Upon sandwiching a K+-doped solid polymer electrolyte (SPE) in between the Prussian blue (PB) and indium hexacyanoferrate (InHCF) thin films, a precoloring-free electrochromic device (ECD), undergoing reversible blue-to-yellowish electrochromism when charged, was fabricated and studied. The K+-doped SPE was prepared through dissolving KCl into the monomer solution of 2-acrylamido-2-methylpropane sulfonic acid (AMPS), which was then polymerized to form the KCl-doped poly-AMPS (PAMP). It was demonstrated that this ECD shows better at-rest stability than a precolored system. Also, it was found that the KCl doping level of SPE (defined by the molar ratio of KCl to AMPS, (KCl)/(AMPS)) affects the electrochemical behaviors of PB and InHCF in different ways. Thus, the KCl doping level was optimized for attaining ECD's best performances. It was found that the widest transmittance window, fastest response, and highest coloration efficiency are obtained when (KCl)/ (AMPS)= 0.220, while it is the saturated doping level leads to the longest cycle life. In brief, this work aims at optimizing the design on the K+-SPE-based PB-InHCF ECD. (C) 2003 Elsevier B.V. All rights reserved.