Bias-dependent current efficiency in polymer light-emitting diodes

The current efficiency (CE) of single-layer poly-p-phenylene vinylene-based light-emitting diodes (PLEDs) at low voltages is poor and strongly bias dependent, which has been attributed to the quenching of excitons at the metallic cathode. In the absence of exciton quenching the CE is expected to be voltage independent. We have verified this hypothesis by investigating the electro-optical properties of double-layer PLEDs, where an additional polymeric electron transport layer (ETL) is inserted between the light-emitting layer and the cathode. It is confirmed that for ETLs with thicknesses of 40 nm or larger, where exciton quenching is strongly weakened, the conversion efficiency is indeed voltage independent. Reducing the ETL to 20 nm leads to a pronounced improvement of both quantum and power efficiencies of the PLED at low voltages.

Growth of phenylene vinylene thin films via surface polymerization by ion-assisted deposition

Surface polymerization by ion-assisted deposition was used to grow phenylene vinylene films (SPIAD-PPV) using the evaporation of 2methoxy-5-(2'-ethylhexyloxy)-1,4-bis((4',4 ''-bisstyryl) benzene) (MEH-OPV5) and the simultaneous deposition of non-mass-selected 10-200 eV thiophene or acetylene ions. Images recorded by scanning electron microscopy showed differences in SPIAD-PPV film morphology versus evaporated MEH-OPV5 that indicated changes in the underlying chemical structure. Spectroscopic differences between MEH-OPV5 and SPIAD-PPV films suggested preferential attack of vinylene groups and/or hydrocarbon side chains. A possible explanation for the changes in morphology and electronic structure was cross-linking at vinylene sites of adjacent MEH-OPV5 molecules which could lead to changes in molecular packing. Although morphological and electronic differences were seen for SPIAD-PPV films, these films still showed valence bands characteristic of phenylene vinylenes. (c) 2008 Elsevier B.V. All rights reserved

Tuning of photo‐ and electroluminescence in alkylated polythiophenes with well‐defined regioregularity

Color tuning of luminescence via molecular engineering of the Active polymer is important for the commercial application of pi-conjugated polymers in photonic devices. The synthesis of a series of regiospecific alkylated polythiophenes is described. in which the effective conjugation length could be varied by altering the length of the coplanar blocks between head-to-head dyads. It is demonstrated that the color-tuned materials obtained by this novel approach are potentially suitable for application in displays and other devices

Electric field effects on internal conversion:An alternative mechanism for field-induced fluorescence quenching of MEH-PPV and its oligomers in the low concentration limit

In a previously published study (J. Phys. Chem. B 2006, 110, 7732-7742), we reported field-induced fluorescence quenching in both poly[2-methoxy,5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrices at high dilution (<0.1% by weight). The observed quenching is not readily explained by field-induced exciton dissociation or by the presence of free charges, two mechanisms that have been invoked to explain this phenomenon by previous authors. A model is developed here that ascribes the observed fluorescence quench in dilute samples to an energetic shift of the relaxed excited state caused by the electric field resulting in increased nonradiative relaxation. To determine whether the relevant nonradiative pathway is intersystem crossing or internal conversion, analytical expressions are derived for each of these two mechanisms. Only the expression derived for the Stark effect on the rate of internal conversion quantitatively predicts the magnitude of quench observed in MEH-PPV and in the oligomeric species

Retention Time and Depolarization in Organic Nonvolatile Memories Based on Ferroelectric Semiconductor Phase-Separated Blends

Resistive switches have been fabricated using a phase-separated blend film of ferroelectric random copolymer poly(vinylidene fluoride-co-trifluoroethylene) with the organic semiconductor regio-irregular poly(3-hexylthiophene) (rir-P3HT). Spin-coated blend films have been contacted with symmetrical Ag top and Ag bottom electrodes, yielding switching diodes. The ferroelectric polarization modulates the injection barrier, yielding an injection-limited OFF-state and a space-charge-limited ON-state. To study the effect of depolarization, an additional polyphenylenevinylene-type semiconductor layer with the highest occupied molecular orbital energy that is comparable to that of rir-P3HT has been inserted in the diode stack. When the ad-layer is the injecting contact, the current modulation ratio goes to unity. The origin is a decrease in the effective band bending at the contact with increasing ad-layer thickness. When the counter electrode at the blend interface is the injecting contact, the diode can be switched, but the ON-state is only stable when an electric field that is larger than the coercive field is applied. Upon field removal, the ferroelectric depolarizes, and the current drops to that of an unpoled pristine diode. The depolarization is confirmed by capacitance–voltage and retention time measurements. To realize bistable diodes with excellent retention times, the thickness of the semiconducting wetting layer may not be at most 10 nm.

Novel high efficiency copolymer laser dye in the blue wavelength region

The lasing performance and wavelength tunability of a novel high efficiency copolymer poly[(2,5,2",5"-tetraoctyl)-p-terphenyl-4,4"-ylene vinylene-p-phenylene vinylene], TOP-PPV, pumped with third harmonic radiation of a Nd:YAG laser, was studied in various organic solvents. The results were compared with Coumarin 120 and Coumarin 47 in ethanol under identical experimental conditions. The efficiency of the TOP-PPV copolymer in hexane exceeds that of both coumarin dyes with more than 50%. The laser emission of the polymer dye in hexane is tunable in the wavelength region between 414 and 456 nm (Δλ=38 nm).

Improved Synthesis of 2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde, a Key Intermediate for Drimane-related Sesquiterpenes.

The titel compound is conveniently prepared in 65% overall yield by a two step synthesis starting from he commercially available β-ionone.