Browsing by Author "Shih, PI"

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Highly efficient light-emitting diodes based on fluorene copolymer consisting of triarylamine units in the main chain and oxadiazole pendent groups
(2005-11-01)
Wu, FI
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Shih, PI
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Shu, CF
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Tung, YL
;
Chi, Y
We have synthesized a fluorene-based copolymer, PFA-OXD, which possesses bipolar charge-transporting functionalities, through the incorporation of electron-rich triphenylamine moieties and electron-deficient oxadiazole pendent groups into the polymer backbone and onto the C9 position of each fluorene repeating unit, respectively. The propeller-shaped triphenylamine units in the main chain and the rigid cardo-type OXD side chains endow this polymer with a very high glass transition temperature (306 degrees C) and good thermal stability, without sacrificing its good solution processability. Both in dilute solution and as a thin film, PFA-OXD exhibits a bright blue emission upon excitation of either the main chain or oxadiazole side chains; this phenomenon indicates that efficient energy transfer occurs from the photoexcited pendent groups to the polymer backbone. An organic light-emitting device incorporating this copolymer as the emitting layer exhibits a voltage-independent and stable blue emission having color coordinates (0.14, 0.15) at 9 V; the maximum brightness was 7128 cd m(-2) and the maximum luminance efficiency was 2.07 cd A(-1). In addition, doping PFA-OXD as a host material with 2.6 wt % of a red-emitting phosphorescent dye, Os(fppz), led to a high-brightness, highly efficient red electroluminescent device having CIE coordinates of (0.65, 0.34), a maximum brightness of 28 440 cd m(-2), and a maximum external efficiency of 9.30%.
Highly efficient red-electrophosphorescent devices based on polyfluorene copolymers containing charge-transporting pendant units
(2005-07-28)
Wu, FI
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Shih, PI
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Tseng, YH
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Chen, GY
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Chien, CH
We have systematically examined the photoluminescence (PL) and electroluminescence (EL) behavior of blends comprising two efficient red phosphors doped, respectively, into the blue-emitting polyfluorene derivatives PF-TPA-OXD and PF-OXD. The host polymers, which contain both hole- and electron-transporting or merely electron-tran sporting side chains, are capable of facilitating charge injection and transport. After determining the HOMO and LUMO energy levels of these materials, we were able to match the dopant with its most suitable host to achieve the direct formation and confinement of an exciton at the dopant. This configuration also leads to a reduction in the electrical excitation of the host polymer, which in turn decreases the degree of exciton loss arising from nonradiative decay of the host triplet. Using this approach, we were able to realize the production of high-performance red-electrophosphorescent devices. For Os(fppz)-doped devices, we obtain a balanced charge recombination in conjunction with higher current and luminance when using PF-TPA-OXD as the host matrix; this device reached a maximum external quantum efficiency of 8.37% with a peak brightness of 16 720 cd/m(2). The absence of charge-transporting pendant units, i.e., the device fabricated from poly [9,9-dioctylfluorene-2,7-diyl] (POF), led, however, to relatively poor electroluminescence characteristics (5.81% and 2144 cd/m(2)).
Novel distyrylcarbazole derivatives as hole-transporting blue emitters for electroluminescent devices
(2005)
Wu, FI
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Shih, PI
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Yuan, MC
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Dixit, AK
;
Shu, CF
We have synthesized three novel distyrylcarbazole derivatives for use as simultaneously hole- transporting and light- emitting layers in blue light- emitting diodes. Each compound, which contains a rigid carbazole core and two 2,2- diphenylvinyl end groups substituted through either the 3,6- or the 2,7- position, forms films satisfactorily and exhibits a blue emission with its PL maximum in the range 459 - 470 nm. Photophysical measurements indicate that twisting of the adjacent C - C bonds in the 3,6- position of the carbazole core in dilute solutions causes an efficient nonradiative relaxation to occur, yielding a much smaller quantum yield for fluorescence in 3,6- linked carbazoles. As intense emissions of 2,7- linked carbazoles are observed, such deactivation from an excited- state is inefficient therein. Electrochemical studies revealed that incorporation of the carbazole core increased the HOMO energy effectively; this feature facilitates hole injection. These distyrylcarbazole derivatives are promising bifunctional, blue- emitting, holetransporting molecules for use in simple double- layer devices of a general structure ITO/ emitting layer/ TPBI/ Mg : Ag, in which TPBI - 1,3,5- tris( N- phenylbenzimidazol- 2- yl) benzene - serves as a hole- blocking and electron- transporting material. The devices prepared using 2,7-distyrylcarbazole as the emitter produced bright blue emissions having activating voltages below 3.0 V. A DPVTCz- based device attained a luminance efficiency of 3.11 cd A(-1) at 5V, a brightness of 3062 cd m(-2), and CIE color coordinates of ( 0.14, 0.22).
Poly(p-phenylenevinylene) presenting pendent pentaphenylene dendron groups for light-emitting diodes
(2005-11-01)
Tseng, YH
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Wu, FI
;
Shih, PI
;
Shu, CF
;
應用化學系
We have synthesized, using the Gilch method, a novel poly(p-phenylenevinylene) derivative (PPV-PP) containing two pendent pentaphenylene dendritic wedges, and have characterized its structure and properties. The incorporated side chain pentaphenylene dendrons serve as solubilizing groups, prevent pi-stacking interactions from occurring between the polymer main chains, and suppress the formation of excimers in the solid state. Photoluminescence studies indicate that efficient intramolecular energy transfer occurred from the photoexcited pentaphenylene groups to the poly(p-phenylenevinylene) backbone. The polymer film exhibits a maximum emission at 510 nm and had a photoluminescence efficiency of 46%, which is similar to that measured in dilute solution. The photoluminescence spectra remained almost unchanged after thermal annealing at 150 degrees]C for 20 h, and displayed inhibited excimer formation. Polymer light-emitting diodes that we fabricated in the configuration ITO/PEDOT/PPV-PP/Mg:Ag/Ag exhibited a maximum emission peak at 513 nm, corresponding to the green region [x = 0.30 and y = 0.62 in the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates]. The maximum brightness and maximum luminance efficiency were 1562 cd/m(2) and 1.93 cd/A, respectively. (c) 2005 Wiley Periodicals, Inc.
Stable organic blue-light-emitting devices prepared from poly[spiro(fluorene-9,9'-xanthene)]
(2005-11-29)
Tseng, YH
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Shih, PI
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Chien, CH
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Dixit, AK
;
Shu, CF
A short, efficient synthetic route has been developed for the preparation of 2,7-dibromospiro-[fluorene-9,9'-(2',7'-di-n-octyloxyxanthene)], which was subsequently polymerized with the corresponding diborolane through Suzuki coupling to afford a fluorene-based homopolymer, PSFX, that contains a spiroxanthene group functionalized on the C-9 position of each fluorene repeat unit. Single-crystal X-ray diffraction analysis of the dibromo monomer revealed that the spiro-fused fluorene and xanthene moieties lie perpendicular to one another. As a consequence of the incorporation of the spiro pendant group, PSFX possesses a high glass transition temperature and good thermal stability while displaying its stable blue emission in the solid state. A light-emitting diode device prepared using PSFX as the emitting layer exhibits an efficient, stable blue emission; it has a turn-on voltage of 6 V and a maximum external quantum efficiency of 1.74% ph/el. Even when the brightness was increased up to 10(3) cd/m(2) (ca. 11 V), the CIE color coordinates of the resulting EL spectrum remained in the deep-blue region (0.15, 0.07) and the device's efficiency was 1.33% ph/el.