An efficient white‐light‐emitting polymer (W3) is realized by covalently attaching a green fluorophore and a red phosphor into the backbone and the side chains, respectively, of polyfluorene at a concentration of 0.04 mol %. In addition, charge‐transporting pendant units are included to improve carrier injection and transport. White‐electrophosphorescent devices with the structure ITO/PEDOT:PSS/W3/CsF/Al (ITO: indium tin oxide; PEDOT:PSS: poly(styrenesulfonate)‐doped poly(3,4‐ethylenedioxythiophene)) exhibit a low turn‐on voltage of 2.8 V and a luminance of ca. 103 cd m–2 at below 6 V. The peak luminance and power‐conversion efficiencies are 8.2 cd A–1 and 7.2 lm W–1, respectively. Furthermore, the device shows relatively stable white emission: the Commission Internationale d'Éclairage (CIE) chromaticity coordinates of the devices change only slightly from (0.35,0.38) at 10 mA cm–2 to (0.33,0.36) at 100 mA cm–2, with an almost constant color render index (CRI) value of 82 at all measured current densities.
An efficient orange‐light‐emitting polymer (PFTO‐BSeD5) has been developed through the incorporation of low‐bandgap benzoselenadiazole (BSeD) moieties into the backbone of a blue‐light‐emitting polyfluorene copolymer (PFTO poly{[9,9‐bis(4‐(5‐(4‐tert‐butylphenyl)‐[1,3,4]‐oxadiazol‐2‐yl)phenyl)‐9′,9′‐di‐n‐octyl‐[2,2′]‐bifluoren‐7,7′‐diyl]‐stat‐[9,9‐bis(4‐(N,N‐di(4‐n‐butylphenyl)amino)phenyl)‐9′,9′‐di‐n‐octyl‐[2,2′]‐bifluoren‐7,7′‐diyl]}) that contains hole‐transporting triphenylamine and electron‐transporting oxadiazole pendent groups. A polymer light‐emitting device based on this copolymer exhibits a strong, bright‐orange emission with Commission Internationale de L'Eclairage (CIE) color coordinates (0.45,0.52). The maximum brightness is 13 716 cd m–2 and the maximum luminance efficiency is 5.53 cd A–1. The use of blends of PFTO‐BSeD5 in PFTO leads to efficient and stable white‐light‐emitting diodes—at a doping concentration of 9 wt %, the device reaches its maximum external quantum efficiency of 1.64 % (4.08 cd A–1). The emission color remains almost unchanged under different bias conditions: the CIE coordinates are (0.32,0.33) at 11.0 V (2.54 mA cm–2, 102 cd m–2) and (0.31,0.33) at 21.0 V (281 mA cm–2, 7328 cd m–2). These values are very close to the ideal CIE chromaticity coordinates for a pure white color (0.33,0.33).
A series of efficient and bright white light-emitting diodes were fabricated using the blends of two fluorene-derived fluorescent dyes, (4,7-bis-(9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′]bifluoren-7-yl)-benzo[1,2,5]thiadiazole) (FFBFF-emits green) and (4,7-bis-[5-(9,9-dihexyl-9H-fluoren-2-yl)-thiophen-2-yl]-benzo[1,2,5]thiadiazole) (FTBTF-emits red) in an efficient blue-emitting polyfluorene-derived copolymer (poly[(9,9-bis(4-di(4-n-butylphenyl)aminophenyl))]-stat-(9,9-bis(4-(5-(4-tert-butylphenyl)-2-oxadiazolyl)-phenyl))-stat-(9,9-di-n-octyl)fluorene) (PF-TPA-OXD). The resulting white light-emitting device reaches a maximum external quantum efficiency of 0.82% and a maximum brightness of 12900cd∕m2 at 12V. The Commission Internationale d’Énclairage chromaticity coordinates of the device remain very close to that of pure white emission at a relatively broad bias range from 6V(x=0.36,y=0.37)to12V(x=0.34,y=0.34).
We have synthesized a novel polyfluorene copolymer polyfluorene–bis[4‐(diphenylamino)styryl]fluorene (PF–DPAS) by orthogonally attaching an amino‐substituted distyrylarylene dye bis[4‐(diphenylamino)styryl]fluorene, onto the C9 position of a fluorene unit. We have investigated this polymer's thermal properties, electronic properties (viz., absorption and photoluminescence), and electrochemical behavior. Photoluminescence studies indicate that color tuning can be achieved through efficient Förster energy transfer from the higher‐energy polyfluorene backbone to the lower‐energy pendant DPAS units. We have fabricated light‐emitting diodes with the structure indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene) (PEDOT)/emitting layer/1,3,5‐tris(N‐phenylbenzimidazol‐2‐yl)benzene (TPBI)/Mg:Ag. The devices, based on blends of PF–DPAS in polyfluorene–triphenylamine–oxadiazole (PF–TPA–OXD), exhibit significant improvements in device performance relative to that of the pure PF–TPA–OXD device; we attributed this improvement to both a red‐shift of the electroluminescence (EL) spectra and an enhancement in quantum efficiency. At a blend ratio of 1:20, the EL spectrum is voltage‐independent and stable, and exhibits the characteristic emission of a DPAS moiety: a peak at 461 nm and Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.18). The maximum external quantum efficiency is 2.08 % (2.87 cd A–1) at a bias of 9 V (86.1 mA cm–2) with a brightness of 2467 cd m–2; the maximum brightness (6916 cd m–2) occurred at an applied voltage of 13 V and a current density of 361 mA cm–2.
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 103 cd/m2 (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.
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