Interfacial redox centers as origin of color switching in organic electrochromic device

Abstract

In an attempt to understand the color switching mechanism of organic electrochromic devices, live spectroscopy of a viologen based device has been done. Role of redox reactions taking place at the electrode/electrolyte interface has been identified using Raman and UV–Vis spectroscopies carried out during the device operation. In-situ Raman and transmission/absorption studies establish the origin of bias induced color change, between a transparent and navy blue color, in the electrochromic device. The origin of color change has been attributed to the bias induced redox switching between its dication and free radical forms which have different optical properties from each other. Raman spectra collected from negative and positive electrodes of the device reveal that blue color species (free radical) are present at the negative electrode which is created due to reduction of the dicationic form. In-situ UV–Vis spectra reveals that the navy blue color of the device under biased condition is not due to increase in the transparency corresponding to the blue wavelength but due to suppression of its transparency corresponding to the complementary colors as studied using a from CIE (Commission Internationale de l’Eclairge International Commission on Illumination) chart. Absorption modulation has been reported from the device with good ON/OFF contrast of the device.