Properties of electrochromic devices made with Nb2O5 and Nb2O5:X (X=Li,Ti or Mo) as coloring electrode

Abstract

Electrochromic windows using Nb2O5, Nb2O5- Li2O, Nb2O5-TiO2 and Nb2O5-MoO3 sol-gel layers as cathodic coloring electrodes and CeO2- TiO2 as anodic ion-storage electrode have been assembled and tested. In agreement with a theoretical estimation, the switching behavior of these devices is strongly dependent on the difference between the internal potential of the electrochromic and the ion-storage electrodes. Only the configuration using a Nb2O5-MoO3 layer enters in consideration for long-time switching within the safe voltage range -2.5 V < V < +2.5 V. All other cells require larger negative potentials. The 8 X 4 cm2 devices built with Nb2O5:Mo are transparent with a slightly yellow color in the bleached state (+2.5 V) and dark gray in the colored state (-2.5 V). For a fixed thickness of the EC electrode (180 nm) the photopic transmittance increases with the thickness of the ion storage layer dIS in agreement with the model prediction. It reaches a maximum of ca. 0.3, corresponding to a Li+ intercalated charge density of 18 mC/cm2. For dIS is congruent to 950 nm, such devices are stable up to at least 12,000 cycles under potentiostatic cycling between +/- 2.5 V, 120 s.