Comprehensive role of the ITO conductivity playing in WO3 based electrochromic devices from electrochromic behavior to electrochromic performance

Electrochromic (EC) devices as a special member of the electrochemical family, have many applications because of their capability of simultaneously achieving energy storage and color changing. A critical component in an EC device is the transparent conductor electrode (TCE) that both acts as the current collector and the conductive agent. ITO is the gifted option for TCE, and its resistance was generally expected the lower the better to achieve fast responding of the EC device. However, our knowledge about effects of the ITO conductivity on EC behavior and performance is very limited, not to mention the comprehensive underlying mechanism. Herein, taking the most widely-used EC-material amorphous WO3-x and five ITO TCE with sheet resistances widely ranging 1 Omega/square similar to 100 Omega/square, systematic experiments were carried out with ensuring all samples being fully colored and bleached. COMSOL simulations and XPS depth-profiling results revealed that the ITO conductivity had no influence on the electric-field distribution across the WO3 electrode nor on the injected cations' final distribution in WO3. However, the charge storage mechanism, the coloring/bleaching current distribution, the ohmic resistance R-Omega and the charge-transfer resistance R-ct of the WO3-y film electrode were all ITO-conductivity-dependent. Particularly, with the increasing ITO conductivity, contribution of the capacitive process in EC behavior increased comparing to the diffusion-controlled process, resulting into the accelerating response speed and the decaying memory effect. This research is an important supplement to the existing design rules of EC devices.