Synthesis and Properties of Novel Polymers to Increase the Electrochromic Service Life of Poly(3-hexylthiophene)
Poly(3-alkylthiophene)s (P3ATs) are a promising class of electrochromic materials because of their convenient processability and environmental stability. The adhesion of electroactive polymer coatings on a transparent conducting substrate is a significant factor affecting the durable colour-changing life of electrochromic devices such as visual displays and switchable windows. Traditional electrochromic coatings on transparent ITO glass electrodes often have poor cycling lifetimes, due to incompatibility between the polymers and the inorganic substrates. However, providing a covalently-bonded network between the active electrochromics and the ITO glass is an efficient method to increase the adhesion and hence to extend the service time; the research described here used Suzuki-Miyaura coupling of N-(3-trimethoxysilylpropyl)pyrrole (TMP) onto poly(3-hexylthiophene) (P3HT), as a precursor to a sol-gel reaction. The Si-O-metal network produced between the siloxane groups and metal oxide after the sol-gel reaction served to enhance the adhesion of the poly(3-hexylthiophene) onto ITO glass. With this stronger bonding, the colour-switching service time was found to be extended considerably by the observation of repeated electrochemical cycling tests. In addition, the electro-optic and electrochemical properties of copolymers with various molar substitution ratios of TMP on P3HT were investigated by cyclic voltammetry and UV-visible spectroscopy. The π-π* transition energy of P3HT increased with the degree of bromo-substitution, but diminished again after substituting with TMP. The hysteresis in electrochromic switching also became more noticeable with increased bromo-substitution on the P3HT, but it decreased when TMP was attached to the thiophene rings.