Browsing by Author "Dixit, AK"

Now showing 1 - 2 of 2

Novel distyrylcarbazole derivatives as hole-transporting blue emitters for electroluminescent devices
(2005)
Wu, FI
;
Shih, PI
;
Yuan, MC
;
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).
Stable organic blue-light-emitting devices prepared from poly[spiro(fluorene-9,9'-xanthene)]
(2005-11-29)
Tseng, YH
;
Shih, PI
;
Chien, CH
;
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.