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.

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

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: 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 ...