Petra Herguth

Bright white light electroluminescent devices based on a dye-dispersed polyfluorene derivative

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’Enclairage 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).

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.

Poly(fluorene-co-benzothiadiazole)s: Effect of structure, molecular weight, and polydispersity on their performance in polymer light-emitting diodes

Copolymers based on fluorene and benzothiadiazole exhibit high brightness and quantum efficiencies when incorporated into polymer light-emitting diodes (PLEDs). Their emission wavelength is strongly dependent on the benzothiadiazole-containing segment of the polymer. However, the chain structure and charge-transport and -transfer processes in these materials are not well studied. We report the synthesis of a structural-random (r-PF3B) and a structural-defined (s-PF3B) copolymer, poly-fluorene-co-benzothiadiazole whereas the ratio of the two co- monomers was chosen to be 75% and 25%, respectively. We have systematically investigated the effect of structure on their photoluminescence (PL) and electroluminescence (EL) properties. Furthermore we have also studied the effect of molecular weight and its distribution on the performance of the material in PL and EL. We have found that the absorption and emission spectra (PL and EL) of these polymers are quite independent of their structures, molecular weights, and polydispersity. However, the PL and EL efficiencies do vary with the materials studied. These materials were fabricated into a series of double-layer devices. Their external quantum efficiencies (ranging from 0.097% to 1.7%) and maximum brightness (ranging from 153 to 23300 cd/m2) are highly dependent on the structure as well as the molecular weight and polydispersity. The higher the molecular weight and the narrower the distribution are, the higher the efficiency and brightness of the devices. Likewise, the structure of the polymer also influences the efficiency. It was found that the structural-random copolymer (r-PF3B) exhibits higher efficiencies and brightness when compared with the structural-defined one (s-PF3B) in the same molecular weight range.

Absorption and Luminescence Properties of Sequentially Random- and Defined Copolymers Based on Poly(fluorene-benzothiadiazole)

The emission wavelength in conjugated copolymers is determined by the various monomers used, their sequence in the polymer backbone, the effective conjugation length of the material, and intra- and intermolecular interactions of the different units. This paper will discuss the influence of conjugation length as well as the influence of intramolecular charge-transfer on the absorption and emission properties. Fluorene, benzothiadiazole and a third comonomer (fluorene, xylene or triphenylamine) were used as building blocks. The influence of monomer sequence is probed as well by comparing sequentially random copolymers to their sequentially defined ones with identical monomer ratios. Model oligomers were also made for comparison.

Development of efficient electron-transporting polymers for light-emitting diodes

Polyfluorenes are a class of very efficient conjugated polymers used in the development of LEDs that exhibit very high hole mobility. In order to balance the charge transport and enhance quantum efficiency of the LEDs, fluorene-based copolymers were synthesized based on the statistic copolymerization between fluorene and 2,5-dicyanobezene. By attaching two electron-withdrawing cyano groups onto the phenylene ring, both the electron affinity and the electron conduction of these copolymers are greatly enhanced comparing to the fluorene homopolymer. LED devices using the cyano-containing fluorene copolymers show very bright emission and low turn-on voltages. The emission color of these polymers could be also tuned by exciplex formation between the polymers and amine-containing hole transporting materials.

Highly Efficient Electron-Transporting Polymers for Light-Emitting Diodes

A series of well-defined oligomers were synthesized to provide insight on the effect of the position of cyano substituents on the electronic properties of conjugated molecules and polymers. Two copolymers PF3CNP1 and PF1CNP1 composed of 9,9-di-n-hexylfluorene and 2,5-dicyanobenzene have been synthesized based on this model study. By attaching two electron-withdrawing cyano groups onto the phenylene ring, the electron affinity and electron transporting ability of polyfluorenes are greatly enhanced. The devices employing a conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT) as the hole injecting layer, and the cyano- containing copolymers as the emitting and electron-transporting layer showed a much improved electroluminescence (EL) efficiency and brightness compared to that of the homopolymer, poly(9,9-di-n-hexylfluorene) (PHF) with the same device configuration. It is ascribed to the smaller energy barrier for electron injection and higher electron mobility due to increased electron affinity of the polymers through the cyano substituents. The device based on PF3CNP1 has the best performance with a low turn-on voltage (3.4 V), bright blue emission (5430 cd/m2 at a bias voltage of 7.4 V), and a maximum external quantum efficiency of 0.50 % due to both optimized electron affinity and charge carrier mobility.