ELECTROCHROMIC PERFORMANCE OF A NOVEL POLY(NAPHTHALENEDIIMIDEQUATERTHIOPHENE)

Abstract

Conjugated polymers (CP) display several interesting properties which enable them their application in different fields such as solar cells, light emitting diodes, field effect transistors, electrochromic devices to cite a few. One of the most interesting properties of this class of materials is their ability of switching from a conductive to an insulating state and vice versa by changing their redox state. The possibility to switch from an oxidized to a neutral or reduced state induces variations of the optical spectrum due to changes in the electron excitation energy configuration resulting in electrochromism. Nowadays, electrochromic conjugated polymers are stimulating the development of transmissive and reflective technologies such as transmissive and reflective displays, smart windows, e-papers, optical memories, and optical shutters. In this work we prepared and investigated the electrochromic properties of a new, air stable polythiophene derivative, namely poly-naphthalenediimidequaterthiophene (PNDIT4). PNDIT4 thin films have been obtained by the electrochemical oxidation of the chemically synthesized monomer on transparent ITO electrodes supported on PET in two solvents mixtures having different volume ratios. The morphological and chemical investigation of the thin film have been performed by a number of surface tools including microscopy and spectroscopy techniques allowing to correlate their structure with the electrochromic performances. Spectroelectrochemical experiments have been performed in a two electrodes electrochemical cell endowed with quartz windows. Both absorbance spectra under different polarization potentials and transmittance vs time plots under different excitation wavelengths have been recorded. Figure 1 shows a typical optical contrast of a PNDIT4 film on ITO, obtained by applying a square wave potential of ±0.8V (anode vs cathode) at 900 nm. The polymer displayed a colour variation from green to beige when switched from the doped to the undoped states, respectively, along with a good optical memory in the explored time interval.