ELSEVIER Synthetic Metals 71 (1995) 2121-2124 Thiophene polymers in light emitting diodes: Making multicolour devices O.Ingan&sa, M.Berggrena, M.R.AnderssonbJ G.Gustafssona, T.Hjertbergc, 0. Wennerstr6mb, P.Dyrekleva and M.Granstr6ma aLaboratory of Applied Physics, IFM, Linkaping University, S-631 33 Linkoping, Sweden bDept. of Organic Chemistry and cPolymer Technology, Chalmers University of Technology, S-412 96 G6teborg, Sweden Abstract We can control the bandgap of thiophene polymers over 2 eV by choosing the nature, position and regularity of side chain substitutions. Electroluminescence from these polymers cever the full visible spectrum, from the blue into the near i&a-red. Blends of these polymer materials allow us to construct voltage controlled variable colour light sources. A newly developed transfer technique allow us to mount thin oriented films of the polymers in polymer LEDs to obtain polarised light sources giving polarisation anisotropy’s of up to 3. Sub-h light sources have been constructed Tom these polymer materials using nanometer polymer electrodes. I. INTRODUCTION The development of electroluminiscent devices based on conjugated polymers will greatly extend the functionality of such devices, compared to devices based on inorganic material. The ease of molecular engineering to control bandgap and light emission properties of the polymers, the means of forming the thin polymer films used in the devices, the possibility to align the quantum wire-like polymers to obtain polarised light sources, the modes of construction using polymer self-organisation - all these aspects opens up new avenues for devices. We report progress in all these areas, leading to a family of substituted polythiophenes with electroluminescence from the blue into the near infrared. These have been synthesised with the goal of using steric hindrance due to substituents, to control the planarity of the polythiophene main chain. We have obtained new functions in blends of these polymers, as voltage controlled colours are obtained in such diodes. The phase separation normally found in polymer blends here gives us the advantage of forming a multitude of parallel polymer LEDs, each giving its own colour. We have used this family of polymers to build polarised electroluminescent light sources. Stretch orientation of the polymers in the form of thin films supported on stretchable polymer substrates gives appreciable orientation. We can transfer these films, in the thickness range 60-100 run, onto electrodes and leave them aligned on the electrode after detaching the substrate. The quality of these films is such that we 0379-6779/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)03194-B can then evaporate metal contacts on top of the film to form oriented polymer diodes, in which the electroluminescence is polarised. The highest anisotropy reached so far is 3.1, between the intensity of light emitted with polarisation parallel to the orientation direction and that transversely polarised. The versatility of the polymers, and conjugated polymers in general, is exploited in our nano&EDs. These are built using a hole-injecting polymer electrode, which has been formed by electropolymerization in the form of narrow cylinders of width 10 run -10 pm. These define the electrodes. Electroluminescent polymer layers are formed on top of these and an electron ir$&ing contact on top of the polymer layer. We obtain light from the micro- and nano-LEDs formed from these structures and have been able to image the larger diodes with ordinary light microscopes. At smaller dimensions than &, this will not be possible and we have yet to prove that the dimension of the small electrodes de6nes the size of the light source. 2. POLYMER MATERIALS The substituted polythiophenes have ail been designed to give varying degrees of main chain planarity. In this way, the conjugation length and band gap are controlled in a systematic manner1‘6. The design principles have been inspired by the physics driving thermochromism in the poly(3- alkylthiophenes)‘, but we have extended the family of substituent to include alkyl, cycloalkyl and alkylphenylene groups (Fig.1). In this way we have