Electrochromic Properties of Perovskite NdNiO₃ Thin Films for Smart Windows

Semiconductors with electrically tunable band gaps are of great interest in controlling transparency to electromagnetic radiation. Thin films of perovskite nickelate NdNiO₃ (NNO), a class of correlated oxides, were deposited on single-crystal (LaAlO₃ (LAO)) and polycrystalline (fluorine-doped tin oxide-coated glass (FTO)) substrates by magnetron sputtering, chemical solution deposition (CSD), and atomic layer deposition (ALD). Their electrochromic behaviors were investigated using a three-electrode setup in basic (KOH solution, pH = 12) electrolyte. During bleaching/coloration process, the proton intercalation/deintercalation and simultaneous electron compensation in the NNO lattice under electrical bias led to crossover of the material between the pristine-conducting phase (Ni³⁺) and the strongly correlated insulating phase (Ni²⁺), which serves as the working principle for electrochromic (tunable opacity in the visible range) behavior. Cyclic voltammetry (CV) scans demonstrate that NNO films are electrochemically stable in basic solutions for all three film deposition methods explored here. CV scans at varying rates enabled the extraction of diffusion coefficient of protons in thin film NNO, which is ∼10^–7 cm² s^–1 among all films studied. Large light transmittance modulation by bleaching and coloration was observed on films grown on both LAO and FTO substrates, suggesting its potential as an electrochromic material candidate for smart windows and optical shutter applications. Porous NNO films obtained by chemical solution deposition tend to demonstrate stronger electrochromic activity than dense films grown by sputtering or ALD.