Electrochromic switching and nanoscale electrical properties of a poly(5-cyano indole)-poly(3,4-ethylenedioxypyrrole) device with a free standing ionic liquid electrolyte

Abstract

Poly(5-cyano indole) or PCIND and poly(3,4-ethylenedioxy pyrrole) or PEDOP films have been electro-synthesized for the first time in a hydrophobic ionic liquid: trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate. PCIND, is an anodically coloring electrochrome, and exhibited a reversible switching between a transmissive yellow and a saturated green hue, with an absorption maximum at 650 nm in the fully oxidized state. Conducting atomic force microscopy studies revealed the PCIND film to be composed of an ensemble of segregated high current islands with a nanoscale electronic conductivity of 0.1 S cm−1 and a band gap of 1.41 eV. The cathodically coloring PEDOP film comprised of uniformly distributed and inter-connected high current carrying domains with a band gap of 1.82 eV and a conductivity of 5.4 S cm−1. Prototype electrochromic devices were fabricated using PEDOP and PCIND as cathode and anode with a thermally stable ionic liquid based, free standing polymeric gel film with a high ionic conductivity of 1.19 × 10−3 S cm−1 as the electrolyte. The device showed large coloration efficiencies of 480 and 796 cm2 C−1 at visible and NIR wavelengths of 475 and 1100 nm respectively which far exceeded the coloration efficiencies of the individual electrochromes, thereby demonstrating the synergy between the two colorants. The performance attributes of the device, which switched reversibly between red, green and blue hues, are an outcome of an interplay between the high nanolevel electron conduction capabilities (enable fast charge transport) and high ion storage capacities (increase optical contrast as more number of electrochemically addressable sites are accessed by the electrolyte ions) of the PEDOP and PCIND films. Our studies demonstrate the applicability of PCIND films as anodic electrochromes in energy efficient windows.