OLEDs with enhanced high temperature operational stability

Temperature dependence of electroluminescence degradation is studied in organic light emitting devices containing an emitting layer composed of a mixture of different hole transport molecules and tris(8-hydroxyquinoline)aluminum (AlQ(3)) electron transport and emitter molecule. The emitting layer is sandwiched between hole and electron transport layers. Devices containing the hole transport molecule N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB), doped with quinacridone (DMQ) green emitter showed remarkable temperature stability. For these devices, a half-life of about 78,500 hours, 18,700 hours, and 8,600 hours can be projected for operating temperatures of 22degreesC, 700degreesC and 100degreesC, respectively, at an initial device luminance of 100 cd/m(2). Activation energies for device degradation were determined for devices with different hole transport molecules and it was found that devices with higher activation energy show better high temperature stability. These results are consistent with the recently proposed degradation mechanism based on the unstable cationic AlQ(3) species

A modeling approach to understanding OLED performance improvements arising from spatial variations in guest:host blend ratio

Phosphorescent organic light emitting diodes (OLEDs) suffer from efficiency roll off, where device efficiency rapidly decays at higher luminance. One strategy to minimize this loss of efficiency at higher luminance is the use of non-uniform or graded guest:host blend ratios within the emissive layer. This work applies a multi-scale modeling framework to elucidate the mechanisms by which a non-uniform blend ratio can change the performance of an OLED. Mobility and exciton data are extracted from a kinetic Monte–Carlo model, which is then coupled to a drift diffusion model for fast sampling of the parameter space. The model is applied to OLEDs with uniform, linear, and stepwise graduations in the blend ratio in the emissive layer. The distribution of the guests in the film was found to affect the mobility of the charge carriers, and it was determined that having a graduated guest profile broadened the recombination zone, leading to a reduction in second order annihilation rates. That is, there was a reduction in triplet–triplet and triplet-polaron annihilation. Reducing triplet–triplet and triplet-polaron annihilation would lead to an improvement in device efficiency

Noise and charge transport in polymer thin-film structures

The low frequency noise (LFN) properties of. the field-effect transistors (FETs) using polymers as the semiconducting material in thin-film transistor (TFT) structures are investigated and discussed in terms of the charge carrier transport. Results obtained from several research groups are summarized. Injection-drift limited model (IDLM) for charge transport in amorphous PFETs is discussed. IDLM has some advantages in comparison to the commonly used metal-oxide-semiconductor (MOS) transistor models. A general trend of proportionality between noise power density and the DC power applied to the polymer FET's (PFET's), channel is observed in the data from several research groups. This trend implies mobility fluctuation in PFET as the dominant noise source

Luminescence from host-guest conjugated polymers

A host-guest system leading to enhanced luminescence is investigated using a self-forming, solid-state solution of a conjugated polymer. Poly(fluorene-alt-thiophene)s (PFTs) were synthesized with varying ratios of 2,5- and 3,4-thiophene linkages. Copolymers possessing small quantities of the former gave rise to a pseudo host-guest system in which a matrix of 3,4-thiophene linked conjugated polymer transfers electronic excitation energy to isolated, emitting 2,5-thiophene linked conjugated segments

Instability of the noise level in polymer field-effect transistors with non-stationary electrical characteristics

The low frequency noise (LFN) properties of field-effect transistors (FETs) using polymers as the semiconducting substrate material are investigated and explained in terms of the charge carrier transport in polymer thin-film structure. Three mechanisms contribute to the carrier transport - charge injection from source electrode into polymer, charge hopping between polymer molecules for drift transport toward the drain, and charge buildup, probably at polymeroxide interface. Charge buildup is responsible for non-stationary electrical characteristics, but does not contribute significantly to the LFN. Charge hopping determines the maximum value of the mobility and the minimum value of mobility 1/f noise. The variations of the PFET characteristics are mainly due to dispersion in the injection barrier of source-to-polymer contact. High disorder in the polymer at the source contact can increase the leakage current in PFET and can introduce number fluctuation S-GN in the polymer conduction on top of the mobility fluctuation. SGN is proportional to the DC power applied to the injection barrier and should be assumed as a voltage source, since carrier hopping in the polymer reduces the effect of the injection noise. At present, the physical origin of SGN is not fully understood

Low-frequency noise in polymer thin-film transistors

The low-frequency noise (LFN) properties of the polymer field effect transistors (PFETs) using polymer semiconducting material are investigated and discussed in terms of the charge carrier transport. Results obtained from several research groups are summarised. A general trend of proportionality between noise power density and the DC power applied to the PFET channel is observed in the data from publications. This trend implies that the mobility fluctuation in the PFET is the dominant noise source. Copyright IEE, 200

Toxic effects of polyethylene terephthalate microparticles and Di(2-ethylhexyl)phthalate on the calanoid copepod, Parvocalanus crassirostris

© 2017 Elsevier Inc.Large amounts of plastic end up in the oceans every year where they fragment into microplastics over time. During this process, microplastics and their associated plasticizers become available for ingestion by different organisms. This study assessed the effects of microplastics (Polyethylene terephthalate; PET) and one plasticizer (Di(2-ethylhexyl)phthalate; DEHP) on mortality, productivity, population sizes and gene expression of the calanoid copepod Parvocalanus crassirostris. Copepods were exposed to DEHP for 48 h to assess toxicity. Adults were very healthy following chemical exposure (up to 5120 µg L-1), whereas nauplii were severely affected at very low concentrations (48 h LC50value of 1.04 ng L-1). Adults exposed to sub-lethal concentrations of DEHP (0.1–0.3 µg L-1) or microplastics (10,000–80,000 particles mL-1) exhibited substantial reductions in egg production. Populations were exposed to either microplastics or DEHP for 6 days with 18 days of recovery or for 24 days. Populations exposed to microplastics for 24 days significantly depleted in population size (60±4.1%, p<0.001) relative to controls, whilst populations exposed for only 6 days (with 18 days of recovery) experienced less severe depletions (75±6.0% of control, p<0.05). Populations exposed to DEHP, however, exhibited no recovery and both treatments (6 and 24 days) yielded the same average population size at the termination of the experiment (59±4.9% and 59±3.4% compared to control; p<0.001). These results suggest that DEHP may induce reproductive disorders that can be inherited by subsequent generations. Histone 3 (H3) was significantly (p<0.05) upregulated in both plastic and DEHP treatments after 6 days of exposure, but not after 18 days of recovery. Hsp70-like expression showed to be unresponsive to either DEHP or microplastic exposure. Clearly, microplastics and plasticizers pose a serious threat to zooplankton and potentially to higher trophic levels

Explosive sensing with fluorescent dendrimers: The role of collisional quenching

We have investigated a series of branched fluorescent sensing compounds with thiophene units in the arms and triphenylamine centers for the detection of nitrated model compounds for 2,4,6-trinitrotoluene (TNT) and the plastic explosives taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB). Stern-Volmer measurements in solution show that the fluorescence is more efficiently quenched by nitroaromatic compounds when compared to a non-nitrated quencher, benzophenone. Simple modification of the structure of the sensing compound was found to result in significant changes to the sensitivity and selectivity toward the nitrated analytes. A key result from time-resolved fluorescent measurements showed that the chromophore-analyte interaction was primarily a collisional process. This process is in contrast to conjugated polymers where static quenching dominates, a difference that could offer a potentially more powerful detection mechanism. © 2010 American Chemical Society