Summary: Carbazole‐based oligomeric and polymeric materials have been studied for almost 25 years for their unique electrical, electrochemical and optical properties. Interestingly, carbazole units can be linked in two different ways leading to either poly(3,6‐carbazole) or poly(2,7‐carbazole) derivatives. While the former class seems to be very interesting for electrochemical and phosphorescence applications, the latter shows very promising optical properties in the visible range for light emitting diodes (LED). The major intrinsic difference between these two classes is the effective conjugation length: poly(2,7‐carbazole) materials having the longer one, due to their poly(p‐phenylene)‐like structure. Using different synthetic strategies and substitution patterns, the physico‐chemical properties of both classes can be fine‐tuned, leading to high performance materials for a large number electronic applications.Chemical structures for poly(3,6‐carbazole) and poly(2,7‐carbazole) and the materials used as the starting points for their respective syntheses.magnified imageChemical structures for poly(3,6‐carbazole) and poly(2,7‐carbazole) and the materials used as the starting points for their respective syntheses.

No abstract

PACS. 73.30.+y Surface double layers, Schottky barriers, and work functions, 73.61.Ph Polymers; organic compounds,

No abstract

: Non-doped white organic light-emitting diodes using an ultrathin yellow-emitting layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on either side of the interface of a holetransporting α-NPB (4,4'-bis[N-(1-naphtyl)-N-phenylamino]biphenyl) layer and a blue-emitting DPVBi (4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl) layer are described. Both the thickness and the position of the rubrene layer allow fine chromaticity tuning from deep blue to pure yellow via a bright white (WOLED) with CIE coordinates (x= 0.33, y= 0.32), a η ext of 1.9%, and a color rendering index (CRI) of 70. Such a structure also provides an accurate sensing tool to measure the exciton diffusion length in both DPVBi and NPB (8.7 and 4.9 nm respectively). Organic light emitting devices (OLEDs) are a promising technology for fabrication of full-color flatpanel displays. The development of OLEDs relies on the capability to obtain emission spanning the full visible spectrum. In particular, White OLEDs (WOLEDs) are of foremost interest for lighting and display applications 1 . To achieve white emission, various methods have been used, such as e.g. excimer/exciplex emission 2 , mixing of three (red, blue, green) or two (complimentary) colors in a single host matrix  or in physically separate layers 3 . Among these various devices, numerous doped-type WOLEDs using two mixed complimentary colors to produce white have been fabricated 4,5 . Although the co-evaporation process allows to a certain extent a control of the emitted radiation color via the different evaporation rates, it remains technologically difficult to accurately control the concentration. Hence, fine tuning of the color and achievement of bright white emission remain problematic. We report in this letter on a way to finely tune the color, including balanced white emission, in a multilayer non-doped OLED 6,7 based on blue matrices, in which an ultrathin yellow emitting layer was inserted. We show that by adjusting both the thickness and position of this layer, a very accurate control of the emitted color can be obtained, from deep blue with CIE coordinates (0.17, 0.15) to pure yellow (0.51, 0.48), via a bright white (0.32, 0.31) close to the equi-energy white point (0.33, 0.33), and a quite good Color Rendering Index (CRI) of 70. The external quantum efficiencies, the chromaticity coordinates and the luminance values are investigated for various thicknesses and positions of the yellow-emitting layer. Finally, the device structure, sometimes referred as "delta doping" 8 , allows a better understanding of the emission process through an experimental determination of the exciton lengths.The 0.3 cm 2 -active-surface OLED-structure consists of the different layers described in fig. 1. The Indium Tin-Oxide (ITO)-covered glass substrate was cleaned by sonication in a detergent solution, then in deionized water and prepared by a UV-ozone treatment. Organic compounds were deposited onto the ITO anode by sublimation under high vacuum (10 -7 Torr) at a rate of 0.1 -0.2 nm/s. An ...

Novel solid-state blue light-emitting devices are precipitously finding their niche in the present market-driven information technology. Among their projected applications, ['] optical recording and large-area flat-panel displays have attracted much attention due to their impact on the development of user-friendly interfaces that demand higher data-storage capacity and better on-screen communication links. With optical storage density constrained by the diffraction-limited recording-spot size, a short-term approach is to replace the currently available red gallium arsenide diode lasers with shorter wavelength blue lasers. Of the three primary colors required for full-color display applications, blue has been the most elusive to obtain. While the materials for blue light-emitting diodes (LEDs) are scarce the prospective candidates are, nevertheless, promising. For instance, the long search for a wide-bandgap inorganic semiconductor that emits blue lightL2] has now led to the commercialization of the most efficient blue LED based on a gallium nitride double-heterostructure quantum well. [3] This LED emitted blue light had a maximum at 1 = 450 nm with a full width at half maximum of 70 nm and an external quantum efficiency of 2.7 YO. On another front, due to their less stringent fabrication requirements, organic materials have recently been gaining interest as cost-effective alternatives to inorganic-based LEDs. The brightest blue organic LEDs previously reported consisted of a multilayer organic device structure that displayed blue light had maxima near i = 410 nmL4] and 480 nm[5,61 with a luminance better than 1000 cd/m2.In this communication, we report blue light-emission from a single-layer organic LED based on carbazole dimers. The device displays a narrow electroluminescence (EL) band, easily seen under normal room lighting, peaking just below 1 . = 420 nm, and has a modest external quantum efficiency of 0.07 YO. The EL spectrum is due to emission from excited states localized within the dimer structure. However, extra long-wavelength sidebands are also seen in the spectrum. Through chemical tuning of the interface barrier height, we identified these sidebands as light emission near the interface with the hole-injecting electrode. We argued that, due to the high ionization potential

No abstract

Introduction.Since the first report on electroluminescence in poly(p-phenylenevinylene) 1 (PPV), a wide variety of conjugated polymers used as emissive layers in light-emitting diodes (LEDs) have been studied. Over the past few years, progress in LED research has been made concerning the control of color, efficiency, longterm emission stability, brightness, processability, and the adjustment of the electrode work function. 2a-d Recently, poly(N-vinylcarbazole) (PVK) has attracted attention in utilization as a hole-transporting matrix of polymer blends composed of a PPV derivative as the electron-transporting layer. LEDs in which the emitting layer is constituted by such PVK blends have shown remarkable increase of the luminescence efficiency as compared to those PVK-free. 3,4 We previously reported on multilayer organic diodes based on either poly(Nalkyl-3,6-carbazolylene) 5 (PCZ), a polymer with an allcarbazole skeleton, or derivatives of the PCZ constituting dyads, i.e., bicarbazyl. 6,7 These LEDs display a bright blue light. Recently, we studied the possibility to ally advantages of the transporting features of both the PPV and the PCZ into a sole polymeric structure. In such a way, we synthesized by Knoevenagel condensation a new alternating copolymer, i.e., poly[bicarbazolylene-alt-phenylenebis(cyanovinylene)] 8 (PCPV). Unfortunately, the obtained materials were mostly insoluble in organic solvents and were not sufficiently processable to be incorporated as a thin layer in LED. Insolubility of these materials was attributed to the rigidity of the macromolecular chains and the relatively high molecular weights obtained (corresponding to DP n g 40) together with a slight cross-linking arising from Michael or Thorpe additions onto the vinylene or the cyano groups, respectively. In the conditions used only a small amount of oligomers (DP n ≈ 6) was obtained.We report herein on the synthesis conditions in which Knoevenagel condensation between the aldehyde monomer and the appropriate diacetonitrile can be controlled. In such a way, a readily soluble well-defined polymer with DP n ≈ 17 was obtained. Furthermore, the electroluminescence feature of LEDs based on PCPV as an emissive layer are described and compared with those of bicarbazyl taken as a model compound. In particular, it was found that this copolymer exhibits an internal charge transfer between the electron-donor carbazole subunits and the electron-acceptor cyanovinylene moi-