Yunzheng Wang

1.54 μm infrared photoluminescence and electroluminescence from an erbium organic compound

Infrared emission at 1.54 μm excited optically and electrically from an erbium organic compound tris(acetylacetonato)(1,10-phenanthroline) erbium [Er(acac)3(phen)] is observed. The rare-earth complex is dispersed into a polymer matrix of poly(N-vinylcarbazole) (PVK) to fabricate an electroluminescent (EL) device with an ITO/PVK:Er(acac)3(phen)/Al:Li/Ag structure, where ITO represents indium–tin–oxide-coated glass. The device shows infrared EL emission at 1.54 μm, which suggests a simple and cheap method to obtain a light source for 1.54-μm-wavelength devices in optical communications.

Novel light-emitting diodes involving heterocyclic aromatic conjugated polymers

We report the fabrication and properties of three-, four-, and five-layer electroluminescent devices fabricated from light emissive N-based heterocyclic novel polymeric derivatives of PPP and PPV with which they are isoelectronic. They include poly(pyridine vinylene), (PPyV), and poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene), (PPV.PPyV). Some of the devices operate in both forward and reverse bias modes thus enabling operation in an ac mode. One type of device has the general construction: M/I/polymer/I/ITO where M equals Cu or Al, I equals polyaniline (emeraldine base, EB) or poly(3-hexylthiophene), (P3HT), and polymer equals PPV.PPyV. Under low frequency ac (sinusoidal) driving, light pulses with twice the driving frequency were observed in a device where M equals Al or Cu, I equals EB and polymer equals PPV.PPyV; and in a device where M equals Al, I equals P3HT and polymer equals PPV.PPyV. In the latter device the electroluminescence spectrum in the reverse bias mode differed from that in the forward bias mode. It was also shown that blends of PPyV in Nylon 6,6 exhibit a lower operating voltage than the pure polymer.

High luminescent efficiency in light-emitting polymers due to effective exciton confinement

Highly efficient light-emitting polymers have become possible by molecular engineering. Photoluminescence (PL) quantum yield above 90% in the solid state is reported for the alternating block copolymer of distyrylbenzene. We conclude that the alternate arrangement of conjugated and nonconjugated segments with surrounding side groups for chromophores effectively confine the excitons for radiative emission. The effectiveness of the exciton confinement is confirmed through the temperature independence of the PL quantum yield. The time-resolved PL decay measurement supports this model through the independence of the PL yield on temperature and emission wavelength. The synthesized copolymers have been employed for the fabrication of electroluminescent (EL) devices, demonstrating high external EL efficiency with low operation threshold.

Self-organization of core-shell and core-shell-corona structures in small liquid droplets

It was recently discovered that core-shell and core-shell-corona microstructures in immiscible liquid alloys, which were previously obtained only in outer space, can be fabricated under gravity condition on earth using conventional gas atomization. The origin was attributed solely to Marangoni motion driven by temperature-dependence of interfacial energy. We found in this letter, with the aid of computer simulation, that coupled processes of spinodal decomposition, decomposition-induced fluid flow, collision and collision-induced-collision among second-phase droplets all play critical rules at different stages in the formation of these structures. Their contributions relative to the Marangoni effect are analyzed as function of system size.

Phase-field simulation of thermally induced spinodal decomposition in polymer blends

A phase-field simulation of thermally induced phase separation with various initial average concentrations under different quench depths is systematically carried out in this paper. The resultant morphology of the phase-separated materials is determined by the quench temperature and the average concentration. A detailed understanding of the effects of quench depth during the phase separation process of spinodal decomposition (SD) is presented. The coarsening mechanism with various average concentrations is investigated. In addition, hydrodynamic effects play an important role in phase separation in liquid immiscible alloys. Therefore, a systematic comparison between systems with and without hydrodynamic effects under different conditions is also considered. The model developed and the results obtained could shed light on using SD in immiscible polymer systems to obtain desired nanostructures for advanced applications.

Concurrent spinodal decomposition and surface roughening in thin solid films

A computational model based on the phase field method is developed to investigate the morphological evolution of a thin solid film undergoing concurrent spinodal decomposition and surface roughening. It is found that there is a strong link between developing two-phase microstructures, surface roughness, and film break-up. A non-traditional film-to-island morphological transition is observed due to the presence of coherency stress in the film caused by phase separation.

Five-layered symmetrically configured ac light-emitting (SCALE) devices and their three-layered variations: use of gold as an electron and hole injection electrode

Symmetrically configured ac light-emitting (SCALE) devices based on conjugated polymers utilizing indium-tin oxide (ITO) and aluminum as electrodes have been demonstrated recently. Here we report the fabrication of SCALE devices using a more stable high workfunction metal, such as gold, as a charge (both electron and hole) injection electrode. Also, a variation of such devices in which the electroluminescent polymer, instead of being separated from the insulating polymer, is dispersed in the insulating polymer to form a unified emitter-insulator is reported. These devices emit light in both forward and reverse dc bias with symmetric current- voltage characteristics. Under low frequency ac (sinusoidal) driving voltage, light pulses with double the driving frequency are observed. A model is proposed to account for the device operation.

Direct and Alternating Current Light-Emitting Devices Based on Pyridine-Containing Conjugated Polymers

In the past decade, there has been great interest in organic molecular and polymeric electroluminescent devices, particularly conjugated-polymer-based light-emitting devices (LEDs).1–4 Electroluminescence (EL) combined with other unique properties of polymers, such as solution processability, bandgap tunability, and mechanical flexibility, make conjugated polymers excellent candidates for low-cost large-area display applications.

Interface Control of Polymer Based Light Emitting Devices.

Bilayer and multilayer polymer structures provide opportunities for new photophysics and new types of light emitting devices. Photoluminescent and electroluminescent studies of bilayer heterojunctions formed from a poly(pyridyl vinylene phenylene vinylene) (PPyVPV) derivative and poly(vinyl carbazole) (PVK) show an emission peak which cannot be ascribed to either the PPyVPV derivative or PVK layer. Through studies of absorption and photoluminescence excitation (PLE) spectra we demonstrated that the additional feature results from an exciplex at the bilayer interface. The photoluminescence efficiency of the exciplex is greater than 20%. We also discuss here the fabrication of color variable bipolar/ac light-emitting devices based on conjugated polymers. The devices consist of blends of pyridine-phenylene and thiophene-phenylene based copolymers sandwiched between the emeraldine base form and the sulfonated form of polyaniline. ITO and Al are used as electrodes. The devices operate under either polarity of driving voltage with different colors of light being emitted, red under forward bias, and green under reverse bias.