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ABSTRACT: Time-of-flight mobility measurements on a first generation, bis(fluorene)-cored dendrimer are reported. A charge generation layer was used enabling measurements to be performed on spin-coated films, comparable to those used in devices such as organic light-emitting diodes. The results are compared with spin-coated polyfluorene films. The temperature and electric field dependence of the mobility of the dendrimer was studied and found to be in excellent agreement with the Gaussian disorder model, with an energetic disorder parameter, σ, of 74 meV and a positional disorder parameter, Σ, of 2.6. Copyright © 2007 Elsevier B.V. All rights reserved.
Organic Electronics 02/2013; · 4.05 Impact Factor
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ABSTRACT: We show that it is possible to produce an efficient solution-processable phosphorescent poly(dendrimer) OLED with a 32 lm/W power efficiency at 100 cd/m2 without using a charge transporting host or any improvements in light extraction. This is achieved by using the dendrimer architecture to control inter-chromophore interactions. The effects of using 4,4',4″-tris(N-carbazolyl)triphenylamine (TCTA) as a charge transporting host and using a double dendron structure to further reduce inter-chromophore interactions are also reported.
Optics Express 03/2012; 20 Suppl 2:A213-8. · 3.59 Impact Factor
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Advanced Functional Materials 01/2012; · 10.18 Impact Factor
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ABSTRACT: A family of star-shaped conjugated oligofluorene molecules based around a central benzene core is studied with the aim of identifying how changes in molecular structure can affect the laser performance of organic materials. As the oligofluorene arm length increases the optical transitions are found to move to longer wavelength, there is an increase in photoluminescence quantum yield and a corresponding reduction in the excitation density for amplified spontaneous emission. Distributed-feedback lasers based on these materials are tunable across 402-462 nm with lasing thresholds as low as 1.1 kW cm(-2) and efficiencies as high as 6.6%. The laser performance is compared with that of family of star-shaped molecules with different core structure. This shows that a reduction in intermolecular interactions is very important to achieving high performance lasing in organic semiconductors.
Semiconductor Science and Technology. 01/2012; 27(9).
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ABSTRACT: We have carried out absorption, time-resolved fluorescence, and fluorescence quantum yield measurements of four new soluble anthracene derivatives. They show natural radiative lifetimes in the range of 2.5-4.4 ns, which is 5-10 times shorter than those reported for unsubstituted anthracene. The 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives show the largest fluorescence transition dipoles, which is attributed to extended π-conjugation between anthracene and phenyls through acetylene linkages. Spin-cast films of the BPEA derivatives show strong fluorescence quenching by weakly emitting low energy excitations, which is attributed to excimer-like traps. Quenching is significantly reduced when bulky dendrons are attached so that they give maximum coverage of the emitting chromophore and prevent their aggregation. The results show that anthracene derivatives can be developed into efficient solution-processable fluorescent emitters for the blue and green spectral regions.
The Journal of Physical Chemistry A 06/2011; 115(26):7401-5. · 2.95 Impact Factor
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ABSTRACT: Semitransparent organic bulk-heterojunction solar cells based on poly(3-hexylthiophene) (P3HT):phenyl-C -butyric acid methyl ester (PCBM) with a bottom indium tin oxide (ITO) and semitransparent top metal electrode are presented. The solar cell devices are characterized under two different conditions-top illumination and bottom illumination. The photovoltaic performance is compared with a bottom-illuminated reference device having a fully reflecting aluminum electrode. Optical simulations based on the transfer matrix approach were performed in order to understand the device physics of these semitransparent solar cells. We found that unlike bottom illumination, the exciton generation in top-illuminated devices takes place close to the metal electrode giving rise to high recombination losses in such devices.
IEEE Journal of Selected Topics in Quantum Electronics 01/2011; · 3.78 Impact Factor
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ABSTRACT: We report the observation of a spin-dependent dark transport current, exhibiting spin coherence at room temperature, in a π-conjugated polymer-fullerene blend using pulsed electrically detected magnetic resonance. The resonance at g = 2.0028(3) is due to polarons in the polymer, and exhibits spin locking at high microwave fields. The presence of an excess of fullerene, and the operating voltage (1 V) used, suppresses negative polaron formation in the polymer. It is concluded that spin-dependent transport is due to the formation of positive bipolarons.
Physical Review Letters 10/2010; 105(17):176601. · 7.37 Impact Factor
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Applied Physics Letters 03/2010; 96. · 3.84 Impact Factor
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ABSTRACT: A method employing conjugated polymer thin film blends is shown to provide a simple and convenient way of greatly enhancing the ultraviolet response of silicon photodetectors. Hybrid organic semiconductor/silicon photodetectors are demonstrated using fluorene copolymers and give a quantum efficiency of 60% at 200 nm. The quantum efficiency is greater than 34% over the entire 200-620 nm range. These devices show promise for use in high sensitivity, low cost UV-visible photodetection and imaging applications.
Optics Express 02/2010; 18(4):3219-25. · 3.59 Impact Factor
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ABSTRACT: Organic semiconducting materials have been researched for novel optoelectronic devices due to their efficient light emission and high gain properties. Such devices range from organic light-emitting diodes and solar cells to lasers and amplifiers. This paper explores the research carried out on one of these devices: organic semiconductor amplifiers that have shown to give high gains of 20-40 dB in solution and solid state.
Proceedings of the IEEE 10/2009; · 6.81 Impact Factor
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ABSTRACT: We report the trace vapor detection of the molecule 1,4-dinitrobenzene, a model analyte for the explosive substance 2,4,6-trinitrotoluene, via fluoresence quenching of a first generation conjugated dendrimer containing a 2,2<sup>′</sup> -bis[9,9-di- n -hexylfluorene] core. We show that much greater sensitivity can be obtained by using the material as a surface emitting distributed feedback laser. We find that the slope efficiency of the laser is a convenient and sensitive indicator of the presence of the analyte. The slope efficiency decreases by a factor 50 in the presence of 1,4-dinitrobenzene.
Applied Physics Letters 09/2009; · 3.84 Impact Factor
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ABSTRACT: A detailed study of the photophysics of a family of bisfluorene-cored dendrimers is reported. Polarized time-resolved fluorescence, singlet-singlet exciton annihilation and fluorescence quantum yield measurements were performed and used to understand how the dendron structure affects the light-emitting properties of the materials. The exciton diffusion rate is similar in all films studied. An increase in the nonradiative deactivation rate by nearly one order of magnitude is observed in films of dendrimers with stilbenyl and carbazolyl based dendrons as compared to solutions, whereas the dendrimers with biphenyl and diphenylethylenyl dendrons showed highly efficient emission (photoluminescence quantum yields of 90%) in both solution and the solid state. The results of the materials that show fluorescence quenching can be explained by the presence of quenching sites at a concentration of just a fraction of a percent of all macromolecules. A possible explanation of this quenching is hole transfer from the emissive chromophore to the dendron in a face-to-face geometry. These results are important for the design of efficient blue emitters for optoelectronic applications.
The Journal of chemical physics 06/2008; 128(20):204703. · 3.09 Impact Factor
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ABSTRACT: We demonstrate indirect electrically pumped lasing in a hybrid polymer laser. The lasers comprise a corrugated fluorene copolymer waveguide on an InGaN light-emitting diode and were driven under nanosecond pulsed operation. We observe the onset of distributed feedback lasing at 568 nm for peak drive currents above 144 A. Angle-resolved photoluminescence measurements identify the lasing mechanism as band edge feedback from a photonic stopband in the TE0 waveguide mode.
Applied Physics Letters 04/2008; 92(16):163306-163306-3. · 3.84 Impact Factor
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Advanced Materials 04/2008; · 13.88 Impact Factor
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ABSTRACT: A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay.
Physical Review Letters 02/2008; 100(1):017402. · 7.37 Impact Factor
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ABSTRACT: We report a study of photoinduced charge separation in poly(1,4-phenylenevinylene) (PPV) derivatives with enhanced exciton dissociation using light-induced electron paramagnetic resonance (EPR). Four polymers were studied; all contained alkoxy and fluorenyl side chain units, and two of them also contained a nitro group on the 7-positon of the fluorenyl moieties. Pulsed EPR detected a light induced polaron for all the polymers with the intensity significantly higher for those with nitro groups. A second center was observed for these materials and spectra could be fitted assuming a radical anion was localized on the nitro groups. The fraction of radical anion and polaron centers was comparable, consistent with intramolecular exciton dissociation for the PPV derivatives containing the nitro groups
Organic Electronics 01/2008; · 4.05 Impact Factor
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ABSTRACT: We report polymer distributed feedback lasers with dramatically extended operational lifetimes by using a simple encapsulation process. The lasers are configured as surface emitting, two-dimensional distributed feedback lasers based on the polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]. The microstructure is transferred to the polymer surface through solvent assisted micromolding. Once encapsulated, a 2500-fold improvement in lifetime is demonstrated under ambient conditions, compared to the unencapsulated device. A blueshift of the emission wavelength observed during operation is characterized by absorption and ellipsometry measurements and attributed to a change in effective index due to a loss of conjugation in the polymer.
Applied Physics Letters 12/2007; 91(26):261104-261104-3. · 3.84 Impact Factor
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Advanced Materials 11/2007; 19(22):4054 - 4057. · 13.88 Impact Factor
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ABSTRACT: Conjugated dendrimers are promising optoelectronic materials as their modular architecture enables independent tuning of the optical, electronic, and processing properties. Dendrimer distributed feedback lasers are demonstrated, and an improvement in slope efficiency from 0.3 % to 1.6 % is seen as the dendrimer generation is increased. Simple fabrication of the lasers by soft lithography is also shown, giving a high slope efficiency of 14 %. The combination of high efficiency dendrimer gain media and soft lithographic fabrication techniques makes conjugated dendrimers attractive materials for organic semiconductor lasers.
Advanced Materials 09/2007; 19(19):3000 - 3003. · 13.88 Impact Factor
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ABSTRACT: Amplification of three short light pulses in a 140 ps time window at 5 kHz repetition rate has been demonstrated using a compact amplifier based on the conjugated polymer poly(9,9′-dioctylfluorene-co-benzothiadiazole). The amplifier was optically pumped and gratings were used to couple the signal into and out of the film. A gain of 22 dB was observed for a signal pulse temporally aligned with the pump pulse in a 1 mm waveguide. For a signal pulse delayed by 140 ps, the maximum gain achieved was 14 dB. The results are a step towards the use of polymer amplifiers in data communications.
Applied Physics Letters 07/2007; 91(1):011105-011105-3. · 3.84 Impact Factor
