.