A complementary electrochromic device (ECD) based on a Prussian blue (PB) thin film and a heptyl viologen (HV(BF4)(2)) solution was assembled and studied. An equal-volume mixture of deionized water and isopropanol solution saturated with potassium ions was used as the electrolyte solution. The structure of a PB-HV(BF4)(2)-based ECD (PHECD) can be formulated as (ITO/PB/HV(BF4)(2) in solution/ITO), and can be switched reversibly between blue and colorless states. The time required to darken the device depends on the applied potential and the charge capacity of the PB film. A blue film was formed evenly on the bare ITO surface by deposition of HV+center dot during darkening of the PHECD. Under application of 0 V. it took a longer time to bleach the PHECD, due to re-oxidation and diffusion of the HV+center dot species back into the solution. A maximum transmittance window of 59% was obtained at 609 nm with the operating potentials of 1.0 V for darkening and 0 V for bleaching. No pre-darkening or pre-bleaching of the electrodes was needed for assembly of the PHECD. (C) 2011 Elsevier B.V. All rights reserved.