Eur. Phys. J. E (2003) 11: 293-300
https://doi.org/10.1140/epje/i2002-10158-1

Original Paper

Conducting polymers as driving electrodes for Polymer-Dispersed Liquid-Crystals display devices: On the electro-optical efficiency

¹ Laboratoire de Thermophysique de la Matière Condensée, UMR CNRS 8024 - Université du Littoral-Côte d'Opale, MREID, 59140, Dunkerque, France
² Laboratoire de Thermophysique de la Matière Condensée, University of Science and Arts of Oklahoma, OK 73018, Chickasha, USA

^* e-mail: frederick.roussel@purple.univ-littoral.fr

Abstract

Intrinsically conducting polymer (ICP) thin films are used as driving electrodes for Polymer-Dispersed Liquid-Crystals (PDLC) display devices. In order to investigate the electro-optical efficiency of these organic electrodes, three different kinds of conducting polymers, i.e. polyaniline doped with 10-camphorsulfonic acid (PANI(HCSA)), polypyrrole doped with dodecylbenzenesulfonic acid (PPY(DBSA)), and polyethylenedioxythiophene doped with polystyrenesulfonate (PEDOT(PSS)), were prepared or purchased, and coated either on glass or plastic substrates. Optical absorption studies in the UV-Vis range of the conducting polymer-coated substrates were first performed showing the presence of conducting species for the three types of polymers. The electrical characteristics of the resulting films were measured with the four-probes technique. PANI(HCSA) exhibits a higher conductivity $\sigma\sim 122{\rm S\cdot cm^{-1}}$ ($R_{\rm S}=1.2 \; 10^3\; \Omega\cdot\Box^{ -1}$) compared to PPY(DBSA) $\sigma\sim 2.6{\rm S\cdot cm^{-1}}$ ($R_{\rm S}=150.7 \;10^3\;\Omega\cdot\Box^{-1}$), and PEDOT(PSS) $\sigma\sim 1.6{\rm S\cdot cm^{-1}}$ ($R_{\rm S}=637.3 \; 10^3\; \Omega\cdot\Box^{-1}$). It is also shown that for a given conducting polymer, its electrical conductivity decreases when a plastic substrate is used. These observations have been related to significant morphological changes observed by scanning electron microscopy (SEM). A mixture of Norland Optical Adhesive 65 and nematic liquid-crystal E7 in the weight ratio (35:65) was used as precursor of the PDLC material. Better electro-optical responses (transmission properties, drive voltages and switching times) of PDLC films were obtained for devices prepared with (PPY(DBSA))-based electrodes. The electro-optical performances of the PDLC display devices also depend on the nature of the ICP substrate used.