Cyclic voltammograms were taken on the various films. In general, the shapes of the voltammograms changed depending on the specific additive, but the main features characteristic of the pure oxide tended to prevail. The charge capacity—and hence the magnitude of the electrochromism—is influenced by the potentiodynamic range, particularly the magnitude of the voltage for full coloration, Ucol. It appeared that similar charge capacities (from 15 to 20 mC/cm2) could be obtained provided that Ucol was varied by 0.05 to 0.1 V when additives were present. This shift is insignificant for electrochromic device applications.

The electrochromism of the films was characterized in terms of a coloration efficiency CE obtained from voltammograms and optical data according to [14]

(1- Rc )2 T

(1- R )2 T,

m

where ДО is the total charge that is exchanged and subscripts b and c refer to bleached and colored states, respectively. The total exchanged charge was

measured for the cathodic bleaching rather than the anodic coloration in order to minimize the effect of oxygen evolution.

Figure 1(a) shows spectral CEs for a nickel-oxide and a nickel-aluminum-oxide film. Both of these films were optimized by an additional introduction of some hydrogen during the deposition. It is noteworthy that the films have CEs that are much larger than those reported in the literature [1]. It should be emphasized that the CEs in nickel oxide and nickel-based oxides are intimately related to the conditions under which the depositions take place, which implies that the results of the enhanced CEs are related to features such as crystallinity, grain size, porosity, and contents of oxygen and hydrogen in the films. The film with the largest effective grain size presents the lowest CE, thus supporting the idea that a large inner surface area of the film is connected with the electrochromic activity, and that the coloration process takes place in the outermost parts of the grains. This fact—together with good crystallinity and high porosity, along with optimized quantities of oxygen and

(a) (b)

Wavelength (nm) Wavelength (nm)

Figure 1. Spectral coloration efficiency (CE) of nickel-oxide-based (a) and iridium-oxide — based films (b) of the shown compositions. NiAlO and IrTaO indicate that Al and Ta are present in the oxides but do not specify the amount.

hydrogen—gives highly efficient electrochromic films [15].

Figure 1(b) shows spectral CEs for two different iridium-oxide-based films. Clearly iridium-tantalum oxide has a lower CE than pure iridium oxide for some wavelengths. However it does not follow that Ir oxide is superior to IrTa oxide in applications, since a more crucial property may be the bleached state transmittance, as we elaborate below.