Sputtered Si3N4 and SiO2 electron barrier layer between a redox electrolyte and the WO3 film in electrochromic devices

Abstract The application of a redox electrolyte in hybrid type electrochromic devices allows a high charge capacity, thereby a high optical contrast can be achieved. However, the current between the redox electrolyte and the electrochromic layer (tungsten oxide, WO 3 ) interface in equilibrium at colored state of device is critical for a window with large area. This loss current should be maintained below 10 μA/cm 2 to avoid an inhomogeneous coloration of a large window caused by an ohmic voltage drop in transparent conductive oxide (TCO). The power consumption of such a loss current can be neglected. In this regard, we have investigated silicon nitride (Si 3 N 4 ) and silicon oxide (SiO 2 ) films as an intermediate electronic barrier layer on the top of the WO 3 with thicknesses of 12, 35, 80 and 180 nm. These films were coated by direct current reactive magnetron sputtering technique. The electron barrier properties were studied for the iodide/triiodide ( I − / I 3 − ) and tetramethylthiourea/tetramethylformaminium disulfide dication (TMTU/TMFDS 2+ ) redox couples for application in electrochromic devices. For both redox couples, Si 3 N 4 showed an effective electronic barrier layer. The thickness of 80 nm of Si 3 N 4 reduced the loss current from 240 down to 20 μA/cm 2 for the I − / I 3 − redox electrolyte with the visual (solar) transmission from 45% (33%)down to 0.7% (0.5%)at 1 V.Similarly, the loss current was reduced from 70 down to 7 μA/cm 2 with the visual (solar) transmission from 71% (54.5%) down to 3% (2%) for TMTU/TMFDS 2+ redox electrolyte. However no significant reduction of loss circuit current was achieved with SiO 2 barrier layer for both redox couples.