Low-energy driven electrochromic devices using radical polymer as transparent counter electroactive material

Electroactive and transparent organic radical polymers offered a novel design of materials for electrochromic (EC) devices. A radical polymer containing 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) groups as redox active sites per repeating unit was spin-coated on a counter ITO/glass electrode of the EC device which was also comprised of Prussian blue (PB) as an electrochromic material on ITO and ion-conducting polymer gel between the two electrodes. Electrochemical switching of the cell was monitored using the visible absorption of PB (λ_max = 700 nm) that appeared in the oxidized (mixed-valence) state, while the radical polymer was transparent in the visible region in both redox states. PB and the radical polymer were concurrently reduced and oxidized, respectively, on each electrode during the charging process, which corresponded to the decoloration of the cell. The coloration was effected by a discharging process. The electrochromic switching and stability of the cell was characterized by a low driving voltage ΔV and, consequently, a small driving energy ∫ΔVi(t)dt, as a result of a small potential gap between PB and the radical polymer. The optical switch was fast and fully reversible by virtue of the large heterogeneous electron transfer rate constant of the TEMPO center (k₀ ≈ 10^-1 cm/s). The polymeric counter electrode material, without dissolution into the electrolyte layer, led to a good open circuit memory that did not require refreshing charges to maintain the redox states of PB.