Photoelectrochromic devices with cobalt redox electrolytes

Abstract

In this work, the use of cobalt redox electrolytes in partly covered photoelectrochromic devices is investigated experimentally for the first time. The fabricated devices consist of a conductive glass photoanode coated with an electrochromic WO3 film of optical quality, including a mesoporous TiO2 layer (sensitized by the MK2 organic dye) that covers 20% of the device area. The liquid electrolyte is composed of 0.22 M Co(II)(bpy)3(PF6)2, 0.5 M LiClO4 and 0.5 M 4-tert-butylpyridine. A platinized conductive glass cathode completes the cell set-up. The fabricated devices are almost transparent in the bleached state with a Tlum value above 50%. They exhibit coloration speeds in the order of minutes, with a maximum contrast ratio of 2.9:1 after 21 min of illumination at 1000 W m−2 under open circuit conditions (OC), and high reversibility to fully bleached state in short circuit conditions. They provide a maximum measured power conversion efficiency of 0.28% due to limitations imposed by conflicting requirements of the photovoltaic and electrochromic elements, which is nonetheless sufficient to drive the coloration process. Since only the reduced specie Co2+ is present, initial illumination under OC for 3 min at 1000 W m−2 is necessary, prior to measurements, to oxidize Co2+ to Co3+ through the dye regeneration process in the electrolyte. The higher recombination losses of the Co2+/3+ redox shuttle compared to I−/I3−, which lead to a considerable reduction in coloration depth 20 days post fabrication due to loss of photoelectrons at the WO3/electrolyte interface, are suppressed by the use of a 35 nm thick ZnS barrier deposited on top of WO3. Remarkably, it results in a stabilized contrast ratio of 1.5:1, 23 days post fabrication. In addition, the color coordinates of the present devices resemble those of typical electrochromics: they exhibit blue coloration, as a result of the lack of the absorbing iodine in the electrolyte that produces a green tint.