Absolute Photoluminescence Quantum Efficiency Research Articles

Highly and Deep Red-Luminescent Bisphenylamine-Appended Benzocarcogendiazole Fluorophores.

Benzocarcogendiazole units have been frequently utilized for optoelectronics such as organic solar cells because of their robustness, rigidity, and band-gap tunability based on their strong electron-withdrawing properties. Focusing on the luminescent characteristics, these molecules have been utilized to demonstrate highly sensitive chromisms because of the potential of charge transfer. Here, we demonstrate deep red-emissions in bis(4-tert-butylphenyl)amine-appended benzocarcogendiazole-based donor-acceptor-donor (D-A-D) fluorophores, namely 1 and 2. Because benzocarcogendiazole and bis(4-tert-butylphenyl)amine serve as strong electron acceptor and donor, respectively, strong intramolecular charge transfer (ICT) enables long wavelength of photoluminescence (PL) even in the small molecular weight. Although photoluminescence (PL) in long wavelength tends to exhibit quite low absolute PL quantum efficiency (ΦPL), the values of solutions 1 and 2 are quite high (up to 50 %). According to X-ray crystallographic characterizations and DFT calculations, these high ΦPL values are attributable to the segregated π-planes of benzocarcogendiazole units, which is induced by the bulky substituents of bis(4-tert-butylphenyl)amines.

Ytterbium-doped oxyfluoride nano-glass-ceramic fibers for laser cooling

Ytterbium (Yb3+)-doped oxyfluoride SiO2-Al2O3-CdF2-PbF2-YF3 glass and nano-glass-ceramic (i.e. glass-ceramics containing nanocrystals) single-index optical fibers were fabricated using two methods: by traditional glass preform drawing and by the crucible technique (‘direct-melt process’). The latter technique permitted the fabrication of perfectly vitreous optical fibers (of about 200 µm diameter), leading subsequently to the fabrication of nano glass-ceramic fibers by a well-controlled heat-treatment process above the glass transition temperature. Structural characterizations have (i) confirmed the vitreous state (absence of crystals) of the glass preforms and the glass fiber obtained from the ‘direct-melt process’ and, (ii) evidenced the formation of Pb1-x-y-zCdxYyYbzF2 (x + y + z ≈0.3-0.4) fluorite nanocrystals in the final glass-ceramic fibers. In particular, the nanocrystal size was found to be rather homogenous and smaller than 10 nm from TEM measurements for the nano-glass-ceramic fibers produced by controlled crystallization. The absolute photoluminescence quantum efficiency, mean fluorescence wavelength and anti-Stokes fluorescence of the Yb3+-doped fibers were measured upon laser excitation at wavelengths of 940 nm, 975 nm and 1030 nm, respectively. As expected, higher photoluminescence quantum yield in the near infrared (0.95, close to unity) was obtained for the nano-glass-ceramic fiber when compared with the glass-fiber. Theoretical calculations were also carried out, showing that optical refrigeration would be achievable from these chemically stable and durable nano-glass-ceramic fibers provided that 95% (segregation ratio) of Yb3+ ions are incorporated into the fluorite nanocrystals.

Enhancement and Controlling the Signal of Circularly Polarized Luminescence Based on a Planar Chiral Tetrasubstituted [2.2]Paracyclophane Framework in Aggregation System

Optically active π-conjugated oligo(p-phenylene ethynylene) dimers with a planar chiral 4,7,12,15-tetrasubstituted [2.2]paracyclophane were synthesized. We investigated the optical and chiroptical properties of the racemic and optically active compounds by UV–vis absorption, photoluminescence (PL), circular dichroism (CD), and circularly polarized luminescence (CPL) measurements in the ground and excited states. Aggregates were prepared by the self-assembly in the mixed CHCl3/MeOH solution, a spin-coated film, a drop-cast film, and the annealed films. In the dilute solution, the dimers exhibited good chiroptical properties, such as dissymmetry factors (gabs and glum) in 10–3 order, large absorption coefficient, and high absolute PL quantum efficiency. In the aggregation state, UV–vis absorption and PL measurements suggested that one of the dimers formed J-aggregates and the others constructed parallel H-aggregates or inclined H-aggregates as kinetically stable self-assemblies. Those differences were cause...

Highly efficient green PLED based on triphenlyaminesilole-carbazole-fluorene copolymers with TPBI as the hole blocking layer

Triphenylamine-containing silole was copolymerized with fluorene and carbazole by Suzuki coupling reactions to obtain a series of copolymers. The chemical structures, electrochemical properties, and optoelectronic properties including absorption, photoluminescence, and electroluminescence of these copolymers were well characterized. The highest absolute photoluminescence quantum efficiency of these polymers in films was up to 81%. 1,3,5-Tri(N-phenylbenzimidizol-2-yl)benzene hole blocking layer greatly boosts the external quantum efficiency up to 4.3% from 0.06%. The external quantum efficiency was further raised to 5.4% by blending copolymer with 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD), and the largest luminous efficiency as high as 13.6 cd/A was realized at the same time.

Temperature‐dependent luminescence of Tb3+ and Eu3+ single‐doped glasses for LED applications

AbstractTb3+ and Eu3+ single‐doped barium borate glass and ZBLAN glass are investigated for their photoluminescence (PL) and absolute PL quantum efficiency. In addition, both glass systems are characterized with respect to their color stability in the temperature range from 25 °C to 400 °C.The Tb3+ doped glass shows high color stability over the entire temperature range investigated, whereas the Eu3+ doped glass changes its color appearance with increasing temperature: Eu3+ in ZBLAN glass changes its color appearance from the orange to the red spectral range, whereas the color appearance in borate glass is shifted from the red to the orange‐red spectral range. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Absolute photoluminescence quantum efficiency of P3HT/CHCl3 solution by Thermal Lens Spectrometry

Thermal Lens Spectrometry (TLS), as well as all photothermal methods, rely on the determination of the fraction of absorbed energy converted in heat (φ), which in turn is related to photoluminescence quantum efficiency (η). In this study, we have used TLS to measure η of Poly(3-hexylthiophene) (P3HT) dissolved in chloroform as a function of concentration and excitation wavelength. For samples synthesized by oxidative polymerization (∼74% HT content), η=0.39±0.02 was found to be constant with excitation energy (UV–vis, 335–514nm) and concentration, from 10−4 to 0.6mg/mL. The value of η was practically the same as that of a commercial regioregular P3HT (>95% HT content), measured under the same conditions. The photodegradation effect was also quantitatively analyzed. Although the average emission wavelength of the photo-degraded sample depends on excitation wavelength, both φ and η are constant in the UV–vis range.

A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-xEuxTa2O7 nanosheets

La–Eu solid solution nanosheets La2/3−xEuxTa2O7 have been synthesized, and their photoluminescence properties have been investigated. La2/3−xEuxTa2O7 nanosheets were prepared from layered perovskite compounds Li2La2/3−xEuxTa2O7 as the precursors by soft chemical exfoliation reactions. Both the precursors and the exfoliated nanosheets exhibit a decrease in intralayer lattice parameters as the Eu contents increase. However, there is a discontinuity in this trend between the nominal Eu content ranges x≤ 0.3 and x ≥ 0.4. This discontinuity is attributed to the difference in degree of TaO6 octahedra tilting for the La- and Eu-rich phases. La2/3−xEuxTa2O7 nanosheets exhibit red emission, characteristic of the f–f transitions in Eu3+ photoactivators. The photoluminescence emission can be obtained from both host and direct photoactivator excitation. However, photoluminescence emission through host excitation is much more dominant than that through direct photoactivator excitation, and this behavior is consistent with that of all the other rare-earth photoactivated nanosheets reported previously. The absolute photoluminescence quantum efficiency of the La2/3−xEuxTa2O7 nanosheets increases as the experimentally determined Eu contents increase up to x=0.45 and decrease above it. This result is in good agreement with the optimum photoactivator concentration expected from the percolation theory. These solid solution La2/3−xEuxTa2O7 nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.

Two novel oligomers based on fluorene and pyridine: Correlation between the structures and optoelectronic properties

To investigate the fine optoelectronic difference of the target oligomers with or without peripheral fluorene moieties, two dumbbell-shaped oligomers (F0 and F1) were designed and convergently prepared via Suzuki coupling reaction. The molecular structures of the oligomers were fully characterized by 1H NMR, 13C NMR, and MALDI-TOF mass spectra, respectively. The absorption, photoluminescent behavior, and energy band gaps of the oligomers were examined through UV–vis, photoluminescent spectra and cyclic voltammetry. The experimental results demonstrate that the absorption and photoluminescent properties are little affected by molecular architecture, while the absolute photoluminescence quantum efficiency of films and energy band gaps derived from cyclic voltammetry in solution are strongly correlated with the molecular frameworks. The observed energy band gaps of oligomers are further validated by the different molecular orbital contours of the HOMO energy levels from theoretical calculations. Preliminary electroluminescent investigations for F1 have also been conducted and discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1548–1558, 2008

Tuning the Optical Properties of a Water-Soluble Cationic Poly(p-Phenylenevinylene): Surfactant Complexation with a Conjugated Polyelectrolyte

In this work, we investigate the emission and absorbance properties of the novel water-soluble cationic conjugated polymer poly{2,5-bis[3-(N,N,N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene}, denoted here as P2, in the presence of varying amounts of the anionic surfactant sodium dodecylsulfate (SDS). We show that the absolute photoluminescence quantum efficiency (PLQE), the absorption wavelength, and the emission wavelength of an aqueous solution of P2 can be adjusted according to the surfactant/polymer ratio in aqueous solution. In particular, we show that the addition of SDS to P2 increases the polyelectrolyte's PLQE to approximately 40%. An observed red shift in the emission spectra upon addition of the surfactant is attributed to the reduction in electrostatic repulsive interactions between side chains that minimize the benzene ring twisting along the backbone structure. At the surfactant's critical micelle concentration, the P2 chains wrap around the outer surface of the SDS micelles. This work has implications on the development of new stable poly(p-phenylenevinylene)-based photovoltaic and electroluminescent materials with tunable optical properties.

Absolute photoluminescence quantum efficiency measurement of light-emitting thin films

We developed an integrated monochromatic excitation light source integrating sphere based detection system to accurately characterize the absolute photoluminescence quantum efficiency of commonly used polymer light emitting films without using a reference sample. Our methodology is similar to the method reported by de Mello et al. [Adv. Mater. 9, 230 (1997)] In this Note, we show that the absolute photoluminescence quantum efficiency might only be measured when an appropriate calibration of the spectral variation of the measurement system is done. This calibration is especially important when employing a short excitation wavelength (<400 nm) for common silicon-based detector.

Correlations between Crystallite/Particle Size and Photoluminescence Properties of Submicrometer Phosphors

Correlations between crystallite/particle size and the luminescent characteristics of submicrometer phosphors were investigated. Spray pyrolyzed europium doped yttrium oxide (Y2O3:Eu3+) particles were selected as a model material. Crystallite size and the particle size were controlled independently. The morphology and crystallite structure were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and selected area electron diffraction. Photoluminescence (PL) properties were examined by spectrofluorophotometry and an absolute PL quantum efficiency (QE) measurement system. Chemical analyses and elemental mapping were conducted by Fourier transform infrared spectrophotometry and STEM equipped with energy dispersive spectroscopy, respectively. The results revealed that the PL properties were strongly dependent on crystallite size, particle size, surface chemistry, and the distribution of europium inside the phosphor particles. The P...

Simple Accurate System for Measuring Absolute Photoluminescence Quantum Efficiency in Organic Solid-State Thin Films

We accurately measured the absolute photoluminescence (PL) quantum efficiency (ηPL) of organic solid-state thin films using an integrating sphere. We particularly measured the ηPL of conventional organic materials used in organic light emitting diodes, such as a tris(8-quinolionolato)aluminum(III) complex (Alq3), and a 2 wt%-fac-tris(2-phenylpyridyl)iridium(III):4,4'-bis(carbazol-9-yl)-2,2'-biphenyl [Ir(ppy)3:CBP] co-deposited film. Alq3 and Ir(ppy)3:CBP showed a ηPL=20±1% and 97±2%, which corresponded well to external electroluminescence efficiency using these materials. We also measured their transient PL decay and determined the radiative rate constants with their ηPL.

Effect of emitter disorder on the recombination zone and the quantum yield of organic electroluminescent diodes

The electroluminescence (EL) quantum yield (QY) of double- and triple-layer organic electroluminescent diodes based on a N,N′-diphenyl-N,N′bis(3-methylphenyl)-1,1′-biphenyl4,4′-diamine /tris (8-hydroxyquinolinato) aluminum III (Alq3) junction has been measured as a function of the electric field and the emitting guest (6,13-diphenylpentacene) concentration in the host Alq3. The well-resolved maxima of the QY plots versus applied field for neat and low dopant concentration emitter layers (EMLs) shift strongly toward high fields and disappear at high dopant concentrations. Based on the EL QY data and the measured absolute photoluminescence quantum efficiency of the emitter, the recombination zone width is determined and shown to be a decreasing function of electric field for all of the diodes. The dopant reduces the width of the recombination zone at low dopant concentrations and increases at high dopant concentrations (&amp;gt;0.5 mol %). The results are discussed in terms of a two-step recombination mechanism, assuming disorder-controlled charge carrier mobilities. The dopant concentration effect on the recombination zone width and EL QY can be explained using the disorder formalism that predicts low dopant concentrations create a high degree of positional (off-diagonal) disorder whereas energetic (diagonal) disorder dominates at higher doping levels in the EMLs. The latter makes the recombination zone width as well as EL QY practically field independent.

Surface recombination measurements on III–V candidate materials for nanostructure light-emitting diodes

Surface recombination is an important characteristic of an optoelectronic material. Although surface recombination is a limiting factor for very small devices it has not been studied intensively. We have investigated surface recombination velocity on the exposed surfaces of the AlGaN, InGaAs, and InGaAlP material systems by using absolute photoluminescence quantum efficiency measurements. Two of these three material systems have low enough surface recombination velocity to be usable in nanoscale photonic crystal light-emitting diodes.

A novel triarylamine-based conjugated polymer and its unusual light-emitting properties

A new soluble triphenylamine-based conjugated polymer, poly[N-(4′-butylphenyl)imino(1,1′∶4′,1 ″-terphenyl-4′4″-ylene)], (PBPITP), has been synthesized, which emits blue photoluminescence (PL) with high absolute PL quantum efficiency (45 ± 3%) but exhibits red electroluminescence (red shifted by as much as 157 nm from the PL spectrum).

High-efficiency oligothiopene-based light-emitting diodes

We report investigations of the photoluminescence (PL) and electroluminescence (EL) of a thiophene oligomer for which we have devised a variety of substitutions aimed at enhancing the solid-state efficiency. We find that the absolute PL quantum efficiency in the solid state is up 37% for both powders or spin-coated thin films of the compound. The material thus becomes competitive for applications in organic light-emitting diodes (LEDs). EL efficiencies up to 1.2 cd/A are demonstrated in LEDs prepared with indium–tin–oxide and Ca–Al electrodes.