Search

Six new four-coordinate tetrahedral boron complexes, containing 9-borafluoren-9-yl and diphenylboron cores attached to orthogonal fluorine- and chlorine-substituted 8-quinolinolato ligand chromophores, have been synthesised, characterised, and applied as emitters in organic light-emitting diodes (OLEDs). An extensive steady-state and time-resolved photophysical study, in solution and in the solid state, resulted in the first-time report of delayed fluorescence (DF) in solid films of 8-quinolinolato boron complexes. The DF intensity dependence on excitation dose suggests that this emission originates from triplet–triplet annihilation (TTA). Density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies give insight into the ground and excited state geometries, electronic structures, absorption energies, and singlet–triplet gaps in these new organoboron luminophores. Finally, given their highly luminescent behaviour, organic light-emitting diode (OLED) devices were produced using the synthesised organoboron compounds as emissive fluorescent dopants. The best OLED displays green-blue (λ maxEL = 489 nm) electroluminescence with an external quantum efficiency (EQE) of 3.3% and a maximum luminance of 6300 cd m−2. [ABSTRACT FROM AUTHOR]

A group of new boron complexes [BPh2{κ2N,N′-NC4H3-2-C(H)＝N-C6H4X}] (X = 4-Cl 4c, 4-Br 4d, 4-I 4e, 3-Br 4f, 2-Br 4g, 2-I 4h) containing different halogens as substituents in the N-aryl ring have been synthesized and characterized in terms of their molecular properties. Their photophysical characteristics have been thoroughly studied in order to understand whether these complexes exhibit an internal heavy-atom effect. Phosphorescence emission was found for some of the synthesized halogen-substituted boron molecules, particularly for 4g and 4h. DFT and TDDFT calculations showed that the lower energy absorption band resulted from the HOMO to LUMO (π–π*) transition, except for 2-I 4h, where the HOMO−1 to LUMO transition was also involved. The strong participation of iodine orbitals in HOMO−1 is reflected in the calculated absorption spectra of the iodine derivatives, especially 2-I 4h, when spin–orbit coupling (SOC) was included. Organic light-emitting diodes (OLEDs) based on these complexes, in the neat form or dispersed in a matrix, were also fabricated and tested. The devices based on films prepared by thermal vacuum deposition showed the best performance. When neat complexes were used, a maximum luminance (Lmax) of 1812 cd m−2 was obtained, with a maximum external quantum efficiency (EQEmax) of 0.15%. An EQEmax of ca. 1% along with a maximum luminance of 494 cd m−2 were obtained for a device fabricated by co-deposition of the boron complex and a host compound (1,3-bis(N-carbazolyl)benzene, mCP). [ABSTRACT FROM AUTHOR]

The effect of very thin polymer interlayers made of either acid-initiated cross-linkable polyfluorenes or the parent non-cross-linkable polyfluorenes, on top of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, on the efficiency of light-emitting diodes based on three different electroluminescent (EL) polymers is studied. We find that the use of non-cross-linkable polymers leads to the formation of interlayers with a thickness not exceeding 1 nm and their effect on the light-emitting diode characteristics is not directly correlated with the energetic position of the frontier levels, indicating that other effects, such as interfacial morphology and contact area, which may depend on the specific interlayer/EL polymer combination, play a significant role. When cross-linkable polymers are used, thicker interlayers are formed (up to about 7 nm), and their effect is better related to their frontier level energy. In particular, we found that with these interlayers, a quite significant EL efficiency increase can be obtained. [ABSTRACT FROM AUTHOR]

The efficiency of light-emitting diodes (LEDs) based on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-1,4-benzo-{2,1′-3}-thiadiazole)], F8BT, is optimized upon simultaneous doping with a hole and an electron trapping molecule, namely, N,N′-Bis(3-methylphenyl)- N,N′-diphenylbenzidine and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, respectively. It is shown that, for devices with poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonic acid as hole-injection layer material and magnesium cathodes, the efficiency is nearly doubled (from ca. 2.5 to 3.7 cd/A) upon doping with ca. 0.34% by weight of both compounds. [ABSTRACT FROM AUTHOR]

Abstract: Light-emitting diodes (LEDs), based on blue-emitting polyfluorenes are usually prone to the appearance of a contaminant green emission (centered around 520nm), leading to an apparent whitish light emission. We find that, for LEDs based on poly(9,9-dioctylfluorene), PFO, the blending with the hole transporting polyvinylcarbazole, PVK, can suppress such green emission. LEDs based on a PFO/PVK blend with a 1:2 weight ratio and with aluminum cathodes show a quite stable blue emission. This result reveals the important role played by the interchain interactions on the observed contaminant green emission. In addition, we observe that in Al-based devices blending causes a decrease in EL efficiency while in Mg-based devices we obtained higher efficiencies with the blend PFO:3PVK when compared with neat PFO-based devices. [Copyright &y& Elsevier]

Abstract: The authors report on the performance of polymer-based light-emitting diodes, LEDs, using amorphous zinc oxide-doped indium oxide, IZO, as anode. In particular, LEDs with poly[(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] as electroluminescent layer and aluminium cathodes, show higher efficiency with this IZO anode (0.015cd/A) than with indium-tin oxide (ITO) (0.010cd/A). Inspite of the higher resistance of this IZO electrode, compared with ITO, the fact that it can be processed at lower temperatures and allows similar or even higher efficiency values for polymer LEDs make this material a good candidate for display and other optoelectronic applications. [Copyright &y& Elsevier]

Abstract: Novel electroluminescent materials combining three functionalities were used as active layers in light-emitting diodes. These functionalities are brought about by the presence of three moieties: diphenylstylbeneamines, for hole transport, quinoxaline, for electron-transport, and a dehydroabietic acid methyl ester, to prevent crystallisation. The devices prepared with these materials and with magnesium cathodes show efficiencies up to 0.03cd/A, which is about one order of magnitude higher than the efficiency obtained with the related diphenylstylbeneamines. [Copyright &y& Elsevier]

Abstract: Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and cross-linked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures. [Copyright &y& Elsevier]

We report on the straightforward synthesis, characterization and photovoltaic and light-emission applications of two series of BODIPY molecules with aryloxy or alkoxy groups bonded to the boron centre. The crystal structures of two of these new dyes are also presented. The BODIPYs have absorption features similar to those of the analogous boron-fluorinated BODIPYs used as starting materials. However, the fluorescence properties vary significantly with the nature of the aryloxy/alkoxy group. The photovoltaic performance in bulk heterojunction solar cells of the newly synthesized compounds does not, however, outperform that of the parent boron-fluorinated BODIPYs. The maximum power conversion efficiency was obtained with two non-fluorescent BODIPYs, reaching 0.9%, compared with 2.79% obtained for the cells based on the corresponding boron-fluorinated precursor. The fluorine replacement by aryloxy/alkoxy groups had instead a positive effect on the light-emitting properties of the compounds of one of the series, leading to higher maximum luminance (114 cd/m2) and efficiency (0.057 cd/A), compared to devices based on the corresponding boron-fluorinated dye (35 cd/m2 and 0.015 cd/A, respectively). • Straightforward synthesis and characterization of a new O-BODIPY dyes series. • Crystal structure of two O-BODIPY dyes. • Performance evaluation of the BODIPY dyes as electron-donors in bulk heterojunction photovoltaic cells. • Application as emissive materials in light-emitting diodes (LEDs). [ABSTRACT FROM AUTHOR]