Michelle S. Liu

Crosslinkable hole-transporting materials for solution processed polymer light-emitting diodes

One of the most challenging tasks in fabricating multilayer polymer light-emitting diodes (PLEDs) by solution processes is to avoid the interfacial mixing between different layers because most of the commercially available emissive and charge-transporting materials are soluble in common organic solvents. To overcome this difficulty, extensive efforts have been invested in developing novel crosslinkable hole-transporting materials (HTMs). After thermo- or photo-crosslinking, all these crosslinked HTMs possess very good solvent resistance which greatly facilitates the subsequent processing of the emitting layer. By taking advantage of these HTMs, high efficiency red–green–blue (RGB)-emitting PLEDs, as well as white- and quantum dot based PLEDs, have been realized. This article provides a brief overview of the recent development of crosslinkable HTMs and their unique advantages in enhancing the performance of LEDs.

Bright red-emitting electrophosphorescent device using osmium complex as a triplet emitter

A series of efficient and bright double-layer light-emitting devices have been fabricated using the osmium (Os) complex as the triplet emissive dopant in both a blue-emitting polyfluorene derivative (PF–TPA–OXD) containing hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) as side chains and a blend of 2-(4-t-butylphenyl)-5(4-biphenylyl)-1,3,4-oxadizole (PBD) in poly(N-vinylcarbazole) (PVK). Due to a balanced charge injection and transport in PF–TPA–OXD and very efficient energy transfer from this polymer to the Os complex, the resulting device (indium tin oxide/HTL/OsCF3:PF–TPA–OXD/Ca/Ag) reaches a maximum external quantum efficiency of 2.1% with a peak brightness of 2920 cd/m2. These results are significantly higher than those obtained from the commonly used host, PVK:PBD (0.49% and 1270 cd/m2).

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.

Semiconducting Polymer Photodetectors with Electron and Hole Blocking Layers: High Detectivity in the Near-Infrared

Sensing from the ultraviolet-visible to the infrared is critical for a variety of industrial and scientific applications. Photodetectors with broad spectral response, from 300 nm to 1,100 nm, were fabricated using a narrow-band gap semiconducting polymer blended with a fullerene derivative. By using both an electron-blocking layer and a hole-blocking layer, the polymer photodetectors, operating at room temperature, exhibited calculated detectivities greater than 1013 cm Hz1/2/W over entire spectral range with linear dynamic range approximately 130 dB. The performance is comparable to or even better than Si photodetectors.

Organic light-emitting diodes using an in situ thermally polymerized hole transporting layer

We have fabricated highly efficient organic light-emitting diodes (LEDS) using in situ thermally polymerized hole transporting materials containing triphenylamine/tetraphenyldiaminobiphenyl as side chains. Double-layer LEDs made with these thermally polymerized polymers as the hole transporting layer and tris(8-hydroxyquinoline) aluminum as the emitting layer showed comparable brightness but lower turn-on voltages and better quantum efficiencies than the device made with the conventional hole transporting molecule, N-N′-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine.

Time-Resolved Photoluminescence Spectroscopy of Surface-Plasmon-Enhanced Light Emission from Conjugate Polymers

The authors have experimentally verified that the light emission from conjugated polymers can be enhanced through the use of surface plasmon coupling layers. Carrier dynamics of such plasmon-enhanced organic light emitters were studied and a recombination rate increase due to surface plasmon polaritons was experimentally observed. Internal quantum efficiency data from the polyfluorenes studied follow the trend supported by the time resolved photoluminescence measurements.

Lithium salt doped conjugated polymers as electron transporting materials for highly efficient blue polymer light-emitting diodes

Highly efficient blue polymer light-emitting diodes (PLEDs) are fabricated using a conjugated polymer, poly[9,9-bis(2-(2-(2-diethanol-amino-ethoxy) ethoxy) ethyl) fluorene-alt-4, 4′-phenylether] as an electron transporting layer (ETL). It was found that the performance of these blue-emitting devices could be greatly improved if the ETL was doped with LiF or Li2CO3 salts. A bis[(4,6-di-fluorophenyl)-pyridinato-N, C2] (picolinate) Ir(III) (FIrpic) complex based blue phosphorescent PLED exhibited a maximum luminance efficiency of 20.3 cd/A with a luminance of 1600 cd/m2 at the current density of 7.9 mA/cm2 and drive voltage of 8.0 V.

Bright and efficient exciplex emission from light-emitting diodes based on hole-transporting amine derivatives and electron-transporting polyfluorenes

We report highly efficient and bright emission from exciplexes generated between a series of hole-transporting amine derivatives and two electron-transporting fluorene–dicyanophenyl (FCNP) copolymers. These exciplexes were formed at either the interface between tetraphenyldiamine-containing perfluorocyclobutane polymers and the FCNP copolymers, or in the blends of the FCNP copolymers with small molecule amine derivatives such as triphenylamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine, and N,N′-diphenyl-N,N′-bis(1-naphthyl)-[1,1′-biphenyl]-4,4′-diamine. The exciplex emission is largely dependent on the composition of the hole-transporting materials. The best device derived from these exciplexes demonstrated a very low turn-on voltage (2.8 V), a high external quantum efficiency (0.91%), and a high brightness of 3370 cd/m2. The desirable properties of these devices were attributed to the excellent electron transport ability of the FCNP copolymers.

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.

Synthesis and characterization of polyquinolines for light-emitting diodes

A novel light-emitting polymer containing both a highly electron-affinitive segment, biquinoline, and a good hole transporting segment, dialkoxyphenylenevinylene, was synthesized and characterized. This polymer possessed excellent film-forming properties, good thermal stability, charge injection properties and electroluminescence efficiency. A greenish-yellow light emitting diode was fabricated using this copolymer as an emitter layer.