Shaw H. Chen

Effects of active layer thickness and thermal annealing on polythiophene: Fullerene bulk heterojunction photovoltaic devices

The effect of thermal annealing on photovoltaic devices comprising poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) with thicknesses up to 1200 nm was investigated. Without thermal annealing, the efficiency of the as-prepared devices decreased with increasing active layer thickness, reflecting largely a reduction in the short-circuit current density and an inverse photocurrent spectral response. Thermal annealing of the full devices was found to substantially recover thick-film device efficiencies while reducing the thin-film device efficiencies. The profound variations in photovoltaic characteristics were interpreted in terms of vertical phase separation in the P3HT:PCBM blend film and Li+ diffusion from the LiF/Al contact.

A new class of non-conjugated bipolar hybrid hosts for phosphorescent organic light-emitting diodes

Comprising hole- and electron-transporting moieties with flexible linkages, representative non-conjugated bipolar hybrids have been synthesized and characterized for a demonstration of their potential use as host materials for the fabrication of phosphorescent organic light-emitting diodes. The advantages of this material class include solution processing into amorphous films with elevated glass transition temperatures, stability against phase separation and crystallization, and provision of LUMO/HOMO levels and triplet energies contributed by the two independent moieties without constraint by the electrochemical energy gap. While exciplex formation between the hole- and electron-transporting moieties is inevitable, its adverse effects on spectral purity and device efficiency can be avoided by trapping charges on triplet emitters, as demonstrated for Ir(mppy)3 in TRZ-3Cz(MP)2, and TRZ-1Cz(MP)2. With these two bipolar hybrids and hole-transporting Cz(MP)2 as the host, the maximum current efficiency of the bilayer PhOLED is achieved with TRZ-3Cz(MP)2, but the driving voltage decreases monotonically with an increasing TRZ content.

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.

Robust organic lasers comprising glassy-cholesteric pentafluorene doped with a red-emitting oligofluorene

Doped with a red-emitting oligofluorene, fluid and glassy cholesteric liquid crystal (CLC) films are characterized by similar lasing thresholds and efficiencies. With picosecond excitations the output from a glassy CLC laser is temporally stable, but that from a fluid CLC laser decays with time. The difference in stability is attributable to external perturbations on supramolecular structure in the fluid but not the solid state, such as heating through optical pumping, light-induced pitch dilation, and laser-induced flow.

Circularly polarized fluorescence from chiral nematic liquid crystalline films: theory and experiment

A theory was formulated for the description of circularly polarized fluorescence (CPF) from a chiral nematic film in the spectral region outside the selective reflection band. The CPF theory incorporates: (1) the ability of a chiral nematic film to accomplish both circular dichroism and circular polarization and (2) linearly polarized fluorescence from chromophores unaxially aligned in the nematic sublayers comprising the film. Chiral nematic fluid films consisting of a nematic fluid, BDH 18523, and a chiral dopant, cholesteryl oleate, were prepared for hosting 1,6-diphenylhexatriene as a fluorescent dye. The experimentally determined dissymmetry factor using both left- and right-handed circularly polarized excitations was found to be in good agreement with the theoretical prediction with all the system parameters determined a priori. The theory was also employed to furnish insight into the effects of the concentration of the fluorescent dye and chiral nematic film thickness on the dissymmetry factor.

NOVEL GLASS-FORMING LIQUID CRYSTALS. IV. EFFECTS OF CENTRAL CORE AND PENDANT GROUP ON VITRIFICATION AND MORPHOLOGICAL STABILITY

Abstract To unravel the effects of the volume-excluding central core and the mesogenic pendant group on both the glass-forming ability and morphological stability of the thermally quenched glass, nine model compounds were synthesized that contain various nematogenic and cholesteryl pendant groups. The glass-forming ability of the melt and morphological stability of the thermally quenched glass were assessed using the DSC, XRD, and hot-stage POM techniques. With cyanobiphenyl as the pendant group, the following descending order in morphological stability against thermally activated recrystallization was established: trans-cyclohexane < all-exo-bicyclo [2.2.2] oct-7-ene < cubane < cis-cyclohexane < benzene. While the cyclohexane compound containing three cyanoterphenyl groups showed a strong tendency to crystallize upon quenching, the chiral nematic system in which one of the cyanoterphenyl groups is substituted by a cholesteryl group showed superior glass-forming ability and morphological stability. Additi...

High-permeability fluorinated polyimide microcapsules by vapor deposition polymerization

Spherical microcapsules ({approx} 1 mm in diameter and {approx} 1 mm in wall thickness) to be used as inertial confinement fusion targets were prepared from 6FDA-ODA polyimide by vapor deposition polymerization. Compared with the previously developed PMDA-ODA polyimide microcapsules, the 6FDA-ODA microcapsules were {approx} 50-fold more permeable to gases including He, D2, O2, N2, Ar, and Ne, considerably more transparent in the UV-visible spectrum, and of marginally lower Youngs modulus, tensile strength, and elongation at break. The microcapsules possessed amorphous morphology with an amorphous d-spacing of 5.8 {angstrom} as determined by wide-angle x-ray diffraction. The cryogenic permeability of helium was measured between 133 and 295 K, and the activation energy for permeation was determined to be 12.3 kJ/mol.

Thermotropic chiral nematic side-chain polymers and cyclic oligomers

Abstract For all the theoretical, experimental and computational research on chiral nematic systems over the last four decades, understanding of cholesteric mesomorphism from the molecular perspective is still rather limited at the present time. Nevertheless, the unique property of selective wavelength reflection accompanied by circular polarization has presented tremendous potential for various optical applications. Theories governing optical birefringence and selective reflection bandwidth are outlined to serve as a foundation for molecular design. Both polymeric and low molar mass glass-forming chiral nematics are surveyed in terms of chemical structure, thermotropic and optical properties. Characterized by relative ease of processing, the low molar mass approach is illustrated with our recent results in terms of design concept, materials synthesis and characterization. We also highlight outstanding issues that need to be addressed to help advance chiral nematics as a class of advanced optical materials.

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.

Mechanical Properties and Gas Permeability of Polyimide Shells Fabricated by the Vapor Deposition Method

Abstract Spherical polyimide (PMDA-ODA) shells with high aspect ratio (OD = 900 to 1020 μm and wall thickness = 0.7 to 8.0 μm) were successfully fabricated by the vapor deposition method. These shells were characterized in terms of gas permeability, Young’s modulus, tensile strength, and ultimate elongation. The measured properties of the shells agreed with those of commercial films (Kapton®). Post-coating treatments of the shells, including thermal imidization in air and biaxial expansion, effectively increased gas permeability. Air-curing resulted in shells that were more brittle and twice as permeable as those cured in nitrogen. Since no difference in chemical composition was found between the N2- and air-cured shells, the effect of air-curing may be attributed to morphological or microstructural changes due to oxygen or water vapor in air. Shells that were expanded plastically showed over 300-fold increase in gas permeability, while retaining half of the original Young’s modulus. The effect of expansion on permeability may have arisen from localized plastic deformation. Both treatments will be useful for preparing ICF targets that allow a rapid fill with DT fuel for cryogenic experiments.