Improved electrochromic performance of NiO-based thin films by lithium and tantalum co-doping

Nickel oxide (NiO) is one of the most promising counter electrode materials for complementary electrochromic (EC) devices. Its charge storage capacity affects the coloration depth of the device. However, NiO films still suffer from some problems like inferior optical modulation and poor bleached-state transparency. Doping of multiple additives is a typical way to enhance the EC performance of NiO films. Here, Li-Ta co-doped NiO films were fabricated by direct-current and radio-frequency magnetron co-sputtering from Ni metallic and LiTaO₃ ceramic targets. The influences of different LiTaO₃ deposition power on film structure, morphology, composition, optical and EC characteristics were investigated. It was found that the crystal structure and surface morphology of the NiO films could be readily tuned by varying LiTaO₃ sputtering deposition power level. The charge capacity and EC performance of NiO films showed a strong dependence on crystal structure variation and surface morphology. When the LiTaO₃ deposition power was 100 W, Li-Ta co-doped NiO films exhibited the optimal EC performance with an optical modulation of 62% and coloration efficiency of 35 cm²/C at 550 nm.