Yu-Hua Niu

High-efficiency light-emitting diodes using neutral surfactants and aluminum cathode

High-efficiency polymer light-emitting diodes were fabricated by spin-coating a layer of neutral surfactant on top of the poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] electroluminescent (EL) layer to facilitate the electron injection through the high-work-function aluminum cathode. The external luminous efficiency of the device can reach 3.59 cd/A, which is higher than the control device (1.89 cd/A) using calcium as cathode. It was found that when the combination of surfactant and aluminum was used as cathode the abundant hole-injection through a hole-transporting layer and hole pile-up at the inner side of the EL/surfactant interface causes an effective electric field to enhance electron injection.

Thermally crosslinked hole-transporting layers for cascade hole-injection and effective electron-blocking/exciton-confinement in phosphorescent polymer light-emitting diodes

A bilayer hole-injection/transport structure was prepared by thermally crosslinking two separate hole-transport layers (HTL). The resulting films possess excellent optical quality and solvent resistance. Cascade hole-injection and effective electron-blocking/exciton confinement can be achieved for light-emitting diodes (LEDs) using blue phosphorescent emitters, such asbis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate. The first HTL was based on tetraphenyldiamine (TPD) has its highest occupied molecular orbit (HOMO) level lies at −5.3eV, and the second HTL with 4,4′,4″-tri(N-carbazolyl)triphenylamine has its HOMO level lies at −5.7eV. The preliminary results from blue LEDs using these cascade HTLs showed much improved device performance than those only use a single layer hole-transporting polymer.

Highly efficient red electrophosphorescent devices based on an iridium complex with trifluoromethyl-substituted pyrimidine ligand

Highly efficient red-emitting electrophosphorescent devices were fabricated by doping an iridium (Ir) complex containing trifluoromethyl (CF3)-substituted pyrimidine ligand into a conjugated bipolar polyfluorene with triphenylamine and oxadiazole as side chains. The device efficiency can be enhanced through effective exciton confinement using a layer of 1,3,5-tris(N- phenylbenzimidazol-2-yl)benzene on the cathode side and a layer of in situ polymerized tetraphenyldiamine-perfluorocyclobutane on the anode side. For a blend with 5wt% of the Ir complex, a maximum external quantum efficiency of 7.9 photon/electron % and a maximum brightness of 15800cd∕m2 are reached with Commission Internationale de L’Eclairage chromaticity coordinates of x=0.65 and y=0.34.

Efficient ultraviolet-blue polymer light-emitting diodes based on a fluorene-based non-conjugated polymer

Efficient UV-blue polymer light-emitting diodes based on a fluorene-based nonconjugated polymer, poly[2,7-(9,9-dihexylfluorene)-alt-4,4′-phenylether] (PFPE), are fabricated. The device with PFPE as emitting layer shows a very narrow ultraviolet-blue electroluminescence emission with a peak at 397nm and a maximal external quantum efficiency of 1.07%. By blending PFPE into poly(N-vinylcarbazole) (PVK), the device performance can be further improved. A maximum external quantum efficiency of 1.81%, with a maximum irradiance power density of 1223μW∕cm2, was reached by using a blend of PVK and PFPE in the weight ratio of 95:5 as emitting layer.

Efficient and stable blue light-emitting diodes based on an anthracene derivative doped poly(N-vinylcarbazole)

Single-layer blue light-emitting diodes (LEDs) are fabricated by spin coating a blend of 9,10-bis(3′,5′-diaryl)phenyl anthracene in poly(N-vinylcarbazole) (PVK) or in the mixture of PVK and an electron-transporting molecule, 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole. The Commission Internationale de I’Eclairage coordinates of the resulting LEDs are very close to that of the blue standard from the National Television Standards Committee. These devices also show excellent color stability when operated at a voltage span from 6to22V. High external quantum efficiency (>1.5%) and brightness (>3000cd∕m2) can be obtained in these devices.

Material and Interface Engineering for Highly Efficient Polymer Light Emitting Diodes

Polymer light‐emitting diodes (PLEDs) have great potential to compete with LCD displays that are currently used for computer and television screens. The efficiency and stability of PLEDs still need to be improved in order to fully realize the advantages of low cost and ease of fabrication provided by organic materials. Our effort in improving the PLEDs performance have been focused on two parallel approaches: 1) Modify the interface between polymer and the charge‐injection electrodes for more efficient device structures; 2) Enhance the efficiency of PLEDs through the development of new conjugated materials with balanced charge‐transporting properties. In this paper, we review our recent progress on the interface engineering between polymer and electrodes to optimize charge‐injection, ‐transport, and ‐recombination in PLEDs, as well as on the material engineering to tune the emission color, electron affinity, and charge mobility.

A highly electroluminescent molecular square

Chiral molecular triangles and squares containing the Pt(diimine) metallocorners were synthesized and characterized, and used as the triplet MLCT luminophore in highly efficient light-emitting devices.

P-187: Crosslinkable Hole-Transporting Polymers for High Efficiency Blue and White Phosphorescent Light-Emitting Diodes

Second hole-transport layer (HTL) with perfect solvent-resistance is formed via spin-coating then thermally crosslinking on top of a pre-crosslinked HTL or p-type conducting polymer layer. By judiciously choosing the energy levels of the hole-transport moieties, cascade hole-injection can be realized for a light-emitting layer with phosphorescent blue emitters (and white thereafter).

Colloidal Quantum Dots: Light-Emitting Diodes, Ligands, and Lifetimes

We describe multilayer hybrid organic/inorganic colloidal quantum dot light-emitting diodes with narrow electroluminescence spectra and good efficiencies at display brightness. We discuss the role of device structure and quantum dot surfactants on device performance. Article not available.