Photochemical stability of electrochromic polymers and devices

Abstract

The stability of fully printed flexible organic electrochromics based on 11 different conjugated polymers is explored from the fundamental chemical degradation level to the operational device level. The photochemical stability of the electrochromic polymers (ECPs) is studied enabling an analysis of the influence that the chemical constitution of the conjugated polymer backbone has on the photochemical stability. Based on changes in the UV-visible absorption and IR spectra, the polymers were categorized into two distinct groups, each with a separate degradation mechanism. During irradiation (1000 W m⁻², AM 1.5G) under ambient conditions the majority of the polymers degraded within 4–5 hours. Three polymers showed increased stability with degradation rates from 0.44 to 1.58% per hour measured as loss of absorption. Application of oxygen and UV barrier foils was found to drastically slow the photochemical decomposition of the polymer films, such that after 2200 hours of continuous irradiation the less stable polymer films were degraded 27% on average, while the degradation of the most stable polymer films was immeasurable thus indicating that such materials can be sufficiently stable for device operations for many years under indoor conditions and for a few years under outside conditions. Finally, functioning electrochromic devices (ECDs) were made and the effect of illumination on the response time and optical contrast was established. This report shows that encapsulated electrochromic devices based on flexible barrier substrates exhibit increased stability and are indeed viable in devices such as shading elements, light management systems, displays with low switching speed requirements and signage.