Mixed cerium/titanium and cerium/zirconium oxides as thin ®lm counter electrodes for all solid state electrochromic transmissive devices Guglielmo Macrelli, Elisabetta Poli* Isoclima SpA ± R&D Department via A. Volta, 14, 35042 Este (PD), Italy Received 7 September 1998; received in revised form 3 November 1998 Abstract Optically passive thin ®lms of mixed cerium/titanium and cerium/zirconium oxides have been prepared by electron beam reactive evaporation. Dierent oxidation levels have been achieved using dierent oxygen ¯ows in the deposition process. The samples have been optically and electrochemically characterized. Performances are discussed in view of a future utilization in a electrochromic device as thin ®lm counter electrode. Dierent materials have been tested to identify the best solution to be used as an optically passive ion storage layer in an existing electrochromic device in alternative to an optically active V 2 O 5 counter electrode. Process conditions and materials performances are reported, related to each other and discussed. Development studies are still in progress towards the optimization of the device and its future scaling up. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Electrochromism; Counter electrode; Mixed oxides; Thin ®lm; e-beam evaporation 1. Introduction This paper reports preliminary results of a research work performed to develop optically passive mixed oxide counter electrode for electrochromic devices. As previously reported [1] we developed a medium area (300 by 300 mm 2 ) electrochromic device based on an all solid state inorganic thin ®lm multilayer structure. In that solution the counter electrode was a thin ®lm of Vanadium pentoxide deposited by plasma assisted electron beam evaporation. There were at least two main drawbacks to overcome: . The counter electrode was an optically active cathodic material that reduced the optical modulation window, . The counter electrode was a thin ®lm deposited by plasma assistance to achieve the correct stoichi- ometry: this complicated greatly the deposition technology. Recently, studies have been reported [2] in which stoichiometrically correct vanadium pentoxide was deposited without plasma assistance, but, even in this case, the optical activity of Vanadium pentoxide is still a problem. These motivations together with the increasingly interest in mixed oxide thin ®lm [3±7] were the background to start this research work. We focused our attention to cerium/titanium and cerium/ zirconium mixed oxides materials. The deposition method we have used is electron beam vacuum evapor- ation; we ®xed the ratio of the starting oxides and we changed the oxygen partial pressure at two dierent levels. Electrochimica Acta 44 (1999) 3137±3147 0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0013-4686(99)00031-6 * Corresponding author.