Xuezhong Jiang

Effect of carbazole–oxadiazole excited-state complexes on the efficiency of dye-doped light-emitting diodes

Interactions between hole-transporting carbazole groups and electron-transporting 1,3,4-oxadiazole groups were studied by photoluminescence and electroluminescence (EL) spectroscopy, in blends of poly(N-vinylcarbazole) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD) and in random copolymers with carbazole and oxadiazole groups attached as side chains. Different excited-state complexes form in the blends, which exhibit exciplexes, and in the copolymers, which manifest electroplexes, due to topological constraints on the position of carbazole and oxadiazole units in the polymer. Both types of complex red-shift the EL spectra of the matrices compared with pure PVK homopolymer, although the shift is significantly greater for the electroplex. The presence of these complexes has a profound effect on the external quantum efficiency of dye-doped organic light-emitting diodes employing the blends or copolymers as matrices, as it strongly affects the efficiency of Forster energy transfer from the matr...

Red electrophosphorescence from osmium complexes

We report red electrophosphorescence from light-emitting diodes based on osmium (Os) complexes. Efficient red emission was achieved using an in situ polymerized tetraphenyldiaminobiphenyl-containing polymer as the hole-transporting layer and Os complexes doped blend of poly(N-vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole as the emitting layer. The emission peaks of the reported Os complexes, ranging from 620 to 650 nm, can be tuned by changing the structures of the ligands because the emission originates from triplet metal-to-ligand-charge-transfer excited state. The Os complexes trap both electrons and holes, which facilitates the direct recombination of holes and electrons on the complex sites. The peak external quantum efficiency and brightness achieved from the complexes were 0.82% and 970 cd/m2, respectively. The Commission Internationale de I’Eclairage chromaticity coordinates (x,y) for the best red emission from the complexes are (0.65, 0.33).

High-performance blue light-emitting diode based on a binaphthyl-containing polyfluorene

We report efficient blue electroluminescence (EL) from multilayer polymer light-emitting diodes using a binaphthyl-containing polyfluorene as the emitting layer and a series of thermally polymerized triphenylamine/tetraphenyldiaminobiphenyl-containing polymers as the hole transport layer. The polymer possesses a high photoluminescence quantum efficiency of 44%. The polymer light-emitting diodes exhibit EL emission peak at 420 or 446 nm, depending on the hole transporting materials. The EL reaches maximum brightness and external quantum efficiency of 3070 cd/m2 and 0.82%, respectively.

Red-emitting electroluminescent devices based on osmium-complexes-doped blend of poly(vinylnaphthalene) and 1,3,4-oxadiazole derivative

Efficient red electrophosphorescence was achieved from double-layer light-emitting devices using osmium (Os)-complexes-doped blend of poly(vinylnaphthalene) (PVN) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PVN:PBD) as the emitting layer. Blending PVN with PBD greatly suppresses the electromer emission of PVN. The PVN:PBD blend emanates a short-wavelength electroluminescence emission peaking at around 375 nm, which overlaps well with the absorption spectra of the Os complexes and ensures very efficient energy transfer to the Os-complex dopants. The best external quantum efficiency of the double-layer devices was 2.2%, with a photometric efficiency of 1.9 cd/A.

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.

The effect of ligand conjugation length on europium complex performance in light-emitting diodes

Abstract Novel europium complexes containing either symmetrical or unsymmetrical diphenanthryl β-diketone ligands were synthesized and characterized. The ligand conjugation length has a profound effect on the spectral properties of the complexes and the device performance of the complexes as emitting dopants in light-emitting diodes. The unsymmetrical ligand has a longer conjugation, which red-shifts the absorption of the complex. The longer wavelength absorption facilitates the choice of host material that can efficiently transfer energy to the complex. However, the longer conjugation also results in a less efficient energy transfer from the ligand to the central Eu3+ ion.

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 emission from a europium complex containing dendron-substituted diketone ligands

A new europium (Eu) complex with dendron-substituted diketone ligands was synthesized and found to exhibit a photoluminescence efficiency of 45%. Double-layer light-emitting diodes based on polymer matrices doped with the Eu complex were fabricated. An electroluminescence external quantum efficiency of 0.80% was achieved when a copolymer containing side-chain carbazole and 1,3,4-oxadiazole groups was used as the matrix. The results are analyzed in the context of Forster energy transfer.

Efficient green polymer light-emitting diodes with microcavity effect in electroluminescence spectrum but constant quantum efficiency

We report the efficient green polymer light-emitting diodes (LEDS) that exhibit a strong microcavity effect in the electroluminescence (EL) spectrum. The LEDs employ a double-layer structure, with poly-(3,4-ethylenedioxythiophene): polystyrene sulfonic acid as the hole-transporting layer and with a highly efficient polyfluorene-based green-emitting polymer as the electron-transporting and emitting layer. The EL spectra of the LEDs demonstrate a strong resonance effect with the thickness of the emitting layer varying from 30 to 280nm. The turn-on voltage of the device increases with the increasing thickness of the emitting layer. However, the brightness and especially the external quantum efficiency of the devices are largely independent of the thickness from 100 to 280nm, when the emitting layer is thick enough to avoid cathode quenching of the electroluminescence.

Organic light-emitting diodes containing fluorinated asymmetrical europium cored beta-diketone complexes

Novel luminescent materials based on europium-cored complexes have been synthesized and incorporated into light emitting diodes using poly (N-vinyl-carbazole) and poly (vinyl naphthalene) blends as doping hosts. The complexes consists of fluorinated β-diketone ligands chelated to europium. Excitation of the ligands and efficient transfer of energy from the excited ligands to the metal core results in the emission of optically pure red light. The ligands were designed such that they include a polycyclic aromatic compound, phenanthrene, and a second substituent to improve processibility. Phenanthrene is used to so that the ligand energy will match with the energy of the metal center. Partially fluorinated substituents were also used to help improve the efficiency and charge transfer capability of the resulting metal complex. The complex consisted of one equivalent of europium and three equivalents of the ligand. One equivalent of either 1,10-phenanthroline or 4,7-diphenyl-1,10-phenanthroline was also chelated to enhance the stability of the complex. Double and triple layer devices were synthesized with the configuration of ITO/BTPD-PFCB/Europium complex in a polymer blend/Ca/Ag for the double layer device and ITO/BTPD-PFCB/Europium complex in a polymer blend/PBD/Ca/Ag for the triple layer device. The double layer devices made with a polymer blend of PVN outperformed the devices made from PVK as the emission bands of the PVN better match the absorption bands of the ligands. A maximum brightness of 178 cd/m2 with a maximum external quantum efficiency of 0.45% was measured for the double layer device.