Electrochromic properties of pure and doped Nb2O5 coatings and devices

Coatings of pure and Zr, Sn, Li, Ti and Mo doped niobium pentoxide have been prepared by the sol–gel process and deposited on ITO-coated glass using the dip coating technique. Their structure, morphology and electrochromic (EC) properties have been studied. The coatings are transparent and, depending on the nature and amount of the doping and the sintering temperature, they present a brown, gray or blue color after Li insertion. The EC properties of 4×8 cm2 size coatings, tested up to 5000 cycles, are highly stable with a coloration efficiency measured at 550 nm ranging between 16 and 28 cm2/C. Same size devices have been built with the configuration glass/FTO/pure or doped Nb2O5/liquid electrolyte/TiO2–CeO2/FTO/glass. In agreement with a theoretical estimation and with this choice of counterelectrode (IS), only Nb2O5:0.4 Mo coatings could be considered. For a fixed thickness (180 nm) of the IS electrode, the transmittance response of the device was found to increase with the thickness of the EC layer. The maximum change of the photopic transmission was 0.3 with an Li+ charge exchange of 18 mC/cm2. The devices were stable up to about 15,000 potentiostatic cycles performed between ±2.5 V, 120 s.