Improved electrochromic performance of nickel oxide porous films by regulating their semiconductor type by titanium doping

Electrochromic (EC) materials have been regarded as important energy-saving materials in recent decades. However, some shortcomings, such as short cycle life, low light modulation, and slow response, have restricted the large-scale application of existing EC materials. To rectify this situation, elemental doping is a promising approach. In this work, titanium (Ti)-doped nanostructured nickel oxide (NiO) porous films have been prepared, and their EC performance and mechanism have been explored based on electrochemical tests and energy-band structure analysis. The results show that an appropriate level of Ti doping not only leads to improved EC performance, but also plays a certain role in regulating the microstructure, resistivity, semiconductor type, energy-level structure, and interfacial heterojunction structure of NiO films. Moreover, evaluation of the energy levels and band gaps of NiO films has indicated that Ti doping reduces the interfacial potential barrier of FTO/NiO. An NiO film doped with 2.0 wt% Ti showed optimum EC properties, including a short response time (bleaching time 0.8 s; coloring time 2.9 s), large light modulation (54.4%), high coloration efficiency (45.6 cm2 C−1), and long EC cycling life (50000 cycles). The Ti doping converted NiO from a p-type to an n-type semiconductor.