ELSEVIER Synthetic Metals 85 (1997) 1253-1254 Blue Light Emitting Diodes based on a partially conjugated Si-containing PPV-copolymer in a multilayer configuration F. Garten, A. Hilberer, F. Cacialli”, F.J. Esselink, Y. van Dam, A.R. Schlatmann, R.H. Friend”, T.M. Klapwijkb and G. Hadziioannoub University of Groningen, Materials Science Center (MSC) Applied Physics Department and Polymer Chemistry Department, Nijenborgh 4, 9747 AG Groningen, The Netherlands “University of Cambridge, Cavendish Laboratory, Madingly Road, Cambridge BC3 OHE, United Kingdom bCorresponding authors Abstract Efficient blue Light Emitting Diodes (LEDs) based on a novel partially conjugated co-polymer (SiPPV) have been realized by a combination of techniques known to enhance the quantum efficiency of organic devices. The copolymer is homogeneously blended in a PVK-matrix to reduce the number of non-radiative decay paths. In single-layer devices the high barrier at the Al top electrode limits efficient electron-injection into the device. In double-layer structures with an evaporated PBD-film at the cathodic side, electron and hole currents are much more balanced, resulting in an internal quantum efficiency of 3.2 %. The average turn-on field of a double-layer device is reduced compared to a single-layer LED. Keywords: LED, polymer, PPV, electroluminescence, multilayer 1. Introduction Light Emitting Diodes (LEDs) based on conjugated polymers ” offer the attractive possibility to tune the color of the emitted light by appropriate modification of the chemical structure.3*4 The most difficult part of the spectrum to reach is the blue part, since the reduced conjugation length required to achieve blue luminescence results in high injection barriers at the metal/polymer interfaces. Hence, blue LEDs characteristically have high turn-on fields and low quantum efficiencies. Several methods are known to enhance these efficiencies. One is to blend the active luminescent polymer in an inert polymer matrix,’ hereby reducing the availability of alternative pathways that lead the exciton to quenching sites. Inherent to this approach is the occurrence of phase separation between the polymers involved. Another method is to add a hole-blocking layer between the cathode metal and the active polymer,6Z7 which reduces the unipolar loss current of holes through the device and enhances the recombination probability for minority carriers. A drawback of the latter method is that both layers in the double-layer are soluble in common organic solvents like toluene or chloroform, which generally leads to rough interfaces between the two layers upon spin coating. Here, we report on the fabrication of efficient blue LEDs based on a novel partially conjugated Si-containing copolymer, homogeneously dispersed in a PVK-matrix. A PBD transport layer is evaporated on top of the blend, leading to a homogeneous interface between the two organic layers. The 0379-6779/97617.00 Q 1997 Elsevier Science S.A All rights reserved PII SO379-6779(96)04344-5 internal quantum efficiency can be as high as 3.2 %, in spite of problems with reproducibility of the results. 2. Experimental Details The LED-structure consists of a single-layer or double-layer configuration, in which the bottom electrode is formed by an ITO-covered transparent glass substrate. After thorough cleaning of the substrate the polymer poly(dimethylsilylene-p-phenylene- vinylene-(2,5-dibutoxy-p-phenylene)-vinylene-p-phenylene) (SiPPV)* (see inset figure 2) is spincoated from a 3 % solution in toluene (0.2 Km filters). After spincoating the films were dried in an oven (< 50°C) for 1-2 hours to remove all toluene fractions still present in the film. Blends of this active polymer in a PVK matrix (5% SiPPV by weight) are also spincoated from toluene solution. All processing steps so far are done under cleanroom-conditions to avoid incorporation of dust into the films. The metal top electrodes were evaporated through a mask by thermal evaporation of Al or Au (p < 2.10” mbar). In a double-layer a PBD-film was evaporated on top of the spin- coated blend prior to evaporation of the top electrode. More details about the fabrication procedure and measurement setup can be found elsewhere.g 3. Results In figure 1 the absorption and photoluminescence (PL) spectra of the PPV-based copolymer are shown. Also, the