Shi Tang

Highly efficient deep-blue electroluminescence based on the triphenylamine-cored and peripheral blue emitters with segregative HOMO–LUMO characteristics

To obtain highly efficient and stable blue electroluminescence, two novel star-shaped compounds are purposefully designed and synthesized, which are composed of two functional groups, central triphenylamine (TPA) and peripheral anthracene (AN) or phenanthrene (PA) with obviously segregative electronic density distribution characteristics between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO and LUMO). As a light-emitting layer, they exhibit not only highly efficient deep-blue electroluminescence, but also good stability in organic light-emitting diodes (OLEDs) due to the combination of several advantageous properties (electrochemical, thermal and morphological stability). In particular, for triphenylamine-cored phenanthrene (TPA-PA), the external quantum efficiency of 7.23% is the best reported result for non-doped deep-blue OLEDs, and the lifetime of the device is also improved, being twice that of 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) under the same device conditions.

The Origin of the Improved Efficiency and Stability of Triphenylamine-Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation†

Herein, we describe the molecular electronic structure, optical, and charge-transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light-emitting diodes (OLEDs) with triphenylamine (TPA)-substituted anthracene derivatives. The high performance of OLEDs with TPA-substituted anthracene is revealed to derive from three original features in comparison with aryl-substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority-carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole-electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built-in electric field to prompt the balance of charge-carrier injection.

A new kind of peripheral carbazole substituted ruthenium(II) complexes for electrochemical deposition organic light-emitting diodes

A new kind of peripheral carbazole substituted ruthenium(II) complexes [Ru(bpy)2(tkdp-bpy)(AsF6)2] (4) (bpy = bipyridine, tkdp-bpy = 4,4′-(3,5-bis(6-(9H-carbazol-9-yl)hexyloxy)phenyl)methoxycarbonyl-2,2′-bipyridine) and [Ru(bpy)2 (dkte-bpy)(ClO4)2] (5) (dkte-bpy = bis(4-(9H-carbazol-9-yl)butyl)-2,2′-bipyridine-4,4′-dicarboxylate) were synthesized and their photophysical, electrochemical, as well as electroluminescent (EL) properties were studied. They display intense deep red phosphorescent emission centered at 660 nm and 658 nm, respectively, both in solution and in the solid state at room temperature. The peripheral carbazole in the complexes acts as an electrochemical coupling unit providing the ability to carry out electrochemical deposition (ED) and to form an ED film on an ITO electrode under a positive potential. Highly luminescent ED films are obtained under a carefully controlled ED process, and used as emitting layers in organic light-emitting diodes (OLEDs). The diodes gives out strong pure red emission and a luminous efficiency of 3.9 cd/A. These are the first electropolymerized phosphorescent OLEDs to be reported.

Synthesis and Electrochemical Properties of Peripheral Carbazole Functional Ter(9,9-spirobifluorene)s

A facile approach for synthesis of spirobifluorene trimers with peripheral carbazole functional groups by utilizing Suzuki coupling as the key reaction has been developed. These novel compounds exhibit blue emission with high quantum yields in solution and thin films, and excellent spectral stability upon photoirradiation and annealing in air. By the introduction of carbazole groups, the oxidation potentials of spirobifluorene trimers S TCPC-6 and STCPC-4 were significantly lower than that of model compound STHPH without peripheral carbazole groups, which reflect that the title compounds process higher HOMO energy level and better hole-injection ability. Highly luminescent films were obtained by electrochemical coupling between carbazole units. Pure blue-emission single-layer LEDs based on electrochemical deposition films as light emitting layers were achieved.

Organic pure-blue-light-emitting devices based on terfluorenes compounds

A series of organic pure-blue-light-emitting devices based on terflurorenes (TF) compounds are reported. In these double heterojunctions (DHJ) devices, two TF compounds act as a blue emitter, alternatively. Highly efficient pure-blue emissions are obtained from these devices. The maximum luminous efficiency of 1.52cd∕A (corresponding to an external quantum efficiency of 2.7%) with 1931 Commission International De L’Eclairage coordinates of (0.165, 0.072) is obtained. It is indicated that the DHJ device structure is beneficial to the performance of blue devices by means of effective confinement of excitons and carriers in the large energy gap blue emitter.

The Counter Anionic Size Effects on Electrochemical, Morphological, and Luminescence Properties of Electrochemically Deposited Luminescent Films

Luminescent films electrodeposited (ED) on indium tin oxide glass are considered for application in low-cost luminescent devices. During ED processes, the anions play an important role in the quality of resultant ED films. Three species of supporting electrolytes TBA-X (TBA = n-tetrabutylammonium, X = BF - 4 , PF - 6 , and AsF - 6 ) with different size of anions were used for the preparation of ED films from a branched carbazyl luminescent precursor TCPC. Increasing the size of anions (BF - 4 < PF - 6 < AsF - 6 ) resulted in an increase of growth rate and improved the morphology of ED films. The ED film prepared using TBA-AsF 6 as supporting electrolyte showed strong fluorescence with efficiency of 65%, which was significantly higher than that of ED films from the smaller anions BF - 4 and PF - 6 . By utilizing the ED films as emitting layer, the light-emitting device also showed improved performance with increasing size of anions in the electrolytes. The origin of observed counter anionic size effects on electrochemical, morphological, and luminescent properties of ED films can be attributed to the difference in interaction between the carbazyl cation and anions, which induced an easier (i.e., for the larger size ions) or difficult (i.e., for the smaller size ions) dedoping process.

Electrochemical deposition of patterning and highly luminescent organic films for light emitting diodes

This paper describes a simple electrochemical deposition (ED) technique to prepare luminescent and patterned films for light-emitting devices (LEDs). The luminescent films are deposited directly on the patterned ITO (indium tin oxide) electrodes through an oxidation coupling reaction of an electroactive and luminescent precursor. The ED films deposited on the ITO strips (width of 200 µm) exhibit smooth surface morphology (roughness of morphology surface of 3.1 nm), small roughness in electrode edge of 1–2 µm and high fluorescence quantum efficiency (>60%). The LEDs with structure ITO/ED film/Ba/Al show pure blue emission (CIE coordinates of (0.16, 0.07)), high brightness of 3080 cd m −2 and the maximum external quantum efficiency of 0.60%. (Some figures in this article are in colour only in the electronic version)

A novel amorphous oligo(phenylenevinylene) dimer with a biphenyl linkage center and fluorene end groups for electroluminescent devices

A new type of oligo(phenylenevinylene) dimer, 2,5,2′,5′-tetra(9,9′-dihexylfluorenyl)biphenyl (TFB), with a biphenyl linkage center and four fluorene end groups, has been synthesized by the Wittig reaction. The full characterization of its structure and optical properties, as well as the performance of its electroluminescent devices are presented. TFB shows strong blue fluorescence both in solution and as a solid film. High-quality films of TFB for light-emitting devices (LEDs) can be fabricated both by vacuum evaporation and the spin-coating technique, which is very special and interesting. Single-layer and multi-layer light-emitting devices using TFB as the active layer all show efficient blue emission.

Efficient pure blue electroluminescence from ter(9,9, 9″,9″-bihexyl-9′,9′-diphenyl)-fluorenes

An oligomer of polyfluorene, ter(9,9, 9″,9″-bihexyl-9′,9′-diphenyl)fluorenes (THPF), is used as a blue-emitting material to achieve efficient pure blue electroluminescence. Based on a 4,4′,4″-tri(N-carbazolyl)-triphenylamine hole-transporting layer, a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline hole-blocking layer, an (8-hydroxyquinoline) aluminum electron-transporting layer, and a LiF∕Al cathode, the device has two emission peaks at 404 and 424nm, chromaticity coordinates at (0.18, 0.09), and a maximum efficiency of 1.27cd∕A. By using a submonolayer of 5,6,11,12-tetraphenylnaphthacene as a carrier detector, we demonstrate that THPF functions well as an electron-transporting layer, similar to Alq.