LAUSR

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.