Rapid switching and high contrast electrochromic property by electrochemical reduction of an alternating copolymer of fluorene and oxadiazole

摘要

While stable polymer electrochromic properties are mainly realized from the electrochemical oxidation of p-type conjugated polymers, in this paper we report that stable electrochromic properties can also be easily achieved by electrochemical reduction of n-type polymers, such as PF4Ox, an alternating copolymer of quater(9,9-dioctylfluorene) and oxadiazole. The introduction of the strong electron-accepting oxadiazole unit into the poly(9,9-dioctylfluorene) main chain promoted the stability of the polymer in the electrochemical reduction. The copolymer film displayed an excellent elecrochromic property upon electrochemical reduction, which turned the colorless film at the neutral state to rosy-red at the electron charged state with a switching time of 1.1 s. The film exhibited a high stability during the electrochromic test, having only 30% loss of the transmittance change in 500 potential square-wave cycles. Fourier transform infrared (FT-IR) study revealed that decay of the electrochromic performance of the film was associated with the formation of fluorenone and quinone defects on the fluorene unit in the copolymer chain. The laser flash photolysis test showed the monomer unit had a high potential to localize an electron and became highly vulnerable to be attacked by oxygen to form the defects. These defects would act as charge carrier traps and produced a pair of inner peaks at -1.84 and 1.03 V in the cyclic voltammetry test of the copolymer film.